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 bas ed on quantum chemical data (HF/6-31C (d, p) and MP2 (full)/6-31G (d, p) ap proximations) of 13 isomers of cytosine. It is closely related to investiga tions of stability and energetics of all isomers of the more common nucleic acid bases, which revealed correlations between conformation (geometric is omerism) of -OH and =NM substituents on the one hand and conversion energie s, 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 str etching, 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 dom inating the mode frequencies. For identification of group modes 'fractional ' mass (other than 'natural' mass) isotope shifts of frequency and normal v ectors proved useful. Likewise, estimation of effects of force constant and structural parameter alterations on frequency shifts by first order pertur bation theory of the FG problem of partial structures contributed valuable insight into the origin of the shifts. (C) 2001 Elsevier Science B.V. All r ights reserved.