Ab initio calculations at the SCF/4-31G level are carried out to study proton transfers in (A-H-B)+ where B = NH3 and A is one of its alkylated derivatives MeNH2, EtNH2, or Me2NH. Also examined are the oxygen analogues composed of H20 and its methyl, ethyl, and dimethyl derivatives. Marcus theory predicts quite well the quantum mechanical dissociation energies of the (A-H-OH2)+ complex to either (AH+ + OH2) or (A + OH3+). The agreement is slightly poorer in the N cases due to the presence of two wells in the (H3N-H-NH3)+ potential but can be improved markedly by an extrapolation technique involving the asymmetric single-well (A-H-NH3)+ potentials. Proton transfers within an intramolecular H bond are modeled by holding fixed the distance between groups. Progressive degrees of alkylation of A produce increases in the barrier to proton transfer from A to B and smaller decreases in the reverse transfer barriers. These changes are quantitatively reproduced by the Marcus equation for all systems. Fair agreement is noted as well between the Bronsted a computed as the slope of the Marcus curve and as the position of the transition state along the reaction coordinate. Evaluation of the "intrinsic" barrier in (A-H-B)+ as the arithmetic average of barriers in (A-H-A)+ and (B-H-B)+ appears to provide a convenient and accurate means of treating fundamentally asymmetric systems, as confirmed by calculations involving (Me20-H-NH3)+.
Quantum Mechanical Test of Marcus Theory. Effects of Alkylation upon Proton TransferThe Journal of Physical Chemistry
PublisherAmerican Chemical Society
Citation InformationQuantum Mechanical Test of Marcus Theory. Effects of Alkylation upon Proton Transfer S. Scheiner and P. Redfern J. Phys. Chem., 1986 90 (13), 2969-2974.