Ab initio calculations on the H₂ +D₂=2HD four-center exchange reaction. I. Elements of the reaction surface

We present the results of ab initia calculations on some interesting regions of the reaction surface for the four-center exchange reaction H ₂+D₂ = 2HD. These calculations, which use a minimum basis set of Slater orbitals, indicate that for all geometries appropriate to the transition state of the reaction, a barrier height of at least 148 kcal/mole is present. This is far greater than the energy required to produce free radicals and more than three times the experimental energy of activation, 42 kcal/mole. Considering the sources and magnitudes for errors due to correlation and basis set restrictions, we estimate the barrier height for this exchange reaction to be 132±20 kcal/mole exclusive of zero-point energies. In this paper we discuss the surface as determined by configuration interaction techniques. We find that the most favorable geometries for the exchange reactions are the square, rhombus, and kite configurations. However, all of these states are unstable with respect to H₃+2H. In addition we find no evidence of collision complexes for any of the likely transition state geometries. In the following paper we will examine the Gl wavefunctions for this system in order to obtain an understanding of the factors responsible for the shape of the surface.