Silver Nyambo Department 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, Kevin, E et al J. Phys. Chem. A 2007, 1111, Metrangolo, P et al Acc. Chem. Res. 2005, 38,

Experimental Details Laser beam Signal to amplifier S1S1 S0S0 S1S1 λ 1 ( nm) (9.03 eV) 1 R2PI of Bromobenzene EXP SETUP 1. Sergeev, Akopyan, et al. Photoionization processes in phenyl halides,Opt. i Spektroskopiya, 1970, 29, 119 λ 1 ( nm)

R2PI TOF mass spectrum of bromobenzene monomer and dimer (C 6 H 5 79 Br) 2 (C 6 H 5 81 Br) 2 (C 6 H 5 79 Br…..C 6 H 5 81 Br)

R2PI spectra for clusters 1.Lloyd Muzangwa et al, J. Chem. Phys. 137, (2012); doi: /  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

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 method 1.  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 (2011). 2.Zhao, Y.; Truhlar, D.G. Theor. Chem. Account 2008, 120, 215. DOI

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: /C3CP00054K Peter Politzer et al. J. Mol. Model 2007 DOI /s 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.

S1S1 S0S0 π …… πBr…….Br and CH……BrCH…….π TD-DFT: Excited states The electronic transition is a π to π* transition The transition leads to a change in geometry in the S 1

Mixed clusters: Bromobenzene and benzene 13.45KJ/mol 5.86KJ/mol

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 Urler Lloyd Muzangwa Aimable Kalume Lisa George Funding: National Science Foundation