Theoretical study of spin-orbit coupling and intersystem crossing in the two-state reaction between Nb(NH2)3 and N2O

Abstract

The two-state mechanism of the reaction of Nb(NH2)3 with N2O on the singlet and triplet potential energy surfaces has been investigated at the B3LYP level. Crossing points between the potential energy surfaces have been located using different methods. Analysis of the strain model shows that the singlet state of the four-coordinate (N2O)Nb(NH2)3 complex with N2O bonded via terminal N atom coordination (12) is more stable in the initial stage of reaction, since the bending of the N2O fragment [Edef(N2O) = 86.1 kcal mol−1] results in an energy splitting of the doubly degenerate LUMO; the low-energy LUMO can now strongly couple with the occupied Nb-localized d orbitals, forming a back-bond and transferring charge (q = 0.82 e) from Nb(NH2)3 to the N2O ligand. Going from 32 to 12, the reacting system changes spin multiplicity near the MECP (minimal energy crossing point) region, which takes place with a spin crossing barrier of 9.6–10.0 kcal mol−1. Analysis of spin-orbit coupling (SOC) indicates that MECP will produce a significant SOC matrix element. The value of SOC is 111.52 cm−1, due to the electron shift between two perpendicular ϕ orbitals with the same rotation direction, and the magnitude of the spin-multi-plicity mixing increases in the small energy gap between high- and low-spin states, greatly enhancing the probability of intersystem crossing. The probabilities of single (P1ISC) and double (P2ISC) passes estimated at MECP (SOC = 111.52 cm−1) are approximately 1.17×10−2 and 2.32×10−2, respectively.