H. Ihmels et Jr. Scheffer, The Norrish type II reaction in the crystalline state: Toward a better understanding of the geometric requirements for gamma-hydrogen atom abstraction, TETRAHEDRON, 55(4), 1999, pp. 885-907
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.