Infrared Spectroscopy of Protonated Benzene Clusters Biswajit Bandyopadhyay

Infrared Spectroscopy of Protonated Benzene Clusters Biswajit Bandyopadhyay, Timothy Cheng and Michael A. Duncan www. arches. uga. edu/~maduncan/ University of Georgia, Athens, GA 30602 Funding: National Science Foundation

Motivation • Pi-stacking interactions play key roles • Clusters • Biomolecules • Organic Crystals : Self assembly of aromatic rings by piinteractions. • Proteins: aromatic rings of Trp, His, Tyr, Phe bind to other aromatic rings by pi-pi or pi-H interactions • Designing functional nonmaterial – Mechanical extraction of inner-shells from multi-walled carbon nanotube. – Controlled flapping motion of molecular flippers as a precursor of nanovehicles. • Protonated systems in the gas phase are ideal model systems to understand molecular structure in absence of any other interactions

Experimental Setup Arc Discharge(1 -2 k. V) to create ions Mass Selection IR Excitation

Mass Spectrum 20% H 2 -80% Ar mixture 1 -2 k. V discharge

sp 3 CH 2 Stretch

Theoretical Calculations were done by Steven Wheeler, Ken Houk’s Group in UCLA. M 05 -2 x/6 -31+G(d, p) Calculated Structures and energies (k. Cal/mol) Relative Energy 0. 0 Complexation Energy -11. 1 Ar- Binding energy 1. 0 0. 1 1. 7 -11. 0 -9. 4 1. 7 1. 3

Infrared spectrum of the Protonated Benzene Dimer Bz 2 H+-Ar Loss of Ar channel ΔE=1. 7 ΔE=0. 1 ΔE=0

Infrared spectrum of the Protonated Benzene Trimer Bz 3 H+-Ar Loss of Ar channel Bz 3 H+ Loss of Bz channel

Infrared spectra of the protonated benzene clusters (trimer and tetramer) (Bz)4 H+Ar Loss of Ar channel (Bz)4 H+Ar Loss of Bz channel (Bz)3 H+Ar Loss of Ar channel

Conclusion • Protonated benzene dimer: The isomer with higher energy is exclusively found in the cluster source • Protonated benzene trimer: the CH 2 moiety interacts with the pi-cloud of the neutral benzene, making the sp 3 CH 2 stretch broad and red shifted • Infrared spectrum of the tetramer is almost identical to trimer, showing that neutral benzene is interacting with the other benzene Acknowledgement • Steven Wheeler • Ken Houk • Funding from NSF

Theory with and without Ar