Thermokinetic Study of Triplet CH₃OH + CH₂: A New Approach for Complexation Free Energy

This study provides novel insights into the methanol (CH₃OH) + methylene (CH₂) reaction dynamics, introducing the complexation energy and the complexation free energy to characterize prereactive complex formation in the entrance channel. High-level W1BD calculations reveal a prereactive complex with complexation energies (and free energies) of -0.74 (+20.51), +6.30 (+108.07), and +15.11 (+214.57) kcal mol^-1 at 300, 1500, and 3000 K, respectively. Hydrogen atom transfer from methanol's OH group (TS1) is kinetically favored over the CH₃ group, aligning with trends in the analogous CH₃OH + OH reaction. At 300 K, the branching ratio favors CH₃O + CH₃ (63.10%) over CH₂OH + CH₃ (36.90%). The room-temperature total low- and high-pressure limit rate constants are 1.67 × 10^-23 and 7.54 × 10^-18 cm³ molecule^-1 s^-1, respectively, derived from the competitive canonical unified statistical model and transition state theory. Rate constants, activation energies, and activation free energies exhibit positive temperature dependence across 200-3000 K. Energetics and kinetics were validated with higher-level methods such as BD(TQ), CCSD(T), MP4, G4, G4MP2, G3B3, CBS-QB3, and DFT approaches, confirming M08-HX and M06-2X as reliable for this system. This work underscores the critical role of methodology in accurately describing complex formation, pathways, and kinetics in complex chemical systems.