Experimental and theoretical studies of the gas-phase reactivity of the (HO)(2)P=O+ phosphonium ions towards methanol

Ion-molecule reactions between the (HO)(2)P=O+ phosphonium ions and methano l were performed in a quadrupole ion trap mass spectrometer. The (HO)(2)P=O + phosphonium ions, formed by electron impact from neutral trimethyl phosph ite ions were found to react with methanol according to three consecutive r eactions, via sequential methanol addition/water elimination, to yield prot onated trimethyl phosphate. To confirm the experimental results, and to sta te the mechanism for the formation of the ionic species, a theoretical stud y by using the density functional theory (DFT) approach has been carried ou t. According to calculations performed at the B3LYP/6-311+G(2df,p) over B3L YP/6-31G* optimized geometries, the overall reaction leading to protonated trimethyl phosphate occurs by an exothermic process of 365 kJ/mol. The isom erization barriers connecting the different intermediates have been also ca lculated in order to have a more complete description of the reaction proce sses, In addition, the proton affinity (PA) and the gas-phase basicity (GB) of the molecular species related to the reactions of the (HO)(2)P=O+ catio ns with methanol namely: monomethyl phosphate, dimethyl phosphate, and trim ethyl phosphate (TMP) have been evaluated to be 855, 875, and 892 kJ/mol (f or PAs) and 823, 843, acid 862 kJ/mol (for GBs), respectively. The excellen t agreement between the theoretical (892 kT/mol) and the experimental value (891 kJ/mol) of the PA of TMP shows the reliability of our DFT calculation s. (C) 2000 Elsevier Science B.V.