Influence of small-amplitude motions on two-dimensional N.M.R. powder patterns

We have analysed the effects of anisotropic molecular vibrations on 13C-1H dipolar/13C chemical shift two-dimensional (2D) powder patterns. The 2D powder pattern visualizes the subtle vibrationally-induced asymmetry of the dipolar coupling tensor, and moreover, indicates the direction of anisotropic vibration. Spectra previously obtained for calcium formate have been reinterpreted by considering the anisotropic vibrations. The amplitudes of the in-plane and out-of-plane vibrations for each of the two crystall-ographically-inequivalent formate ions have been evaluated; the latter has been found to be larger than the former. These amplitudes are elucidated almost in terms of the CH bond bending vibrations, and partially in terms of the overall rotational vibrations of the ions.