Translational/rotational coupling of the hindered CH3 quantum-rotor in lithium acetate dihydrate

摘要

The multiplet structure of the rotational-tunneling peaks in the inelastic neutron-scattering spectrum of lithium acetate is widely regarded as evidence of coupled pairs of CH3 quantum rotors. However, our molecular-mechanics study, which has no adjustable parameters, reveals that rotor/rotor coupling is less important than translational/rotational coupling. In order to remain at a potential-energy minimum during CH3 group reorientation the center-of-mass of this group follows an almost circular path around its time-average position. The energy levels from this dynamical model reproduce the observed tunneling-transitions reasonably well, and the predicted rectangular density distribution of the three methyl H-atoms is in good agreement with that measured by single-crystal neutron diffraction. (C) 1998 American Institute of Physics. [References: 26]