Some general aspects of torsional sensitivity and the GG-effect

Abstract

The geometries of 28 compounds of type X–C1–C2–C3–Y, with X,Y=CH3, F, Cl, OH, NH2, COH, and COOH, were fully optimized by ab initio HF/4-21G calculations at 30° grid points in their respective φ(X–C1–C2–C3), ψ(C1–C2–C3–Y)-torsional spaces. The results make it possible to construct parameter surfaces and their gradients in φ,ψ-space. The magnitude of the gradient, |∇P|=[(∂P/∂φ)2+(∂P/∂ψ)2]1/2, of a structural parameter P (a bond length, bond angle, or non-bonded distance) in φ,ψ-torsional space is a measure of torsional sensitivity (TS); i.e. a measure of the extent to which bond lengths, bond angles, and non-bonded distances change at a point in φ,ψ-space with backbone torsional angles. It is found that TS is not constant throughout the conformational space of a molecule, but varies in a characteristic way. It seems that, regardless of the nature of X or Y, extended forms are typically in regions of low TS; puckered conformations, of high TS. Conformations with two sequential gauche torsional angles (GG sequences) are characterized by high TS of 1,5-non-bonded distances concomitant with relatively low TS of other internal coordinates. This property of GG sequences is the source of a stabilizing and cooperative energy increment that is not afforded by other torsional sequences, such as trans–trans or trans–gauche. A structural data base, consisting of thousands of HF/4-21G structures of X–C–C–Y and X–C–C–C–Y systems has been assembled and is available on a CD.