A density functional study of NBO, NICS and 14N NQR parameters of 5-methylcytosine tautomers in the gas phase

The NBO (natural bond orbital analysis), NICS (nucleus independent chemical shift) and 14N NQR parameters of the most stable tautomers of 5-methylcytosine in the gas phase were predicted using density functional theory (DFT) method. The NBO analysis revealed that the resonance interaction between lone pair of the nitrogen atom and empty (σ* or π*) non-Lewis NBO increases with increasing the p character of the nitrogen lone pair. The present investigation indicated the π clouds in the considered heterocyclic ring contained six electrons, and these tautomers have the aromatic character. The NICS study utilizing the gauge-invariant atomic orbital (GIAO) method showed that there are diatropic currents in the heterocyclic rings of the tautomers, so we determined the order of aromaticity of these tautomers. The results of NQR parameter calculations showed three parameters are effective on nuclear quadrupole coupling constant; the p character value of lone pair electrons of nitrogens, and the related occupancies and the portion of lone pair electrons interfering in magnetic currents of ring and whenever, the lone pair electrons of nitrogens participate in the formation of chemical bond and/or π system of the ring, the qzz and consequently its χ decreases.