P. Schiebel et al., TRANSLATIONAL ROTATIONAL COUPLING OF THE HINDERED CH3 QUANTUM-ROTOR IN LITHIUM-ACETATE DIHYDRATE/, The Journal of chemical physics, 108(6), 1998, pp. 2375-2382
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.