Reorientations in pivalic acid (2,2-dimethyl propanoic acid). - I. Incoherent neutron scattering law for dynamically independent molecular and intramolecular reorientations

Reorientations of the pivalic acid molecule and/or of its parts have been studied in the two solid phases (phase transition at 279.9 K) by incoherent quasielastic neutron scattering (IQNS). Different jump-models are developed for comparison with the experimental results. For methyl-group or carboxylic-group motion, simple reorientational jump models among N sites over a circle can be used. When t-butyl rotations together with internal methyl rotations occur simultaneously, a more sophisticated jump model is developed based upon dynamically uncorrelated reorientational motions about two different axes. Finally we consider the case when a tumbling motion of the whole-molecule is added to the two previous motions and an approximate scattering law is also derived.