Structure and dynamics of the excited CH–chromophore in (CF3)3CH

Abstract

The absorption spectra of gaseous (CF₃) ₃CH (1,1,1,3,3,3‐hexafluoro, 2‐trifluoromethyl propane) were recorded in the IR between 800 and 12 000 cm⁻¹ by high resolution interferometric Fourier transform techniques and in the visible from 12 000 to 17 000 cm⁻¹ by laser photoacoustic spectroscopy. Instead of single bands in the CH overtone region, complex multiplet structures were observed. Thirty‐nine bands were assigned as arising from the interacting CH‐stretching and CH‐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. Close agreement is obtained for the positions and intensities of the observed spectral features using only seven spectroscopic parameters. The experimental and theoretical results are summarized in Tables II, III, and IV. The Hamiltonian can be used to calculate and understand the time‐dependent redistribution of vibrational energy between the coupled CH‐stretching and CH‐bending vibrations. The role of broad vibrational band shapes and the possible exponential decay of CH excitation into a background of states from low‐frequency vibrations is discussed.