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CHAPTER 8

HYDROGEN BONDING ANALYSIS AND CHARGE TRANSFER INTERACTIONS OF 4-BENZYLOXY-2-NITROANILINE INSECTICIDE: A DENSITY FUNCTIONAL THEORETICAL STUDY IN COMBINATION WITH SPECTROSCOPIC TECHNIQUES

A widespread exploration on the intra-molecular charge transfer interaction through an

efficient π-conjugated path from a strong electron-donor group (amino) to a strong electron-

acceptor group (nitro) has been carried out using FTIR, FT-Raman, UV-vis, fluorescence and

NMR spectra on insecticide material 4-Benzyloxy-2-nitroaniline. Density functional theory

methods are used to determine optimized molecular geometry, bond orders, atomic charges,

harmonic vibrational wavenumbers and intensities using 6-31 G (D) basis set by means of

Gaussian 09 program suit. The observed vibrational wavenumbers were compared with the

calculated results. The assignments of the experimental spectra were made with the help of

normal coordinate analysis (NCA) following the scaled quantum mechanical force field

(SQMFF) methodology. A comprehensive investigation on the sp2 to sp3 hybridization and non-

planarity property has been performed. Natural bond orbital analysis is used to study the

existence of C-H…O, N-H…O and C-H…π proper and improper hydrogen bonds by

considering the inter-atomic distance, interaction energy, and charge transfer, which leads to the

molecular stability. The HOMO and LUMO analysis reveals the possibility of charge transfer

within the molecule. A complete assignment of the experimental absorption peaks in the

ultraviolet region of the electronic spectra of has been attempted. The solvent effect with

theoretical UV-visible spectra has been calculated using the time-dependent density

functional theory in combination with the polarized continuum model, and the results are

in good agreement with experimental values. Isotropic chemical shifts of 13C, 1H, 15N and

18O NMR and nuclear spin-spin coupling constants were calculated using the gauge-invariant

atomic orbital method.

169

8.1 Introduction

The benzothiozole system is a common component of many compounds with a wide

range of biological activities [Wei & Guang 2006]. Substituted amines are important

intermediates in the synthesis of benzothiozole derivatives. The title compound 4-Benzyloxy-2-

nitroaniline (BONA) is one such amine derivative [Wei & Hong 2007]. Aniline and its

derivatives have been studied extensively [Whysner et al. 1996; Krueger 1970; Palafox et al.

1993], since they are widely used in commercial and industrial purposes, including pesticide,

pharmaceuticals manufacturing and chemical dye industries. Nitro anilines share the common

feature of having an electron-rich (donor, -NH2) and an electron-deficient (acceptor, -NO2)

substituent, connected by a conjugated π electronic system. σitro anilines belong to push-pull

molecules due to the intra-molecular charge transfer (ICT) from the electron-donor group

through the phenyl ring to the electron-acceptor group.

The change in electron density (E.D) in the * antibonding orbitals and E(2) energies

were calculated by natural bond orbital (NBO) analysis using the DFT method to give clear

evidence about stabilization of various molecular interactions. The combined use of B3LYP

functional and standard basis set 6-31G (D) aided by the scaled quantum mechanical force

field (SQMFF) technique provides an excellent agreement between accuracy and

computational efficiency of vibrational spectra of bioactive molecules [Robert & Weitao

1994; Zhou et al. 2000; 2000a]. NMR spectroscopy is the most valuable tool for structure

elucidation in solution and a large body of data is available to rationalize the trends between

chemical shifts and molecular structure [Kalinowski et al. 1984]. The GIAO method was

utilized for calculating nuclear magnetic shielding tensors. Fluorescence spectroscopy is an

important investigational tool in many areas of analytical science, due to its extremely high

sensitivity and selectivity [Jihad 2007]. Different types of Hydrogen bonding, such as C-H…O,

N-H…O and C-H…π. are possible in the title compound either in its monomeric or polymeric

170

forms, which stabilizes the crystal structure of the compound [Juan & Fernando 2000;

Tarakeshwar et al. 2001; Hayvali et al. 2010].

Main scope of this work is to analyze the hydrogen bonding capabilities, intra-

molecular charge transfer (ICT), non-planarity and sp2 to sp3 hybridization in monomer,

dimer, trimer and stacking forms of the insecticide molecule BONA using UV-vis,

fluorescence, FT-Raman and FTIR spectra together with HOMO and LUMO energy

calculations and natural bond orbital (NBO) analysis together with the π-conjugation, electron

delocalization, hyper conjugation, steric effect and solvent effect, etc. All spectroscopic

properties examined by the experimental techniques were supported by computed results

using the density functional theory (DFT) and the time-dependent density functional theory

(TD-DFT) calculations. This paper also reports the NMR chemical shift () together with

nuclear spin-spin coupling constant (nJHH and nJCH).

8.2 Experimental

4-Benzyloxy-2-nitro aniline procured from Sigma-Aldrich (St. Louis, MO, U.S.A) was

used without further purification. The UV-vis absorption spectrum of the sample was recorded

in methanol, acetone and water solvents using Varian Cary 100 B10 UV-vis spectrophotometer.

Fluorescence spectrum was measured by means of F96-fluorospectrophotometer. 1H and 13C

NMR spectra were taken in BRUKER TPX-400 FT-NMR spectrometer. The IR spectrum of the

sample was recorded using a Perkin-Elmer Spectrum One FTIR Spectrometer in the region 400-

4000 cm-1 with samples in the KBr pellet. The resolution of the spectrum is 2 cm-1. NIR FT-

Raman spectrum in the range 50-3500 cm-1 was recorded using the powdered sample with 1064

nm line provided by Nd: YAG laser as the excitation source and analyzed on Bruker Vertex 70

FTIR-RAM-II FT-Raman module. The detector was a Ge diode cooled to liquid nitrogen

temperature. One thousand scans were accumulated with a total registration time of about 30

min. The spectral resolution after apodization was 2 cm-1.

171

8.3 Computational details

All the electronic structure calculations have been carried out using Gaussian '09

program package [Frisch et al. 2009]. The geometry is fully optimized at the Becke3-Lee-

Yang-Parr (B3LYP) level with standard 6-31G (d) basis set [Kurt et al. 2011]. Normal

coordinate analysis (NCA) was performed including the calculation of vibrational modes and

potential energy distribution in local symmetry coordinates, as well as IR intensities and Raman

activities corresponding to the scaled quantum mechanical force field (SQMFF). These

calculations were done with the MOLVIB program version 7.0 written by Sundius, 1990 &

2002. Following the procedure of SQMFF, the harmonic force field for this compound was

evaluated at B3LYP/6-31G(D) level of theory. The Raman activities (Si) calculated by Gaussian

‘0λ are converted to relative Raman intensities (Ii) using the following relationship derived from

the basic theory of Raman scattering [Keresztury et al. 1993; Keresztury et al. 2002] (eqn 4.1).

The natural bonding orbital calculation was done at the B3LYP/ 6-31G (d) in order to

investigate the optimized geometry corresponding to hydrogen bonding. The hyperconjugative

interaction energy was deduced from the second-order perturbation approach [Chocholousova et

al. 2004] (eqn 3.20). The HOMO-LUMO energy gap has been computed and analyzed. The

TD-DFT method is usually found to be a strong and accurate method for describing low-lying

excited states of conjugated molecules and has consequently been applied to solve countless

chemical and physical problems. Electronic transitions for the molecule were calculated using

TD-DFT method. Solvent effects were considered using the polarizable continuum model

(PCM) [Cossi et al. 1996], which takes in to account of the solute cavity in a wide range of

solvents of different polarity. 1H, 13C, 15N and 18O NMR chemical shifts together with nuclear

spin-spin coupling constants were computed using the gauge-invariant atomic orbital (GIAO)

method [Ditchfield 1972; Krzysztof et al. 1990]. Average polarizibility, heat of formation and

ionization potential were calculated by using MOPAC 09 program [James & Stewart 2009].

172

8.4 Results and Discussion

8.4.1 Optimized Geometry

Understanding the interaction of aniline with a substituent group in determining the

structural and vibrational properties of aniline derivatives [Mariana et al. 1999; Altun et al.

2003] are of great importance. The optimized molecular structures of BONA in different forms

are shown in figs. 8.1-8.4. The complete optimized geometrical parameters in monomer,

dimer, trimer and stacking form along with experimental values [Wei & Hong 2007] are

depicted in supplementary table 13. The asymmetry of the benzene ring is evident from the

negative deviation in bond angles of C1-C2-C3, C5-C7-C8 and positive deviation of the

remaining angles from the normal value of 120° due to the presence of NO2 and NH2 groups.

But in the benzyloxy group the ring is symmetric; the CCC angles are not affected from the

standard value 120°.

Fig. 8.1: The optimized structure of BONA monomer

173

Fig. 8.2: The optimized structure of BONA dimer

Fig. 8.3: The optimized structure of BONA trimer

174

Fig. 8.4: The optimized structure of BONA stacking

The dihedral angles H27-N25-C2-C1 (≃ -11.5°) and H26-N25-C2-C3 (≃ 15.0°) in monomer,

dimer and stacking structures show that amino group is not co-planar with phenyl ring, while in

trimer in order to accommodate two more similar molecules to form inter-molecular hydrogen

bonding with NO2 and NH2 groups the dihedral angles H89-N88-C2-C1 (0.0°) and H90-N88-C2-C3

(0.0°) adopt a perfect co-planar structure. This asymmetric interaction between the amino group

and the aromatic ring causes a small displacement in the nitrogen atom out of the benzene ring.

The nitro group is approximately co-planar with dihedral angles O30-N24-C1-C8 (- 178.7) and

O30-N24-C1-C2 (0.6°) in BONA. But in 4-methoxy-2-nitroaniline (MONA) [Ravikumar &

Hubertjoe 2010] the nitro and amino group are perfectly co-planar. The O29 -C7 bond of BONA

is almost co-planar with the phenyl ring with the dihedral angle 2.2° (C8-C7-O29-C10). In

MONA, the corresponding bond is exactly co-planar with the methyl group. The aromatic ring

of the nitrophenylamine group in BONA is twisted with respect to the benzyl ring system, with

a dihedral angle of 65.8° (O29-C10-C13-C22) between the ring planes [Wei & Hong 2007]. The

deviation from co-planarity in BONA indicates the phenoxy group involvement in the geometry

of the compound. The steric interaction of H9 with H11/H12 [H9…H12 ≃ 2.314 Å], the

delocalization electron at C10 due to the hyperconjugative interaction of H11 /H12 and with

175

benzene ring, the probable hydrogen bonds involvement in crystal structure, the presence of

phenoxy group and variation in optimized parameters contribute to the loss of co-planarity in

BONA molecule. This can be observed from the inter-atomic distance H9…H12 ≃ 2.314 Å and

bond angle C8-C7-O29 ≃ 125.6°. This leads to an increase in charge at benzyl ring hydrogen H12

(0.147 e) in comparison to nitro aniline hydrogen H9 (0.160 e) (table 8.1).

Table 8.1: The Mulliken and Natural charge distribution of BONA

Atoms

Atomic charges (Mulliken)(e)

Natural Charges (NBO)(e)

Mono mer

Dimer Trimer Stack ing

Mono mer

Dimer Trimer Stack ing

C1 0.163 0.151 0.169 0.162 0.031 0.011 0.031 0.031 C2 0.179 0.173 0.196 0.178 0.180 0.177 0.193 0.180 C3 -0.102 -0.118 -0.116 -0.104 -0.227 -0.219 -0.227 -0.229 H4 0.122 0.147 0.152 0.121 0.203 0.215 0.225 0.203 C5 -0.085 -0.093 -0.092 -0.079 -0.186 -0.191 -0.182 -0.184 H6 0.135 0.126 0.131 0.135 0.217 0.210 0.215 0.218 C7 0.110 0.104 0.112 0.098 0.293 0.286 0.292 0.295 C8 -0.130 -0.131 -0.126 -0.141 -0.278 -0.284 -0.285 -0.279 H9 0.160 0.152 0.159 0.187 0.245 0.238 0.243 0.249 C10 -0.073 -0.064 -0.080 -0.095 -0.045 -0.041 -0.046 -0.053 H11 0.136 0.132 0.135 0.180 0.177 0.171 0.177 0.192 H12 0.147 0.141 0.146 0.127 0.181 0.176 0.181 0.177 C13 -0.143 -0.154 -0.137 -0.177 -0.045 -0.035 -0.045 -0.047 C14 -0.072 -0.070 -0.071 -0.062 -0.196 -0.198 -0.195 -0.192 H15 0.113 0.111 0.114 0.112 0.200 0.197 0.201 0.205 C16 -0.125 -0.126 -0.125 -0.133 -0.191 -0.189 -0.191 -0.194 H17 0.120 0.117 0.120 0.127 0.202 0.198 0.202 0.205 C18 -0.106 -0.106 -0.106 -0.111 -0.197 -0.198 -0.196 -0.197 H19 0.121 0.118 0.121 0.142 0.201 0.198 0.201 0.218 C20 -0.125 -0.126 -0.124 -0.130 -0.190 -0.188 -0.191 -0.197 H21 0.121 0.118 0.120 0.121 0.201 0.197 0.201 0.203 C22 -0.043 -0.040 -0.042 -0.088 -0.190 -0.191 -0.188 -0.199 H23 0.117 0.117 0.116 0.111 0.210 0.206 0.207 0.201 N24 0.090 0.105 0.09 0.089 0.476 0.503 0.473 0.476 N25/N88 -0.513 -0.533 -0.538 -0.512 -0.723 -0.715 -0.704 -0.722 H26/H90 0.232 0.231 0.255 0.232 0.371 0.385 0.403 0.371

H27/H89 0.260 0.250 0.265 0.259 0.411 0.399 0.408 0.410 O28/O87 -0.242 -0.248 -0.272 -0.249 -0.385 -0.396 -0.436 -0.398 O29/O26/O30 -0.278 -0.273 -0.276 -0.274 -0.515 -0.513 -0.514 -0.513 O30/O59/O25/O29 -0.287 -0.283 -0.294 -0.287 -0.431 -0.430 -0.454 -0.430

176

In benzyloxy part O-C distance is increased due to extension of conjugation of

nitroaniline group to ether oxygen. The C-C bonds on the either side of the amino group (C1-C2

≃1.417/ C2-C3 ≃ 1.421 Å) are appreciably greater than the other C-C bonds in the ring (C3-C5

≃ 1.376; C5-C7 ≃ 1.412; C7-C8 ≃ 1.382; C8-C1 ≃ 1.411 Å) (supplementary table 13) due to

the resonance effects between the amino group and the phenyl ring. Since benzyloxy ring is

free from substitution, all C-C bond lengths are almost same. Also it is observed that the

carbon-carbon bond lengths of both benzene rings on the molecular skeleton are basically

intermediate between typical C-C (1.54 Å) and C=C (1.34 Å), it indicates that the π-

electrons in the molecule are delocalized.

The elongation of C-NO2 bond is due to single bond character. Conjugation of the

nitro group lone pair with the ring is particularly well illustrated by the lengthening of the

C1-N24 bond in monomer, dimer, trimer and stacking structures (≃1.443 Å). However, the

shortening of C-NH2 bond is due to the transfer of lone pair of electrons from the N atom to the

C atom, and thereby the C-N bond shows significant double bond character. For the C-NH2

moiety, an increase of C-N bond length favors sp3 hybridization, whereas the shortening of the

C-N bond length favors sp2 hybridization (table 8.2) in Nitrogen [Syed & Pant 1979]. The N25-

H27 bond is longer than the N25-H26 bond due to the intra- and inter-molecular N-H…O

hydrogen bonding.

Table 8.2: Formation of Lewis and non-Lewis orbitals

Nature Bond (A-B)

ED (e)

EDA % EDB % NBO s % p %

Monomer C1-C2 1.968 -0.645

49.96 50.04 0.7068 (sp1.67)C

+ 0.7074 (sp1.82)C

37.49 35.48

62.48 64.49

Dimer C1-C2 1.969 -0.636

49.96 50.04 0.7068 (sp1.67)C

+ 0.7074 (sp1.82)C

37.49 35.48

62.48 64.49

Trimer C1-C2 1.969 -0.638

49.96 50.04 0.7068 (sp1.67)C

+ 0.7074 (sp1.82)C

37.49 35.48

62.48 64.49

177

Stacking C1-C2 1.968 -0.644

49.97 50.03 0.7069 (sp1.67)C

+ 0.7074 (sp1.82)C

37.41 35.46

62.55 64.50

Monomer C3-C5 1.976 -0.653

50.10 49.90 0.7078 (sp1.67)C

+ 0.7064(sp1.70)C

37.47 37.07

62.49 62.88

Dimer C3-C5 1.975 -0.642

50.10 49.90 0.7078 (sp1.67)C

+ 0.7064(sp1.70)C

37.47 37.07

62.49 62.88

Trimer C3-C5 1.976 -0.651

50.10 49.90 0.7078 (sp1.67)C

+ 0.7064(sp1.70)C

37.47 37.07

62.49 62.88

Stacking C3-C5 1.976 -0.652

50.13 49.87 0.7080 (sp1.67)C

+ 0.7062(sp1.70)C

37.48 37.06

62.48 62.90

Monomer C7-O29 1.988 -0.818

32.49 67.51 0.5700 (sp2.98)C

+0.8217 (sp2.06)O

25.09 32.63

74.72 67.31

Dimer C7-O29 1.988 -0.815

32.33 67.67 0.5686 (sp2.99)C

+0.8226 (sp1.98)O

25.02 33.57

74.80 66.37

Trimer C7-O26 1.988 -0.815

32.46 67.54 0.5697 (sp2.98)C

+0.8218 (sp2.07)O

25.07 32.57

74.74 67.37

Stacking C7-O30 1.988 -0.815

32.62 67.38 0.5711 (sp2.96)C

+0.8209 (sp2.08)O

25.22 32.48

74.59 67.46

Monomer C10 -O29

1.987 -0.744

31.74 68.26 0.5634 (sp3.86)C

+0.8262(sp2.57)O

20.54 27.98

79.22 71.97

Dimer C10 -O29

1.988 -0.757

32.01 67.99 0.5657 (sp3.72)C

+0.8246(sp2.39)O

21.14 29.44

78.63 70.51

Trimer C10 -O26

1.987 -0.741

31.76 68.24 0.5635 (sp3.86)C

+0.8261(sp2.58)O

20.52 27.89

79.24 72.06

Stacking C10 -O30

1.986 -0.735

31.63 68.37 0.5624 (sp3.93)C

+0.8269(sp2.59)O

20.25 27.85

79.52 72.09

Monomer C13-C22 1.972 -0.621

50.67 49.33 0.7118 (sp1.88)C

+ 0.7024 (sp1.77)C

34.75 36.10

65.21 63.87

Dimer C13-C22 1.971 -0.617

50.56 49.44 0.7110 (sp1.87)C

+ 0.7032 (sp1.77)C

34.83 36.13

65.13 63.83

Trimer C13-C22 1.972 -0.620

50.63 49.37 0.7115 (sp1.88)C

+ 0.7024 (sp1.77)C

34.68 36.15

65.28 63.82

Stacking C13-C22 1.972 -0.624

50.55 49.45 0.7110 (sp1.85)C

+ 0.7032 (sp1.74)C

35.06 36.51

64.90 63.46

Monomer N24-O30 1.994 -0.963

48.32 51.68 0.6951 (sp2.24)N -0.7189 (sp3.09)O

30.80 24.40

69.10 75.47

Dimer N24-O59 1.994 -1.011

47.79 52.21 0.6913 (sp2.18)N -0.7226 (sp2.61)O

31.44 27.69

68.46 72.18

Trimer N24-O25 1.994 -0.963

48.23 51.77 0.6945 (sp2.29)N -0.7195 (sp3.10)O

30.37 24.36

69.52 75.52

Stacking N24-O29 1.994 48.31 51.69 0.6950 (sp2.25)N 30.72 69.17

178

-0.964 -0.7190 (sp3.09)O 24.43 75.45

Monomer C2-N25 1.989 -0.767

40.53 59.47 0.6366 (sp2.38)C -0.7712 (sp1.56)N

29.59 39.02

69.10 60.92

Dimer C2-N25 1.989 -0.750

40.72 59.28 0.6381 (sp2.34)C -0.7700 (sp1.56)N

29.94 39.01

70.00 60.93

Trimer C2-O88 1.989 -0.771

40.62 59.38 0.6374 (sp2.31)C -0.7706 (sp1.55)N

30.21 39.26

69.73 60.68

Stacking C2-N25 1.989 -0.766

40.50 59.50 0.6364 (sp2.38)C -0.7713 (sp1.56)N

29.57 39.02

70.36 60.93

8.4.2 Vibrational spectral analysis

The observed IR (fig.5.8a) and simulated IR (fig. 5.8b) and observed Raman (fig.5.9a)

and simulated Raman (fig. 5.9b) spectra of BONA are used for vibrational spectral analysis. A

non-redundant set of internal coordinates for DMPAA was defined (tables 8.3 & 8.4), which

was similar to the ‘natural coordinates’ recommended by Pulay et al. 1979. The calculated

wavenumbers were selectively scaled according to the SQM procedure incorporating a set of

transerable scale factors. The SQM wavenumbers related to the observed bands are presented in

table 8.5 along with detailed assignments. The calculated vibrational wavenumbers of monomer,

dimer, trimer and stacking structures are given in supplementary table 14. The detailed

analysis of vibrational wavenumbers for various functional groups is discussed below.

Fig. 8.5: IR spectra of BONA-a) experimental, b) computed

179

Fig. 8.6: Raman spectra of BONA-a) experimental, b) computed

8.3 Definitions of local symmetry coordinates for BONA

No Symbol Type Definition Stretching

1-3 ri CH(R1) C3-H4, C5-H6, C8-H9 7-9 Ri CC(R1) C1-C2, C2-C3, C3-C5, C5-C7, C7-C8, C8-C1 10 Qi CarO C7-O29

11, 12 Qi CN C2-N25, C1-N24 13 Qi CH2O C10-O29

14-19 Ri CC(R2) C13-C22, C22-C20, C20-C18, C18-C16, C16-C14, C14-C13 20-24 ri CC(R2) C22-H23, C20-H21, C18-H19, C16-H17, C14-H15

25 Ri CarC C10-C13 26, 27 Qi NH2 N25-H27, N25-H26 28, 29 Qi NO2 N15-O14, N15-O14 30, 31 ri CH2 C10-H12, C10-H11

Bending

32-37 i bCH(R1) C5-C3-H4, C2-C3-H4, C7-C5-H6, C3-C5-H6, C1-C8-H9, C7-C8-H9

38-41 i bCN(R1) C3-C2-N25, C1-C2-N25, C2-C1-N24, C8-C1-N24 42-51 i bCH(R2) C13-C22-H23, C20-C22-H23, C22-C20-H21, C18-C20-H21,

C20-C18-H19, C16-C18-H19, C18-C16-H17, C14-C16-H17, C16-C14-H15, C13-C14-H15

52, 53 i bCC(R2) C10-C13-C14, C10-C13-C22 54-59 i TRIDCC(R2) C14-C13-C22, C22-C20-C18, C18-C16-C14, C13-C22-C20,

C20-C18-C16, C16-C14-C13 60 i bOC C7-O29-C10

61-66 i TRIDCC(R1) C8-C1-C2, C2-C3-C5, C5-C7-C8, C1-C2-C3, C3-C5-C7, C7-C8-C1

67, 68 i CH2SCI H12-C10-H11, O29-C10-C13 69-72 i CH2ROC O29-C10-H12, C13-C10-H11, O29-C10-H11, C13-C10-H12 73-75 i NO2SCI O30-N24-O28, C1-N24-O30, C1-N24-O28

180

76-78 i NH2SCI H26-N25-H27, C2-N25-H27, C2-N25-H26 79, 80 i bOC(R1) C8-C7-O29, C5-C7-O29

Out-of-Plane

81-86 i WAGG(R1) H4-C3-C2-C5, H6-C5-C3-C7, H9-C8-C7-C1, O29-C7-C5-C8,N24-C1-C8-C2, N25-C2-C1-C3

87-92 i WAGG(R2) H23-C22-C13-C20, H21-C20-C22-C18, H19-C18-C20-C16,

H17-C16-C18-C14, H15-C14-C16-C13, C10-C13-C14-C22 93 i WAGGNO2 C1-N24-O28-O30

Torsion

94-99 i PUCK(R1) C8-C1-C2-C3, C2-C3-C5-C7, C5-C7-C8-C1, C1-C2-C3-C5 , C3-C5-C7-C8, C7-C8-C1-C2

100-104 i CNT1(R1) O28-N24-C1-C8, O28-N24-C1-C2, O30-N24-C1-C2, O30-N24-C1-C8

105-108 i CNT1(R1) H27-N25-C2-C1, H27-N25-C2-C3, H26-N25-C2-C3, H26-N25-C2-C1

109,110 i COT(R1) C8-C7-O29-C10, C5-C7-O29-C10 111-113 i CH2COT C7-O29-C10-H11, C7-O29-C10-H12, C7-O29-C10-C13 114-119 i CCT(R2) O29-C10-C13-C22, O29-C10-C13-C14, H11-C10-C13-C14,

H12-C10-C13-C22 , H11-C10-C13-C22, H12-C10-C13-C14 120-125 i PUCK(R2) C14-C13-C22-C20, C22-C20-C18-C16, C18-C16-C14-C13,

C13-C22-C20-C18, C20-C18-C16-C14, C16-C14-C13-C22

8.4 Definitions of internal valence coordinates of DMPAA

No Symbol Definition 1-3 R1CHST r1, r2, r3,

4-9 R1CCST R4, R5, R6, R7, R8, R9 10 R1OCST Q10

11,12 NH2CNST Q11, Q12

13 R2OCST Q13

14-19 R2CCST R14, R15, R16, R17, R18, R19

20-24 R2CHST r20, r21, r22, r23, r24 25 R2CCS2 R25 26 NH2SS (Q26+Q27)/ 2 27 NH2AS (Q26-Q27)/ 2 28 NO2SS (Q28+Q29)/ 2 29 NO2AS (Q28-Q29)/ 2 30 CH2S (r30+r31)/2

31 CH2AS (r30-r31)/2

32-34 R1BCH (32-33)/2, (34-35)/2, (36-37)/2

35-36 R1BCN (38-39)/2, (40-41)/2 37-41 R2BCH (42-43)/2, (44-45)/2, (46-47)/2,(48-49)/2, (50-51)/2

42 R2BCC (δ52-δ 53)/2 43 R2TRD (δ54+δ 55+ δ56-δ 57- δ58-δ 59)/6 44 R2ASD1 (2δ54+2δ 55+ δ56-δ 57- δ58-δ 59)/12 45 R2ASDO (δ57+δ 56-δ 54-δ 59)/4 46 OCHBE1 δ60 47 R1TRD (δ61+δ 62+ δ63-δ 64- δ65-δ 66)/6

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48 R1ASD1 (2δ61-δ 62- δ63-δ 64+2δ65-δ 66)/12 49 R1ASDO (-δ 62+ δ63+δ 64-δ 66)/4 50 CH2SCI (3.92267-68)/16 51 CH2XY (67-3.92268)/16 52 CH2ROC (69+70-71-72)/4 53 CH2WAG (69+71-70-72)/4 54 CH2TW (69-70-71+72)/4 55 NO2SCI ( 73- 74+ 75)/3 56 NO2ROC ( 74- 75)/2 57 NH2SCI (2 76- 77- 78)/6 58 NH2ROC ( 77- 78)/2 59 R1BOC ( 79- 80)/2

60-65 R1WAG 81, 82, 83, 84, 85, 86 66-1 R2WAG 87, 88, 89, 90, 91, 92 72 NO2WAG, 93 73 NH2WAG 94 74 R1PUCK (95+96-97-98-99-100)/6 75 R1ASTI (95-96+99-100)/4 76 RIASTO (-95-96+297+298-99-100)/12 77 R1CNTI (101+102+103+104)/ 4 78 R1CNT2 (105+106+107+108)/ 4 79 R1COT (109+110)/ 2 80 CH2COT (111+112+113)/ 3 81 R2CCT (114+1115+116-117-118-119)/ 6 82 R2PUCK (120+121+122-123-124-125)/ 6 83 R2ASTI (120-121-124-125)/ 4 84 R2ASTO (-120-121+122+123-2124-125)/12

Table 8.5: Vibrational assignment of BONA by NCA method

Calculated Wavenumber

(cm-1)

Experimental wavenumber(cm-1)

Intensity (a.u) Assignment with PED (%)

IR Raman Ai IR Ii R 3560 3476s 14 5 υNH2AS(98) 3432 3352vs 3357w 21 18 υNH2SS(98) 3150 2 4 βυC5H6R1(99) 3095 3162vw 1 16 βυC14H15R2(99) 3089 3117vw 3 39 βυC3H4R1(99) 3079 3091vw 9 7 β0aυC16H17R2(99) 3076 6 13 β0aυC14H15R2(99) 3065 3060w 4 14 β0aυC3H4R1(99) 3063 3041w 0 14 υCH2AS(88)+υCH2SS(11) 3056 3031vw 3037vw 2 4 υCH2SS(88)+υCH2AS(11) 2925 2926m 7 4 1γυC14H15R2(92) 2889 2890vw 6 20 1γυC14H15R2 (90) 1636 1640m 1639m 7 19 8bυC1C8R1(54)+υNO2AS(28)+R1ASDO(11) 1604 1604w 0 29 8aυC13C14R2(65)+R2BCH(21)

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1585 1596m 15 4 NH2SCI(54)+ NH2WAG(19)

1584 1576m 1573m 2 6 8bυC13C14R2 (56)+ NH2SCI

(16)+R2BCH(11) 1558 1533vw 59 14 8aυC1C2R1(50)+υNO2AS(13)+R1BCN(11) 1539 1513vs 1523vw 100 2 υNO2AS(65)+ NH2SCI(13)+ NH2WAG(11) 1491 3 1 CH2 sci (38)+R2BCH(33) 1486 2 4 CH2 sci (42)+ R2BCH(33)+R1BCH(11) 1475 1467w 1469w 21 2 CH2SCI (52)+R1BCH(23) 1445 1419s 4 2 19bυC14C16R2 (52)+R2BCH(35) 1415 1417m 18 11 19bυC1C8R1(46)+R1BCH(20)+NH2CNS(11) 1367 1387vs 1386vs 29 100 CH2WAG(69) 1357 27 38 14υC1C8R1(35) 1331 1336vs 1336vs 38 83 υC2N25(41)+υNO2SS(23)+14 υC7C8R1(13) 1315 1 1 3 H15C14C16R2(62) 1301 1 5 14υC13C22R2 (57)+ CH2ROC(26) 1276 1286m 1286s 31 40 3 H4C3C2R1(45)+υC2N25(27)+ NH2ROC(11) 1252 1252vs 1251m 44 14 υC2N25 (33)+R1BCH(24) 1229 1219vs 37 24 υO29C7R1(14)+ CH2ROC(34) 1222 1205vs 1203s 66 46 υO29C7R1(37)+ CH2ROC(26) 1191 1184vw 0 16 υC10C13 (55)+R2TRD(12)+ CH2WAG(11) 1167 1 8 λa H15C14C16R2 (79) 1148 1156s 1155m 0 6 15 H17C16C18R2 (83) 1143 9 6 15 H4C3C2R1(59)+ NH2WAG(22) 1078 1091s 1091m 2 1 15 H15C14C16R2(62)+CH2TW(23) 1071 1055vw 1055m 12 3 υC1N24(33)+ CH2WAG(31)+R1TRD(28) 1026 1026vw 5 6 NH2ROC(35)+NO2CNS(32) 1018 4 27 NH2ROC(31)+18b H15C14C16R2 (29) 1015 1016s 1004m 23 4 υC5C7R1(51) + NH2ROC(11) 995 990w 2 4 CH2TW(55)+R2WAGG(21) 979 1 35 1β C1C8C7 (51) 959 953m 952vw 0 1 5τH15C14C16C18R2(88)+R2PUCK(13) 937 5 21 11τH4C3C5C7R1(35)+NO2CNS(11) 932 0 0 17aτH15C14C16C18 R2(78)+R2ASTO(11) 922 923vw 0 1 11τH4C3C2C1R1(98)+R1PUCK(13) 889 908vw 1 3 17bτH15C14C16C18R2(84) 851 846s 852m 4 4 10aτH6C5C7C8R1(λ0)+ NO2SCI (11) 829 830s 0 9 10aτH15C14C16C18R2(98) 824 821m 5 23 10aτH15C14C16C18 (69) +CH2XY(22) 808 1 59 υC2N25(42)+ NO2SCI(33)+R1TRD(11) 794 795m 798m 7 13 10bτH4C3C5C7R1(76)+R1ASTO(23) 782 762w 9 7 NO2ROC(60)+R2ASDI(32) 734 748s 748m 5 2 NO2WAG(66)+R1WAGG(22)+R1PUCK(11) 730 8 1 11τH15C14C16C18R2(63)+ R2PUCK(13) 685 698s 700vw 4 4 11τH15C14C16C18 R2(51)+R2PUCK(11) 674 692vw 678vw 1 7 4τC13C14C16C18R2(48)+R1ASDI(26) 666 0 1 4τC1C8C7C5R1(67)+R1PUCK(11) 604 613m 611m 0 13 6aτC16C18C20C22R2(80)

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599 2 14 6bτC16C18C20C22R2(72)+OCHBE1(23) 564 578vw 1 3 τH26N25C2C1(48)+R1CNT2(23) 551 555w 0 6 τH26N25C2C1(68)+R1BOC(32) 528 507w 511vw 13 3 NH2TW(96) 493 486m 1 6 16bτC13C14C16C18R2(41)+R2ASDI(34) 475 0 23 16bτC13C14C16C18R2(25)+R2WAGG(23) 434 443vw 1 10 16aτC13C14C16C18R2(β6)+ NO2ROC (12) 424 414m 0 4 τC5C7O29C10 (66)+R1ASTI(23) 400 0 1 16aτC13C14C16C18R2(76)+R2WAGG(13) 393 1 34 NH2ROC(69)+ NO2ROC(13) 389 380vw 4 8 NH2ROC(53)+ NO2ROC(23) +NO2CNS(11) 354 0 7 NO2 rock (66)+R1PUCK(24) 317 13 5 τC2C1N24C8(58)+R2BCC(28) 294 289vw 53 20 τH26N25C2C1(43)+R1CNT2(13) 245 246w 7 29 C7O29C10 (48)+R1BCN(22)+R2ASTI(13) 229 192vw 0 31 τC10C13C14C16(48) +R2ASTI(11) 173 164vw 0 11 τC1C8C7O26(52) +R1WAGG(11) 138 152vw 1 28 τC8C5O29C7(48) +CH2COT(22) 128 100s 0 22 τC10C14C22C13(33)+CH2XY(25) 84 0 39 τC8C5O29C7(49)+R2CCT(26) 65 0 64 τC1C8C7O26(57)+R2WAGG(11) 53 0 52 τH26N25C2C1 (48)+R1BOC(27) 28 0 64 τC7O29C10C13 (72)+R1ASTO(11) 18 0 72 τC14C13C10O29 (69)+R2CCT(11)

8.4.2.1 Nitroaniline ring vibrations-Ph1

The possible normal modes of the asymmetric trisubstituted benzene rings have been

extensively studied by adopting Wilson’s numbering convention [Varsanyi 1969]. The

selection rule for asymmetric trisubstituted benzene derivatives allows three C-H stretching

vibrations. Modes 2 and 20a are expected in the region 3000-3120 cm-1. The mode 2

appears as a weak band at 3117 cm-1 in IR (table 8.3). The corresponding calculated values of

monomer, dimer, trimer and stacking structures are 3089, 3270, 3110 and 3096 cm-1 with a PED

of 99% (supplementary table 14). The mode 20 is observed in Raman at 3091 and 3060 cm-1.

The normal vibrations 8a, 8b, 19a, 19b and 14 are classified as the C-C stretching

vibrations. Normally in trisubstituted benzene, the mode 8a is lower than that of 8b and is

expected in the range 1560-1642 cm-1. The medium band observed at 1640 cm-1 in IR is

attributed to 8b mode and its counterpart in Raman occurs at 1639 cm-1. The 8a mode

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appears in Raman at 1533 cm-1. The mode 19b is identified as a strong band at 1419 cm-1 in

IR and medium band in Raman at 1417 cm-1. The mode 14 is expected in the range 1240-

1290 cm-1. In BONA mode 14 is observed as a very intense band at 1336 cm -1 in both IR

and Raman. The strong hyperconjugative interaction between the lone pair electron of O29 and

* (C7-C8) (table 8.6) antibonding orbital is reflected in both IR and Raman by showing an up

shift in C-C stretching mode 14. The simultaneous activation of modes 8a, 8b, 14 and 19b

provide evidences for the charge transfer interaction.

Table 8.6: Second order perturbation theory using NBO analysis of BONA

Nature Donor(i) ED(i)(e) Acceptor (j) ED(j)(e) E(2)kJ/mol

Monomer

π(C1-C2) 1.615 π*(C3-C5) 0.281 43.9 π*(C7-C8) 0.357 65.7

π(C3-C5) 1.758 π*(C1-C2) 0.482 64.7 π*(C7-C8) 0.357 53.2

π(C7-C8) 1.713 π*(C1-C2) 0.482 52.5 π*(C3- C5) 0.281 60.0

π(C13- C14) 1.662 π*(C16- C18) 0.326 63.5 π*(C20- C22) 0.315 61.8

n1(O28) 1.981 *(N24-O30) 0.065 7.2 n2(O28) 1.883 *(N24-O30) 0.065 71.6 n1(O28) 1.981 *(C1-N24) 0.098 17.8 n2(O28) 1.883 *(C1-N24) 0.098 46.2 n1(O29) 1.960 *(C7- C8) 0.031 27.5 n1(O30) 1.976 *(N24-O28) 0.057 4.2 n2(O30) 1.891 *(N24-O28) 0.057 70.2 n1(N25) 1.724 *(C1-C2) 0.033 160.5

Dimer

π(C1- C2) 1.639 π*(C3- C5) 0.257 41.9 π*(C7- C8) 0.348 64.4

π(C3- C5) 1.774 π*(C1- C2) 0.469 60.7 π*(C7- C8) 0.348 53.2

π(C7- C8) 1.739 π*(C1- C2) 0.469 48.5 π*(C3- C5) 0.257 57.0

π(C13- C14) 1.664 π*(C16- C18) 0.327 65.0 π*(C20- C22) 0.316 62.6

n1(O28) 1.980 *(N24-O59) 0.065 7.9 n2(O28) 1.881 *(N24-O59) 0.065 76.8 n1(O28) 1.980 *(C1-N24) 0.098 17.7 n2(O28) 1.881 *(C1-N24) 0.098 56.4 n1(O29) 1.958 *(C7- C8) 0.030 28.1 n1(O59) 1.976 *(N24-O28) 0.057 5.8 n2(O59) 1.890 *(N24-O28) 0.057 77.0 n1(O57) 1.975 *(N53-O56) 0.057 7.1

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n2(O57) 1.888 *(N53-O56) 0.057 72.9 n1(N25) 1.712 *(C1-C2) 0.033 177.7

Trimer

π(C3- C5) 1.675 π*(C7- C8) 0.249 53.4 π(C7- C8) 1.675 π*(C3- C5) 0.345 56.9

π(C13- C14) 1.663 π*(C16- C18) 0.325 63.3 π*(C20- C22) 0.314 62.3

n1(O25) 1.976 *(N24-O87) 0.059 4.8 n2(O25) 1.890 *(N24-O87) 0.059 68.4 n1(O26) 1.961 *(C7- C8) 0.029 27.3 n1(O87) 1.975 *(N24-O25) 0.062 9.5 n2(O87) 1.888 *(N24-O25) 0.062 61.7 n1(O87) 1.976 *(C1-N24) 0.085 14.2 n2(O87) 1.890 *(C1-N24) 0.085 46.1 n1(O54) 1.975 *(N50-O55) 0.062 9.8 n2(O54) 1.888 *(N50-O55) 0.062 61.9 n1(O84) 1.975 *(N80-O85) 0.065 6.9 n2(O84) 1.888 *(N80-O85) 0.065 70.8

Stacking

π(C1- C2) 1.613 π*(C3- C5) 0.280 43.6 π*(C7- C8) 0.356 65.0

π(C3- C5) 1.758 π*(C1- C2) 0.484 65.1 π*(C7- C8) 0.356 52.9

π(C7- C8) 1.710 π*(C1- C2) 0.484 53.0 π*(C3- C5) 0.280 60.3

π(C13- C22) 1.710 π*(C14- C16) 0.484 61.8 π*(C18- C20) 0.280 62.0

n1(O28) 1.980 *(N24-O29) 0.064 7.1 n2(O28) 1.885 *(N24-O29) 0.064 70.9 n1(O28) 1.980 *(C1-N24) 0.096 17.7 n2(O28) 1.885 *(C1-N24) 0.096 45.3 n1(O30) 1.961 *(C7- C8) 0.030 27.7 n2(O30) 1.827 *(C10-H11) 0.028 17.5 n2(O30) 1.827 *(C10-H12) 0.028 21.7 n1(N25) 1.724 *(C1-C2) 0.033 159.8

8.4.2.2 Benzyloxy ring vibrations-Ph2

The C-H stretching wavenumbers of mono substituted phenyl rings are expected in the

region 3120-3010 cm-1. Normal vibrations 2, 7a, 7b, 20a and 20b are classified as the C-H

stretching vibrations. The weak band in IR at 3162 cm-1 corresponds to mode 2. Blue shifting

(42 cm-1) of the C22-H23 stretching wavenumber is due to the presence of intramolecular

hydrogen bonding obtained by computed wavenumbers in dimer, trimer and stacking structures.

The mode 20a is observed as a weak band in Raman at 3091 cm-1. The weak bands observed at

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2926 and 2890 cm-1 in IR are attributed to mode 13. Most of the C-H stretching modes are found

to be weak, which is due to the charge transfer from the hydrogen atom to the carbon atom. The

selection rule for mono substituted benzene ring allows five C-C stretching modes: 8a, 8b, 19a,

19b and 14. The vibrational mode 8a is identified in Raman at 1604 cm-1. The DFT computation

shows that the corresponding wavenumbers for monomer, dimer, trimer and stacking lie at

1604, 1772, 1611 and 1602 cm-1. The blue shifting of C-C stretching 8a mode in dimer clearly

exhibits the higher degree of conjugation in the side chain. The medium band observed at 1573

and 1576 cm-1 in Raman and IR is identified as 8b mode. The mode 19b of mono substituted

phenyl rings are expected in the region 1470-1440 cm-1. In Raman spectrum the medium band

observed at 1417 cm-1 is correlated to the 19b mode. The simultaneous activation of 8a, 8b and

19b mode clearly exhibits the intra-molecular charge transfer, which leads to the enhanced

insecticidal activity of the compound.

8.4.2.3 Amino group vibrations

Generally hydrogen bonding leads to a lowering in NH2 stretching wavenumber, an

increase in NH2 bending wavenumber and an increase in intensity [Smith 1999]. The

stretching of aromatic primary amines have two bands, which occurs at 3500 and 3400

cm-1 for asymmetric and symmetric stretching vibrations. The asymmetric NH2 stretching is

observed in IR as a very strong band at 3476 cm-1. Symmetric NH2 stretching is observed as a

strong band in IR at 3352 and weak in Raman at 3357 cm-1. The red shifting (48 cm-1) of NH2

stretching wavenumber is due to the formation of strong intra- and inter-molecular N-H…O

hydrogen bonding (supplementary table 15). Further, red shifting is supported by the reduction

in the N-H bond order values (table 8.7), occurring due to donor-acceptor interaction. The NH2

scissoring and rocking deformations occur at 1600-1700 and 900-1150 cm-1. The intense

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band observed at 1513 in IR and weak band in Raman at 1523 cm-1 and medium band at 1576

(IR) and 1573 cm-1 (Raman) (table 8.3) are ascribed to NH2 scissoring vibration.

Table 8.7: Bond order of BONA

Bond Bond Order

Monomer Dimer Trimer Stacking C1-C2 1.273 1.082 1.246 1.271 C2-C3 1.270 1.082 1.242 1.271 C3-C5 1.542 1.209 1.573 1.542 C5-C7 1.274 1.047 1.247 1.272 C7-C8 1.438 1.188 1.464 1.436 C8-C1 1.303 1.052 1.272 1.300 C1-N24 0.997 0.830 1.044 1.002 N24-O30/ N24-O59/N24-O25/N24-O29 1.406 1.079 1.386 1.401 N24-O28/N24-O87 1.464 1.101 1.405 1.453 C2-N25/C2-N88 1.227 1.017 1.287 1.227 N25-H26

/N88-H90 0.839 0.724 0.785 0.839

N25-H27/N88-H89 0.786 0.723 0.783 0.786 C3-H4 0.923 0.806 0.909 0.923 C5-H6 0.917 0.807 0.918 0.917 C8-H9 0.896 0.798 0.898 0.894 C7-O29/C7-O26/C7-O30 1.004 0.853 1.004 1.011 O29-C10/O26-C10/O30-C10 0.887 0.733 0.885 0.878 C10-H11 0.926 0.801 0.927 0.913 C10-H12 0.912 0.796 0.914 0.928 C10-C13 1.015 0.848 1.014 1.014 C13-C14 1.409 1.136 1.410 1.387 C14-C16 1.430 1.134 1.429 1.451 C16-C18 1.442 1.147 1.443 1.422 C18-C20 1.426 1.135 1.425 1.442 C20-C22 1.448 1.146 1.450 1.428 C22-C13 1.390 1.119 1.389 1.410 C14-H15 0.924 0.810 0.924 0.922 C16-H17 0.925 0.787 0.925 0.924 C18-H19 0.925 0.787 0925 0.917 C20-H21 0.925 0.787 0.925 0.925 C22-H23 0.921 0.808 0.922 0.924

8.4.2.4 Nitro group vibrations

Aromatic nitro compounds have strong absorptions due to the asymmetric and

symmetric stretching vibrations, which appears in the range 1485-1570 and 1320-1370 cm-1

respectively [Socrates 2001]. The bands at 1640 and 1513 cm-1 (table 8.3) in IR and 1639 and

1523 cm-1 in Raman are correlated to NO2 asymmetric stretching mode. Observed strong band

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at 1336 cm-1 in IR and Raman are attributed to NO2

symmetric stretching. The intensity

enhancement of NO2 stretching wavenumbers is due to conjugation with the phenyl ring.

8.4.2.5 O-CH2 group vibrations

The methylene vibrational band positions are most consistent, when this group is

attached to other carbon atoms. When the CH2 group is directly attached to an oxygen atom, the

stretching and bending bands can shift the position due to electronic effects. For the aryl

methylenoxy group, the methylene asymmetric stretching bands occur in the region 2920-2955

cm-1 [Smith 1999]. The methylene asymmetric stretching vibration is observed as weak band in

Raman at 3041 cm-1, the corresponding calculated values of monomer, dimer, trimer and

stacking lie at 3063, 3231, 3067 and 3057 cm-1 respectively. The methylene symmetric

stretching bands are generally observed in the region 2835-2878 cm-1. In BONA the

methylene symmetric stretching vibration is observed as weak band at 3031 (IR) and 3037 cm-1

(Raman). There is a blue shifting of 86 (asymmetric) and 153 cm-1 (symmetric) in CH2

stretching mode. The lone pair electron of oxygen interact strongly with the nitroaniline ring, in

order to par with the decrease in electron density in oxygen, the C-H bond provide electron to

O-C bond by which there is hyperconjugation. Due to this there is an extended conjugation in

benzene rings and the angle in these places (CH2 and O) deviate from 109o to 120o

(supplementary table 13), the bonds become stronger and then there is a probability for blue

shifting. The blue shifting of 102 (asymmetric) and 169 cm-1 (symmetric) in the calculated C-H

stretching of stacking structure shows that there is weak C-H…π interaction (supplementary

table 15). The O-CH2 wagging vibrations are observed generally in the region 1360-1400 cm-1

[Lin et al. 1991]. The O-CH2 wagging vibrations are observed as very intense band in IR and

Raman at 1387 and 1386 cm-1. NCA predicts the wavenumber at 1367 cm-1 with very intense in

Raman band (100) and PED (69%)

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8.4.3 NBO analysis

The NBO analysis provides a description of the structure of a compound by a set of

localized bond, antibond and Rydberg extravalence orbitals to identify and confirm the possible

C-H…O inter-molecular, N-H…O inter- and intra-molecular and C-H…π stacking interactions

between the units that would form the proper and improper hydrogen bonding. In order to

investigate the various second-order interaction between the filled orbitals of one subsystem and

vacant orbitals of another subsystem the DFT level computation has been used, and it predicts

the delocalization or hyperconjugation [Yang et al. 2006]. The second-order perturbation theory

analysis of Fock matrix in NBO shows strong intra- and inter-molecular hyperconjugative

interactions, and is presented in supplementary table 15.

The interactions result in a loss of occupancy from the localized NBO of the idealized

Lewis structure into an empty non-Lewis orbital, which are presented in tables 8.2 & 8.6. A

number of C-H…O and N-H…O complexes have been investigated [Herrebout et al. 2005;

Hiroatsu et al. 2003; Harada et al. 2002], in which one of the interesting aspects is the

wavenumber red shift in the N-H stretching band and blue shift in the C-H stretching band.

The NBO analysis clearly shows the existence of strong N-H…O and C-H…O inter-

molecular hydrogen bonds in BONA dimer and trimer structures. The inter-molecular N-H…O

hydrogen bonding is formed owing to the orbital overlap between n2(O57) and *(σ25-H27) with

E(2) value 1.3 kJmol-1 (supplementary table 15) in dimer and between n1(O54) and *(σ88-H90)

with E(2) value 7.2 kJmol-1 in trimer, which results in ICT, causing stabilization of H-bonded

systems. Thus, the nature and strength of inter-molecular hydrogen bonding can be explored by

studying the changes in electron density (ED) in the vicinity of N-H…O hydrogen bonds, which

leads to an increase in ED of N-H antibonding orbital. The increase in population of the N-H

antibonding orbital weakens the N-H bond, which is due to the accumulation of the electron

density in the N-H bond drawn not only from n(O) of the hydrogen acceptor but also from the

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entire molecule leading to its elongation and the concomitant red shift in the N-H stretching

wavenumber [Alabugin et al. 2003] which is substantiated by vibrational analysis. The existence

of inter-molecular C-H…O hydrogen bonds due to the interaction between the lone pair of

oxygen n(τ) and antibonding orbital *(C-H) has been confirmed by the results of σBτ

analysis. Notwithstanding the fact that the energetic contribution of hyperconjugative

interactions is weak, the E(2) values for dimer, trimer and stacking structures are 0.8, 2.1 and 0.3

kJmol-1 respectively. These values are chemically significant and are used as a measure of the

inter-molecular delocalization. The strengthening and contraction of the C-H bonds is due to

rehybridization [Alabugin et al. 2003], which is revealed by the low value of electron density

≃0.016 e (table 8.1) in the *(C3-H4) orbital with a blue shifting in wavenumber as

substantiated by vibrational analysis. The most important interaction (n→*) energies related to

the resonance in the molecule are electron donation from the n1(N) atoms of the electron

donating group to the antibonding acceptor *(C-C) of the phenyl ring i.e. n1(N25) → *(C1-C2)

= 160.5 kJ mol-1 with low electron density (1.724 e) due to small energy differences between

donor and acceptor in monomer. In dimer the corresponding value is 177.7 kJ mol-1. This

increase in energy is due to strong inter-molecular N-H…O hydrogen bonding.

In the present study, a six-membered ring involving the strong intra-molecular N-H…O

hydrogen bonding is exposed in the NBO analysis result (supplementary table 15) by the

interaction between the oxygen lone pair and N-H antibonding orbital n1(O30) → *(σ25-H27)

having hyper conjugative interaction energy ≃7.2 kJmol-1. In MONA this value is much reduced

(3.2 kJmol-1) due to the presence of methyl group in the place of benzene ring. Red shifting in

N-H stretching wavenumber as shown by vibrational spectral analysis is revealed by the high

value of electron density ≃ 0.0β8 e in the *σ-H orbital. The NBO results, optimized

geometrical parameters and corresponding down shift in N-H stretching wavenumber predict the

191

red shift of hydrogen bonding. The intra-molecular H27…O30 distance is ≃ 1.915Å. This

distance is significantly shorter than that of the van der Waals separation between the τ atom

and the H atom (2.72 Å) [Harumi 2005] indicating the existence of the strong N-H…O

interaction in BONA.

In addition to the six-membered intra-molecular N-H…O hydrogen bonding there is

intra-molecular *C-H hyperconjugative interactions observed by the NBO analysis. This is due

to the interaction between lone pair of oxygen atom n1(O29) of benzyloxy group and antibonding

orbital *(C10-H11) bond of CH2 group having E(2) value ≃2.4 kJmol-1. In MONA the

corresponding value is found to be low (1.63 kJmol-1). Due to the presence of methylene group

the hydrogen atom acts as an electron acceptor so that there is hyperconjugation. Blue shifting in

C-H stretching wavenumber in the *(C10-H11) orbital is due to rehybridization. These

interactions have been shown to be of greater importance in biological systems in order to

elucidate the structure-activity relationship [Ribeiro et al. 2002a]. The intra-molecular H…O

distance of H11…O29 is ≃ 2.080Å.

The interactions between π-system have been the focus of deep analysis both at

experimental and theoretical levels [Seiji et al. 2002]. The C-H…π weak hydrogen bond is the

main inter-molecular force stabilizing the molecule. The X-H…Y(π) contacts can be

characterized by blue shifts of X-H stretching wavenumber. Juan & Fernando 2000 have shown

that C-H…π contacts are proper hydrogen-bonds. The stacking structure shows that the

hydrogen of methylene interacts with the π cloud of benzene forming H…π complex, where

methylene is the electron acceptor. The C-H distance between the two sub molecules in the

methylene-benzene complex is about 3.1 Å. C-H…π complexes show a high symmetryν the

bond path connects the C-H hydrogen with the centre of the C-C bond or the middle of the ring.

The C-H…C contacts in BONA can then be characterized as C-H…π interactions. A C-H…π

192

interaction with C…C distance, 3.884 Å between each benzene ring of the CH2 (C-H unit) and

the phenyl (π unit ) components are observed with a blue shifting of CH2 stretching in the

stacking structure, which is substantiated by vibrational analysis.

The intra-molecular hyper conjugative interactions are formed by the orbital overlap

between π(C-C) and π*(C-C) bond orbitals, which results in an ICT causing stabilization of the

system. In monomer, dimer, trimer and stacking interactions of nitro aniline ring of BONA, the

contributions of stabilization energies for the π(C3-C5)→π*(C7-C8) are ≃ 53.2 kJmol-1 (table

8.6). But in MONA the subsequent energy is 37.2 kJmol-1. In monomer, dimer, trimer and

stacking interactions in benzyl ring of BONA, the contributions of stabilization energies for the

π(C13-C14)→π*(C16-C18) are ≃ 63.5 kJmol-1. The hyperconjugative interaction between ether

oxygen and the π*(C7-C8) antibonding orbital is ≃ 27.5 kJmol-1. But in MONA the consequent

value is 55.6 kJmol-1. This variation is due to the presence of electron donor methyl group in

MONA, but in BONA there is electron accepting benzyl group. The p-character of (σ-O) in

monomer, dimer, trimer and stacking structures are ≃ 2.2 (table 8.2), the corresponding N-O

bond length of nitro group closer to NH2 group (≃ 1.234 Å) (supplementary table 13) is shorter

while comparing to other N-O bond. This result suggests that the N-O bond length of BONA is

essentially controlled by the p-character of these hybrid orbitals and also by the nature of the N-

O bond.

8.4.4 Electronic absorption spectral analysis

The electronic absorption spectrum of BONA was measured at room temperature in

methanol, acetone and water solvent environment. The experimental absorption bands were

assigned using the spin-allowed singlet-singlet electronic transition calculated in the framework

193

of PCM-TDDFT. The experimental and computed UV-visible absorption spectrum of BONA in

different solvents is shown in figs. 8.7a (experimental) & 8.7b (computed).

Fig. 8.7: UV-visible absorption spectra of BONA in (I) acetone, (II) methanol and (III) water

Fig. 8.8: HOMO-LUMO plots of BONA

The distribution and energy levels of 3D plots of the frontier orbitals [Lewis et al. 1994;

Gokhan 2008] for the ground state are shown in fig. 8.9. Each calculated transition is

194

represented by a Gaussian function with the height (c) equal to the oscillator

strength and b equal to 0.04 nm-2.

Table 8.8: Electronic absorption spectral data of BONA

Solvents

Transi tion

feature

Oscillator strength

C.I Expansion Coefficient

Excita tion

Energy (e.V)

Calcul ated

Wave length (n.m)

Exper imental Wave length (n.m)

Dipole Moment

(D)

Assign Ments

Vacuum H →δ 0.11 0.70 2.63 471 -

6.03 n→π*

H-5→ δ 0.00 0.69 3.82 324 - π→π* H-1→δ 0.00 0.70 3.91 314 - π→ *

Acetone H→δ 0.11 0.70 2.63 470 441

5.98 n→π*

H-5→δ 0.00 0.68 3.81 325 - π→π* H-1→δ 0.00 0.70 3.91 317 - π→ *

Acetonitrile

H →δ 0.11 0.70 2.63 471 - 6.03

n→π* H-5→δ 0.00 0.69 3.82 325 - π→π* H-1 → δ 0.00 0.71 3.91 317 - π→ *

Benzene H →δ 0.11 0.70 2.70 459 -

5.27 n→π*

H-5→δ 0.00 0.69 3.75 330 - π→π* H-1→δ 0.00 0.71 4.01 309 - π→ *

Chloroform

H→δ 0.11 0.70 2.66 465 - 5.65

n→π* H-5→δ 0.00 0.48 3.78 328 - π→π* H-1→δ 0.00 0.71 3.95 314 - π→ *

Ethanol H→δ 0.11 0.70 2.63 471 -

6.00 n→π*

H-5→ δ 0.00 0.69 3.82 325 - π→π* H-1→δ 0.00 0.70 3.91 317 - π→ *

Toluene H→δ 0.11 0.70 2.70 459 -

5.30 n→π*

H-5→δ 0.00 0.69 3.75 330 - π→π* H-1→ δ 0.00 0.70 4.01 309 - π→ *

Methanol H→δ 0.10 0.70 2.63 471 444

6.02 n→π*

H-5→δ 0.00 0.69 3.82 325 284 π→π* H-1→δ 0.00 0.70 3.91 317 214 π→ *

Water H→δ 0.10 0.70 2.63 471 410

6.06 n→π*

H-5→δ 0.00 0.69 3.82 324 270 π→π* H-1→δ 0.00 0.70 3.91 317 - π→ *

Chloro Benzene

H→δ 0.11 0.70 2.64 468 - 5.72

n→π* H-5→δ 0.00 0.60 3.79 327 - π→π* H-1→δ 0.00 0.70 3.94 314 - π→ *

Diethyl Ether

H→δ 0.11 0.70 2.68 463 - 5.60

n→π* H-5→δ 0.00 0.38 3.78 328 - π→π* H-1→δ 0.00 0.70 3.96 313 - π→ *

Dichloro Methane

H→δ 0.11 0.70 2.64 469 - 5.84

n→π* H-5→δ 0.00 0.67 3.80 321 - π→π* H-1→δ 0.00 0.70 3.93 316 - π→ *

Cyclohexane

H→δ 0.11 0.70 2.72 456 - 5.19

n→π* H-5→δ 0.00 0.69 3.75 331 - π→π* H-1→δ 0.00 0.71 4.03 305 - π→ *

195

Nitro-substituted amines show absorption peaks in the range 400-700 nm [Pinheiro et

al. 2004]. Experimental UV-Vis spectrum shows three prominent bands at 444, 284 and 214

nm in methanol solvent (table 8.8). In ground state, the charge density is mainly accumulated

on the NO2 fragment in case of HOMO. The broadening and intensity enhancement of UV-vis

absorption spectra clearly exhibits the CT interaction. The calculation shows that the frontier

level Hτετ is at molecular orbital number 64 with ‘A’ symmetry and δUετ is at 65 with the

same ‘A’ symmetry. In BONA under methanol environment it is observed that the first peak

originates from the n-π* transition due to the intra-molecular charge transfer from NH2 to NO2.

The first intense peak at 471nm (64A→65A) dominantly describable with Hτετ-LUMO

excitation, has large oscillator strength (f) of 0.1 with a bathochromic shift (red shift) of 27 nm

due to the presence of nitro group. The next allowed transitions are at 325 nm (60A→65A)

and 317 nm (63A→65A) with very less oscillator strengths. In case of acetone

environment a strong transition is observed at 441 nm. The corresponding calculated value lies

at 470 nm with a bathochromic shift (red shift) of 29 nm. In MONA the absorption position is

identified as broad intense band at 440 nm. Also for BONA in water environment the intense

transition is observed at 410 nm. The calculation shows that the corresponding value lies at 471

nm. In water solvent bathochromic shift (red shift) is more (61 nm). In the dipole moment

calculation it is observed that there will be a strong interaction when the dipole moment is more.

This can be observed when the solvent is more polar (water).

In BONA it is observed that the first intense dipole allowed transition is calculated at

471 nm (H→δ) in vacuum, acetonitrile and ethanol by a number of transitions with high

oscillator strength (0.11). In cyclohexane, benzene, toluene, diethyl ether, chloroform,

chloromethane and dichloromethane the same transition is calculated at 456, 459, 459, 463, 465,

469 and 469 nm respectively. This shift in wavelength does not agree with the change in the

polarity of the organic solvents and therefore, it can be considered as a result of combination in

several solvent characteristics such as polarity, basicity and hydrogen bond accepting ability.

196

Generally, the energy values of LUMO and HOMO and their energy gap reflect the chemical

reactivity [Mills et al. 2004] and kinetic stability of the molecule. Spatial distribution of

molecular orbitals, HOMO and LUMO, are excellent indicators of electron transport in

molecular systems. A molecule with a small frontier orbital gap is more polarizable and is

generally associated with a high chemical reactivity, low kinetic stability and is also termed as

soft molecule [Fleming 1976]. The HOMO-LUMO energy gap of BONA in the monomer,

dimer, trimer and stacking structures are given as 258, 272, 231 and 257 kJmol-1 (table 8.9).

The calculated HOMO and LUMO energies clearly show that the charge transfer occurs within

the molecule. The conjugated molecules are characterized by a small HOMO-LUMO separation,

which is the result of a significant degree of ICT from the end-capping electron-donor groups to

the efficient electron-acceptor groups through -conjugated path. The large stabilization of the

LUMO due to the strong electron-accepting ability in BONA with low HOMO-LUMO gap (258

kJmol-1) account its bioactivity by which the insecticidal activity. Since MONA is a non-linear

optical crystal with high efficiency the HOMO-LUMO energy gap is calculated at higher value

(276 kJmol-1) than BτσA. It is worth noting that Hτετs have an overall π-bonding character

and LUMOs have antibonding π* character and are highly delocalized over the whole molecule.

The strong charge transfer interaction through -conjugated bridge results in substantial ground

state donor-acceptor mixing. Consequently, an ED transfer occurs from the more aromatic part

of the -conjugated system in the electron-donor side to its electron-withdrawing part. The

HOMO is mainly located on NH2, whereas the LUMO is on NO2 as depicted in fig. 8.8. Plot

of the Hτετ and δUετ show that π-electron transfer takes place from the region of NH2 to

NO2 followed by redistribution of electron charge. This may lead to an increase in the C 1-N24

bond length and a decrease in the C2-N25 (supplementary table 13) bond length on electronic

excitation due to ICT from NH2 to NO2, where the electron density increases mainly in the

NO2 moiety and decreases in NH2 moiety.

197

Table 8.9: SCF Energy and other parameters of BONA

Nature Zero point Vibrational energy(kJmol-1)

Dipole Moment (D)

S C F Energy (kJmol-1)

HOMO-LUMO energy Gap kJmol-1

Monomer 614.6 4.1 -2199329 258 Dimer 1324.6 9.4 -4372245 272 Trimer 1853.7 20.2 -6597094 231

Stacking 1231.4 - -4397713 257

The interpretation of the observed spectral features are greatly assisted by molecular orbital

calculations, which provide orbital energies and furnish a detailed description of orbital

including spatial characteristics, nodal patterns and individual atom contributions. The lowest

singlet transition for BONA is the transition from the HOMO orbital to LUMO orbital, and

the geometrical relaxation can be understood by analyzing the nodal patterns of the HOMO

and LUMO orbitals. The nodal patterns of the HOMO and LUMO orbitals are shown in fig.

8.8. The analysis of the electron density of HOMO and LUMO of BONA can throw some

light on the ground and excited state proton transfer process. HOMO of BONA predicts that

intra-molecular hydrogen bonded ring system is primarily of bonding character over the C5-

C7-C8, C1-C2-C3, N25-H26-H27 bonds and antibonding character over N24. All oxygen atoms

have bonding character with larger electron density at O30 (-0.287 e) (table 8.1) due to intra-

molecular hydrogen bonding. Electronic charge distribution in LUMO shows that C2-C3, C1-

N24, N25, C5, C8, O28 and O30 atoms have bonding character and C7 and O29 have antibonding

character. This antibonding character of O29 is due to hyperconjugation as mentioned in NBO

analysis, and this also shows that charge transfer takes place from amino group to nitro

group, but LUMO has nodes across these bonds. As a consequence C5-C7, C7-C8, C3-C2 and

C2-C1 bonds (0.006, 0.018, 0.006 and 0.015 Å) (supplementary table 13) are elongated,

whereas N24 is having anti-bonding character.

198

8.4.5 Fluorescence emission spectral analysis

Fluorescence usually occurs from the S1 to the S0 electronic state by emission of

photons. The 00 transitions is generally the same for absorption and fluorescence. According

to Stoke’s rule, the fluorescence maximum is always located at higher wavelengths than the

absorption maximum because of the loss in energy due to vibrational relaxation. The gap

between the maximum of the first absorption band and the maximum of the corresponding

fluorescence band is called the Stokes shift. Most fluorophores undergo an intra-molecular

charge transfer up on excitation. In such cases, the relaxed excited state S1 will be energetically

stabilized relative to the ground state S0 and a significant red shift of the fluorescence band will

be observed. Fluorescence quenching [Ana et al. 2001; Jonathan 2005] is referred to any process

which decreases the fluorescence intensity of a given substance. The absorption spectrum for

BONA shows a broader peak observed at 441 nm. The fluorescence intensity of this molecule is

at 471 (Counts per sec) (fig. 8.9). The corresponding Stoke’s shift is γ0 nm. The calculated

intense (f=0.111) transition (6465) wavelength is 468nm (table 8.10). This fluorescence

emission is assigned to n-π* state, which may be attributed to the essence of the σ-H…O and C-

H…O hydrogen bond that increases the inter- and intra-molecular charge transfer (ICT). In the

analyte-free form, upon excitation by photons the electron of HOMO in the fluorophore is

promoted to the local LUMO; subsequently, the electron seated on the HOMO of the donor

transfers to the HOMO of the fluorophore synergistically (electron transfer), which causes

fluorescence quenching.

Table 8.10: Fluorescence emission spectral data of BONA

Solvent Tran sition

feature

Oscill ator

strength

C.I Expansion Coefficient

Excitation Energy (e.V)

Calculated Wavellength

(nm)

Experimental Wavelength

(n.m) Methanol H →δ 0.111 0.70 2.65 468 471

H-5→ δ 0.00 0.69 3.82 324 - H-1 → δ 0.00 0.70 3.89 318 -

δ → σo.65 δUετ τrbital & H → σo.64 Hτετ τrbital

199

Fig. 8.9: Fluorescence emission spectra of BONA.

8.4.6 NMR spectral analysis

The isotropic chemical shifts are frequently used as an aid in the identification of bioactive

compounds. It is recognized that accurate predictions of molecular geometries are essential for

reliable calculations of magnetic properties. Therefore, full geometry optimization of BONA

was performed at B3LYP/6-311G (d, p) method. Then, gauge-including atomic orbital [Szabo et

al. 2009] (GIAO) 1H, 13C, 15N and 18O NMR chemical shift and nuclear spin-spin coupling

constant calculations of the compound have been made. GIAO procedure is superior to other

methods, since it exhibits a faster convergence of the calculated properties upon extension of the

basis set used. Taking into account the computational cost and the effectiveness of calculation,

the GIAO method seems to be preferable from many aspects at the present state of this subject.

The experimental and calculated values for 1H, 13C, 15N and 18O chemical shift and nuclear spin-

spin coupling constant are listed in tables 8.11 & 8.12. The 1H NMR along with the assignment

200

of hydrogen is given in fig. 8.10. The chemical shift value observed at 6.27 ppm for the H27 is

comparably different from the calculated value 7.20 with a degeneracy 1, which can be

attributed to the strong intra-molecular N-H…O hydrogen bonding. The calculated value for

H26 is only 3.29 and the experimental value shows the degeneracy with H27, since in solution

condition the intra-molecular hydrogen bonding is so weak due to solvent interaction and the

NH2 group is free to rotate with respect to the C-N bond axis. The degeneracy expected for H11

and H12, H23 and H15 and H17, H19 and H21 are observed both in theoretical and experimental

values.

Fig. 8.10: 1H NMR spectrum of BONA

The calculated 13C NMR chemical shift were comparable to the 13C NMR experimental

values [Bernd Schmidt et al. 2010], which is >100ppm [Mohammed & Colin 2003; Kalinowski

et al. 1984], usually observed for carbons of aromatic rings. There is a very good agreement

between experimental and theoretical chemical shift results in the title molecule. Moreover, the

pair C14/C22 and C16/C20 is expected to be isochronous due to signal averaging. The calculated

results are in agreement, even though the experimental values show alteration indicating the

interactions of the other groups. The variation in calculated and experimental values are

maximum for those carbon were hetero atoms are attached C7, C2 and C1, however C10 showed

comparatively lower shift from calculated value was electron withdrawal from benzene ring and

201

ether oxygen is expected and electron donation is expected from H11 and H12 hydrogen’s due to

hyper conjugative interactions. While comparing to other oxygen atoms low value of 112.08

ppm is calculated at O29, which is due to the hyper conjugative interaction between CH2 group

and O29 atom. Because of this effect, chemical shift at C10 (71.10 ppm) is very much reduced.

The C1 signal located near to electron-withdrawing NO2 group resonated at higher field (136.60

ppm) than the C2 adjacent to electron donating NH2 group (140.20 ppm). Coupling constants JCH

in the benzyl ring of BONA are in the range 149.64-151.73 Hz, which is expected for the sp2

hybridized carbon atoms. An examination of the couplings of 1JC3H4 shows the electron donor

group NH2 in the nitro aniline ring leads to a decrease of 1JC3H4 (151.41 Hz), where as electron-

withdrawing group NO2 increases the 1JC8H9 (156.36 Hz) coupling constant. The JHH values

calculated are in the range 6.35-6.54 and that observed experimentally are 2.9 and 9.1 [Bernd et

al. 2010].

Table 8.11: Chemical shifts (13C, 1H) of BONA [δ(ppm)]

Atom Experimental

(ppm)l Computed

(ppm)

Difference (ppm)

Degeneracy

C7 150.10 136.16 13.94 1 C2 140.20 126.59 13.61 1 C1 136.60 123.00 13.6 1 C13 131.80 122.56 9.24 1 C22 128.90 118.44 10.46 1 C14 128.40 118.46 9.94 2 C18 127.90 115.95 11.95 1 C16 127.40 115.37 12.03 2 C20 120.30 115.36 4.94 2 C5 120.30 115.36 4.94 1 C3 108.40 104.87 3.53 1 C8 108.40 95.78 12.62 1 C10 71.10 65.06 6.04 1 H27 6.27 (5.87) 7.20 -0.93 1 H15 7.47 (7.63) 7.13 0.34 6 H23 7.47 (7.63) 7.13 0.34 6 H9 7.24 (7.48) 7.11 0.13 6 H17 7.38 (7.26) 7.10 0.28 6 H19 7.38 (7.26) 7.10 0.28 6 H21 7.38 (7.26) 7.10 0.28 6 H6 7.13 (7.11) 6.48 0.65 1 H4 6.78 (6.74) 5.99 0.79 1

202

H11 5.16 (5.01) 4.45 0.71 2 H12 5.16 (5.01) 4.45 0.71 2 H26 6.27 (5.87) 3.29 2.98 1 N24 382.43 1 N25 75.56 1 O28 604.99 1 O30 582.24 1 O29 112.08 1

l taken from Bernd et al. 2010

Table 8.12: Calculated 1JCH and 1JHH coupling constants (Hz) of BONA

1JCH / 1JHH Calculated

1JC8H9 156.36 1JC5H6 154.04 1JC3H4 151.41 1JC14H15 149.66 1JC16H17 151.73 1JC18H19 151.69 1JC20H21 151.72 1JC22H23 149.64 1JH4H6 7.74 1JH15H17 ,

1JH21H23 6.54 1JH17H19 ,

1JH19H21 6.35 1JH26H27 6.03

8.4.7 Bond order analysis

The bond order idea is used in molecular dynamics and bond order potentials. The

magnitude of the bond order is associated with the bond length. According to Pauling in 1931,

the bond order is experimentally described by the expression

Sij = exp [Rij - dij / b], where Rij is the bond length experimentally measured, dij is the single

bond length and “b” is a constant depending on the atoms. Pauling suggested a value of 0.γ5γ

for “b”. Dealing with molecular orbital theory, the bond order, which is referred to as the

amount of charge located on a bond is of great importance in the calculation of various

molecular parameters like dipole moment. It also gives an insight into the understanding of the

chemical properties associated with the bond. Addition of electrons to bonding molecular

orbitals will increase the bond order by ½ for every electron added. So, the stability of the

molecule will increase, the bond length will decrease and the frequency of vibration will

203

increase. Removal of an electron from a bonding molecular orbital will have the opposite effect.

Addition of electrons to antibonding molecular orbitals will decrease the stability and the bond

order will decrease by ½ for every electron added and removal of electrons from antibonding

orbitals will have the opposite effect.

The bond orders calculated for BONA are given in table 8.7. The C-C bond order

slightly increases by increasing the hydrogen bond strength. In monomer structure the six bonds

in the benzene ring have bond orders in the range 1.270-1.542. The bond order values between

directly bonded atoms are seen to be close to classical values i.e., near unity for a single bond, 2

for a double bond and approximately 1.5 for a partial double bond. All the substituent groups /

atoms i.e. oxygen is electronegative than hydrogen and there is a flow of electronic charge from

the C-H region to the attractive C-O region. This reduces the C-H bond order to a value slightly

less than unity. The bond order of N-H bond (≃ 0.786) is lower than that of C-H bond (≃

0.923). The bond order of N-H decreases due to H-bonding and that for C-H decreases due to

hyperconjugation. In the case of dimer structure bond order is still lower in N-H and C-H bonds.

The C1-N24 bond order is ≃ 0.997 and the C2-N25 bond order is ≃ 1.227. This shows the

conjugation from the donor amino group to the acceptor nitro group through the ring π-system.

On the other hand, the N25-H27 bond (≃ 0.786) is weakened compared to the N25-H26 bond (≃

0.839) due to the intra- and inter-molecular N25-H27…O30 hydrogen bonding, which is

substantiated by NBO analysis (supplementary table 15). The bond order value in dimer

structure is small while comparing to monomer, trimer and stacking structure due to more

charge transfer interaction.

8.4.8 Charge distribution analysis

The charge distribution on the molecule has a significant influence on the vibrational

spectra. Mulliken and natural charges computed at the DFT level is collected in table 8.1. N25 of

the amino group is identified as the preferred site for protonation [Kolandaivel et al. 2005; Kim

204

et al. 1977; Ram et al. 2000], due to this fact that the N24-O30, involved in intra-molecular

hydrogen bonding with hydrogen atom H27 is double bonded. Mulliken charge analysis show

that the hydrogen atoms attached to N25 i.e. H27 (≃ 0.260 e) has more electron deficient than H26

atom (≃ 0.232 e). This is in good agreement with the criterion for hydrogen bond formation and

hence loss of charge by the hydrogen atom. The Mulliken population analysis shows that the

presence of two large electronegative oxygen atoms imposes positive charges on the nitrogen

atom (N24) of the nitro group, by which there is more positive charge on the C1 atom, while the

negative charges on the amino nitrogen (N25) produced by hydrogen resulting in enormous

positive charge on C2 atom. As a result more electronegative amino nitrogen atom pulls the C2

atom towards itself leading to the shortening of C2-N25 bond, which might distort the relevant

bond angle. Particularly, the C3 = C5 and C7 = C8 bonds lengthen by ≃ 0.036 and ≃ 0.042 Å

(table 8.1) respectively, whereas the C1-C2, C2-C3, C5-C7 and C8-C1 bonds undergo a shortening

by ≃ 0.123, 0.119, 0.128 and 0.129 Å, and this show good conjugation throughout the ring. The

calculated results showed that the NH2 moieties in the compound possess the net negative

charge of ≃ -0.021 e in monomer, trimer and stacking structures, which shows that the charge-

transfer is strongly enhanced. But in dimer the value is very low (≃ -0.007e). When the charges

of carbon atoms are considered, the benzyloxy ring carbon atoms C13, C14, C16, C18, C20 and C22

and all the aniline ring carbon atoms C3, C5 and C8 attached to hydrogen atoms have negative

charge. Very similar values of positive charges are observed in hydrogen atoms connected with

carbon atoms of the benzyloxy ring also. It is worth mentioning that decrease in

electronegativity and increases in N25-H27 bond length are good evidences for very strong intra-

and inter-molecular N-H…O hydrogen bonding. The dipole moment of the compound is

calculated to be 4.1 Debye (table 8.9).

205

DFT calculation predicts an overall Mulliken and natural charge 0.092 and 0.197 e in

the monomer structure (table 8.1) over the aniline moiety (including amino and nitro group) and

-0.022 and 0.005 e over the phenyl moiety by which it places a negative charge at C10 (-

0.073and -0.045 e) and positive charge at C7 (0.110 and 0.293 e) with respect to negative charge

at O29 (-0.278 and -0.515 e). Hence the bond length C7-O29 (1.366) (supplementary table 13)

contracts and C10-O29 (1.429 Å) elongates. NBO analysis indicates that in C-O bond sp orbital of

C is greater than that of O, but in N-O bond sp orbital of N is less than that of O. Large positive

heat of formation (ΔHf) is due to nitro group present in the compound (table 8.13). The average

polarizability and ionization potential are calculated to be 191 a.u and 8.65 e.V.

Table 8.13: Ionization Potential and other parameters obtained using PM6 method

Form

Po

lari

zabi

lity

(A.U

)

Hea

t of

form

atio

n (k

J)

Tot

al E

nerg

y (k

Jmol

-1)

Ele

ctro

nic

Ene

rgy

(kJm

ol-1

)

Cor

e-co

re

repu

lsio

n (e

.V)

Ioni

zatio

n Po

tent

ial (

e.V

)

Monomer 191 6.8 -288008 -1759115 1471203 8.65 Dimer 386 -14 -575919 -4714740 4138628 8.18 Trimer 582 -41 -864023 -8503127 7639103 8.03

Stacking 377 -7 -570615 -5215689 4639674 8.65 8.4.9 Electrostatic potential analysis

The molecular surface electrostatic potential (MSEP) [Soni et al. 2010] has been employed as an

informative tool of chemistry to describe different physical and chemical features, including

non-covalent interactions in complex biological system. The different values of the electrostatic

potential at the surface are represented by different colors; red represents region of most

negative electrostatic potential, blue represents region of zero electrostatic potential and white

represents region of most positive electrostatic potential. The three dimensional MSEP

isodensity surfaces with potential isocontour surface plotted by ArgusLab software [Mark 2007]

(fig. 8.12) for BONA clearly suggest that the potential swings wildly between nitro group atoms

206

(dark red) and amino group (white). The nitro group oxygen atoms reflect the most

electronegative region and have excess negative charge (-0.040λ → -0.0500 e) and the hydrogen

atom attached to the amino group bears the brunt of positive charge (0.0500 → 0.040λ e).

Whereas the MSEP also shows that the positive charge is primarily concentrated over the two

rings (blue and violet). In the stacking structure MSEP map is almost similar to the monomer

structure. In the dimer structure the negative charge (red) is fully concentrated in one molecule

and positive charge (white) in the other molecule. In the trimer structure negative charge (red) is

accumulated in the central molecule and the nitroaniline part of third molecule and positive

charge (white) is distributed in the first molecule. The total charges were summed up for donor

group, acceptor group and π-conjugation (aromatic ring) in the ground state. The π-electron is

largely localized in the aromatic ring. The red regions refer to an area that would favor

interactions with approaching electrophiles. V(r) minima are in lone pair region in a plane

bisecting the HNH angle predicting a site of hydrogen bonding acceptor. The white regions

interact extensively with incoming nucleophiles and thus the sites of hydrogen bond donor

groups.

Fig. 8.11: Electrostatic potential plot of BONA

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8.5 Conclusion

Structural study of BONA has been performed by experimental spectroscopic

methods along with quantum chemical calculations. The NCA performed on BONA

reproduces its experimental geometry and harmonic vibrational wavenumbers excellently.

The theoretical model of the crystal structure of BONA obtained by DFT calculation

describes the possibility of weak C-H…O and C-H…π, and strong σ-H…O hydrogen

bonding. The vibrational spectral analysis also reveals the existence of strong intra- and inter-

molecular N-H…O interaction by red shifting in N-H stretching and blue shifting in C-H

stretching due to C-H…π and C-H…O hydrogen bonding. The lowering of HOMO and LUMO

energy gap indicates charge transfer interaction occurs within the molecule, by which it leads to

an insecticidal activity. TD-DFT calculation clearly explicates the charge transfer

interaction involving donor to acceptor groups. The donor-acceptor interaction causes the charge

redistribution in the molecule, so that there is an increase in C-N bond order and decrease in N-

H bond order. Nuclear spin-spin coupling constant shows that it increases near electron-

withdrawing group and decreases near electron-donating group.