Reexamination of the [formula omitted] microwave and infrared spectra

Abstract

The high-resolution infrared spectrum of C 2 H 3 + recorded in the 3μ CH stretching region (J. Phys. Chem. 99 (1995) 15611–15623) is re-analyzed using an effective internal axis method (IAM) Hamiltonian to account for tunneling splittings associated with the large amplitude hydrogen migration motion. The line position analysis carried out with this approach allows us to fit 63% of the data with a standard deviation of 0.05 cm −1, using eight parameters, including the band center, five semi-rigid-rotor spectroscopic constants, and two parameters describing the magnitude of the tunneling splitting and its rotational dependence. The rotational dependence of the observed tunneling splittings is described by an angular offset θ, which can also be calculated theoretically from ab initio equilibrium and transition-state structures already in the literature. The agreement between fitted and calculated values of θ, as well as agreement with the value determined from a previous treatment of splittings in the vibrational ground state, gives strong support for the validity of the model. Additional support is provided by the barrier height of 1488 cm −1 determined here from the ground state splittings, which agrees well with an ab initio estimate of 1300 ± 450 cm −1. The principal problem in the present treatment of the infrared data is the fact that observed minus calculated residuals for 37% of the assigned lines are greater than 0.05 cm −1. This is believed to be due to random perturbations by dark states.