Exploring the dynamics of reaction N(2D)+C2H4 with crossed molecular-beam experiments and quantum-chemical calculations

Abstract

We conducted the title reaction using a crossed molecular-beam apparatus, quantum-chemical calculations, and RRKM calculations. Synchrotron radiation from an undulator served to ionize selectively reaction products by advantage of negligibly small dissociative ionization. We observed two products with gross formula C(2)H(3)N and C(2)H(2)N associated with loss of one and two hydrogen atoms, respectively. Measurements of kinetic-energy distributions, angular distributions, low-resolution photoionization spectra, and branching ratios of the two products were carried out. Furthermore, we evaluated total branching ratios of various exit channels using RRKM calculations based on the potential-energy surface of reaction N((2)D)+C(2)H(4) established with the method CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p)+ZPE[B3LYP/6-311G(d,p)]. The combination of experimental and computational results allows us to reveal the reaction dynamics. The N((2)D) atom adds to the C=C π-bond of ethene (C(2)H(4)) to form a cyclic complex c-CH(2)(N)CH(2) that directly ejects a hydrogen atom or rearranges to other intermediates followed by elimination of a hydrogen atom to produce C(2)H(3)N; c-CH(2)(N)CH+H is the dominant product channel. Subsequently, most C(2)H(3)N radicals, notably c-CH(2)(N)CH, further decompose to CH(2)CN+H. This work provides results and explanations different from the previous work of Balucani et al. [J. Phys. Chem. A, 2000, 104, 5655], indicating that selective photoionization with synchrotron radiation as an ionization source is a good choice in chemical dynamics research.

The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan

“…The reaction of N( 2 D) with CH 4 also mainly leads to HCN formation (with low CH 2 NH steady state concentrations). The N( 2 D) + C 2 H 4 reaction does not give H + CH 3 CN products, yielding H + CH 2 NCH and H + c-CH 2 NCH (cyclic compound) (Balucani et al, 2012;Lee et al, 2011), CH 2 NCH and c-CH 2 NCH leading quickly to HCN in Titan's atmosphere conditions. The reaction of N( 2 D) with C 2 H 2 produces HCCN (Takayanagi et al, 1998;Herron, 1999) which is quickly transformed into CCN through reaction with H atoms Takayanagi et al, 1998), initiating the formation of complex nitriles in Titan's upper atmosphere.…”

“…c-CH 2 NCH is a closed-shell molecule and much less reactive than CH 2 NCH. Balucani et al (2012); Lee et al (2011) found a barrier close to 14 kJ mol À1 for H addition leading to c-CH 2 (N) CH 2 or c-CH 2 (NH) CH. The c-CH 2 (N) CH 2 production channel seems to have no bimolecular exit channel and as c-CH 2 NCH is formed at high altitude where the pressure is low and stabilization is weak then we neglect this channel as c-CH 2 (N) CH 2 will back-dissociate into c-CH 2 NCH + H. The other adduct, c-CH 2 (NH) CH, is described in the theoretical calculations of Balucani et al (2012) and should isomerize toward CH 2 CHNH and then lead to CH 2 CNH + H through a loose barrier or isomerize toward CH 3 CNH and then CH 3 CN + H. CH 2 CNH formation will be favored due to loose transition state.…”

“…From an ionospheric model, they obtained a mole fraction at 1100 km equal to 3:0 Â 10 À6 (with an estimated uncertainty factor of 3). Cui et al (2009) proposed an average value of 3:0 Â 10 À5 around 1100 km from the INMS/Cassini analysis of 15 flybys, arguing that some important reactions were probably missing in The CH 3 CN molecule is not produced in our model by the N( 2 D) + C 2 H 4 reaction but through the association reaction H + CH 2 CN (Balucani et al, 2012;Lee et al, 2011). We calculated the rate constant from our semi-empirical model for association reactions (Hébrard et al, 2013), the CH 2 CN radical being produced by the H + C 2 H 4 CN reaction (this work, C 2 H 4 CN itself being the product of the H + C 2 H 3 CN association) and the N + C 2 H 3 reaction (Payne et al, 1996).…”

“…We then introduced these new compounds and developed their chemical networks relevant for Titan chemistry. CH 2 NCH is a diradical with a singlet ground state (H 2 C Á AN@C ÁÁ H $ H 2 C@NAC ÁÁ H) and should react quickly with atoms and radicals, particularly with H atoms as shown by theoretical calculations (Balucani et al, 2012;Lee et al, 2011) probably leading to CH 3 + HCN. As there is likely to be no barrier in the entrance valley for the CH 2 NCH + H reaction we consider a rate constant deduced from capture rate theory.…”

“…The N( 2 D) + C 2 H 4 reaction has been studied experimentally and theoretically Takayanagi et al, 1998;Lee et al, 2011;Balucani et al, 2000;Balucani et al, 2012). Contrary to what is considered in usual models of Titan's atmosphere (Lebonnois et al, 2001;Wilson and Atreya, 2004;Lavvas et al, 2008a;Krasnopolsky, 2009), the products are not H + CH 3 CN, but rather H + CH 2 NCH and H + c-CH 2 NCH (cyclic compound) (Balucani et al, 2012;Lee et al, 2011). This non-production of CH 3 CN is of crucial importance as it was thought to be the main (if not only) source of CH 3 CN.…”