Calculating the Rate Constant for the NH2 center dot + CO reversible arrow NH2CO center dot reversible arrow H plus NHCO Reactions and Thermodynamic Properties of NH2CO center dot

摘要

The Delta H-f(298)0(NH2CO center dot) = -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 reversible arrow NIT2 + CO and NH2CO reversible arrow 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,K-infinity, = 8.2 x 10(10) ( T/298)(118)e(115/RT) s(-1), which were obtained within the UB3LYP approach, are in closest agreement with available experimental data. The constants k(2,infinity) 4.0 x 10(7)(T/298)(1.7)e(-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 NIT2 + CO reaction at P = 1 atm and T= 304 K (k(-1)= 2.2 x 10(-18) cm(3) molecule(-1) s(-1)) suggests that this reaction should make a significant contribution to the removal of NH2 from the atmosphere under pre-abiogenesis conditions. The resulting NH2CO center dot 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 x 10(-7) s(-1) and k(2) = 5.8 x x 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 x 10(-21) cm(3) 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 center dot under the abiogenesis conditions.