15min:
ON THE OUTCOME OF THE REACTIONS BETWEEN HYDROCARBON RADICALS AND O2 IN HELIUM NANODROPLETS.

A. M. MORRISON AND G. E. DOUBERLY, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556.

Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH3 + O2 and C3H3 (propargyl) + O2 reactions. The hydrocarbon radicals are generated in an effusive pyrolysis source located upstream from a differentially pumped O2 gas pick-up cell. In this experimental configuration, the reaction occurs between sequentially picked-up and presumably cold fragments. The CH3 + O2 reaction leads barrierlessly to the methyl-peroxy radical, and despite having to dissipate an energy of approximately 30 kcal/mol, the infrared spectra reveal a large abundance of droplets containing the cold CH3O2 radical. Theoretical studies have predicted an approximately 2-4 kcal/mol barrier in the entrance channel of the C3H3 + O2 reaction. Therefore, we initially expected to see a weakly bound “entrance channel” C3H3--O2 van-der-Waals complex, given the rapid cooling provided by the dissipative helium environment. However, only the trans-acetylenic isomer of the propargyl-peroxy radical is observed. The dipole moment of this species is measured with infrared laser Stark spectroscopy.