The Norrish type II reaction in the crystalline state: Toward a better understanding of the geometric requirements for gamma-hydrogen atom abstraction

By correlating the success or failure of the Norrish type II reaction for a series of compounds in the crystalline state with geometric data derived f rom X-ray crystallography (the so-called Crystal Structure-Solid State Reac tivity Method), the distance and angular requirements for photochemical gam ma-hydrogen atom abstraction have been established. These studies indicate that abstractions are preferred when the C=O ... H-gamma, distance is close to the sum of the van der Waals radii of hydrogen and oxygen (2.72 Angstro m), and that the gamma-hydrogen atom can deviate by as much as 50-60 degree s from the plane containing the oxygen n-orbital responsible for abstractio n. Because molecular conformations and interatomic distances and angles can be calculated readily by using empirical force field methods, such data pe rmit predictions to be made concerning the outcome of hypothetical Norrish type II processes. (C) 1999 Elsevier Science Ltd. All rights reserved.