The absence of the OOP-CI pathway in αTp can be attributed to the moments of inertia imposed on the dynamics by the isopropyl and methyl substituents, which disfavor strong out-ofplane motion. The slight out-of-plane motion needed to enter the pericyclic minimum seems to be the limiting motion in αTp to provide access to the RO-CI. This makes further out-of-plane bending unlikely. In contrast, the out-of-plane motion is favored in αPh with respect to the rehybridization of the methylene carbon with the bulky isopropyl substituent based on the same considerations. Thus, the observed differences in the population of the OOP-CI pathway in αPh and αTp can be directly related to effects from the moments of inertia of the substituents.