Kinetics of the four basic thermal three-centre exchange reactions of the hydrogen isotopes

Rate constants have been determined for the reaction D + D2= D2+ D. These, together with our earlier results for the reactions H + H2= H2+ H, H + D2= HD + D and D + H2= DH + H, are considered in comparison with ab initio and semiempirical calculations. The experimental results for all four reactions can be treated in a satisfactory manner by a modified form of transition state theory, and suggest a barrier height of approximately 9.2 kcal mole–1 and a reaction path which, even for the heteronuclear complexes, is essentially coincident with the normal mode co-ordinate for the asymmetric vibration of the complex. On the basis of this treatment the force constants for the symmetric and asymmetric vibrations of the complex are approximately 2.6 × 105 and –0.27 × 105 dyne cm–1, respectively.