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

Abstract

The Δ f H 298 0 (NH2CO•) = −8.6 ± 1 kJ/mol and S 298 0 = 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 298 0 = 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.