Ab initio molecular orbital methods are used to study transfer of the central proton in the asymmetric (H3NHOH2)+ system. Calculations are performed at the Hartree-Fock level with basis sets of split-valence (4-31G) and double-S plus polarization function (DZP) quality. The effects of electron correlation upon the transfer potentials are computed via the generalized valence bond (GVB) and polarization configuration interaction (POL-CI) techniques. The barrier to proton transfer between NH3 and OH2 is observed to heighten as the distance between the latter two molecules is increased. At the equilibrium R(N0) hydrogen-bond length, the transfer potential contains a single minimum, (H3NH...OH2)+, in which the central proton is more closely associated with NH3. In both the rapid and adiabatic models of proton transfer, there is no energy barrier to decay of (H3N...HOH2)+ to the equilibrium (H3NH...OH2)+ structure. While all the calculations agree on the above points, there are some notable quantitative discrepancies between the various methods. Enlargement of the basis set at the HartreeFock level results in higher transfer barriers while subsequent inclusion of electron correlation (POL-CI) leads to barrier reductions. The GVB procedure, with its partial treatment of correlation, produces changes in the potentials opposite to those observed for the more complete POL-CI treatment.
Molecular Orbital Study of Proton Transfer in (H3NHOH2)+The Journal of Physical Chemistry
PublisherAmerican Chemical Society
Citation InformationMolecular Orbital Study of Proton Transfer in (H3NHOH2)+ S. Scheiner and L. B. Harding J. Phys. Chem., 1983 87 (7), 1145-1153.