LINEAR FREE-ENERGY OF ACTIVATION RELATIONSHIP FOR BARRIERLESS ASSOCIATION REACTIONS

An analysis of variational transition state theory rate constants, for the association reactions Cl- + CH3- Cl, Cl- + CH3Br, H + CH3, H + di amond{111}, CH3 + CH3, and Al + Al-2 shows that the free energy of act ivation Delta G(double dagger) varies nearly linearly with temperature , over a broad temperature range, for each reaction. This suggests tha t association rate constants can be parametrized by the free energy of activation at 300 K, Delta G(300)(double dagger) Delta G(double dagge r) with temperature, Delta Delta G(double dagger)/Delta T. The near li near dependence of Delta G(double dagger) with temperature is supporte d by a semiempirical model for association kinetics. The rate constant s for Cl- + CH3Cl, Cl- + CH3Br, and CH3 + CH3 decrease with increase i n temperature, while those for H + CH3, H + diamond{111}, and Al + Al- 2 slightly increase. For the six association reactions considered here , the linear free energy relationship gives semiquantitative rate cons tants over a rather broad temperature range of 200-2000 K. To calculat e accurate rate constants over a broader temperature range, particular ly at low temperatures, nonlinear terms must be included in the free e nergy expansion.