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.

The scattering densities of the orientationally disordered protons and deuterons in Ni(NX,)bY? (X = H or D, Y = Br. 1. NO, or PFd %e obtained from neutron single-crystal diffraction data al room temperature. While we observe a nearly crrcular density distribution for Y = PF4. a nuclear density distribution with four maxima on a square shows up for Y = 51.I and Nos. This is 3 pronounced deviation from the circular distribution that is expected from uniaxial rotational diffusion or reorientational jump models for the dynamics of the Orientational disordered ammonia groups. All observed density distributions m consistently explained as the consequence of rotation-tmslationn coupling in an Mharmonic crystal potential. The calculated potential parameten. i.e. anharmonicily and strength of the coupling, depend on the type of anion present in the crystal frame. The we& anisompy in the Y=NOI and PF, compounds is the consequence of a nearly eightfold non-cvstallographic symmehy of the atom establishing the hindrance potential. P Schiebel et ai Potential reftnementWe determine the potential parameters A, B , C and the distance dp between the protons and the CM of the proton triangle by a least-squares analysis. by minimizing the residual = ~[ P B a l a m u n n ( x i ~ J'i) -&'ohs(&.

Bragg intensities from neutron and X-ray diffraction data of C6o single crystals were used to determine the nuclear-and electron-density distributions of C60 at room temperature. The anisotropic density distribution is reconstructed by the maximum-entropy method and evaluated in terms of symmetry-adapted spherical harmonics. From this analysis, the orientational probability density function f(w) has been calculated and the rotational potential V(w) that is experienced by a C60 molecule in the cubic surrounding at 295 K has been obtained, f(w) shows strong deviations from the uniform orientational probability density function that would result from isotropic rotation. Accordingly, V(w) exhibits well developed minima. The absolute potential minimum is found at an Euler-angle set Wl and a second set of minima at slightly higher energy at w2. The potential difference between V(wl) and V(w2) is 313 K, whereas the overall rotational potential barrier height amounts to 522 K. w~ and w2 are comparable with the major and minor orientations that are adopted by the molecules in their low-temperature arrangement. The angles t,o 1 and w2 are fixed by the intrinsic geometry of the Euler-angle space (a,/3, ,y) under the combined action of the cubic site and the icosahedral molecular point group.

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Symmetry-adapted functions (SAF's) of order L can be generated recursively from powers of a seed function or from products of SAF's having lower order 1. The algorithm uses a symmetry-adapted version of the decomposition of products of spherical harmonic functions via Wigner 3-j symbols. The totally symmetric SAF's are calculated for the point group 2/m35 up to L = 30. For L --30, the icosahedral group has two independent SAF's; a simple method is suggested for making a unique choice for these two SAF's.