Vibrational spectrum and potential energy surface of the CH chromophore in CHD3

Abstract

The rovibrational spectrum of trideutero-methane has been measured at resolutions mostly close to the Doppler limit on an interferometric Fourier transform spectrometer from the lowest fundamental vibration to high overtones of the CH stretching vibration (wave numbers from 900 to 12 000 cm−1). The CH chromophore spectrum is fully assigned and interpreted by means of the tridiagonal Fermi resonance Hamiltonian for the coupled CH stretching and bending vibrations. The Hamiltonian predicts and also fits the visible spectrum up to 19 000 cm−1 measured by Scherer et al., Perry et al., and Campargue et al. The effective tridiagonal Hamiltonian is derived ab initio by means of MRD-CI and full CI calculations of the potential surface of methane, a variational vibrational calculation in a normal coordinate subspace of the coupled CH stretching and bending motions and an approximate similarity transformation to tridiagonal form. Fits of the experimental results by the tridiagonal and the variational Hamiltonian lead to equivalent spectroscopic constants. A careful experimental estimate of the main Fermi resonance coupling constant gives k′sbb ≂(30±15) cm−1 in agreement with the best current ab initio result (31 cm−1). The ab initio potential in polar normal coordinates agrees with the potential derived from spectroscopic data covering an energy range of about 220 kJ mol−1 (more than half the dissociation energy). Good predictions are obtained for the parameters of the effective Hamiltonian, the spectral patterns, intensity distributions, and rotational constants in the Fermi resonance polyads. Three alternative interpretations of the parameters of the effective Hamiltonian are investigated and rejected on the basis of the available experimental and ab initio data. The results and conclusions are discussed in relation to intramolecular vibrational redistribution on the subpicosecond time scale and the recombination–dissociation kinetics of methane.