Raman study of vibrational dephasing in haloalkanes

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Abstract
The Raman spectra of the carbon–halogen stretching mode was measured as a function of temperature in the five liquids: ethyl bromide, isopropyl chloride and bromide, and t-butyl chloride and bromide. The derived isotropic (vibrational relaxation) correlation times showed a very clear trend. The magnitude of τiso was quite similar in both chloroalkanes (in the range from 1.2 to 1.5 ps), whereas, the values in all three bromoalkanes were virtually superposable and markedly longer (1.8–2.1 ps) than in the chloro compounds. These results, together with those from earlier studies on methyl iodide, provide evidence that vibrational dephasing efficiencies (τ−1iso) of the C–X mode in haloalkanes vary in the order Cl≳Br≳I. It was further observed that, in contrast to the rather unusual temperature dependence of vibrational relaxation in methyl iodide, τiso either remained approximately constant or decreased with temperature in the compounds studied here. Modulation times calculated from the Kubo expression were found to be significantly longer (approximately 1 ps) in the isopropyl halides than in the t-butyl compounds, where τm’s were of the same magnitude as Enskog collision times. Vibrational dephasing times calculated from the Fischer–Laubereau isolated binary collision model were of the same order of magnitude but longer than the experimental correlation times.