VIBRATIONAL-SPECTRA FOR PURINE AND ITS N-15-SUBSTITUTED AND D-SUBSTITUTED ANALOGS - ASSIGNMENT OF NORMAL-MODES FROM AB-INITIO 3-21G FORCE-FIELDS

Fourier transform (FT) IR and FT-Raman spectra for purine and its N-, C-8- and N- and C-8-deuterated analogues and for that N-15-substituted on the pyrimidine ring are reported. Experimental data are interpreted by reference to ab initio-calculated wavenumbers for normal modes of both N7H- and N9H-purines. The latter were obtained from normal coordinate analyses carried out on the basis of ab initio 3-21G force fields using the optimized geometry. The effect of hydrogen bonding on the discrepancy between calculated and observed wavenumbers for normal modes was analysed. Considering the solid N7H-purine, the observed N-15 shifts for FT-IR bands often agree with those calculated for the corresponding normal modes. Several weak bands are tentatively assigned to the N9H tautomer, presumably present in small amounts in solution and also in the solid in the caw of deuterated purines.