Slit Discharge IR Spectroscopy of a Jet-Cooled Cyclopropyl Radical: Structure and

High-resolution infrared spectra of a jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH2 stretch (î7) vibration. In addition to yielding the first precise gas-phase structural information, the spectra reveal quantum level doubling into lower (+) and upper (-) states due to tunneling of the lone R-CH with respect to the CCC plane. The bands clearly reveal intensity alternation due to H atom nuclear spin statistics (6:10 and 10:6 for even:odd Ka+Kc in lower (+) and upper (-) tunneling levels, respectively) consistent with C2V symmetry of the cyclopropyl-tunneling transition state. The two ground-state-tunneling levels fit extremely well to a rigid asymmetric rotor Hamiltonian, but there is clear evidence for both local and global state mixing in the vibrationally excited î7 tunneling levels. In particular, the upper (-) tunneling component of the î7 state is split by anharmonic coupling with a nearly isoenergetic dark state, which thereby acquires oscillator strength via intensity sharing with this bright state. From thermal Boltzmann analysis of fractional populations, tunneling splittings for a cyclopropyl radical are estimated to be 3.2 ( 0.3 cm-1 and 4.9 ( 0.3 cm-1 in the ground and î7-excited states, respectively. This analysis indicates ground-state stereoracemization of the R-CH radical center to be a very fast process [k 2.0(4) 1011 s-1], with the increase in the tunneling rate upon CH2 in-phase asymmetric stretch excitation consistent with ab initio predictions of equilibrium vs transition-state zero-point energies. Modeling of the ground-state-tunneling splittings with high level ab initio 1D potentials indicates an improved V0) 1115 ( 35 cm-1 barrier height for R-CH inversion through the cyclopropyl CCC plane. I