Theoretical density functional and ab initio computational study of the tautomeric, vibrational and H-bond properties of 1,7-di-CH 3-guanine: a comparison with FT-IR data from matrix isolation experiments

Abstract

In this work we report the results of a combined matrix-isolation FT-IR and theoretical computational DFT and ab initio study of the tautomeric and vibrational characteristics of 1,7-dimethyl-guanin (17DMG). The HF, MP2 and DFT methods are all in agreement in predicting that the amino-oxo tautomer of 17DMG is the most stable form, and that the imino-oxo tautomer, which can exist in two different geometrical isomers, is the second most stable form. The energy difference between the amino-oxo and imino-oxo tautomer is rather large: 23, 25 and 33 kJ mol −1, for the HF, DFT and MP2 methods, respectively. The imino-hydroxy tautomer is, according to the calculations, by far the least stable form ( ΔE > 180 kJ mol −1). The FT-IR spectra of 17DMG isolated in an argon matrix are in agreement with the theoretical calculation indicating that only the amino-oxo tautomer should be present in the gas phase in detectable amounts. The experimental IR frequencies agree quite well with the theoretical values. When a single scaling factor is applied to approximately correct the calculated frequencies for various systematic errors in theoretical approach, the mean frequency deviations for the HF 6-31++ G∗∗ , DFT 6-31 G∗∗ and DFT 6-31++ G∗∗ calculated spectra are 17.5 cm −1, 12.2 cm −1 and 16.3 cm −1, respectively. The DFT mean-frequency-deviations decrease to 8.1 cm −1 and 10.1 cm −1 when variable scaling factors are applied. The stabilities of the different water complexes of the amino-oxo tautomer of 17DMG are investigated by optimizing the complexes at the HF 6-31++ G∗∗ level of theory and calculating the MP2 energies at these optimized geometries. The N3⋯HOH complex, which has a closed structure with two H-bonds, is predicted to be the most stable structure. The MP2 calculated energy differences between the most stable complex and the C 6O⋯HOH, the HNH⋯OH 2 and the H 2N⋯HOH hetero-dimers are 14.28 kJ mol −1, 19.54 kJ mol −1 and 20.73 kJ mol −1, respectively.