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

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 deltaE(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 Cs-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 x 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.

Computational study of the spin-forbidden H2oxidative addition to 16-electron Fe(0) complexes

“…the need to reach a MECP. The explicit calculation of the MECP for this reaction has only recently been carried out 33 and a good agreement was obtained between the computed and the experimental activation barriers, in support of the mechanistic proposition. This observation is relevant because the spinforbidden CO additions to other 16-electron triplet complexes have been shown to be limited by a normal steric barrier while the crossing point resides on the down-slope leading to the product.…”

“…39 In our present calculations, we have used this B3PW91* functional throughout, together with the same type of flexible, polarised basis set also used in ref. 33, so that the results should be directly comparable.…”

“…23- 33 The experimental observation that the spin-forbidden CO addition to yield singlet Fe(CO) 5 is ca. 400 times slower than the spin-allowed addition to Fe(CO) 3 has prompted the proposition 15 that this extra barrier is related to the need to † Based on the presentation given at Dalton Discussion No.…”

SPIN-FORBIDDEN AND SPIN-ALLOWED CYCLOPROPENONE (c-H2C3O) FORMATION IN INTERSTELLAR MEDIUM

“…The spin-forbidden pathway of reaction (1) was modeled by a transition probability at the MECP as determined by Landau-Zener theory (Zener 1932;Wittig 2005) and rate constant calculated using nonadiabatic transition state theory (NA-TST; Harvey & Aschi 2003). The Landau-Zener probability of a transition from the triplet to the singlet state at the MECP is…”

“…Reaction (1) is the radical-radical spin-forbidden reaction of atomic oxygen and cyclopropenylidene, reaction (2) is the radical-molecule spinallowed reaction of molecular oxygen and cyclopropenylidene, and reaction (3) is the molecule-molecule spin-allowed reaction of acetylene and carbon monoxide (Figure 1). The rate constant for the spin-forbidden pathway of reaction 1 was calculated using nonadiabatic transition state theory (Harvey & Aschi 2003). The transition probability between the triplet and singlet states was modeled through Landau-Zener theory (Zener 1932;Wittig 2005) by locating the minimum energy crossing point (MECP; Poli & Harvey 2002) of singlet and triplet states, and calculating the spin-orbit coupling between the two states.…”