Kinetic Studies on the N(²D, ²P) + CH₄ and CD₄ Reactions:  The Role of Nonadiabatic Transitions on Thermal Rate Constants

Abstract

Thermal rate constants for the N(2D,2P) + CH4 (CD4) reactions have been measured using a technique of pulse radiolysis−resonance absorption in the temperature range between 223 and 298 K. Activation energies determined from the temperature dependence were about 1.5 and 1.0 kcal/mol for the reactions of N(2D) and N(2P), respectively. The H/D kinetic isotope effects were about 1.8 and 1.6 for N(2D) and N(2P), respectively. The rate constants for N(2P) + CH4 were much smaller than those for N(2D) by a factor of 40−60. Variational transition-state theory calculations of the rate constants for the N(2D) + CH4 (CD4) insertion reaction have been carried out using the reaction path information obtained from ab initio molecular orbital calculations. The comparison between the calculated and experimental rate constants shows that multiple surface coefficients are larger than the statistical value, meaning that nonadiabatic transitions are important for the N(2D) + CH4 reaction.

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

“…An indirect identification of CH 2 NH in the atmosphere of Titan has indeed been reported by Vuitton et al (2006Vuitton et al ( , 2007 and Yelle et al (2010 Table 3). As for N( 2 D) reaction, Takayanagi et al (1999) measured a rate constant for the reaction with CD 4 , which was found to be slightly lower than for CH 4 . Both Umemoto et al (1985) and Takayanagi et al (1999) (±25%) for C 2 H 2 k = 6.25 × 10 −11 cm 3 s −1 (±25%) for C 2 D 2 .…”

“…As for N( 2 D) reaction, Takayanagi et al (1999) measured a rate constant for the reaction with CD 4 , which was found to be slightly lower than for CH 4 . Both Umemoto et al (1985) and Takayanagi et al (1999) (±25%) for C 2 H 2 k = 6.25 × 10 −11 cm 3 s −1 (±25%) for C 2 D 2 . The rate constant has been measured by two groups (Fell et al 1981;Takayanagi et al 1998b; see Table 3) measured the rate constants over the temperature range 223-293 K and reported values of k = 1.6 × 10 −10 exp (−270/T) cm 3 s −1 and 1.4 × 10 −10 exp(−240/T) for C 2 H 2 and C 2 D 2 , respectively.…”

“…Since the collisional deactivation of N ( 2 D) by N 2 is a slow process, the reaction of N ( 2 D) with methane has been soon recognized as an important pathway. The rate constant at 298 K recommended by Herron (4.0 × 10 −12 cm 3 s −1 ) is the average of the values determined by Fell et al (1981), Umemoto et al (1998b) and Takayanagi et al (1999) (see Table 3). The data by Black et al (1969) were disregarded as they differ too much with respect to the other measurements.…”

“…The data by Black et al (1969) were disregarded as they differ too much with respect to the other measurements. Umemoto et al (1998b) and Takayanagi et al (1999) also studied the reaction with CD 4 , measuring at 298 K slightly lower rate constants for CD 4 than for CH 4 . Takayanagi et al (1999) In addition to kinetic data, much work has been done on this system at the level of reaction dynamics experiments and theoretical calculations in recent years.…”

“…Umemoto et al (1998b) and Takayanagi et al (1999) also studied the reaction with CD 4 , measuring at 298 K slightly lower rate constants for CD 4 than for CH 4 . Takayanagi et al (1999) In addition to kinetic data, much work has been done on this system at the level of reaction dynamics experiments and theoretical calculations in recent years. The first hint of the reaction mechanism and primary products came from the ab initio calculations by Kurosaki et al (1998) of the relevant potential energy surface.…”