The C( ³ P) + CH ₂ ( ³ B ₁ ) reaction: accurate electronic structure calculations and kinetics for astrochemical modeling

In this work, we present a theoretical study of the C + CH₂ → H + CCH reaction, which is expected to play a key role in the formation of carbon-chain species in molecular clouds. We employ high-level <i>ab initio</i> electronic structure methods, including the HEAT-345Q composite scheme and multi-reference configuration interaction, to characterize the potential energy surface. Reaction kinetics are investigated through both conventional transition state theory for tight transition states and variational reaction coordinate transition state theory (VRC-TST) for barrierless pathways. The master equation technique is used to account for competing complex formation, isomerization, and dissociation processes, particularly under collisionless conditions relevant to astrochemistry. Rate constants for product formation are calculated over a wide temperature range (10-300 K), and fitted to extended Arrhenius expressions. The results reveal that both singlet and triplet pathways contribute significantly to the overall reaction rate, with the triplet surface dominating under typical ISM conditions.