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

Context. Formaldehyde is a potential biogenic precursor involved in prebiotic chemical evolution. The cold conditions of the interstellar medium (ISM) allow H₂CO 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 H₂CO 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 H₂CO in cold astrophysical regions are: ¹CH₂ + ⋅³O₂ →¹H₂CO + O⋅(³P) in DMCs, ⋅³CH₂ + ⋅³O₂ →¹H₂CO + ⋅O(³P) in DCDMCs, and ⋅CH₃ + ⋅O(³P) →¹H₂CO + ⋅H in region III, ⋅CH₃ +⋅O(¹D) →¹H₂CO + ⋅H in region II, and ¹CH₂ + ⋅³O₂ →¹H₂CO + ⋅O(³P) in region I belonging to CELMPs.

Conclusions. Quantum chemical calculations suggest that the principal carbonaceous precursors of H₂CO in cold regions for the gas-phase are CH₂(a¹A₁), and ⋅CH₂(X³B₁) combined with ⋅O₂(³Σ_g) and ⋅CH₃(²A^”) + ⋅O(³P) / O(¹D). Reactions based on more complex reagents yield less effective thermodynamics in the gas-phase H₂CO molecular formation.