The pre-exponential factor in activated reorientational processes

Different approaches to the pre-exponential factor for processes in which a rotator crosses a potential barrier by thermal fluctuations are compared. It is shown that Eyring's treatment, in its classical limit, is equivalent to that of Bauer and. furthermore, that Eyring's entropy of activation has no intrinsic qualitative significance. For simple two-well potentials explicit pre-exponential factors are given: for the plane rotator the pre-exponential factor is approximately equal to the frequency of libration in the well whereas in the case of the space rotator, it involves the square of this frequency. Finally the assumptions underlying the above-mentioned treatments are discussed, and a word of warning given against the hasty interpretation of constant pressure experimental results.