Quantum chemistry predicted correlations between geometric isomerism (conformation) of -OH and =NH substituents and typical group frequencies of nucleic acid bases: cytosine.

Results of a search for correlations between typical group mode frequencies and conformation of -OH and =NH substituents is reported. The study is based on quantum chemical data (HF/6-31G (d, p) and MP2 (full)/6-31G (d, p) approximations) of 13 isomers of cytosine. It is closely related to investigations of stability and energetics of all isomers of the more common nucleic acid bases, which revealed correlations between conformation (geometric isomerism) of OH and =NH substituents on the one hand and conversion energies, relaxation of internal structural parameters, electric field gradients, etc. on the other hand. In the majority of cases alteration of conformation of these substituents is accompanied by systematic frequency shifts of stretching, in-plane and out-of-plane bending (torsional) modes conventionally assigned to such groups and by alteration of quantum chemically predicted estimates of harmonic valence force constants and structural parameters dominating the mode frequencies. For identification of group modes 'fractional' mass (other than 'natural' mass) isotope shifts of frequency and normal vectors proved useful. Likewise, estimation of effects of force constant and structural parameter alterations on frequency shifts by first order perturbation theory of the FG problem of partial structures contributed valuable insight into the origin of the shifts.