Correlation of Experimental Rate Constants of the Hydrogen Exchange Reactions with a Theoretical H3 Potential Surface, Using Transition-State Theory

The recent ab initio potential surface for H3 obtained by Shavitt, Stevens, Minn, and Karplus is used with one empirical parameter, a uniform scale factor for the energy profile along the reaction path, to compute rate constants for various isotopic forms of the H+H2 exchange reaction. The scale factor is adjusted to give agreement with the high-temperature part of the experimental rates recently measured by Westenberg and de Haas, and the results are compared with all the available experimental data. Tunneling is estimated by fitting a one-dimensional Eckart barrier to the higher 30% of the computed reaction-path barrier. It is concluded that the ab initio barrier needs to be scaled down by only 11% in order to reproduce the best experimental data, and the “experimental” barrier height is accordingly obtained as 9.8 ± 0.2 kcal/mole.