The Gaussian-2 (G2) theoretical procedure based on ab initio molecular orbital theory was applied to study the potential energy surfaces corresponding to the [H2, O, F]+ [1], [H2, S, F]+, [H2, O, Cl]+ and [H2, S, Cl]+ singlet and triplet state cations which can be generated in the gas-phase reactions between F+(3P) and F+(1D) with water or hydrogen sulfide or in the reactions between Cl+(3P) and Cl+(1D) and the aforementioned neutrals, respectively. Important differences between singlets and triplets, regarding both their bonding and their relative stabilities have been found. There are also significant differences between the reactivity patterns exhibit by F+ and Cl+, and between water and hydrogen sulfide when reacting with these halogen cations. A topological analysis of the electron charge distributions of the corresponding molecular cations by means of the atoms-in-molecules theory of Bader, shows that, in general the triplet state cations are essentially ion-dipole complexes, while the singlets are covalently bound species. One of the most important consequences of this dramatic difference in the bonding of triplet and singlet species is, for example, that although the first 1D excited state of F+ is about 2.6 eV. above the 3P ground state, the most stable cations of the reactions between F+ and hydrogen sulfide are singlet state species. The most likely process in the reactions between F+ and water or hydrogen sulfide is the single charge transfer. Very likely, this resonant charge transfer occurs into an excited state of OH2+ and SH2+. The formation of different products as OH+, FH+, SH+, ClH+, etc. has been also investigated, and an estimation of the heat of formation of the most stable ionic species is also given.
This work has been partially supported by the D.G.I.C.Y.T Project No. PB93-0289-C02-01.