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.

The hitherto unknown gas-phase basicity of the tetraphosphorus molecule, P4, has been determined by means of Fourier transform ion cyclotron resonance mass spectrometry using proton transfer equilibrium techniques. A quantum mechanical treatment of P4 and P4H+ in the framework of the G2 level of theory leads to a proton affinity agreeing nicely with the experimental value and, most important, reveals the existence of a novel kind of chemical bond in P4H+:  the symmetrical and essentially covalent (P···H···P) linkage.

The gas-phase proton affinities of 3-thio-5-oxo, 5-thio-3-oxo, and 3,5-dithio derivatives of 2,7-dimethyl-[1,2,4]-triazepine have been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The structures and vibrational frequencies of all the stable protonated tautomers and all the transition states connecting them have been obtained by means of the B3LYP density functional method, together with a 6-31G* basis set expansion. The final energies were obtained at the B3LYP/6-311+G(3df,-2p) level. In contrast with the results from the analogous thiouracils, our results indicate that all of these compounds behave as sulfur bases in the gas phase. For 5-thio-3-oxo-[1,2,4]-triazepine and 3,5-dithio-[1,2,4]triazepine, the thiol-enol and the dithiol forms are the most stable protonated species, respectively. Conversely, for 3-thio-5-oxo-[1,2,4]-triazepine, the thiol-ketone form is the most stable one. For 5-thio-3-oxo-[1,2,4]triazepine and 3,5-dithio-[1,2,4]-triazepine, as it was found for thiouracils, a comparison between theoretical and experimental proton affinities suggests the formation of dimers between protonated and neutral species, which favors proton-transfer mechanisms leading to the formation of the most stable protonated species.

The Oaussian-2 (02) theoretical procedure, based on ab initio molecular orbital theory, as well as other high level ab initio calculations [QCISD(T) and CCSD(T)/6-311 + +0(3df,2pd, and QCISD(T)/6-31I+0(5d2f,2p)] which avoid the additivity scheme of this theory, are used to estimate the heat of formation of HSO. For this purpose, we propose also, as a further improvement of the standard 02 scheme, to evaluate the residual correlation effects by using the QCISD(TQ) method, which is exact in fifth order rather than the usual QCISD(T) procedure. The heat of formation of HSO at 0 K estimated using different reactive processes is -4.2± 1.3 kcallmol. The most significant consequence is that the depletion of ozone by HSO is predicted to be slightly endothermic rather than exothermic, as it has been concluded in other theoretical studies. We have also found that HSO is more stable than SOH, but the energy gap between both isomers predicted by our calculations is smaller than previous reported values.

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The structure and relative stability of methanol complexes with various cyclic ketones, lactones, lactams, and N-methyl lactams from three- to seven-membered rings have been investigated using the density functional theory method. The geometries, harmonic frequencies, and energies were calculated at the B3LYP/6-311+G(d,p) level. Three stable structures, cis-a, cis-b, and trans, with respect to the ring oxygen (nitrogen) atom, were found to be local minima of the potential energy surface. For lactones and N-methyl lactams, the most stable structure is trans; it is stabilized, as in cyclic ketones, through the conventional hydrogen bond (HB) interaction between the basic carbonyl oxygen and the acidic methanolic hydrogen and an unconventional HB interaction between the methanolic oxygen and the CH hydrogen, in the alpha position of the carbonyl group. For unsubstituted lactams, the cis-a structure, stabilized through a HB interaction between the NH group and the methanol oxygen in addition to the conventional HB interaction, is the most stable. The topological properties of the electron density ratify the existence of conventional (N,O-H. . .O) and unconventional (C-H. . .O) hydrogen bonding. A good correlation was found between the HB distances and the electron density at the HB critical point. The unsubstituted lactams yield more stable complexes with methanol than N-methyl lactams, lactones, and cyclic ketones. In the most stable complexes, both components behave simultaneously as a HB donor and as a HB acceptor.