EXPERIMENTAL AND THEORETICAL-STUDIES OF THE GAS-PHASE PROTONATION OF ORTHOPHOSPHORIC ACID

The gas-phase proton affinity (PA) and the gas-phase basicity (GB) of orthophosphoric acid (H3PO4) have been determined by the kinetic metho d to be 833 and 800 kJ/mol, respectively. High level ab initio calcula tions at the G2(MP2) level of theory give a PA of 827 kJ/mol. A good a greement is observed between the G2(MP2) and density functional theory (using B3LYP) results. B3LYP/6-311 + G(2df,p) calculations and chemic al ionization (CI) experiments have been performed in order to clarify the loss of water from protonated H3PO4. The ore-oxygen atom is confi rmed as the most basic site, the hydroxy-oxygen protonated isomer O=P( OH)(2)(OH2)(+) being predicted to be 126 kJ/mol less stable than P(OH) (4)(+). The isomerization barrier connecting both isomers is calculate d as 200 kJ/mol and the dissociation products, water and protonated me taphosphoric acid O=P(OH)(2)(+), are found to be 284 kJ/mol with regar d to P(OH)(4)(+). The proton affinity-of metaphosphoric acid (HPO3) is also evaluated to be 712 kJ/mol at the G2(MP2) level of theory. In ad dition, a proton-bound complex [HO(O)PO ... H ... OH2](+) has been loc ated as an intermediate for the elimination of HPO3 or H2O. The elimin ation of HPO3, which is not observed in CI experiments, was found to b e more endothermic than the loss of water by 23 kJ/mol. (C) 1998 Ameri can Institute of Physics.