Calculating the rate constant for the NH ^•₂ + CO ⇄ NH₂CO^• ⇄ H + NHCO reactions and thermodynamic properties of NH₂CO^•

Abstract

The Δ _f H ⁰₂₉₈ (NH₂CO^•) = −8.6 ± 1 kJ/mol and S ⁰₂₉₈ = 260.6 ± 5.2 J mol⁻¹ K⁻¹ values have been obtained using the energies of isodesmic reactions within the CBS-Q approximation. Use of the CBS-Q, UMP2, UBHandHLYP, and UB3LYP approaches has afforded the most substantiated entropy value, S ⁰₂₉₈ = 258.2 ± 2.8 J mol⁻¹ K⁻¹. The energetics of the NH₂CO^• ⇄ NH ^•₂ + CO and NH₂CO^• ⇄ H + NHCO reactions and their rate constants (k ₁ and k ₂, respectively) have been calculated using the UMP2, UBHandHLYP, and UB3LYP approaches. The rate constant values k _{1, ∞} = 8.2 × 10¹⁰(T/298)^1.18e^−115/RT s⁻¹, which were obtained within the UB3LYP approach, are in closest agreement with available experimental data. The constants k _{2, ∞} = 4.0 × 10⁷(T/298)^1.7e^−149/RT s⁻¹, obtained using the UMP2 approach, are best consistent with indirect experimental evidence. The UB3LYP value of the rate constant of the NH ^•₂ + CO reaction at P = 1 atm and T = 304 K (k ₋₁ = 2.2 × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹) suggests that this reaction should make a significant contribution to the removal of NH ^•₂ from the atmosphere under pre-abiogenesis conditions. The resulting NH₂CO^• adduct is a fairly stable compound capable of participating in the formation of the chemical composition of the prebiogenic atmosphere. This conclusion is supported by the small rate constant values k ₁ = 3.3 × 10⁻⁷ s⁻¹ and k ₂ = 5.8 × × 10⁻¹⁸ s⁻¹ at P = 1 atm and T = 304 K, which were calculated using the UB3LYP and UMP2 approaches. In addition, the k ₋₂ = 2.8 × 10⁻²¹ cm³ molecule⁻¹ s⁻¹ value (P = 1 atm) for the NHCO + H reaction, calculated using the UMP2 approach, indicates that this reaction makes an insignificant contribution to the disappearance of H atoms and to the formation of NH₂CO^• under the abiogenesis conditions.