Thermochemistry of the reactions F+(P-3, D-1)+PH3 in the gas phase

High-level ab initio calculations in the framework of the G2 theory have be en performed for the [H-3, P, F](+) singlet- and tripler-state cations. The bonding characteristics of singlet- and triplet-state cations are rather d ifferent. The latter are weakly bound species involving electrostatic and/o r polarization interactions, while the former present covalent bonds. As a consequence, while in the F+(P-3) + PH3((1)A(1)) reactions in the gas phase the charge-transfer process competes with the formation of HF((1)Sigma(+)) + PH2+(B-3(1)), the main products when the reaction involves the F+ cation in its D-1 first excited state are HF((1)Sigma(+)) + PH2+((1)A(1)). In bot h cases, the reactions are extremely exothermic, and therefore, the prc,duc ts are anticipated to be vibrationally excited. The [H-3, P, F](+) triplet- state cations are good examples of molecular planetary systems, in which a neutral fluorine atom or a neutral HF molecule orbits around a PH3+ or a PH 2+ moiety, respectively. Although the singlet PES lies systematically below the triplet PES, there are regions where both surfaces approach each other significantly. The spin-orbit coupling between them, evaluated at the corr esponding minimum energy crossing point, indicates that a fast transition b etween both PESs should take place, implying the possibility of having "spi n-forbidden" reactions. From our calculations, the heat of formation for FP H2 was estimated to be -58.2 +/- 2.5 kcal/mol.