Torsional damping and solvent friction in liquid n-butane: Experimental estimates from Raman spectroscopy
The isotropic Raman linewidths of the in-phase CCC bending modes of liquid n-butane are analyzed in terms of dephasing by torsional oscillations. A simple, effective Hamiltonian is developed to calculate the coupling between this bending mode and the torsion. For the gauche conformer the coupling is linear in the torsional coordinate and quite strong, but for the trans conformer the coupling is weak. This coupling is used to relate the linewidths of the bending modes to the torsional dynamics, which are modeled by a damped, harmonic oscillator. The damping constant and a related torsional correlation time are then extracted from the experimental linewidths. The resulting correlation times are compared with those calculated assuming either hydrodynamic or collisional (Enskog) friction on the torsional coordinate. Both theoretical models give values that lie below the experimental upper bound, but the Enskog friction compares somewhat better with our best experimental estimates of the torsional damping. © 1989 American Institute of Physics.
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