Analysis of the Principles Governing Proton-Transfer Reactions. Comparison of the Imine and Amine Groups
Journal of the American Chemical Society
American Chemical Society
Proton transfers involving the imine and amine groups are studied by ab initio SCF calculations with use of the 4-31G* basis set with H2C=NH and NH3 serving as respective models of the two groups. The potential energy surface of the imine-H+-amine system contains two distinct minima, corresponding to (H2CHNH--NH3)+ and (H2CHN--HNH3)+, the former being the more stable, separated by a barrier of 5.9 kcal/mol. The energy barrier separating the two minima rises rapidly as the H bond is elongated; nevertheless, the (H2CHNH--NH3)+ configuration remains the more stable regardless of the H bond length. On the other hand, the relative energies of the two minima are profoundly affected by angular distortions of the bond. In general, rotation of either subunit, turning its N lone pair away from the N- -N axis, causes a preferential stabilization of the configuration in which the proton is associated with that group, an effect attributed to a more favorable ion-dipole interaction. An exception to this rule is noted with distortions out of the imine plane; the differing behavior of the amine and imine groups is traced to the opposite sign of the appropriate components of their quadrupole moments. These same principles explain the results obtained for the oxygen analogues carbonyl and hydroxyl after consideration of the additional factor that the dipole moment vectors of these groups do not coincide with the directions of the O lone pairs.
Analysis of the Principles Governing Proton Transfer Reactions. Comparison of the Imine and Amine Groups E. A. Hillenbrand and S. Scheiner J. Am. Chem. Soc., 1985 107 (25), 7690-7696.