CRYSTAL-STRUCTURE AND LOW-TEMPERATURE METHYL-GROUP DYNAMICS OF COBALTAND NICKEL ACETATES

The crystal structures of cobalt and nickel acetate tetrahydrate have been determined at room-temperature and liquid-helium temperature by n eutron powder diffraction of the fully deuterated salts. Molecular mec hanics and ab initio methods based on these structural results have th en been used to calculate the rotational potentials experienced by the methyl groups. We have also used inelastic neutron scattering to meas ure the rotational potential via the rotational tunneling spectrum of the methyl groups, and this has enabled us to compare different method s for the calculation of partial charges in these ionic compounds. Goo d agreement between the observables and calculations has been obtained for both compounds when nb initio methods are used to recalculate par tial charges at every step of the methyl rotation. (C) 1998 American I nstitute of Physics, [S0021-9606(98)00743-0].