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 framework of the G2(MP2) theory, have been carried out to establish the nature of the protonation site. Thiouracils behave as bases of rather similar moderate strength in the gas phase, the 2,4-dithiouracil being the most basic of the three. In all cases, the protonation takes place at the heteroatom attached to position 4, hence although, in general, thiocarbonyls are stronger bases than carbonyls in the gas phase, 2-thiouracil behaves as an oxygen base. For 2-thiouracyl and 2,4-dithiouracil, the most stable protonated conformer is the enol−enethiol form that cannot be formed by either direct protonation of the corresponding neutral or a unimolecular tautomerization of the oxygen or sulfur protonated species. We have shown that alternative mechanisms involving the formation of hydrogen bonded dimers between the protonated form and the neutral form, followed by appropriate proton transfers within the dimer, can be invoked to explain the formation of the most stable conformer.

The first systematic comparison of structural effects on the intrinsic reactivities of carbonyl and thiocarbonyl compounds has been carried out. To this end, the gas-phase basicities (GB) of a wide variety of thiocarbonyl compounds XCSY (as well as of some carbonyl derivatives) were determined by means of Fourier transform ion cyclotron resonance spectrometry (FTICR) and SCF and MP2 ab initio calculations at different levels of accuracy were performed on 27 different neutral compounds and their protonated forms. The same set, enlarged by the inclusion of very large systems such as di-tert-butyl-and bis-( 1-adamanty1)thioketones was also investigated at the AM 1 semiempirical level in order to get a more complete view of structural effects. The agreement between the calculated and the experimental changes in thermodynamic state functions is good in all instances. Correlation analysis of the experimental data shows that (i) substituent effects on the gas-phase basicity of thiocarbonyl compounds are linearly related to those of their carbonyl homologs with a slope of 0.80 and (ii) these effects can be quantitatively analyzed in terms of polarizability, field, and resonance effects (Taft-Topsom model). Comparison of the GBs of thiocarbonyl and carbonyl compounds with solution basicities and nucleophilicities sheds light on differential structural and solvation effects. Substituent effects on both neutral and protonated species were explored by means of appropriate isodesmic reactions. These results confirm that all thiocarbonyl compounds investigated are sulfur bases in the gas phase. The features revealed by correlation analysis can be rationalized in terms of the interactions between the MOs of the substituent and the parent compound.

Intermolecular charge-transfer (CT) spectra of several complexes between thiocarbonyl compounds and molecular iodine were studied in the UV-visible region. Equilibrium constants and Gibbs energy changes of 1:1 charge-transfer complexes were determined in solution. Two different kinds of complexes were detected, those which present the CT band in the 300 nm region and those which absorb around 350 nm. Ab initio calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ*//LANL2DZ* were carried out to clarify their structure. Complexes with the CT band around 300 nm correspond to those where the molecule of iodine lies in the same plane of the CdS group, while in those absorbing in the 350 nm region the I 2 moiety is almost perpendicular to that plane. These perpendicular complexes are formed when the substituents around the thiocarbonyl group are voluminous, due to steric hindrance and to the different nature of the HOMO. In both kinds of complexes, the thiocarbonyl-iodine interaction is essentially electrostatic. The substituent effects were analyzed by Taft-Topsom's model. Experimental data in solution and theoretical estimates were found to follow a good linear relationship. The gas-phase basicity of the set of thiocarbonyl compounds investigated toward proton is linearly correlated with their basicity toward molecular iodine in solution. This finding strongly supports previous conclusions regarding the relationship between gas-phase and solution reactivity data.