Presentation on theme: "Silver Nyambo Department of Chemistry, Marquette University, Wisconsin Resonance enhanced two-photon ionization (R2PI) spectroscopy of halo-aromatic clusters."— Presentation transcript:
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, 85-1095. 2.Kevin, E et al J. Phys. Chem. A 2007, 1111, 1688-1694 3.Metrangolo, P et al Acc. Chem. Res. 2005, 38, 386-395
Experimental Details Laser beam Signal to amplifier S1S1 S0S0 S1S1 λ 1 (270.21 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 (270.21 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, 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
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 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/C3CP00054K Peter 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.
3.637 3.015 3.655 3.059 3.629 3.011 2.674 2.480 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