Journal of Physical Chemistry A, Vol.110, No.9, 3059-3070, 2006
Slit discharge IR spectroscopy of a jet-cooled cyclopropyl radical: Structure and intramolecular tunneling dynamics
High-resolution infrared spectra of a jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH2 stretch (nu(7)) 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 alpha-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 K-a+K-c in lower (+) and upper (-) tunneling levels, respectively) consistent with C-2 nu 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 nu(7) tunneling levels. In particular, the upper (-) tunneling component of the nu(7) 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 nu(7)-excited states, respectively. This analysis indicates ground-state stereoracemization of the alpha-CH radical center to be a very fast process [k approximate to 2.0(4) x 10(11) 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 V-0 = 1115 +/- 35 cm(-1) barrier height for alpha-CH inversion through the cyclopropyl CCC plane.