15min:

TREVOR J. SEARS, Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973; AND PHILIP M. JOHNSON, Department of Chemistry, SUNY, Stony Brook, NY 11794.

First detected in 1988, the -CH₂ wagging fundamental of C₂H₅ recorded at high resolution by diode laser absorption extends from approximately 470 to more than 600 cm^-1. The mode corresponds to the out-of-plane motion at the radical center and this is strongly coupled to the torsion or internal rotation in this species. We published a preliminary analysis of the spectrum in 1996, but at the time recognized many questions remained. Aided by extensive ab initio calculations and the detection of part of the spectrum at 3 µ m in a jet-cooled sample, we have recently made much progress understanding the details of the 20 µ m spectrum and the internal dynamics in the radical. In total, close to 500 transitions have been assigned in 18 rotation-torsion branches. The observation of branches involving levels with m_torsion = 3 and low k_a, in particular, has allowed much more precise estimates of the barrier to internal rotation and its change on vibrational excitation. Calculations of the torsion/wag potential surface have allowed a physical interpretation of these experimental results.

Acknowledgement : This work was carried out at Brookhaven National Laboratory under contract DE-AC02-98CH10886 with the U. S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.