Rotational diffusion in methyl iodide: Bandshape analysis of degenerate vibrational modes

Abstract

The Raman spectral bandshapes of the v,(A ,), v,(A ,) and v4(E) vibrations of CH,I were studied as a function of temperature in the liquid phase. Perpendicular diffusion coefficients calculated from v, agreed well with earlier reported results. The degenerate CH, stretching vibration, v4, was analyzed via fitting by a model incorporating two Lorentzian lineshapes. Values of the parallel diffusion constants, Dll , determined using only the narrower component were in excellent semi-quantitative agreement with those predicted by the free rotor model of molecular reorientation. Analysis of the room-temperature Raman spectrum of vs(@ yielded the same value of Dll as that calculated from v., . The room-temperature infrared spectral intensities of v5 were fitted by a model with a narrow Lorentzian containing contributions from rotational and vibrational relaxation superposed on a second component introduced to account for the broad background absorption. Dll determined from the IR bandwidth was in good agreement with values obtained from the Raman spectra of v, and v5. These results demonstrate the utility of analyzing degenerate vibrational modes to determine the parallel diffusion Coefficients in C,, molecules.Analysis of the isotropic and anisotropic Raman linewidths of totally symmetric vibrations is a well established, reliable technique for the determination of 'tumbling' rotational diffusion coefficients (Dl) of symmetric top molecules in 1 i q~i d s . l~~ More recently, a number of researchers have successfully utilized various methods of estimating vibrational relaxation contributions (A,,) to the bandwidth to extract the 'spinning' diffusion constants ( D 11) from the doubly degenerate vibrations of molecules of D6h4,5 and D,h6-9 symmetry. To date, however, attempts to determine Dll for molecules of C,, or lower symmetry from Raman spectra of the degenerate (E) modes have proved unsuccessful owing to the more complex dependence of the lineshape on D, , Dl, and A, .1--3*10In order to ascertain whether reliable estimates of the parallel diffusion coefficients can be calculated from the degenerate vibrations of C,, molecules, we have measured the Raman bandshapes of the v4(CH, antisymmetric stretch), v,(CH, symmetric stretch) and v,(C-I stretch) modes of methyl iodide as a function of temperature in the liquid phase. The derived values of D II are compared with results from room-temperature Raman and infrared bandshapes of the v5(CH, stretch) vibrational mode and with the predictions of current theories of rotational diffusion. VIBRATIONAL BANDSHAPES OF DEGENERATE MODES IN C,, MOLECULESAssuming a statistically independent, exponentially decaying, vibrational relaxation correlation function [G,(t) = exp( -t/.r,)] and small-step rotational diffusion, the total correlation function for Raman vibrations of E symmetry in C3, molecules is the sum of two exponential terms:2The coefficient A is a function of the spherical components, b',\ and by',, of the polarizability derivative tensor, and the correlation times are gi...

Orientational Diffusion of Methyl Groups in Crystalline CH3F: An Infrared Study

“…Nearly free rotation has been reported for various CH 3 X molecules in the liquid phase and solutions. ,,,, Thus, it is of interest to compare the spinning diffusion constant for crystalline CH 3 F with that for the liquid phase of similar compounds. Extrapolating relation 10 to higher temperatures, we obtained the D ∥ value of 180(30) × 10 10 s -1 for the hypothetical “solid” CH 3 F at 295 K. This, as anticipated, agrees well with the D ∥ (295 K) values determined for liquid CH 3 Br, CH 3 I, and CH 3 CN, which also fall in the 180(30) × 10 10 s -1 region. ,, …”