Combined crossed beam and theoretical studies of the C(1D) + CH4 reaction

Abstract

The reaction involving atomic carbon in its first electronically excited state (1)D and methane has been investigated in crossed molecular beam experiments at a collision energy of 25.3 kJ mol(-1). Electronic structure calculations of the underlying potential energy surface (PES) and Rice-Ramsperger-Kassel-Marcus (RRKM) estimates of rates and branching ratios have been performed to assist the interpretation of the experimental results. The reaction proceeds via insertion of C((1)D) into one of the C-H bonds of methane leading to the formation of the intermediate HCCH(3) (methylcarbene or ethylidene), which either decomposes directly into the products C(2)H(3) + H or C(2)H(2) + H(2) or isomerizes to the more stable ethylene, which in turn dissociates into C(2)H(3) + H or H(2)CC + H(2). The experimental results indicate that the H-displacement and H(2)-elimination channels are of equal importance and that for both channels the reaction mechanism is controlled by the presence of a bound intermediate, the lifetime of which is comparable to its rotational period. On the contrary, RRKM estimates predict a very short lifetime for the insertion intermediate and the dominance of the H-displacement channel. It is concluded that the reaction C((1)D) + CH(4) cannot be described statistically and a dynamical treatment is necessary to understand its mechanism. Possibly, nonadiabatic effects are responsible for the discrepancies, as triplet and singlet PES of methylcarbene cross each other and intersystem crossing is possible. Similarities with the photodissociation of ethylene and with the related reactions N((2)D) + CH(4), O((1)D) + CH(4) and S((1)D) + CH(4) are also commented on.

A combined crossed molecular beams and theoretical study of the reaction CN+C2H4

“…We have performed RRKM calculations using the same code developed by us and described in previous papers [41,42]. The microcanonical rate constant for each elementary step is calculated using the formula kðEÞ ¼ NðEÞ=hqðEÞ where N(E) denotes the sum of states in the transition state at energy E, q(E) is the reactant density of states at energy E and h is Planck's constant.…”

“…In particular, we have used the M06L local density functional method [41]. Restricted calculations were used for closed-shell systems and unrestricted ones for open-shell systems.…”

Dimerization of methanimine and its charged species in the atmosphere of Titan and interstellar/cometary ice analogs

“…We obtained rate constants via a combination of capture theory and RRKM calculations, using the same code we developed and described in previous papers (Leonori et al 2009(Leonori et al , 2013Balucani et al 2015). The rate constant for the formation of the initial intermediate is calculated using capture theory as detailed in Vazart et al (2015).…”

Crossed molecular beam studies of astronomically relevant bimolecular reactions

“…For instance, a beam characterization via Stern-Gerlach magnetic analysis has revealed that a significant fraction of atomic oxygen and nitrogen are formed in their 1 D and 2 D, respectively, electronically excited metastable states, while a fraction of atomic chlorine is formed in the excited spinorbit state, 2 P 1/2 (Alagia et al 1997). We have obtained experimental evidence that also atomic carbon and sulfur are partially formed in their first electronically excited 1 D states (Leonori et al 2008a;Leonori et al 2009). In this way, we have been able to investigate the dynamics of several reactions involving these excited species, with some of them being relevant in the chemistry of planetary atmospheres and cometary comae.…”

“…2 Electron ionization efficiency as a function of electron energy of the HCCS radical product from the S( 1 D) ? C 2 H 2 reaction at E c = 35.6 kJ mol -1 (adapted, with permission, from Leonori et al 2009) reactants requires sources specifically devised for their generation starting from precursor species. Among the various techniques, in our laboratory we make use of a high-pressure, high-power radio frequency (RF) discharge source (Alagia et al 1997).…”