Juan C. Ramal-Olmedo

Context. Cyanamide (NH₂CN) and its tautomer carbodiimide (NHCHN) are believed to have been key precursors of purines and pyrimidines during abiogenesis on primitive Earth. The detection of guanine and cytosine in meteorites and comets provides evidence of their nonterrestrial formation. Although NH₂CN has been found in several molecular clouds, NHCHN has only been detected in Sgr B2(N). Their possible molecular formation mechanisms in the gas phase and therefore their respective molecular precursors remain an open subject of investigation. Aims. The main objective of this paper is to determine which reactions can produce NH₂CN and HNCNH in the amounts observed under the astrophysical conditions of Sgr B2(N). The determination of their most likely precursors could serve to provide new insights into possible routes to purine and pyrimidine synthesis, and by extension to nucleosides, under the astrophysical conditions of dense molecular clouds. Methods. Initially, we proposed 120 reaction mechanisms, 60 being dedicated to NH₂CN formation and the remaining 60 to HNCNH. These mechanisms were constructed using 25 chemical species that were identified in outer space. We calculated the molecular energies of reactants and products at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory, and defined the values of thermodynamic functions using the Maxwell-Boltzmann statistical quantum theory. Via an extensive literature review on the abundances of reactants and products in Sgr B2(N), in addition to a detailed kinetic study for a range of 20–300 K, we identify the most likely reaction mechanisms for both cyanamides of those proposed previously and presently. Results. From the 120 analyzed reactions, only nine for NH₂CN and four for HNCNH could thermodynamically account for their synthesis in Sgr B2(N). The kinetic portion of our study shows that Ra60 (CH₃NH₂ +·CN → NH₂CN +·CH₃), with a modified Arrhenius expression of k_T = 1.22 × 10⁻⁹ (T/300)^−0.038 exp⁻ (−147.34/T) cm³ mol⁻¹ s⁻¹, is the most efficient reaction at low temperatures (2 → NH₂CN +³NH) appears to be the most likely candidate, showing a modified Arrhenius constant of k_T = 2.51 × 10⁻¹¹ (T/300)^−32.18 exp⁻ (−1.332/T) cm³ mol⁻¹ s⁻¹. In the case of carbodiimide production, Rb18 (·H₂NC +·NH₂ → HNCNH +·H) is the most efficient reaction, fitting a rate constant of k_T = 4.70 × 10⁻¹³ (300/T)^−3.24 exp⁻ (36.28/T) cm³ mol⁻¹ s⁻¹ in Sgr B2(N). Conclusions. The detected gas-phase abundances of cyanamide (NH₂CN) in Sgr B2(N) can be explained as: Ra60 (·CN +·CH₃NH₂) from 20 to 60 K; Ra5: (·CN +·NH₂) from 60 to 120 K; and Ra37-2 (·HNCN +·NH₂) from 120 to 300 K. The carbodiimide (HNCNH) synthesis could proceed via Rb18 (·H₂NC +·NH₂). Moreover, the presence of·HNCN and·H₂NC in Sgr B2(N) are predicted here, making them viable candidates for future astronomical observations. The foreseen column density for the cyanomidil radical is ~10¹⁶ cm² s⁻¹ at 150 K or higher, while for amino methylidine, the value is a few 10¹³ cm² s⁻¹ at 100 K.

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.