A non-adiabatic version of RRKM theory is applied to predicting the microcanonical rates for different mechanisms of spin-forbidden dissociation of methoxy cation and its isotopically substituted derivatives, to formyl cation and dihydrogen. The predictions are in agreement with experimental results on this system, and in particular with the occurrence of a “direct’' mechanism for dissociation, rather than of an indirect one via hydroxymethyl cation. Abinitio computations were used throughout to provide the parameters needed to apply the non-adiabatic RRKM theory, and the success of this strategy is shown to be promising for other applications in polyatomic systems. Finally, the kinetic energy release distribution for loss of hydrogen from methoxy and hydroxymethyl cations are computed using abinitio “direct dynamics’' classical trajectories at the HF/6-31G** level, their similarity is also in agreement with experiment.

New density functional theory and ab initio computations on the [Fe(CO)₅] system are reported. Careful exploration of basis set and correlation effects leads to “best” values for the difference in energy ΔE(1,3) between ground state ³[Fe(CO)₄] and the singlet excited state of ca. 8 kcal mol⁻¹, and for the bond dissociation energy BDE(3) of [Fe(CO)₅] with respect to ground state fragments ³[Fe(CO)₄] + CO of ca. 40 kcal mol⁻¹. A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with ³[Fe(CO)₄]. A C_s-symmetric minimum energy crossing point (MECP) between the reactant (triplet) and product (singlet) potential energy surfaces is found, lying 0.43 kcal mol⁻¹ above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.