Proton transfers in hydrogen-bonded systems. 2. Electron correlation effects in diamminehydrogen(1+)

Potentials for transfer of the central proton between the NH3 units of (H3NHNH3)+ are calculated via ab initio methods at several levels of sophistication. Double-well potentials are obtained for all internitrogen distances studied which vary between 2.73 and 3.15 Å. All methods indicate that the energy barrier to proton transfer increases as the two NH3 molecules are further removed from one another. Treatment of electron correlation via the polarization configuration interaction (POL-CI) method leads to barriers substantially smaller than those obtained by restricted Hartree-Fock calculations with use of a basis set of double �� plus polarization function (RHF/DZP) quality. Inclusion of only intrapair correlations involving the two nitrogen lone pairs with use of the generalized valence bond (GVB) method leads to no decreases in the barriers relative to the DZP Hartree-Fock estimates. Use of the smaller split valence 4-31G basis produces barriers intermediate between the RHF/DZP and POL-CI results. Proton transfers in (N2H7)+in the gas phase are studied using both a rigid model and one which includes relaxation of the R(NN) distance during proton motion. The latter relaxation results in a lowering of the POL-CI barrier from 3.5 to 1.5 kcal/mol.