POTENTIAL-ENERGY FUNCTION FOR CH3-REVERSIBLE-ARROW-C2H6 - ATTRIBUTES OF THE MINIMUM ENERGY PATH(CH3)

The region of the potential energy surface for the title reaction in t he vicinity of its minimum energy path has been predicted from the ana lysis of ab initio electronic energy calculations. The ab initio proce dure employs a 6-31G* basis set and a configuration interaction calcu lation which uses the orbitals obtained in a generalized valence bond calculation. Calculated equilibrium properties of ethane and of isolat ed methyl radical are compared to existing theoretical and experimenta l results. The reaction coordinate is represented by the carbon-carbon interatomic distance. The following attributes are reported as a func tion of this distance and fit to functional forms which smoothly inter polate between reactant and product values of each attribute: the mini mum energy path potential, the minimum energy path geometry, normal mo de frequencies for vibrational motion orthogonal to the reaction coord inate, a torsional potential, and a fundamental anharmonic frequency f or local mode, out-of-plane CH3 bending (umbrella motion). The best re presentation is provided by a three-parameter modified Morse function for the minimum energy path potential and a two-parameter hyperbolic t angent switching function for all other attributes. A poorer but simpl er representation, which may be satisfactory for selected applications , is provided by a standard Morse function and a one-parameter exponen tial switching function. Previous applications of the exponential swit ching function to estimate the reaction coordinate dependence of the f requencies and geometry of this system have assumed the same value of the range parameter a for each property and have taken a to be less th an or equal to the ''standard'' value of 1.0 angstrom-1. Based on the present analysis this is incorrect: The a values depend on the propert y and range from approximately 1.2 to approximately 1.8 angstrom-1.