Exploring the potential energy surface of the ethyl cation by new procedures

The MP2/6-31G** potential energy hypersurface (PES) of the chemically interesting molecule ion, C2H5+, is analyzed by new procedures proposed by us recently. The reaction paths (RPs) start from the minima or saddle points by following the so-called reduced gradient procedure and/or its modification, the tangent search concept (TASC). The minimum energy paths (MEP) obtained by TASC are fine approximations of those gradient extremals, which follow the valley floor (or ridge) along the smallest (absolute) eigenvalue of the Hessian. It is the valley floor gradient extremal which is also termed the streambed of the PES. Tracing the streambed uphill we locate saddle points of the H-scrambling in C2H5+. The potential energy surface of this cation is used to systematically explore the properties of RP definitions with particular focus to RP bifurcations. The intrinsic reaction coordinate (IRC), which is mostly used as MEP in chemistry, is included for the purpose of comparison, (C) 2002 Elsevier Science B.V. All rights reserved.