Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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The study of hydrogen bonding and p� � �p interactionsin phenol� � �ethynylbenzene complex by IR spectroscopy

http://dx.doi.org/10.1016/j.saa.2014.04.1491386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +385 1 457 1220; fax: +385 1 168 0195.E-mail address: dvojta@irb.hr (D. Vojta).

Danijela Vojta ⇑, Mario VazdarDivision of Organic Chemistry and Biochemistry, Ru -der Boškovic Institute, Bijenicka 54, 10001 Zagreb, Croatia

h i g h l i g h t s

� IR spectra ofphenol� � �ethynylbenzene hydrogen-bonded complex were presented.� Association constant of

phenol� � �ethynylbenzene complex isdetermined.� The direct involvement of C„C

moiety in hydrogen-bonded complexis exhibited.� The contribution of p� � �p interactions

in the complex is suggested.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 February 2014Received in revised form 17 April 2014Accepted 23 April 2014Available online 5 May 2014

Keywords:EthynylbenzenePhenolEthanolHydrogen bondingp� � �p interactionsIR spectroscopy

a b s t r a c t

Weak hydrogen bonds between phenol and ethynylbenzene in tetrachloroethene were explored by usingFTIR spectroscopy. Association constants (Kc) were determined by high dilution method at two temper-atures, 20 �C and 26 �C, and they are, respectively, 0.54 ± 0.09 mol�1 dm3 and 0.36 ± 0.08 mol�1 dm3. Theposition of ethynylbenzene stretching band, when in hydrogen bonding complex with phenol (C„C� � �), isproposed to be governed by the interplay of OH� � �p (C„C moiety or phenyl ring of ethynylbenzene) andp� � �p (phenyl ring of phenol� � �C„C moiety or phenyl ring of ethynylbenzene) interactions. This conclu-sion is supported by the findings on the complex between ethanol and ethynylbenzene; in the latter,C„C� � � stretching band is shifted to the higher wavenumbers, as expected when ethynylbenzene inter-acts with hydrogen bond donor. Geometries and energies of the presumed complexes, as well as theirvibrational spectra, are predicted by using ab initio calculations. The spectroscopic and thermodynamicdata obtained here offer the missing pieces in the present picture of migration of H-atom of phenolOH group between competing hydrogen bond accepting centers on ethynylbenzene.

� 2014 Elsevier B.V. All rights reserved.

Introduction

In prediction of the structure and thermodynamical descriptorsof hydrogen bonded (HB) complexes a subtle balance betweenweak HB centers on HB acceptor governs their interaction withHB donors. In this context, ethynylbenzene (EB) emerged as one

of the most inspiring systems where usual HB hierarchy patternis not elusive. Although C„C group is considered to be moreattractive for H-atom than phenyl moiety, it is not always the pri-mary site where HB is formed. UV–IR double resonance experi-ments in the gas phase, combined with ab initio calculations atMP2/aug-cc-pVDZ level of theory, shown that HB pattern formedby EB, depending on the other protagonist of the HB complex,was found to be diverse; cyclic structure of water� � �EB complexwas assumed to be the result of an interplay between OAH� � �p

D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14 7

and O� � �HACH = HBs, where one water H-atom form HB with C„Cmoiety of EB, while O-atom of water form HB with one of the orthoatoms of phenyl ring [1]. In contrast, in simple alcohols like meth-anol or ethanol, the complexation was presumed to be dictated bythe interaction of alcohol OH group with p system of phenyl ring,and additional interaction of CH3 group with C„C moiety [2]. Theobserved differences are rationalized in terms of domination of dis-persion over electrostatics when H-atom is replaced with methylor ethyl group and the analogous phenomenon is observed in com-plexes where EB interacts with ammonia and methylamine [3].

The ability of EB to form HB complexes with different partnersin solution was explored as well, although mostly expressed onlyin terms of frequency shifts. In particular, West et al. studied theposition of „CAH� � � stretching band of EB as a function of differ-ent HB acceptors like amides, while its HB donating characterwas explored in interaction with phenol (PH) [4]. Furthermore,the study of PH� � �EB interaction, conducted by Yoshida et al. [5],was more detailed since they explored the temperature dependentbehavior of OH� � � stretching band of PH and estimated the thermo-dynamical parameters of HB system from it.

HB patterns formed by PH and EB present intricate issue eventoday, especially in the context of migration dynamics of PHH-atom among two HB accepting sites (Fig. 1 with some possibleinteraction patterns). When investigated chemical exchange ofD-atom of hydroxyl group of PH (OD group) by 2D IR spectroscopy,Rosenfeld et al. [6] concluded that direct migration of D-atombetween HB accepting sites on a single EB molecule (C„C groupand phenyl ring) during 3–5 ps period is the only mechanism ofchemical exchange (Fig. 1a and b). Contrary to the direct migrationmechanism of PH along EB in CCl4 mixtures, ab initio calculations,together with the simulations of molecular dynamics of PH sur-rounded only with EB molecules [7], imply that indirect migrationmechanism is the main mechanism of chemical exchange and thatit dominates over direct migration. Indirect migration refers to theintermolecular hopping of PH molecule from the HB acceptingsite of one EB molecule (C„C group) to the another HB acceptingsite on the another EB molecule (phenyl group); in such a three-molecule complex, PH interacts with C„C moiety of one EB mole-cule and phenyl ring of the another EB molecule. In turn, theyfound that, when CCl4 is absent, dispersion is main interactionforce, while in the presence of CCl4, interaction between PH andEB was found to be driven by both HB and dispersion forces,p� � �p stacking in particular, and two-molecule complex (dimer)is the most probable structure.

In contrast to the detailed investigation of the chemicalexchange kinetics, thermodynamics of HB between EB and PH israther incomplete. As far as the authors are aware, the only thermo-dynamic values are those obtained by Yoshida et al. [5] and theyshould be revisited due to several reasons. First, in experimentalconditions in which they worked, both of the compounds certainlyundergo self-association, as it has been pointed out in Refs. [8,9]. Inaddition, PH� � �PH species, as well as higher polymers, produce thebroad features that span the part of the spectral range in whichEB� � �PH complex absorbs, making the measurements of the areasbelow the bands disputable. Second, even if the self-association

Fig. 1. Possible interactions of phenol (PH) and ethynylbenzene (EB): (a) OH� � �p (C„C m(phenyl ring) and p� � �p (C„C moiety � � �phenyl ring); and (d) OH� � �p (C„C moiety).

were avoided, the equilibrium concentration of every compound,being determined from the correlation of the integrated absorptionintensities of the investigated bands and their positions [10], is con-sidered not to be very accurate.

Thus, in order to get more accurate and detailed insight intothermodynamics of PH� � �EB HB, high dilution method [11] willbe employed for determination of association constant, Kc. The lat-ter will be measured at two temperatures, 20 �C and 26 �C, whichare considered to be lower and upper limit of usual room temper-ature. Their difference will be briefly discussed and compared withKc values of other systems that have either PH as HB donor or EB asHB acceptor in a HB complex.

Nonetheless, the direct involvement of C„C group in HB will beexhibited and the mechanism that defines the position of C„C� � �stretching band (associated C„C oscillator) will be proposed. Theimportance of p� � �p interaction in PH� � �EB HB will be highlightedby comparison of spectral shift of C„C� � � stretching band in theEtOH� � �EB HB complex. The obtained experimental findings willbe supported with the ab initio calculations. The findings obtainedon the latter will shed more light on its recent research results aswell [2,3].

Experimental

Chemicals

Ethynylbenzene (EB, dark yellow liquid at room temperature,b.p. = 142–144 �C, purity 99%) and tetrachloroethene (C2Cl4, col-orless liquid at room temperature, b.p. = 122 �C, spectroscopicgrade), purchased from Alfa Aesar and, respectively, AcrosOrganics, were used as received. Phenol (PH, white crystallinesolid at room temperature, m.p. = 40–41 �C, p.a. grade)and ethanol (EtOH, colorless liquid at room temperature,q(EtOH)20 �C = 0.7893 g cm�3, b.p. = 78 �C, p.a. grade), purchasedfrom Kemika, were additionally purified by recrystallizationand, respectively, fractional distillation. Densities of EB andC2Cl4 were determined by a densitometer DMA 5000-AntonPaar: (q(EB)20 �C; 26 �C = 0.930187 g cm�3; 0.924700 g cm�3,q(C2Cl4)20 �C; 26 �C = 1.622696 g cm�3; 1.612728 g cm�3).

Preparation of solutions

PH + EB in C2Cl4 mixturesA set of 15 binary mixtures was prepared by dissolving EB in

C2Cl4. The concentrations of EB were: 0.090459 mol dm�36

c0(EB) 6 1.17350 mol dm�3. A set of 15 ternary mixtures was pre-pared from corresponding binary mixture (EB in C2Cl4) and PH.The concentration of PH in all ternary mixtures was in range0.00312 mol dm�3

6 c0(PH) 6 0.00996 mol dm�1.

EtOH + EB in C2Cl4 mixturesA set of 5 binary mixtures was prepared by dissolving EB in

C2Cl4 with concentrations of EB in range 0.19604 mol dm�36

c0(EB) 6 0.90249 mol dm�3. In a set of 5 ternary mixtures, after

oiety) and p� � �p (phenyl ring� � �phenyl ring); (b) OH� � �p (phenyl ring); (c) OH� � �p

8 D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14

addition of EtOH (liquid) in every binary mixture, the EB’sconcentrations were in range 0.19600 mol dm�3

6 c0(EB) 60.90233 mol dm�3. The concentration of EtOH in all ternary mix-tures was c0(EtOH) = 0.0069 mol dm�3.

The first group of mixtures is prepared due to; (i) determinationof association constant (Kc) that characterize PH� � �EB HB by usinghigh dilution method and (ii) the analysis of certain spectral fea-tures upon HB formation. The second group of mixtures is preparedonly for the reason (ii). More details on the high dilution methodand the performed spectral analysis are given in the Supplemen-tary Materials.

FT IR spectra at room temperatures

IR spectra were measured on an ABB Bomem MB102 spectrom-eter, equipped with CsI optics and a DTGS detector with nominalresolution of 2 cm�1 and 10 scans; the latter was found to be suf-ficient for obtaining smooth enough IR spectra, especially OHstretching band, for determination of the band absorbance, and,at the same time, for preventing the accumulation of water fromthe atmosphere which absorbs in the same spectral range and, con-sequently, brings up the unreliability in the absorbance measure-ments (Fig. S3 in Supplementary Materials). PH + EB in C2Cl4

mixtures were recorded at 20 �C and 26 �C (lower and achievedupper limits of common room temperature interval), while EtO-H + EB in C2Cl4 mixtures at 20 �C. The accuracy in temperaturemeasurement was ±1 �C.

IR measurements of solutions at room temperatures underatmospheric pressure in air have been performed by using sealedcells equipped with either KBr or CaF2 windows. In all the mea-surements the cell’s path length was determined by Bertie’s proce-dure using either dichloromethane (CH2Cl2) or benzene (C6H6) as asecondary standard [12]. IR spectra of both binary and ternary mix-tures concerned PH + EB in C2Cl4 mixtures were measured in cellswith d = 5090.25 lm (C6H6), d = 1003.35 lm and 102.25 lm (CH2-

Cl2). The data for each sample were collected three times in orderto calculate the uncertainty in the absorbance of PH’s OH stretch-ing band and, consequently, the uncertainty in Kc. IR spectra ofboth binary and ternary mixtures concerned EtOH + EB in C2Cl4

mixtures were recorded in CaF2 cell of d = 1001.19 lm (CH2Cl2)and IR spectrum of each solution was recorded once. The repeatedmeasurements of the latter mixtures are considered to be unneces-sary since only the positions of certain bands in their IR spectra areto be determined and because the calculation of Kc for EtOH� � �EBsystem is not the aim of this paper. Between measurements of dif-ferent solutions, the cell was emptied, dried with N2, washed twicewith the studied solution; the cell is then refilled and the data wererecollected.

Calculations

Ab initio calculations were done at the MP2/aug-cc-pVDZ levelof theory for EtOH, PH, EB and corresponding complex structures.Geometry optimizations and subsequent vibrational analysis forall molecules was performed to verify that a minimum on thepotential energy surface was obtained. Complexation energies(with included zero point vibrational energy) were calculatedusing the supermolecular approach [13] as the difference betweencomplex and its constituents and corrected for basis set superposi-tion error (BSSE) [14]. Due to harmonic approximation, i.e. theneglect of anharmonicity in vibrational analysis, calculated vibra-tional frequencies were scaled by the factor of 0.9604 in order tobetter match experimental values [15]. All quantum chemical cal-culations were performed employing Gaussian 03 [16].

Results and discussion

Calculations of PH, EtOH and EB as monomers, in HB complexes andtheir vibrational frequencies

Quantum chemical MP2/aug-cc-pVDZ calculations predict thatthree types of complex structures between EB and PH exist(Fig. 2). The most stable complex, with complexation energy of�27.6 kJ mol�1 is formed upon HB between PH hydroxyl groupwith the C„CAH group of EB and simultaneous p� � �p stackinginteraction between benzene rings of EB and PH (structure 1). Instructure 2, only HB between OH group of PH and phenyl moietyexists and there is no additional p� � �p stacking stabilization whichis evidenced by smaller stabilization energy of �18.2 kJ mol�1. Inaddition, OH group is not directed towards the center of phenylring; instead it points to the bond between a pair of C-atoms soit will be designated as OH� � �C@C. It is accompanied by the inter-action of ortho H-atom of PH with a pair of C-atoms opposite to thepair involved in the interaction with OH group. Finally, in structure3 we observe two HB motifs, the first one between OH group of PHwith phenyl ring of EB and the second one where C„CAH group ofEB interacts with phenyl ring of PH. This structure is slightly morestable than structure 2, but is however less stable than structure 1with complexation energy of �19.9 kJ mol�1.

Vibrational analysis of complexes 1–3 shows that there is a low-frequency shift of 68–89 cm�1 (depending on the complex) in~m(OH) frequencies upon complexation of PH with EB compared toneat PH solution (Table 1). This is in agreement with presentedexperimental values where a similar low-frequency shift wasobserved (Table 1). Overall, the comparison between experimentaland calculated frequencies is very good when the scaling factor of0.9604 is used as suggested in [15].

IR spectra of binary mixtures

Alcohols in C2Cl4The most relevant spectral feature used for determination of

association constant is OH stretching of unassociated PH in C2Cl4

observed at 3610 cm�1. Possible self-association of PH can beneglected since, as CCl4 concerns, which is considered to be verysimilar to C2Cl4 [17,18], dimerization of PH starts at 0.1 mol dm�3

[19]. However, according to Widom et al. [20], PH underwentself-association already at c0(PH) = 0.003 mol dm�3. As in themajority of studies where PH act as HB donor the choice of0.005 mol dm�3 was considered as optimal [21], analytical concen-trations used here in all solutions were between0.003117 mol dm�3 and 0.00996 mol dm�3. Since at concentra-tions smaller than c0(PH) = 0.001 mol dm�3 OH stretching pro-duces rather weak band with the absorbance unreliable toquantify, it is not recommended to use smaller concentrations.Molar absorption coefficient of OH stretching band at 3610 cm�1

is e = 235 ± 1 mol�1 dm3 cm�1 [22]. OH stretching of unassociatedEtOH appears at 3635 cm�1 [19], while dimers and polymers, cre-ated at concentrations above, 0.01 mol dm�3 and, respectively,0.2 mol dm�3, would absorb at, respectively, 3509 cm�1 and3336 cm�1 in CCl4 at 25 �C [23]. Self-association is excluded at con-centrations about 0.005 mol dm�3 and, in this research, analyticalconcentration of EtOH in all ternary mixtures was far below thislimit.

Ethynylbenzene (EB) in C2Cl4Engagement of EB in HB is mostly expressed in the changes of

spectral features attributed to the vibrations of C„C moiety (whenin HB complex with HB donors like alcohols) or C„CAH moiety(when in complex with HB acceptors). The former is observed as

Fig. 2. HB complexes between EB and PH (1–3) and EB and EtOH (1a and 2a). Complexation energies (corrected for BSSE) are given in parentheses.

Table 1The positions and the tentative assignment of the vibrational modes of PH, EB, EtOH and HB complexes of PH and EB and, respectively, EtOH and EB in C2Cl4.

System ~ma

OH OH� � � „CAH „CAH� � � C„C C„C� � �

Exp Calc Exp Calc Exp Calc Exp Calc Exp Calc Exp Calc

PH 3610 3656 – – – – – – – – – –EB – – – – 3313 3343 – – 2113 2022 – –EtOH 3634 3679 – – – – – – – – – –

PH + EB 3605 – 3572 3588d 3313 – – 3343d 2113 – 2113 2021c

3547 3579b 3340c 2015d

3533 3567c 3335b 2009b

35243513

EtOH + EB 3633 – 3656 3638e 3313 – – – 2113 – 2114–2115 2021e

�3600 3638f 2016f

a cm�1.b Structure 1 (OH� � �C„C and p� � �p).c Structure 2 (OH� � �C@C and @CAH� � �p).d Structure 3 (OH� � �p and C„C� � �p).e Structure 1a (OH� � �phenyl).f Structure 2a (OH� � �C„C and ACH� � �p).

D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14 9

asymmetric band centered at 2113 cm�1 in C2Cl4, while the latteras complex envelope in the spectral range 3360–3260 cm�1, withmaximum at 3313 cm�1. The assignment and explanation of theobserved band shapes are offered in [24,25] and are out of thescope of this paper. Self-association of EB, accomplished via HBsbetween C„CAH and C„C groups at concentrations higher than1.0 mol dm�3, is evidenced by the appearance of new band atabout 3300 cm�1 [9] assigned as C„CAH� � � stretching, whileC„C stretching band is generally less affected by the HB formation[26]. Interaction between EB and PH studied in this paper was con-ducted at analytical concentrations at which EB does not undergoself-association, so that only PH� � �EB complex in stoichiometry1:1 is expected. However, it should also be emphasized that EBcan participate in HB complex as HB acceptor employing eitherthrough C„C bond or phenyl ring [27].

IR spectra of ternary mixtures

IR spectra of solutions that contain both PH and EB reveal somefeatures absent in IR spectra of solely PH or EB solutions; the OHstretching band shifts for about 5 cm�1 to the lower wavenumbers

when the concentration of EB is gradually increased and novelbroad feature arises at low frequency side of OH stretching withEB’s concentration dependent intensities when compared withsolution containing only PH (Fig. 3a; black line designates PH inC2Cl4 solution, while other lines correspond to the IR spectra ofPH + EB in C2Cl4 mixtures). The former effect may be attributedto the solvent effects [18] because, when EB is present in the PHsolution at such high concentrations (in Fig. 3, in the range c0-

(EB) = 0.22552 mol dm3 (second lowest) �1.0766 mol dm3 (secondgreatest)), the solvent is effectively being changed from pure C2Cl4

(3610 cm�1) to the mixture of C2Cl4 and EB (3605 cm�1). As the lat-ter is concerned, although overall width and flatness of the featuredisables one to determine the number and position of the particu-lar band with certain accuracy, from the second derivative spectra(in the upper part of Fig. 3a with the spectral treatment more thor-ough described in Supplementary Materials), it is estimated that,aside the band at 3610 cm�1, there are five more bands at the fol-lowing positions: 3572 cm�1, 3547 cm�1, 3533 cm�1, 3524 cm�1

and 3513 cm�1. According to the calculations, it is reasonable toassume that the bands observed at 3572 cm�1 (calc. 3588 cm�1),3547 cm�1 (calc. 3579 cm�1) and 3533 cm�1 (calc. 3567 cm�1)

Fig. 3. IR spectra of PH + EB mixtures in C2Cl4 in the spectral ranges: (a) 3635–3505 cm�1 (OH and OH� � � stretching) after C2Cl4 subtraction in 5090.25 lm cell (secondderivative spectra in the upper part of Fig. 3a); (b) 3360–3260 cm�1 (C„CAH stretching) after C2Cl4 subtraction in 102.25 lm cell; and (c) 2170–2052 cm�1 (C„C� � �stretching) after EB + C2Cl4 subtraction in 1003.35 lm cell. In all figures the IR spectrum of PH solution without EB is shown as black line with the initial concentration ofc0(PH) = 0.00469 mol dm�3. The initial concentration of PH in ternary mixtures presented in figures was in range 0.00312 mol dm�3

6 c0(PH) 6 0.00996 mol dm�1, while theinitial concentration of EB, respectively, was in range 0.22552 mol dm�3

6 c0(EB) 6 1.0766 mol dm�3. In Fig. 3b thicker line of the highest intensity designates the IRspectrum of EB solution with the initial concentration of c0(EB) = 1.0766 mol dm3 which generates essentially the same spectrum as when PH is present and absent (thus, onlyone solution is presented).

10 D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14

can be attributed to the stretching of OH oscillator involved inthree types of HBs, respectively: OH� � �p, OH� � �C„C and OH� � �C@C(Table 1). The same assignment of the first two bands is also madeby Yoshida et al. [5] when studied association of PH and EB in CCl4.Other two observed bands at 3524 cm�1 and 3513 cm�1 might beattributed to the other PH� � �EB complexes, possibly with differentstoichiometry like 2:1. Although we have not dissolved PH in pureEB [7], it is still plausible that certain amount of trimers exists.

The position and the shape of vibrational envelope attributed tothe C„CAH stretching band remains unchanged in the exploredconcentration range (Fig. 3b). As, qualitatively, the envelope looksthe same as the one produced by EB in CCl4 solution when thelatter is present at much lower concentration (c0(EB) =0.005 mol dm3, [9]), indicating that self-association of EB mole-cules is avoided. In Fig. 3b, thicker band of the highest intensityof EB in C2Cl4 solution completely overlaps with the spectrum ofPH + EB in C2Cl4 mixtures (c0(EB) = 1.0766 mol dm3), while blackline (PH in C2Cl4 solution) proves that PH does not absorb in thisspectral range. The subtraction of IR spectra of EB in C2Cl4 fromPH + EB in C2Cl4, when C„CAH stretching is to be captured in

the cell of d � 100 lm, however, could not reveal any significantresidual. Thus, the eventual participation of C„CAH in HB com-plex cannot be proved from these experiments. When discussingthe calculated vibrational spectra of three the most relevantPH� � �EB complexes, it is interesting that only the third one (struc-ture 3 in Fig. 2) does not predict the shift of the C„CAH stretchingband upon complexation (at calculated frequency of 3343 cm�1

absorbs both unassociated and associated C„CAH oscillator).Structures 1 and 2 exhibit low-frequency shift of the associatedC„CAH stretching oscillator, calculated to be at 3335 cm�1 and,respectively, at 3340 cm�1 (Table 1).

Although the appearance of new band(s) attributed to thestretching of an oscillator covalently bonded to H-atom and itsconcentration dependence is usually considered to be necessaryand sufficient proof for the HB formation [28], it is interesting tofurther deliberate the unambiguous evidence that C„C moietyparticipates in HB formation.

After subtraction of binary from ternary mixture spectrarecorded in the same cell and at the same conditions(1003.35 lm cell), in spectral range where C„C stretching absorbs

D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14 11

(2170–2052 cm�1), a feature was observed which obviously origi-nates due to the stretching of C„C oscillator directly involved inHB with PH and assigned as C„C� � � (Fig. 3c), where � � � designatesassociation or participation in a HB complex. The position of thisband coincides with the position of unassociated C„C oscillatorwithin the limits of instrument uncertainty in wavenumber posi-tion (2113 ± 1 cm�1). As the change in the position of the bandinvolved in some kind of intermolecular arrangement usually sug-gest the change in the band length upon complexation [29,30], theabsence of the shift, as well as the shift considered as negligiblewhen taking into account instrumental limitations, implies thatthere is no change in the bond length of C„C moiety in spite ofthe fact that HB is accomplished directly employing the latter.

The absence of the shift of C„C� � � stretching band when inter-action is directly accomplished via C„C moiety might occur ifsome other driving forces, aside presumed OH� � �p HBs(OH� � �C„C), appear on the scene. The participation of p� � �p stack-ing interactions between EB (phenyl ring) and PH (phenyl ring), asexplained by Rosenfeld et al. [6], may result with the observedphenomenon. If this is the only event that takes place, then C„Cis effectively both HB donor and acceptor in the same moleculebecause it transfers H-atom to the phenyl moiety and then takesit back (Fig. 1a and b). This dual character may result with the can-cellation of opposite effects of PH� � �EB HB on the C„C bondstretching and lengthening and, in total, with unshifted C„C� � �stretching band. However, the possibility of another scenarioshould be considered as well and it is presented here for the firsttime. It also accounts direct migration mechanism, although withone important exception; the sliding of PH molecule along EB mol-ecule is accompanied with the rotation of PH molecules around theaxis perpendicular to the plane formed by the complex PH� � �EB, sothere is a chance for participation of PH’s phenyl ring in associationas well (Fig. 1c and structure 3 in Fig. 2) In turn, delicate tuningbetween HB and p� � �p interaction anticipates that both C„C andphenyl groups are always involved in certain interaction, eitherOH� � �p (p being C„C or phenyl ring) or p� � �p (phenyl ring ofPH� � �C„C or phenyl ring of EB). Detail band shape analysis of thisband is disabled due to two main reasons: (i) if this was the scopeof this paper the spectra should be collected with much greaterscan numbers. In this way, the noise would be reduced to the levelthat it does not distort the signal in such an extent; (ii) C„Cstretching band profile, even when not involved in the HB complex,is complicated feature by itself. Its natural asymmetry [24,25],without the possibility of forming HB, was a long-lasting problemfor not only IR [31] but Raman spectroscopy as well [31,32]. Hence,when involved in a HB complex, especially from the point of viewof low-resolution IR spectroscopy in solution, it becomes rathertedious to discuss the asymmetry of the band attributed to thestretching of C„C� � � oscillator. Thus, we will focus only on theband position.

In contrast to the experimental observations, calculated vibra-tional spectra of PH� � �EB HB complex, C„C moiety in particular,reveal that C„C� � � stretching shifts to the lower frequencies uponcomplexation; for the structure 2 (Fig. 2) the shift is the smallestðD~m ¼ �1 cm�1Þ, while for structure 3 is D~m ¼ �7 cm�1 and,respectively, the greatest shift is expected for structure 1ðD~m ¼ �13 cm�1Þ. According to the structure 2, C„C moiety of EBis not involved in the HB complex with PH so, effectively, the struc-ture 2 is not presumed to be responsible for the appearance of theC„C� � � stretching band. The structures 3 and 1, due to the directengagement of C„C moiety in HB complex, may reproduce theobserved phenomenon. Moreover, the predictions of the stabiliza-tion energies suggest that these two structures (structures 1 and 3in Fig. 2) are the most stable ones. However, the absence of theshift in experimental IR spectra still remains to be clarified. Oneof the important factors is the influence of explicit solvent

molecules on HB formation (which is neglected here due to com-putational expense) which certainly affects the interaction energybetween two compounds, especially when weak HBs with lowenergies are investigated. Moreover, when the interplay of the var-ious interactions (of which all are regarded as weak) is expected, itfollows that the bond length of an oscillator involved in HB doesnot change. When taking into account mentioned disagreementbetween predicted and experimentally observed spectra, as wellas of their possible explanations, due to smaller shift of C„C� � �stretching in the structure 3 compared to structure 2 is predicted,we attribute the observed phenomenon to the HB complex 3. Thelikelihood that EB’s C„C group form p interaction with phenyl ringis already reported on EB’s dimers, studied with both IR–UV doubleresonance spectroscopy and ab initio calculations at different levelsof theory [33,34], where p-stacked antiparallel structure was foundto be the most stable.

If the influence of the HB and p� � �p stacking interplay on theposition of C„C� � � oscillator is to be unambiguously presented, itis obligatory to conduct an analogous experiment on HB system,similar to PH� � �EB, where p� � �p stacking interactions are elimi-nated. As EtOH does not possess any other HB accepting centerbesides OH group, only OH� � �p (where p designated either C„Cor phenyl group) are expected to be observed. Thus, mixtures ofEB with EtOH are further analyzed.

When EB is added to the solution of EtOH in C2Cl4 and its con-centration is gradually increased, the band attributed to the EtOHmonomer (3634 cm�1) becomes broader on the low-frequency sideand exhibits small shift of about 1 cm�1 to the lower wavenum-bers. Upon gradual increase of EB’s concentration, the asymmetryof the OH stretching band appears suggesting the association ofEtOH and EB via OH group. The position of the OH� � � band isassumed to be at �3600 cm�1 (Fig. 4a). Aside the mentioned fea-ture, a weak band at high-frequency side of monomer appears at3656 cm�1 (Table 1). Calculations found that two HB complexeswith different arrangement of EtOH and EB are the most stableones; the one in which the interaction is accomplished throughOH� � �phenyl interaction, as already suggested in [2] (structure 1ain Fig. 2) and, respectively, through OH� � �C„C interaction (struc-ture 2a in Fig. 2). However, the calculations predicted as well thatOH� � � stretching in both of the complexes produce the bands at3638 cm�1, i.e. shifted to the lower frequencies ðD~m ¼ �41 cm�1Þwhen compared with the band of the monomer EtOH(3679 cm�1). As IR–UV double resonance measurements suggestedthat EtOH� � �EB HB complex is mainly established via OH� � �p (phe-nyl) interaction (structure 1a in Fig. 2 in this paper), with low-fre-quency shift of OH� � � stretching oscillator band [2], we will assignthe band at �3600 cm�1 to the complex of the same geometry (1ain Fig. 2). The band at 3656 cm�1 will be, therefore, attributed tothe OH� � �C„C interaction (2a in Fig. 2) which is only briefly dis-cussed in [2]. The high-frequency shift of the OH� � � stretchingoscillator band may be presumably due to the redistribution ofelectron density, originating from C„C moiety, in EtOH molecule,in a manner that only small amount of charge is kept in r⁄ orbitalof OH bond and that a larger amount is transferred on the ethylgroup [35,36]. As, according to the structure 2a, the H-atoms ofethyl group may exert some kind of interaction with phenyl ring,it might be that this additional interaction assists the transfer ofcharge into molecular parts that are not directly connected withOH group.

The HB formation is additionally confirmed after subtraction ofEB in C2Cl4 mixture from corresponding PH + EB in C2Cl4 mixturewhich reveal additional vibrational feature in spectral range2175–2025 cm�1 most likely attributed to the C„C� � �OH HB com-plex (Fig. 4b). The irregularity of the observed vibrational signaturethat does not resemble to the common shape of vibrational bandsmay be explained as follows: when subtracting binary from

Fig. 4. IR spectra of EtOH + EB mixtures in C2Cl4 in spectral ranges: (a) 3680–3560 cm�1 (OH and OH� � � stretching) after C2Cl4 subtraction in 1001.19 lm cell; (b) 2175–2052 cm�1 (C„C� � � stretching) after EB + C2Cl4 subtraction in 1001.19 lm cell; and (c) second derivative of the feature (C„C� � � stretching band) in (b). The initialconcentration of EtOH in these solutions was c0(PH) = 0.0069 mol dm�1, while the initial concentration of EB was in range 0.19600 mol dm�3

6 c0(EB) 6 0.90233 mol dm�3.

12 D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14

ternary mixture IR spectrum, the differences between them, interms of their intensities, are rather small. In effect, we may expectthe feature that resembles more to the noise than to the physicallymeaningful signal. However, since the obtained difference gener-ates the band profile that becomes negative on the high-frequencyside of the expected band maximum and positive on the low-frequency side, respectively (expected according to the binary mix-ture), it may be concluded that at certain wavenumbers ternarymixture is of greater intensity than binary, while at some otherwavenumbers the situation is opposite. The most simple explana-tion for this phenomenon is simply the appearance of small butnon-negligible shift of the band maximum due to the small differ-ences in the positions of the C„C and C„C� � � oscillators(1–2 cm�1). Thus, the position of the C„C� � � stretching band, beingthe most important spectral parameter of interest, is further esti-mated by employing derivative spectroscopy (see SupplementaryMaterials). After taking second derivative of the spectral feature(Fig. 4c), vibrational band was revealed that absorbs between2114 and 2115 cm�1. Small shift upon HB formationðD~m ¼ 1—2 cm�1Þ is usually registered when weak HBs are con-cerned [26]. The direction of the shift towards higher wavenum-bers implies shortening of C„C bond of EB upon complexationwith EtOH (HB donor).

The comparison with the calculated IR spectra reveals an oppo-site trend; two bands attributed to the stretching of associatedC„C moiety are expected and both are shifted to the lower wave-numbers with respect to the monomer band, for D~m ¼ �1 cm�1

(structure 1a) and D~m ¼ �5 cm�1 (structure 2a). Since the secondone originates due to the complex in which HB is accomplisheddirectly via C„C group, this complex is presumably responsiblefor the appearance of the observed phenomenon. In this example,however, just as in the previous one, we are faced with the situa-tion in which calculations predicted low-frequency shift of theassociated C„C oscillator, while in the reality the situation isopposite. Aside to the possible causes like accounting for the expli-cit solvation as already elaborated, it is also likely that the properinclusion of anharmonic effects may produce more accurate resultswhen the direction of the shift of C„C� � � oscillator is underconsideration.

Determination of the association constant

By employing expression (3) from Supplementary Materialssection, association constants of PH� � �EB HB complex in C2Cl4

obtained here are measured at 20 �C, and, respectively, 26 �C. Thevalues are Kc(20 ± 1 �C) = 0.54 ± 0.09 mol�1 dm3 and, respectively,

D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14 13

Kc(26 ± 1 �C) = 0.36 ± 0.08 mol�1 dm3 and are, as expected, classi-fied as weak HBs. The important implication of obtained Kc valuecomes out when HB basicity of EB in term of Abraham’s bH

2 param-eter [37] is to be determined since, at present, bH

2 descriptor for anyp system is unknown. Thus, from expression log Kc ¼7:354 � aH

2 bH2 � 1:094, and knowing Abraham’s aH

2 parameter forPH (0.596), it is estimated that bH

2 for EB is 0.14 ± 0.09 (from Kc

obtained at 26 ± 1 �C). It has to be emphasized that in EB basicitydesignates overall p system (both C„C group and phenyl ring).Although high dilution method can be applied for determinationof and , it functions by far better for medium strong HBsthan weak ones. Arnett et al. [38] shown that in weak HB com-plexes with small Kc values, such as here, are also small; theyare within range�4.2 kJ mol�1 to�16.7 kJ mol�1, while the accom-panied uncertainties can vary from ±1.7 kJ mol�1 to ±2.1 kJ mol�1.In turn, when the value of is near the lower limit of mentionedrange, as expected here, the uncertainty can vary 40% to 50% of themeasured value, indicating that the results obtained with thismethod would not be very reliable. As an alternative to the highdilution method in determination of , pure base method [38]should be conducted, where the uncertainties are within the range0.4–0.8 kJ mol�1. However, this method requires the dissolution ofPH in pure EB, where EB� � �EB self-association, i.e. competitive reac-tion to the PH� � �EB association, occurs, that is aimed to be avoided.

Since and values of PH� � �EB HB complex in CCl4 and,indirectly, Kc values, are already reported by Yoshida et al. [5]( (PH� � �C„C, PH� � �phenyl) = �3.8 kJ mol�1, �6.9 kJ mol�1;(PH� � �C„C, PH� � �phenyl) = �21.3 J K�1 mol�1, �35.9 J K�1 mol�1;Kc (PH� � �C„C, PH� � �phenyl) = 0.27 mol�1dm3, 0.22 mol�1dm3),they should be commented in the light of the present research:(i) Yoshida’s results are based on the assumption that only PH� � �EBdimers in stoichiometry 1:1 were formed. However, they workedat analytical concentrations at which PH very likely self-associates(c0(PH) = 0.020 mol dm�3). (ii) The absorption intensities of thebands attributed to the stretching of OH� � �p oscillators, as wellas their positions, were obtained by integration. Wherever integra-tion of several weak overlapping bands is to be made, there isenough space for rising up the uncertainties. This is also possiblein the discussed case, especially due to overlapping with neglectedOH� � �O stretching bands. (iii) Absorption coefficients of the bandsbring additional unreliability in the analysis since they are esti-mated from the correlation between frequency shifts and shift inthe absorption intensities [10].

As the Kc reflects overall stability of the complex and is, presum-ably, the result of the interplay of OH� � �p and p� � �p interactions, itis interesting to compare the stability of PH� � �EB HB complex withother weak HB complexes where PH is HB donor, while HB accep-tor is, in a certain fashion, similar enough to EB. In this light, com-parison with PH� � �benzene and PH� � �ethyne is the most useful forthe estimation of the contribution of certain interaction type onoverall stability here studied complex. The value of determinedequilibrium constant is slightly greater than the one obtained fromPH� � �benzene in CCl4 [39] which is 0.26 mol�1 dm3 at 298 K (25 �C)which is considered as intermediate case of interplay of HB andvan der Waals dispersion forces. Calculations at B3LYP/6-31+G(d,p) level predicted that, in gas phase, PH and benzene formdimer in a way that OH group of PH points towards the bondbetween adjacent C-atoms of benzene, in contrast to the expecteddirection to the center of p system [40]. As the planes of PH andbenzene molecule are perpendicular, the additional interaction ofone of the ortho H atoms of PH with the bond between adjacentC-atoms opposite to the former pair is enabled. Slightly greaterKc in PH� � �EB complex than in PH� � �benzene complex implies theimportance of ethyne moiety in overall stability of the former.When PH� � �ethyne interaction is discussed, in spite of the lack ofthe value of Kc, the most recent experimental findings [41],

obtained by using IR–UV double resonance spectroscopy in gasphase coupled with calculations at B97-d/TZVPP and MP2/TZVPPlevels of theory, suggest that OH� � �p(C„C) interaction is responsi-ble for a dimer formation. When the latter was studied in matrixisolation [42], however, experimental evidences indicate that,aside OH� � �p(C„C) interaction, „CAH� � �O interaction operatesin complexation. The former is indicated by the low-frequencyshift of OH stretching of PH when ethyne is present(D � �77 cm�1), while the latter by the shift of „CAH stretchingband, when PH is present, for about 44 cm�1 to the lower frequen-cies when compared the position when PH is absent. This examplealso suggests high impact of the surroundings on the complexationpattern. Associations in which EB is HB acceptor are poorly charac-terized in terms of thermodynamics; the majority of qualitativefindings is, however, available in [26] and in the experimentaland calculated works by Naresh Patwari et al. [1–3,27,33,34].

Conclusions

Hydrogen bonding between PH and EB in stoichiometry 1:1 ischaracterized in term of association constant Kc. The values aredetermined at two temperatures, 20 �C and 26 �C and they are0.54 ± 0.09 mol�1 dm3 and, respectively, 0.36 ± 0.08 mol�1 dm3.The unambiguous participation of C„C group in HB is demon-strated by the appearance of C„C� � � stretching band. The absenceof the shift of associated C„C oscillator, in comparison with unas-sociated C„C oscillator upon complexation, is explained in thecontext of involvement of p� � �p interactions in HB complex. It isproposed that C„C moiety, when is in HB complex, does notshorten or lengthen because it interacts with both OH group andphenyl ring of PH. This geometry of the complex is enabled by rota-tion of PH around the axis perpendicular to the plane formed byPH� � �EB HB complex. According to the ab initio calculations, thepresumed arrangement of monomers has complexationenergy DE = �19.9 kJ mol�1, while the most stable complex(DE = �26.7 kJ mol�1) is the one whose arrangement not accompa-nied by the proposed rotation.

The ultimate importance of p� � �p interactions is further con-firmed when IR spectra of EtOH + EB in C2Cl4 mixtures, wherep� � �p interactions are absent, are analyzed. In the latter, OH� � �C„CHB is certainly formed, as proven from the appearance of bothOH� � � and C„C� � �stretching bands. Upon complexation, C„C� � �stretching band shifts to the higher wavenumber, as expected forfunctional group who acts only as HB acceptor. The comparisonof the C„C� � � stretching band positions in these two examplesshow that, when additional driving force like p� � �p interactions,aside HB, is enabled, properties of molecules like bond length, evenin the weak associations, can be changed.

Acknowledgements

This work was supported by a Grant No. 0982904-2927 fromMinistry of Science, Education and Sport of the Republic of Croatia.D.V. thanks to J. Aleric and T. Parlic-Risovic from Croatian Metrol-ogy Institute for measuring the densities of ethynylbenzene andtetrachloroethene. M.V. acknowledges Croatian Academy of Sci-ences and Arts for financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2014.04.149.

14 D. Vojta, M. Vazdar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 6–14

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