15min:

FENG DONG, SCOTT DAVIS AND DAVID J. NESBITT, JILA, University of Colorado, Boulder, CO 80309.

High-resolution infrared spectra of jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH₂ stretch ( ₇) vibration. In addition to yielding first precise gas phase structural information for this radical, the spectra reveal doubling due to quantum tunneling of the lone -CH with respect to the CCC plane, lift the degeneracy and generating lower (+) and upper (-) tunneling states. The bands clearly reveal intensity alternation (6:10 for even:odd Ka+Kc levels in the lower lower and 10:6 in the upper upper) due to H atom nuclear spin statistics, confirming that the tunneling transition state for cyclopropyl radical is of C_2v symmetry. However, in addition to the two predicted vibrational bands (i.e. lower lower and upper upper), a third band is observed due to IVR mixing of the upper tunneling component ( ₇) with a nearly isoenergetic dark state. From fractional populations in the ground and excited state, the tunneling splittings for cyclopropyl radical in the ground and excited ( ₇) state are estimated to be 3.2\pm0.3 cm^-1 and 4.9\pm0.3 cm^-1, respectively. This indicates that stereoracemization of the -CH radical center is a very fast process (k=2.0x10¹¹ s^-1), and that the barrier decreases upon vibrational excitation of the CH₂ in-phase antisymmetric stretch. The barrier height for -CH inversion through the cyclopropyl plane is also extracted from the analysis of the tunneling dynamics with simple 1D potential energy surface.