AM1 GAS-PHASE PROTON AFFINITIES OF 2-THIOCYTOSINE

For the six tautomeric forms of thiocytosine, the gas phase proton affinities (PA) have been calculated using the AM1 semi-empirical method. The PA reveal that for the thiol derivatives of thiocytosine (TH) the preferred sites of protonation are N1 > N3 > NH2 > SH. The same trend is found for the thione derivatives of TH with the exception that the sulfur proton affinities cannot be considered as the protonation on S7 yields a thiol form already considered. For the protonated predominant species at room temperature TH(1,7,8,8), the localized MO, sigma and pi bond order matrices and composition of the most important MO have also been calculated. The calculation also reveals that the net atomic charges do not correctly predict sites protonation.