Tautomeric transitions of isocytosine isolated in argon and neon matrices induced by UV irradiation

The tautomeric equilibrium of the isocytosine molecules in the low temperature Ar and Ne matrices has been investigated by the FTIR matrix isolation spectroscopy and quantum-chemical calculations at the DFT, MP2, and CCSD(T) levels of theory. Broad-band UV irradiation of matrix isolated isocytosine results in an increase of the abundance of the minor isocytosine tautomer, KA1. After UV irradiation the intensity of the spectral bands assigned to this tautomer increases more than 1.5 times and the bands of other minor conformers are not observed. The values of ΔH (6.6 kJ/mol) and ΔG (6.8 kJ/mol) determined based on the experimental data are close to the ΔE value (8.3 kJ/mol) calculated at the CCSD/aug-cc-pVDZ level of theory. Two Fermi resonances involving the νCO vibration of the minor tautomer KA1 (2-amino-4-oxo-pyrimidine or keto-amino) and four Fermi resonances involving the ring stretching vibrations of the most abundant tautomer EA1 (2-amino-4-hydroxy-pyrimidine or enol-amino) are found. The performance of the different computational methods in predicting the vibrational spectra of isocytosine is evaluated.