Ab initioMO calculations on the acidities of water and methanol,and hydrogen bond energies of the conjugate ions with a water molecule

The acidities, deprotonation energies, of water and methanol were calculated by the use of the ab initio self-consistent-field (SCF ) molecular orbital (MO ) method with electron correlation computed by the thirdorder Møller–Plesset perturbation method and configuration interaction with double excitations. Zero-point vibrational energy correction translational energy change, and the PV work term were included to evaluate the accurate acidities. The calculated acidity difference including these corrections was 7 kcal/mol, which is somewhat smaller than the experimental ones (9.5–12.5 kcal/mol) recently determined. The hydrogen bond energies of the conjugate ions (OH^? and CH₃O^?) with a water molecule were calculated to be 2.3 kcal/mol near the Hartree–Fock limit; this energy only amounts to 25% of the (total) hydration energy difference between the two negative ions. The aqueous solvation effect on the acidity scale was discussed.