TRANSLATIONAL ROTATIONAL COUPLING OF THE HINDERED CH3 QUANTUM-ROTOR IN LITHIUM-ACETATE DIHYDRATE/

The multiplet structure of the rotational-tunneling peaks in the inela stic neutron-scattering spectrum of lithium acetate is widely regarded as evidence of coupled pairs of CH3 quantum rotors. However, our mole cular-mechanics study, which has no adjustable parameters, reveals tha t 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 almos t circular path around its time-average position. The energy levels fr om this dynamical model reproduce the observed tunneling-transitions r easonably well, and the predicted rectangular density distribution of the three methyl H-atoms is in good agreement with that measured by si ngle-crystal neutron diffraction. (C) 1998 American Institute of Physi cs.