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Context.  Formaldehyde is a potential biogenic precursor involved in prebiotic chemical evolution. The cold conditions of the interstellar medium (ISM) allow H 2 CO to be reactive, playing a significant role as a chemical intermediate in formation pathways leading to interstellar complex organic molecules. However, gas-phase molecular formation mechanisms in cold regions of the ISM are poorly understood.   Aims.  We computationally determine the most favored gas-phase molecular formation mechanisms at local thermodynamic equilibrium conditions that can produce the detected amounts of H 2 CO in diffuse molecular clouds (DMCs), in dark, cold, and dense molecular clouds (DCDMCs), and in three regions of circumstellar envelopes of low-mass protostars (CELMPs).   Methods.  The potential energy surfaces, thermodynamic functions, and single-point energies for transition states were calculated at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory and basis sets. Molecular thermodynamics and related partition functions were obtained by applying the Maxwell-Boltzmann quantum statistics theory from energies computed at CCSD(T)-F12/cc-pVTZ-F12 with corrections for zero-point energy. A literature review on detected abundances of reactants helped us to propose the most favorable formation routes.   Results.  The most probable reactions that produce H 2 CO in cold astrophysical regions are:  1 CH 2  + ⋅ 3 O 2  → 1 H 2 CO + O⋅( 3 P) in DMCs, ⋅ 3 CH 2  + ⋅ 3 O 2  → 1 H 2 CO + ⋅O( 3 P) in DCDMCs, and ⋅CH 3  + ⋅O( 3 P) → 1 H 2 CO + ⋅H in region III, ⋅CH 3  +⋅O( 1 D) → 1 H 2 CO + ⋅H in region II, and  1 CH 2  + ⋅ 3 O 2  → 1 H 2 CO + ⋅O( 3 P) in region I belonging to CELMPs.   Conclusions.  Quantum chemical calculations suggest that the principal carbonaceous precursors of H 2 CO in cold regions for the gas-phase are CH 2 (a 1 A 1 ), and ⋅CH 2 (X 3 B 1 ) combined with ⋅O 2 ( 3 Σ  g  ) and ⋅CH 3 ( 2 A  ”  ) + ⋅O( 3 P) / O( 1 D). Reactions based on more complex reagents yield less effective thermodynamics in the gas-phase H 2 CO molecular formation.

Mechanisms for gas-phase molecular formation of neutral formaldehyde (H2CO) in cold astrophysical regions

Mechanisms for gas-phase molecular formation of neutral formaldehyde (H₂CO) in cold astrophysical regions