Combined Matrix-Isolation FT-IR and Theoretical Study of the Intrinsic Tautomeric, Vibrational, and H-Bonding Properties of N4-Methoxycytosine

Abstract

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 KI (anti/syn). Theoretically, two closed H-bonded complexes with water are found for both isomers, i.e., N1−H...O−H...OC2 and N3−H...O−H...OC2, 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 complexe...