Chiroptical studies. Part 90. The circular dichroism of cyclopentanones and related compounds. A proposed octant-dissignate effect of C–H bonds

Abstract

An analysis of c.d. data (n→π*) for chiral cyclopentanone rings suggests that the C_α–H bonds make the dominant contributions, in the octant-dissignate (‘anti-octant’) sense. The magnitude of a C_α–H contribution is considered to be given approximately by the empirical expression δΔε= 6.2 sin²ω, where ω is the torsion angle between the C_α–H and CO bonds. On this basis, all common α-substituents probably contribute in the octant-dissignate sense to the c.d. of a ketone, most appearing to have an octant-consignate effect only because they replace a more dissignate α-hydrogen atom. This new method for calculation of chiroptical effects has been applied to some compounds thought to contain twisted cyclohexanone rings, and to some monocyclic cyclopentanone derivatives.