Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF3H

Abstract

The absorption spectrum of trifluoromethane has been recorded between 900 and 14 000 cm⁻¹ with resolutions between 0.004 and 0.5 cm⁻¹ (pressure broadened). 22 bands were assigned as arising from the interacting CH stretching and bending manifolds, which account for most of the absorption in the overtone region. The results can be understood quantitatively with an effective, tridiagonal many‐level Fermi resonance Hamiltonian. The experimental and theoretical results are summarized in Table II. The Hamiltonian is given in Table III and shows a very large stretching–bending interaction constant ‖k_sbb‖=106 cm⁻¹, which is even larger than the diagonal anharmonic constant for the stretching vibration ‖x^′_ss‖=62 cm⁻¹. This leads to extensive vibrational redistribution between stretching and bending motions at high levels of excitation. The time dependent redistribution is calculated with the spectroscopic Hamiltonian. A rotational analysis is presented for some of the bands involved in the Fermi resonance. The effect of the Fermi resonance on hot bands is investigated using the same Hamiltonian in comparison with experiment. The results are discussed in relation to the universal local dynamics of the isolated alkyl CH‐stretching chromophore and in relation to the vibrational dynamics of highly excited polyatomic molecules as a function of certain elements of molecular structure.