Thermochemistry of the Reactions F⁺(³P, ¹D) + PH₃ in the Gas Phase

Abstract

High-level ab initio calculations in the framework of the G2 theory have been performed for the [H3, P, F]+ singlet- and triplet-state cations. The bonding characteristics of singlet- and triplet-state cations are rather different. The latter are weakly bound species involving electrostatic and/or polarization interactions, while the former present covalent bonds. As a consequence, while in the F+(3P) + PH3(1A1) reactions in the gas phase the charge-transfer process competes with the formation of HF(1Σ+) + PH2 +(3B1), the main products when the reaction involves the F+ cation in its 1D first excited state are HF(1Σ+) + PH2 +(1A1). In both cases, the reactions are extremely exothermic, and therefore, the products are anticipated to be vibrationally excited. The [H3, 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 PH2 + 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 corresponding minimum energy crossing point, indicates that a fast transition between both PESs should take place, implying the possibility of having “spin-forbidden” reactions. From our calculations, the heat of formation for FPH2 was estimated to be −58.2 ± 2.5 kcal/mol.

Reactions of F+(3P) and F+(1D) with Silicon Oxide. Possibility of Spin-Forbidden Processes

“…Gas-phase ion processes are of great relevance in atmospheric and interstellar chemistry. − It is nowadays well established that ion−molecule reactions seem to be in the origin of the different species detected inside interstellar clouds. , However, in general, the only information available in most of these processes is the product distribution, but very little is known about the mechanism leading to these products. − In this respect the role of high-level ab initio molecular orbital techniques − was of great importance in elucidating these mechanisms. − Actually, in many occasions, the ab initio or density functional theory calculations are the only alternative available to investigate the structures of ionic species, which, very often, are elusive to experimental observation. Besides, high-level theoretical methods reach the so-called chemical accuracy , and therefore, many thermodynamic properties of neutral and charged species can be estimated within ±2.0 kcal/mol of the experimental values, when available. − High accuracy is important in this field because in interstellar space only exothermic processes are likely to occur, due to the extreme interstellar conditions of low temperature and density, and therefore an accurate knowledge of the topology of the corresponding potential energy surface (PES) is of critical importance.…”

Theoretical Survey of the Potential Energy Surfaces Associated with the N+(3P,D) + C2H4Reactions in the Gas Phase

“…Quite importantly, however, when the ion is attached to a neutral molecule, a reverse stability order is often found and singlets become lower in energy than triplets, ,,− reflecting the stronger bonds between the ion and the neutral in the former. Accordingly, different crossovers between singlet and triplet PESs can be envisaged, ,, and therefore in many of these processes spin-forbidden reaction mechanisms play an important role, as has been suggested before for N + + SH 2 and N + + H 2 CO reactions. , In this paper we will focus our attention on the reactions between N + ( 3 P, 1 D) and ethylene, because hydrocarbons are ubiquitous in the aforementioned media and therefore they usually play an important role in the generation of new molecular systems. , As a matter of fact, ethylene is one of the main components of Titan's atmosphere, , where both N + ( 3 P) and N + ( 1 D) can be also produced from the existing N 2 molecules via cosmic ray absorption. Actually, in the laboratory both ionic species can be produced by electron impact dissociation of N 2 molecules …”