Faraday Discuss., 2003, (Advance Article)
DOI: 10.1039/b211871h

Modelling spin-forbidden reactions: recombination of carbon monoxide with iron tetracarbonyl

Jeremy N. Harvey and Massimiliano Aschi

FULL TEXT

Abstract:

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 E(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.