Why is Pt₄⁺ the Least Efficient Cationic Cluster in Activating the C−H Bond in Methane? Two-State Reaction Computational Investigation

The two-state reaction mechanism of Pt 4 + with CH 4 on the quartet and doublet potential energy surfaces has been investigated at the B3LYP level. Crossing points between the potential energy surfaces are located using different methods, and possible spin inversion processes are discussed by means of spin−orbit coupling (SOC) calculations. As a result, after the C−H insertion intermediates, two distinct H 2 elimination reaction paths have been found, and the second C−H bond broken is regarded as the rate-determining step for two reaction paths. For the first pathway, there is a minimal energy crossing point (MECP) between the two surfaces, where the reacting system will change its spin multiplicities from the quartet state to the doublet state near this crossing region because the magnitude of the spin multiplicity mixing increases in a small energy gap between high- and low-spin states and will greatly enhance the probability of the intersystem crossing. The single P 1 ISC and double P 2 ISC passes estimated at MECP (SOC = 201.25 cm −1 ) are approximately 0.51 and 0.76, respectively. Not only will the reaction overcome a spin-change-induced barrier (ca. 6 kcal/mol) but it also will overcome an adiabatic barrier (ca. 24.7−20.8 kcal/mol), which will greatly reduced the efficiency of the reaction system. As for the second reaction path, similarly, there is a crossing seam between CP-2 and CP-3, the spin-change-induced barrier is very high, about 15.86−18.86 kcal/mol, which also indicates a less favorable process kinetically. Therefore, the lack of a thermodynamic driving force is an important factor contributing to the low efficiency of the reaction system. These conclusions are consistent with the experimental observations.