Transitional modes in simple unimolecular bond fission and in the reverse recombination reactions are characterized quantitatively by statistical adiabatic channel (SACM) and classical trajectory (CT) calculations. Energy E- and angular momentum J-specific numbers of open channels (or activated complex states) W(E,J) and capture probabilities w(E,J) are determined for a series of potentials such as ion-dipole, dipole-dipole, and various model valence potentials. SACM and CT treatments are shown to coincide under classical conditions. Adiabatic as well as nonadiabatic dynamics are considered. The dominant importance of angular momentum couplings is elaborated. A sequence of successive approximations, from phase space theory neglecting centrifugal barriers E-0(J), via phase space theory accounting for centrifugal barriers E-0(J), toward the final result, expressing the effects of the anisotropy of the potentials by specific rigidity factors f(rigid)(E,J), is described. This approach emphasizes the importance to characterize the employed potentials by their centrifugal barriers E-0(J). The derived specific rigidity factors f(rigid)(E,J) are consistent with previously calculated thermal rigidity factors f(rigid)(T). The present approach properly accounts for angular momentum conservation and, at the same time, facilitates the calculation of specific rate constants k(E,J) and falloff curves for unimolecular bond fission and the reverse radical recombination reactions.