Infrared and Raman spectroscopic study of molecular reorientation of mesitylene in the neat liquid

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...

Section: Perpendicular Diffusion Coefficientsmentioning

confidence: 99%

Section: Perpendicular Diffusion Coefficientsmentioning

confidence: 99%

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

Wang

Yuan

Schwartz

1989

Reorientation of methylbenzene molecules in the liquid phase: A comparative Raman band shape analysis

Ribeiro‐Claro

Costa

Teixerira-Dias

1987

A comparative Raman band shape study of the reorientation of monosubstituted benzene molecules (C6H5X, X = CH,, CH,Cl, CHCl,, CCl, and C=CCH,) in the liquid phase has been performed. Reorientational correlation functions appear to be less sensitive to molecular reorientation about the major axis ( Z axis) than to rotation of the axis itself, approaching a symmetric top case. Molecular motion anisotropy is found to increase in the order C6H5CH3 < C6H5CH2Cl < C,H5CHCl2 < C6H5CC13 < C6H&CCH3 as the rotation about the Z axis becomes increasingly preferred. These results are in agreement with NMR spin-lattice relaxation data.As expected for liquids with important intermolecular interactions, both the free rotor and Kubo's model fail to fit the experimental correlation functions.

Raman and infrared study of reorientational dynamics in trifluorobenzene

Yuan

Chen

Schwartz

1989

The Raman spectral bandwidths of two A,' vibrations of 1,3,5trifluorobenzene (TFB) were measured in the neat liquid as a function of temperature and in the solvents CH,CI, and CS,. The Raman and 1R linewidths of two E modes were also measured in solution. Derived values of the two rotational diffusion constants, D , and D,, , were compared with the predictions of several current theories. It was observed that the Hynes-KapraCWeinberg (HKW) theory is superior to both the microviscosity-free rotor and Hu-Zwanzig 'slip' models of reorientational diffusion. Calculated values of D , and D , , for TFB in solution using the HKW model are in excellent agreement with the experimentally measured diffusion constants