Analysis of molecular motion with allowance for intramolecular torsion

The one-parameter model of Dunitz & White [Acta Cryst. (1973), A29, 93-94] for the analysis of torsional degrees of freedom within an otherwise rigid body has been applied to data for a number of representative compounds. The model gives results not significantly different from those of more elaborate models for librations about bonds, even when the r.m.s, amplitudes of these librations are as great as 24°. The limitations of this kind of analysis and some of the problems in applying it are discussed.