Infrared Spectroscopy of Protonated Benzene Clusters Biswajit Bandyopadhyay, Timothy Cheng and Michael A. Duncan University of Georgia, Athens, GA Funding: National Science Foundation

Motivation Pi-stacking interactions play key roles Clusters Biomolecules Organic Crystals : Self assembly of aromatic rings by pi- interactions. 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(1-2kV) to create ions Mass Selection IR Excitation

Mass Spectrum 20% H 2 -80% Ar mixture 1-2 kV discharge

sp 3 CH 2 Stretch

Theoretical Calculations were done by Steven Wheeler, Ken Houk’s Group in UCLA. M05-2x/6-31+G(d,p) Calculated Structures and energies (kCal/mol) Relative Energy Complexation Energy Ar- Binding energy

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 + Loss of Bz channel Bz 3 H + -Ar Loss of Ar channel

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