Atmospheric chemistry of CH3O: its unimolecular reaction and reactions with H2O, NH3, and HF

Abstract

We have investigated the hydrogen atom transfer processes of CH₃O to CH₂OH without catalyst and with water, ammonia, and hydrofluoric acid as catalysts using ab initio methods, density functional theory (DFT) methods, and canonical variational transition state theory with small curvature tunneling (CVT/SCT). Herein, we have performed the benchmark barrier heights of the title reactions using W3X-L//CCSD(T)-F12a/VDZ-F12 methods. We have also performed the calculations of the combination of MPW-type, PBE-type exchange, M05-type, M06-type functional, and composite theoretical model chemistry methods such as CBS-QB3 and G4. We found that the M05-2X/aug-cc-pVTZ, mPW2PLYP/MG3S, M05-2X/aug-cc-pVTZ, and M06-2X/MG3S methods are performed better in different functionals with the unsigned errors (UEs) of 0.34, 0.02, 0.05, and 0.75 kcal mol⁻¹ for its unimolecular reaction and reactions with H₂O, NH₃, and HF, respectively. The calculated results show that NH₃ exerts the strongest catalytic role in the isomerization reaction of CH₃O to CH₂OH, compared with H₂O and HF. In addition, the calculated rate constants show that the effect of tunneling increases the rate constant of the unimolecular reaction of CH₃O by 10²–10¹² times in the temperature range of 210–350 K. Moreover, the variational effects of the transition state are obvious in CH₃O + NH₃. The calculated results also show that the direct unimolecular reaction of CH₃O to CH₂OH is dominant in the sink of CH₃O, compared with the CH₃O + H₂SO₄, CH₃O + HCOOH, CH₃O + H₂O, CH₃O + NH₃, and CH₃O + HF reactions in the atmosphere. The present results provide a new insight into catalysts that not only affect energy barriers, but have influences on tunneling and variational effects of transition states. The present findings should have broad implications in computational chemistry and atmospheric chemistry.