Infrared Spectroscopy of Protonated Water-Benzene Clusters Tim Cheng Biswajit Bandyopadhyay Michael A. Duncan Department of Chemistry, University of Georgia Athens, GA 30602 NSF
Introduction • Understand solvation at a molecular scale • Importance of protonated species for proton transfer intermediates • Protonated water-benzene clusters in the gas phase can be used to study hydrophilic and hydrophobic interfaces • Neutral benzene-water clusters and cationic benzene clusters studied previously (Zwier, Mikami, etc) • IR spectroscopy of the shared proton between two species has been previously studied by several groups (Asmis, Dopfer, Johnson, Duncan, etc. )
J. R. Roscioli, L. R. Mc. Cunn, M. A. Johnson, Science, 2007, 316, 249 sp 3 CH 2 Stretch
Experimental Setup Laser. Vision OPO/OPA with Ag. Ga. Se 2 crystal. Fully automated, turn key system allows for continuous coverage throughout the midinfrared (4500 -800 cm-1). Linewidth ~2 cm-1 An approximately 2 k. V, 2 us high voltage pulse creates a discharge in the center of a molecular precursor/rare gas pulsed supersonic expansion. Collisional cooling and complexation leads to rare gas “tagged” cations.
Mass Spectrum
Shared Proton Stretch B 3 LYP/6 -311+G(d, p)
Protonated Benzene-Water • Proton affinity of benzene: 179. 3 kcal/mol • Proton affinity of water: 165. 2 kcal/mol • While proton affinity is higher for benzene than water, the spectrum of the mixed complex shows that the proton is closer to the water • Proton affinity is not the only deciding factor in choosing where a proton goes in two competing molecules
B 3 LYP/6 -311+g** No fragmentation seen below 2000 cm-1. DFT calculated binding energies for benzene-water is over 4000 cm-1, However, the energies are not BSSE corrected, which would lower the binding energies
Proton stretch between benzene/Ar
sp 3 stretch of protonated benzene
Conclusion • Proton affinity is not the only determining factor of proton location of mixed complexes • Between benzene and water by itself, the proton is shared asymmetrically, with it closer to the water • In the case of multiple-water with a single benzene clusters, the proton is shared between water, with a neutral benzene on the side • The shared proton stretch shifts to the blue with the addition of multiple waters • In protonated multiple-benzene with a single water cluster, protonated benzene is formed, interacting with both a neutral water and benzene
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