Abstract: Density functional theory with the combined Becke 3-LYP exchange-correlation energy function using the 6-31G^* basis set was applied to study the structures and vibrational IRspectra of am ino-oxo and am ino-hydroxy forms of guanine. The calculated force fields were corrected by scaling the forces fields over a convenient set of local internal coordinates. The scale factors were determined by least-squares fitting of the theoretical frequencies to the experimental ones of the two parent molecules, isocytosine and imidazole. The predicted vibrational frequencies of two tautomers of guanine were compared with the available observed matrix-isolated IRspectra with mean deviations of about 6.6 cm^-1 for amino oxo form and 6.0 cm^-1 for am ino-hydroxy form. The results made it possible to give reliable assignments of the IRspectra of isolated guanine. This study shows that the scaled density functional force field approach enables us to give a good interpretation of vibrational spectra of large molecules through the transferability of scale factors.

Key words: Guanine, DFT calculations, SQM force fields, IR spectra