Structure and decomposition path of the HIF radical

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Abstract
The potential‐energy surface for the inorganic free‐radical, HIF, known only from molecular‐beam studies, is investigated using fourth‐order many‐body perturbation theory [MBPT(4)]. All‐electron calculations with three different basis sets are compared with results using relativistic and nonrelativistic effective potentials on I. Two local minima are found with an H–I–F angle α equal to 137.5° and 82°, with the energy difference ranging from 2.3 to 5.1 kcal/mol depending upon the calculation. The barrier to internal conversion is 1.1 kcal/mol. From the lowest minimum our MBPT(4) calculations predict HIF to be bound to dissociation (De) to IF+H by 27.0 to 32.6 kcal/mol, and to HI+F dissociation by 22.1 to 30.9 kcal/mol, in good agreement with experiment. However, very large superposition errors are shown to be possible in all‐electron calculations because of inevitable basis‐set deficiencies for the I atom. Explanations for the double minima in HIF are presented.