The Determination of Barriers to Internal Rotation by Means of Electron Diffraction. Ferrocene and Ruthenocene.

Abstract

The electron scattering pattern from a mol. undergoing internal rotation is a function of an angular probability function W(j) defined so that the probability of the angle of rotation between j and j + dj is equal to W(j)dj. If the restricting barrier is of the shape V(j) = (V0/2)(1 - cos sj), for low and intermediate values of V0/kT the angular probability function is given by the classical expression W(j) = N exp(V(j)/kT). The angular probability function can be found from classical statistics for all shapes of the restricting barrier, a program for least-sqs. refinement of the barrier to internal rotation on electron diffraction data has been written, and the rotational barriers in ferrocene and ruthenocene have been refined. The equil. conformation of the free ferrocene mol. is prismatic, the barrier to internal rotation is 0.9 +- 0.3 kcal. mole-1. The magnitude obtained for the barrier is insensitive to the shape assumed for V(j). For ruthenocene the exptl. data were not good enough to permit a detn. of V0, but the best agreement was obtained for a prismatic model.