The gas-phase acidity (free energy of deprotonation, Delta G(acid)(0))
of H3PO4, orthophosphoric acid, was determined both experimentally an
d theoretically for the first time. Ion-molecule bracketing experiment
s yielded the result 1372 kJ mol(-1) > Delta G(acid)(0)(H3PO4) > 1331
kJ mol(-1). The most reliable theoretical result, Delta G(acid)(0)(H3P
O4) = 1343 kJ mol(-1), is in agreement with experiment and was obtaine
d using Brueckner methods with large basis sets. (C) 1997 American Ins
titute of Physics.