Some general aspects of torsional sensitivity and the GG-effect

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 calcul ations at 30 degrees grid points in their respective phi(X-C1-C2-C3), psi(C 1-C2-C3-Y)torsional spaces. The results make it possible to construct param eter surfaces and their gradients in phi, psi-pace. The magnitude of the gr adient, \del P\ = [(partial derivative P/partial derivative phi)(2) + (part ial derivative P/partial derivative psi)(2)](1/2), of a structural paramete r P (a bond length, bond angle, or non-bonded distance) in phi,psi-torsiona l space is a measure of torsional sensitivity (TS); i.e. a measure of the e xtent to which bond lengths, bond angles, and non-bonded distances change a t a point in phi,psi-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 conform ations, of high TS. Conformations with two sequential gauche torsional angl es (GG sequences) are characterized by high TS of 1,5-non-bonded distances concomitant with relatively low TS of other internal coordinates. This prop erty 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 H F/4-21G structures of X-C-C-Y and X-C-C-C-Y systems has been assembled and is available on a CD. (C) 1999 Elsevier Science B.V. All rights reserved.