Silver NyamboDepartment of Chemistry, Marquette University, Wisconsin

Resonance enhanced two-photon ionization (R2PI) spectroscopy of halo-aromatic clusters

June 2013

Background and motivation

Non covalent interactions such as π-stacking, C-H/π, halogen bonding, hydrogen bond plays a crucial role in chemical and biological processess.

Halobenzenes can be used as prototypes to study these non-covalent interactions.

These interactions may coexist and compete with each other during formation of large molecular assemblies, hence understanding their relative magnitudes becomes important.

1.Ghosh et al Biol. Crystallogr. 2000, 56, 85-1095.2.Kevin , E et al J. Phys. Chem. A 2007, 1111, 1688-16943.Metrangolo, P et al Acc. Chem. Res. 2005, 38, 386-395

Experimental Details

Laser beam

Signal to amplifier

S1

S0

S1

λ1(270.21 nm)

(9.03 eV)1

R2PI of BromobenzeneEXP SETUP

1. Sergeev, Akopyan, et al. Photoionization processes in phenyl halides,Opt. i Spektroskopiya, 1970, 29, 119

λ1(270.21 nm)

145 150 155 160 165 300 305 310 315 320 3250.00

0.01

0.02

0.03

0.04

0.05

0.06

Inte

nsi

ty

Mass (AMU)

R2PI TOF mass spectrum of bromobenzene monomer and dimer

(C6H579Br)2 (C6H5

81Br)2

(C6H579Br…..C6H5

81Br)

79Br

81Br

R2PI spectra for clusters

1. Lloyd Muzangwa et al, J. Chem. Phys. 137, 184307 (2012); doi: 10.1063/1.4765102

All clusters show broad absorption.

red shifted with respect to monomer transition.

Spectra of different clusters are similar.

No halogen bonding was found for chlorobenzene clusters.

Chlorobenzene Bromobenzene

36400 36600 36800 37000 37200 37400 37600 37800

Energy in cm-1

Dimer Trimer Monomer

Computational Details

Geometry optimizations and binding energies: M06-2X / aug-cc-pVDZ and corrected for BSSE and ZPE.

Electronic absorption and oscillator strength: TD-DFT (M06-2X/aug-cc-pVDZ)

Sherrill and co-workers benchmarked the performance of DFT methods against high level post-HF ab initio single reference methods and they found out M06-2X was a cost effective method1.

Zhao and Truhlar have shown that M06-2X performs well on describing non covalent interaction energies.2

1.L. A. Burns, A. Vazquez-Mayagoitia, B. G. Sumpter, and C. D. Sherrill, Journal of Chemical Physics 134 084107 (2011).2.Zhao, Y.; Truhlar, D.G. Theor. Chem. Account 2008, 120, 215. DOI 10.1007

Dimer 4 (9.99 KJ/mol) Dimer 5 (6.56 KJ/mol) Dimer 6 (3.47 KJ/mol) Dimer 7 (6.97 KJ/mol)

Optimized dimer conformations: M06-2X/aug-cc-pVDZ

Binding energies were corrected for zero point energy and the counterpoise method was used to correct for basis set superposition error (BSSE).

Dimer 2 (19.43 KJ/mol)Dimer 1 (21.34 KJ/mol) Dimer 3(21.57 KJ/mol)

The σ-hole and halogen bonding A region of positive electrostatic

potential of some covalently bonded halogen atom.

The halogen bond strength increases in the order Cl ˂ Br ˂ I .

Peter Politzer et al. PCCP 2013 DOI: 10.1039/C3CP00054KPeter Politzer et al. J. Mol. Model 2007 DOI 10.1007/s00894-006-0154-7

Halogen bonded 1

Halogen bonded 2

Bromobenzene dimer: TD-DFT calculations

• Each dimer has two absorptions

• All dimer absorptions are red shifted .

• Absorption occurs over a broad range consistent with spectra . 36600 36800 37000 37200 37400 37600

0.000

0.002

0.004

Ortho Meta CH-pi Halogen bonded Para Monomer

Osc

illat

or st

rength

Wavelength in cm-1

3.637

3.015

3.655

3.059

3.629

3.011

2.674

2.480

S1

S0

π …… π Br…….Br and CH……Br CH…….π

TD-DFT: Excited states

• The electronic transition is a π to π* transition• The transition leads to a change in geometry in the S1

Mixed clusters: Bromobenzene and benzene

-600 -400 -200 0 200 400 600

Relative Energy in cm-1

bbz monomer mixed dimer

13.45KJ/mol 5.86KJ/mol

-600 -400 -200 0 200 400 600

bbz monomer mixed dimer bbz dimer

Relative energy/ cm-1

Summary

Non-covalent interactions in bromobenzene were probed experimentally and computationally.

Computationally, representative minima dimer structures were optimized confirms the existence of multiple conformers.

TD-DFT calculations were used to support our experimental findings.

Acknowledgements

Advisor: Scott A. Reid

Group members:Brandon UrlerLloyd MuzangwaAimable KalumeLisa George

Funding:National Science Foundation