A Raman study of solvent effects on n-butane: Torsional dynamics, rotational dynamics, and conformational equilibrium
We have measured Raman spectra of n-butane dissolved in four solvents, carbon disulfide, benzene, methanol, and cyclopentene. From the band intensities of the in-phase skeletal bending modes of the gauche (325 cm-1) and trans (430 cm-1) conformers, we have determined the enthalpy difference ΔH between the gauche and trans conformers in these solvents to be 397 ± 32 (carbon disulfide), 435 ± 74 (benzene), 467 ± 41 (methanol), and 523 ± 23 cal/mol (cyclopentene). The line widths of these same two bending bands have been analyzed by using the theory of a previous paper (MacPhail, R. A.; Snyder, R. G. J. Chem. Phys. 1989, 91, 3895) to yield experimental estimates of the correlation time π for torsional oscillations of the gauche conformer in these solvents. This correlation time is proportional to the solvent friction on the torsional coordinate and deviates significantly from the behavior predicted by hydrodynamic models of solvent friction, the deviations being most dramatic in the case of methanol. We have also measured rotational correlation times of n-butane in methanol and in carbon disulfide, and these too deviate from hydrodynamic behavior. While these deviations are not as significant as for the torsional correlation times, they are again most pronounced for methanol. The dependence of the torsional correlation times on solution viscosity can be well represented by the power law τ= bηa, where the value of the exponent varies from 0.4 in the carbon disulfide solution to 0.1 in methanol. Alternatively, the temperature dependence of τ can be reproduced by using a collisional Enskog model for the solvent friction. Our results show that the "well friction" in an isomerization reaction can show deviations from hydrodynamic behavior that are quite similar to those often observed for the "barrier friction". © 1991 American Chemical Society.
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