DIABATIC ORDERING OF VIBRATIONAL NORMAL-MODES IN REACTION VALLEY STUDIES

Diabatic ordering of the normal model of a reaction complex along the reaction path has several advantages with regard to adiabatic ordering . The method is based on rotations of the vibrational normal modes at one Feint, s, of the reaction path to maximize overlap with the vibrat ional modes at a neighboring Feint. Global rotations precede the rotat ions of degenerate modes so that changes in the direction of the react ion path and changes in the force constant matrix, which represent the two major effects for changes in mode ordering, can be separated. Ove rlap criteria identify resolved and unresolved avoided crossings of no rmal modes of the same symmetry. Diabatic mode ordering (DMO) can be u sed to resolve the latter by reducing the step size, thus guaranteeing correct ordering of normal modes in dependence of s. DMO is generally applicable to properties of the reaction complex that depend on s suc h as normal mode frequencies, orbital energies, the energy of excited states, etc. Additional applications are possible using a generalized reaction path vector, which may describe the change in atom masses, ge ometrical parameters, and/or the force constant matrix. In this way, t he vibrational spectra of isotopomers can be investigated or the vibra tional frequencies of different molecules correlated. (C) 1997 by John Wiley & Sons, Inc.