Theoretical and experimental study of the gas-phase reactions of PFn+ (n=1, 2) ions with n-bases. A novel route to phosphorus ions present in interstellar clouds

The reactivity of PFn+ (n = 1, 2) cations toward n-type bases, such as H2O, CH3OH, and NH3, has been investigated using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. Both phosphorus ions attack the n-center of the selected bases, yielding the corresponding excited onium intermediates, which undergo extensive prototropic rearrangements before fragmenting with formal losses of H and HF (with H2O and NH3) or HF, CH3, and CH3F (with CH3OH). Ab initio molecular orbital calculations at the G2 level of theory have been used to study the structures, the relative stability, and the proton loss energies of the ionic species involved in these reactions. The G2 [PF, H2O](+) and [PF, NH3](+) potential energy profiles conform well with the FT-ICR reaction kinetics. Unequivocal assignment of the structure and the multiplicity of several ionic species obtained from the title reactions and potentially involved in the formation of phosphorus-containing molecules in outer space, i.e. (PO)H+, (FPO)H+, (PNH)H+, and (FPNH)H+, is allowed by a comparison of their G2-calculated proton loss energies with the experimental protonation enthalpies, measured in the FT-ICR instrument with the bracketing technique. The emerging picture has been compared with previous theoretical data concerning strictly related species wherein the phosphorus atom is replaced by nitrogen.