Resonance enhanced two-photon ionization (R2PI)
spectroscopy of halo-aromatic clusters
Silver Nyambo
Department of Chemistry, Marquette University, Wisconsin
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 noncovalent 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
R2PI of Bromobenzene
EXP SETUP
Signal to amplifier
S1
(9.03 eV)1
λ1(270.21 nm)
S1
λ1(270.21 nm)
Laser beam
S0
1. Sergeev, Akopyan, et al. Photoionization processes in phenyl halides,Opt. i Spektroskopiya, 1970, 29, 119
R2PI TOF mass spectrum of bromobenzene monomer
and dimer
(C6H579Br…..C6H581Br)
0.06
81
Br
0.05
79
Intensity
0.04
Br
(C6H579Br)2
0.03
(C6H581Br)2
0.02
0.01
0.00
145 150 155 160 165
300 305 310 315 320 325
Mass (AMU)
R2PI spectra for clusters
 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.
Dimer
Trimer
Monomer
36400 36600 36800 37000 37200 37400 37600 37800
-1
Energy in cm
Chlorobenzene
1. Lloyd Muzangwa et al, J. Chem. Phys. 137, 184307 (2012); doi: 10.1063/1.4765102
Bromobenzene
Computational Details

Geometry optimizations and binding energies: M06-2X / augcc-pVDZ and corrected for BSSE and ZPE.

Electronic absorption and oscillator strength: TD-DFT (M062X/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
Optimized dimer conformations: M06-2X/aug-cc-pVDZ
Dimer 1 (21.34 KJ/mol)
Dimer 4 (9.99 KJ/mol)
Dimer 2 (19.43 KJ/mol)
Dimer 5 (6.56 KJ/mol)
Dimer 3(21.57 KJ/mol)
Dimer 6 (3.47 KJ/mol)
Dimer 7 (6.97 KJ/mol)
 Binding energies were corrected for zero point energy and the counterpoise method was used to
correct for basis set superposition error (BSSE).
The σ-hole and halogen bonding
 A region of positive electrostatic
potential of some covalently
bonded halogen atom.
Halogen bonded 1
 The halogen bond strength
increases in the order Cl ˂ Br ˂ I .
Halogen bonded 2
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
Bromobenzene dimer: TD-DFT calculations
• Each dimer has two
absorptions
• All dimer absorptions are
red shifted .
Oscillator strength
0.004
Ortho
Meta
CH-pi
Halogen bonded
Para
Monomer
0.002
0.000
36600
36800
37000
37200
Wavelength in cm
37400
-1
37600
• Absorption occurs over a
broad range consistent
with spectra .
TD-DFT: Excited states
S1
3.015
2.674
3.637
S0
2.480
π …… π
Br…….Br and CH……Br
• The electronic transition is a π to π* transition
• The transition leads to a change in geometry in the S1
CH…….π
Mixed clusters: Bromobenzene and benzene
bbz monomer
mixed dimer
bbz dimer
bbz monomer
mixed dimer
-600
-400
-200
0
200
Relative Energy in cm
13.45KJ/mol
400
600
-600
-400
-200
0
200
400
-1
Relative energy/ cm
-1
5.86KJ/mol
600
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