Anharmonic effects on the structural and vibrational properties of the ethyl radical:: A path integral Monte Carlo study

The structural and vibrational properties of the ethyl radical have been investigated by a series of finite temperature simulations that treat the nuclei as quantum particles. The potential energy surface of the electronic ground state has been described by a nonorthogonal tight-binding Hamiltonian that provides results in reasonable agreement with ab initio methods. The quantum nature of the nuclei has been described by path integral Monte Carlo simulations at temperatures between 25 and 1000 K. Special interest deserves the determination of anharmonic and tunneling effects in the zero-point vibrational structure. In particular, we have studied the influence of anharmonic effects both on the mean value and the quantum fluctuations of equilibrium bond lengths and bond angles. The local structure of the radical center is found to be planar as a result of the zero-point motion of the atomic nuclei, even though the minimum energy configuration exhibits a pyramidal structure for this center. Anharmonic effects in the fundamental vibrational modes of the molecule are studied by a nonperturbative approach based on the centroid density. This function is a path integral concept that provides information on the static response of the system to applied external forces. Our study reveals a softening of the stretching modes associated with the C-H bonds and a hardening of the out-of-plane rocking motion of the methylene group. Both effects are in good agreement with experimental and ab initio data. The softening of the C-C stretching mode predicted by our simulations suggests a revision of the currently accepted experimental assignment for two fundamental vibrations of the ethyl radical. The tunneling of an H atom between the methyl and methylene groups has been investigated. These simulations should contribute to the open question whether or not this process is responsible for the changes in the electron spin resonance spectrum at low temperatures. (C) 2003 American Institute of Physics.