A POSSIBLE TRANSITION STATE FOR THE Hz + D2 EXCHANGE REACTION

A considerable efforr has been devoted to ab initio theoretical studies Cl-161 * of the H4 potential surface in order to locate a structure that might correspond to the transition state for the four-center exchange reaction H, + D2 + 2HD and to characterize the long-range intermolecular interaction. To date a satisfactory structure for the transition state has not been located. Various shock-tube studies [ i7--191 of this reaction have been interpreted in terms of a bimolecular rate process having an activation energy of 39 to 42 kcal/mol. Becent atomic resonance absorption experiments [2(i] have been similarly interpreted, with a reported activation energy of 38 f 5 kcal/moi. By contrast the ab initio barrier heights are typically 100 kcal/mol higher, with some representative values being 124 to 148 kcd/ mol for a square planar structure [3-7,121 and 188 kcal/mol for a tetrahedral structure 171. Of comparatively low energy but not suitable as the transition state is the collinear structure. As often pointed out, either the “correct” region of configuration space has not been explored in the ab initio studies, or the ex-