Measurements of Thermal Rate Constants and Theoretical Calculations for the N(²D, ²P) + C₂H₂ and C₂D₂ Reactions

Abstract

Rate constants for the reactions N( 2 D, 2 P) + C 2 H 2 and C 2 D 2 have been measured using a technique of pulse radiolysis-resonance absorption between 220 and 293 K. Arrhenius parameters have been determined from the temperature dependence of the measured rate constants; the activation energies for the reactions of N( 2 D) were about 0.5 kcal/mol, while those for N( 2 P) were about 0.9 kcal/mol. The H/D isotope effect was found to be very small for both the N( 2 D) + C 2 H 2 and N( 2 P) + C 2 H 2 reactions. The rate constants for N( 2 D) + C 2 H 2 were found to be about 3 times as large as those for N( 2 P) + C 2 H 2 . To understand the overall reaction mechanism of the N( 2 D) + C 2 H 2 reaction, ab initio molecular orbital calculations of the lowest doublet potential energy surface have been performed. It has been found that the initial step of the reaction is the addition of the N atom to the π bond of acetylene. The rate constants have been calculated using conventional transitionstate theory and compared to the experimental results. Possible reaction pathways are discussed on the basis of the ab initio results.

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…RRKM calculations have been conducted at 170 K, typical temperature of Titan's atmosphere, by using the new electronic structure calculations ). These new results are different from those of Takayanagi et al (1998a) and Lee et al (2011), possibly because of the incompleteness of the previous potential energy surfaces (see Table 4). Only the channels with a significant yield have been reported in Table 4.…”

“…According to early ab initio calculations of the relevant potential energy surface (Takayanagi et al 1998a), the initial step is the addition of N ( 2 D) to the π bond of the C 2 H 4 molecule with formation of cyclic H 2 C(N)CH 2 which can either decompose directly to 2H-azirine (cyclic-CH 2 (N)CH) and H ( 2 S) or isomerize (via H-migration) Takayanagi et al (1998a), the channel leading to (2H-azirine (cyclic-CH 2 (N)CH) + H) is by far the most dominant, accounting for 84.8% of the total reaction, followed by the channel leading to (ketene-imine (CH 2 CNH) + H) (13.2%), CH 3 + HNC (1.2%) and CH 3 CN + H (0.8%). The yields of the other energetically allowed reaction channels were found to be negligible.…”

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

“…The simple capture rate constant model based on dispersion only leads to a rate constant close to (2/5) × 4.0 × 10 −10 × (T/300) −0.17 = 1.6 × 10 −10 × (T/300) −0.17 cm 3 molecule −1 s −1 . This rate constant is higher than those for similar reactions, N( 2 D) + singlet state molecules such as N( 2 D) + H 2 O and N( 2 D) + NH 3 reactions which are also likely to occur without a barrier considering their high rate constant values at 300 K, 4.0 × 10 −11 and 5.0 × 10 −11 cm 3 molecule −1 s −1 respectively (Takayanagi et al 1998;Herron 1999). To evaluate more carefully the N( 2 D) + HCN rate constant, we performed MRCI+Q/vtz and R-CCSD(T)/vtz calculations for the N( 2 D) + NH 3 reaction also showing no barrier for 2 B 1 and 2 B 2 surfaces.…”

The interstellar gas-phase chemistry of HCN and HNC

“…It may be produced by the CH + HCN, CH + HNC, C + H 2 CN reactions and by DR of CH 2 CN + and CH 3 CN + involving relatively small fluxes. Moreover, HCCN is supposed to react quickly with H , Takayanagi et al 1998) and very likely also with C, N and O atoms without producing HCN or HNC. We neglect this species in this study and did not introduce it into KIDA.…”