Spin-forbidden reaction mechanism of CO₂ cleavage activated by Ta^＋ in the gas phase was investigated, using density functional theory (DFT, UB3LYP) with the relativistic effective core potential (ECP) of basis sets (SDD) for Ta and the 6-311＋G(3d) basis set for C and O. The calculation results show that Ta^＋ initially close to CO₂ on the quintet surface, then the product, TaO^＋ and CO, exit the channel on the triplet surface. The process involves an intersystem crossing (ISC) from the quintet to the triplet state. The minimum energy crossing point (^3-5MECP) is located by using the methods of Harvey et al. The large spin-orbit coupling (SOC) constant (204.94 cm^－1) calculated shows that the spin-forbidden reaction takes place easily. The intersystem crossing probability estimated at MECP indicate that the spin-forbidden reaction have a rapid rate.

Key words: density functional theory, Ta^＋ activation, spin-forbidden reaction, minimum energy crossing point, spin-orbital coupling, intersystem crossing probability