Conformational geometry functions: additivity and cooperative effects

The conformational geometry changes associated with torsional motion about CC single bonds are reported for some 47 basic organic molecules of the type (X,Y,Z)CC(X′,Y′,Z′), and for the ,ψ space of the model dipeptides N-acetyl N′-methyl amides of glycine and alanine. All structures were determined by ab initio HF4−21G geometry optimizations. Employing natural cubic spline parameters, a program was written for use in empirical modeling parameter development, which calculates the bond lengths and angles of the reported compounds as functions of the associated torsional angles at any point in conformational space. The additivity of the conformational geometry functions is explored and illustrates how cooperative effects emerge in complex molecules. The results are instructive for procedures in which the properties of molecular fragments are used to derive force field parameters for the empirical modeling of complex molecules.