Accurate characterization of the transition state geometry for the HF+H′→H+H′F reaction

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Abstract
The potential surface for the exchange reaction HF+H′→H+H′F has been investigated with various multiconfiguration self‐consistent field (SCF) and configuration interaction (CI) wavefunctions. For the first time, nonlinear geometries have been considered. The calculations demonstrate the importance of diffuse functions on the fluorine for describing nonlinear geometries. A qualitative model is presented to explain the need for diffuse functions. The energy barrier for the exchange reaction is calculated to be ∼45 kcal/mole, which is comparable to the values obtained in previous ab initio calculations on the collinear reaction surface (Refs.1 and 2). More importantly, the calculations show that the saddle point region is very flat, the barrier changing by only 1–2 kcal/mole between collinear (180°) and perpendicular (90°) geometries. The optimum angle for the transition state geometry is calculated to be 106°. Dynamics calculations on the HF+H′→H+H′F reaction have employed semiempirical London–Eyring –Polanyi–Sato (LEPS) surfaces that had artifically small energy barriers, as has been pointed out previously. The present calculations show that not only were the barriers for exchange too low, but that the basic shape of the LEPS surfaces was inappropriate. The implications of this result for future dynamics calculations are discussed.