Some general aspects of torsional sensitivity and the GG-effect
by Yu, C. H.; Schafer, L.; Ramek, M.; Miller, D. M.; Teppen, B. J.
The geometries of 28 compounds of type X-C1-C2-C3-Y, with X, Y = CH(3), F, Cl, OH, NH(2), COH, and COOH, were fully optimized by ab initio HF/4-21G calculations at 30 degrees grid points in their respective phi(X-C1-C2-C3), psi(C1-C2-C3-Y)torsional spaces. The results make it possible to construct parameter surfaces and their gradients in phi, psi-pace. The magnitude of the gradient, \del P\ = [(partial derivative P/partial derivative phi)(2) + (partial derivative P/partial derivative psi)(2)](1/2), of a structural parameter P (a bond length, bond angle, or non-bonded distance) in phi,psi-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 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 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.
- Journal
- Journal of Molecular Structure
- Volume
- 485
- Year
- 1999
- Start Page
- 373-384
- URL
- https://dx.doi.org/10.1016/s0022-2860(99)00053-8
- ISBN/ISSN
- 1872-8014; 0022-2860
- DOI
- 10.1016/s0022-2860(99)00053-8