Theoretical study of spin–orbit coupling and zero-field splitting in the spin-forbidden two-state reaction between cobaltacyclopentadiene and isocyanate

Abstract

The two-state reaction mechanism of CpCo(C4H4) with isocyanate on the triplet and singlet potential energy surfaces has been investigated at the B3LYP level. A study is described for the computation of spin–orbit coupling of triplet state of the minimal energy crossing point (CP) with their singlet states and of the zero-field splitting (ZFS) parameters of the triplet states, including the full one- and two-electron terms of the Breit–Pauli Hamiltonian. There are two key crossing points along this two-state reaction pathway. The first crossing point—CP2 exists near 1B. The reacting system will change its spin multiplicity from the triplet state to the singlet state near this crossing region. Although the spin–orbit coupling interaction and ZFS D-tensor of the CP2 region are very strong, the reaction system will occur the reverse intersystem crossing from T1 to S0. Therefore, its spin-flip efficiency may be lower. The second crossing point, CP3 will again change its spin multiplicity from the singlet state to the triplet state in the Co–Cγ bond activation pathway, leading to a decrease in the barrier height of 1TS(CF) from 19.5 to 9.5 kcal/mol (1 cal = 4.182 J), and the efficiency of intersystem crossing from S0 to T1 is high because the larger spin–orbit coupling (SOC) matrix elements will result in the overpopulations of the three sublevels of T1 (3.30 × 10−1, 3.32 × 10−1, and 3.38 × 10−1, respectively).