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

Ion-molecule reactions between the (HO)(2)P=O+ phosphonium ions and methanol were performed in a quadrupole ion trap mass spectrometer. The (HO)(2)P=O+ phosphonium ions, formed by electron impact from neutral trimethyl phosphite ions were found to react with methanol according to three consecutive reactions, via sequential methanol addition/water elimination, to yield protonated trimethyl phosphate. To confirm the experimental results, and to state the mechanism for the formation of the ionic species, a theoretical study by using the density functional theory (DFT) approach has been carried out. According to calculations performed at the B3LYP/6-311+G(2df,p) over B3LYP/6-31G* optimized geometries, the overall reaction leading to protonated trimethyl phosphate occurs by an exothermic process of 365 kJ/mol. The isomerization barriers connecting the different intermediates have been also calculated in order to have a more complete description of the reaction processes, 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+ cations with methanol namely: monomethyl phosphate, dimethyl phosphate, and trimethyl phosphate (TMP) have been evaluated to be 855, 875, and 892 kJ/mol (for PAs) and 823, 843, acid 862 kJ/mol (for GBs), respectively. The excellent 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 calculations. (C) 2000 Elsevier Science B.V.