Theoretical investigations of the proton transfer reaction in hydrogen-bonded complexes of cytosine with HNO

The potential energy (PE) functions of the lowest singlet and triplet states of the hydrogen-bonded complexes of cytosine with HNO and NOH were theoretically investigated along the proton transfer (PT) coordinate. A full geometry optimization was performed along the PT reaction path at the Hartree-Fock level of theory. The energies at the optimized geometries were calculated with the use of second-order Møller-Plesset perturbation theory (MP2) and with second-order perturbation theory employing the complete active space self-consistent field wavefunction as the reference (CASPT2). It was found that the cyclic complex of the 'native' amino-oxo form of cytosine with NOH can be exothermally transformed on the barrierless PE surface into the complex of the 'rare' imino-oxo form with HNO. The results provide a model of a chemically induced PT reaction in nucleic acid bases which can effectively generate their 'rare' tautomeric forms.