In-Plane Vibrational Modes of Guanine from an ab Initio MO Calculation

Harmonic force constants, in-plane vibrational frequencies, and in-plane vibrational modes of guanine have been calculated by an ab initio MO method. The force constants were calculated by the use of an energy gradient method with STO-3G basis set, and then they were corrected into “4-31G force constants” by the scaling factors given by us previously for the case of uracil. The corrected set of force constants could reproduce the observed frequencies of guanine, guanine-7, 9-15N2, guanine-1,3,amino-15N3 (at positions 1, 3 and amino), guanine-8-d (at position 8), guanine-1,9, amino-d4 (at amino and iminos), and guanine-d5. The calculated vibrational modes could also account for the results of earlier experiments on infrared and Raman spectra of guanine residues in 5′-GMP and other nucleotides.