Theoretical and matrix-isolation experimental studies on 2-thiocytosine and 5-fluoro-2-thiocytosine.

2-Thiocytosine (s2Cyt) and 5-fluoro-2-thiocytosine (f5s2Cyt) were studied by means of IR spectroscopy under different environmental conditions: isolated in low-temperature inert gas matrices, associated in thin amorphous and polycrystalline films. The compounds isolated in matrices were only very slightly influenced by the environment. From the analysis of the IR spectra of both compounds it appears that they exist in inert gas matrices only in the amino-thiol tautomeric form. Strong environmental effects were observed for s2Cyt and f5s2Cyt deposited in the form of thin polycrystalline films. Contrary to matrices, in polycrystalline films the amino-thione form dominates for both s2Cyt and f5s2Cyt. The experimental findings are in agreement with the ab initio quantum mechanical calculations of the relative total energies of the tautomeric forms. Those energies were calculated using the Self Consistent Field method corrected for electron correlation effects with the use of the second-order many-body perturbation theory (SCF+MBPT(2)). The theoretical calculations show that the amino-thiol tautomeric form is more stable than the amino-thione form by 38 kJ mol-1 and 48 kJ mol-1 for s2Cyt and f5s2Cyt, respectively. Both molecules, s2Cyt and f5s2Cyt, may also appear in the uracil-like imino-thione tautomeric form, which is predicted to be only 8 kJ mol-1 less stable than the amino-thione form. A new method of the preparation of f5s2Cyt is reported.