Theoretical density functional and ab initio computational study of the tautomeric, vibrational and H-bond properties of 1,7-di-CH₃-guanine: A comparison with FT-IR data from matrix isolation experiments

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-guanine (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 amino-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 /6-31G** 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 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 close 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 C₆=O...HO-H, the H-NH...OH₂ and the H₂N...HO-H hetero-dimers are 14.28 kJ mol^-1, 1954 kJ mol^-1 and 20.73 kJ mol^-1, respectively.