Are the thiouracils sulfur bases in the gas-phase?

The gas-phase proton affinities of 2- and 4-thiouracil and 2,4-dithiouracil have been measured by means of Fourier transform ion cyclotron resonance ( FTICR) mass spectrometry. High-level ab initio calculations, in the framewo rk of the G2(MP2) theory, have been carried out to establish the nature of the protonation site. Thiouracils behave as bases of rather similar moderat e strength in the gas phase, the 2,4-dithiouracil being the most basic of t he three. In all cases, the protonation takes place at the heteroatom attac hed to position 4, hence although, in general, thiocarbonyls are stronger b ases than carbonyls in the gas phase, 2-thiouracil behaves as an oxygen bas e. For 2-thiouracyl and 2,4-dithiouracil, the most stable protonated confor mer is the enol-enethiol form that cannot be formed by either direct proton ation of the corresponding neutral or a unimolecular tautomerization of the oxygen or sulfur protonated species. We have shown that alternative mechan isms involving the formation of hydrogen bonded dimers between the protonat ed form and the neutral form, followed by appropriate proton transfers with in the dimer, can be invoked to explain the formation of the most stable co nformer.