VARIATIONAL TRANSITION-STATE THEORY AND TUNNELING CALCULATIONS WITH REORIENTATION OF THE GENERALIZED TRANSITION-STATES FOR METHYL CATION TRANSFER

The new RODS algorithm based on optimizing the orientation of the divi ding surface at each point along the reaction path in order to maximiz e the free energy of the generalized transition state containing that point has been used to carry out variational transition state calculat ions and tunneling calculations for two reactions with high-frequency vibrations strongly coupled to the reaction coordinate, (Cl-)(CH3NH3+) --> ClCH3(NH3) and ClCH3 + NH3, and CH3Cl(H2O) + NH3(H2O)-(CH3NH3+)(C l-)(H2O)(2). These reactions, both of which involve the transfer of a methyl cation between Cl- and NH3, show much larger variational-transi tion-state and tunneling effects than were observed in previous studie s of the transfer of methyl cations between anionic centers. However, they are hard to study because the adiabatic potential energy curves o f both reactions and, as a consequence, the corresponding foe energy o f activation profiles show big dips when the minimum energy path (MEP) is followed using standard methods, even when very small step sizes a re taken to compute the steepest-descent path. The application of RODS methodology eliminates those dips, giving rise to smooth free energy of activation profiles and vibrationally adiabatic potential curves. C alculations of variational rate constants and tunneling effects are si gnificantly improved.