Density functional theory studies on molecular structure and IR spectra ofguanine - A scaled quantum mechanical force field approach

Density functional theory with the combined Becke 3-LYP exchange-correlatio n energy function using the 6-31G* basis set was applied to study the struc tures and vibrational IR spectra of amino-oxo and amino-hydroxy forms of gu anine, The calculated force fields were corrected by scaling the forces fie lds 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 imidazol e, The predicted vibrational frequencies of two tautomers of guanine were c ompared with the available observed matrix-isolated IR spectra with mean de viations of about 6.6 cm(-1) for amino-oxo form and 6.0 cm(-1) for amino-hy droxy form, The results made it possible to give reliable assignments of th e IR spectra 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 scal e factors.