An early paradigm for chemical kinetics and molecular dynamics is the reaction HI + HI -> H2 + I2 along with its reverse H2 + I2 -> HI + HI for which Bodenstein in 1894 reported experimental rate measurements and suitable rate expressions (including that for the possible termolecular reaction H2 + I + I -> HI + HI). This was soon followed by theoretical studies of its treatment by collision theory and, somewhat later, by absolute rate theory and by speculations on the energy requirements for the forward and backward reactions. Since then, Pauling's and many subsequent freshman textbooks have pictured two HI molecules colliding at high velocity to form H2 and I2. Those pictures led to molecular beam experiments indicating otherwise. These were followed by more theory: classical trajectory calculations based on Keck's reactive flux approach, an excellent example for the successful prediction of rare events. At the same time other experiments indicated the reverse reaction to be termolecular.  Further calculations suggested this may or may not be correct, but that small variations in the potential surface might favor one route or the other. The back-and-forth interplay of experiment and theory for this reaction may continue for years, but at present we have new theoretical predictions, based on new and better potential energy surfaces, along with reactive flux calculations, to report.

Interplay of experiment and theory for the hydrogen/iodine reaction

247th National Meeting (2014)

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