Computational and matrix-isolation FT-IR study of the tautomeric and vibrational properties of methylated guanines

The tautomeric and vibrational characteristics of 1,7-dimethylguanine (17DMG), 7-methylguanine (7MG) and 9-methylguanine (9MG) are studied by a combined experimental matrix-isolation FT-IR and a computational study. The theoretical methods used are the RHF, MP2 and DFT/B3LYP method with the 6-31+(+) G** basis set. For 17DMG, only the most stable amino-oxo tautomer form can be detected in the FT-IR spectrum. The FT-IR spectra of 7MG and 9MG reveal the presence of two tautomers in the matrix in detectable amounts: the amino-oxo-N1H form and the amino-hydroxy form. The amount of the amino-hydroxy form of 7MG is very small ( approximate to 1%), while the amino-hydroxy form of 9MG, for which the two rotamers can be distinguished in the spectrum, is as abundant as the amino-oxo form. A comparison of the experimental and calculated IR frequencies demonstrates that the most accurate spectral parameters are predicted by the DFT/B3LYP method, especially when this method is used with a set of different scaling factors. (C) 1999 Elsevier Science B.V. All rights reserved.