The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to study the potential energy surfaces corresponding to [H2,0,F]+ singlet and triplet state cations which can be generated in the gas-phase reactions between F+(3P) and F+('D) with water. Important differences between singlets and triplets, regarding both their bonding and their stabilities, have been found. The most outstanding result is that although the first 'D excited state of F+ is about 2.6 eV above the 3P ground state, the most stable product of the reaction between F+ and water corresponds to a singlet state cation, l(S). This is so because, in general, triplet state cations are ion-dipole complexes, while the singlets are covalently bound species. Single charge transfer is the most likely process in F+ + water reactions. Very likely, this resonant charge transfer occurs into an excited state of OHz+. The formations of OH+, FH+, and O+ are also exothermic processes. The estimated heat of formation of the most stable H20F+ species is 21 1.4 kcal/mol.

G2 ab initio molecular orbital calculations have been carried out to study the potential energy surfaces (PESs) associated with the reactions of Cl + in its 3 P ground state and in its 1 D first excited state with water. Similar to what was found for the reactions involving F + , the [H 2 ,Cl,O] + triplet state cations are weakly bound species, while the corresponding singlets are covalently bound species. As a consequence, although the PES of Cl + -( 3 P) lies 33.0 kcal/mol below that of the Cl + ( 1 D), the global minimum of the singlet potential energy surface lies 34.0 kcal/mol below the global triplet minimum. We have also found significant differences between the [H 2 ,Cl,O] + species and their fluorine-containing homologs regarding both their bonding and relative stabilities. These differences are more pronounced in the case of the triplets where the relative stabilities of the [H 2 ,F,O] + minima are completely reversed when fluorine is replaced by chlorine. As a result, most of the products of the reactions of F + with water cannot be formed in Cl + + H 2 O reactions, although also in this case the most likely process is the single charge transfer, in agreement with the experimental evidence. However, the formation of OH + becomes exothermic when Cl + reacts in its 1 D excited state. The estimated heat of formation for the most stable H 2 OCl + singlet state species is 193 ( 2 kcal/mol.

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