Combined matrix-isolation FT-IR and theoretical study of the intrinsic tautometric, vibrational, and H-bonding properties of N4-methoxycytosine

The tautomeric, vibrational, and H-bonding characteristics of N4-methoxycytosine (N4MC) are investigated by a combined experimental, matrix-isolation Fourier transform infrared spectroscopic and computational study. For the theoretical part of the study, the methods RHF, MP2, and DFT (B3LYP) are used with the 6-31++G** basis set. N4MC occurs in a low-temperature Ar matrix exclusively in the oxo-imino form. Two isomers of this tautomer exist. Using IR intensities of characteristic bands, an approximate value of 2.3 × 10^-2 is obtained for the isomerization constant K₁ (anti/syn). Theoretically, two closed H-bonded complexes with water are found for both isomers, i.e., N1-H...O-H...O=C2 and N3-H...O-H...O=C2, as well as one less stable open complex, N7...H-O and O8...H-O, for the syn and anti isomer, respectively. In the experimental FT-IR spectra, both closed H-bonded complexes are clearly identified based on characteristic H-bonding absorptions predicted by the calculations. Trace amounts of the open complexes are detected as well. A correlation between the frequency shift of the bonded O-H stretching mode and the elongation of the O-H distance is presented for N-H...O-H...X type complexes of N4MC and some comparable compounds.