A CROSSED MOLECULAR BEAM, LOW-TEMPERATURE KINETICS, AND THEORETICAL INVESTIGATION OF THE REACTION OF THE CYANO RADICAL (CN) WITH 1,3-BUTADIENE (C₄H₆). A ROUTE TO COMPLEX NITROGEN-BEARING MOLECULES IN LOW-TEMPERATURE EXTRATERRESTRIAL ENVIRONMENTS

Abstract

We present a joint crossed molecular beam and kinetics investigation combined with electronic structure and statistical calculations on the reaction of the ground-state cyano radical, CN(X 2 Σ +), with the 1,3-butadiene molecule, H 2 CCHCHCH 2 (X 1 A g), and its partially deuterated counterparts, H 2 CCDCDCH 2 (X 1 A g) and D 2 CCHCHCD 2 (X 1 A g). The crossed beam studies indicate that the reaction proceeds via a long-lived C 5 H 6 N complex, yielding C 5 H 5 N isomer(s) plus atomic hydrogen under single collision conditions as the nascent product(s). Experiments with the partially deuterated 1,3-butadienes indicate that the atomic hydrogen loss originates from one of the terminal carbon atoms of 1,3-butadiene. A combination of the experimental data with electronic structure calculations suggests that the thermodynamically less favorable 1-cyano-1,3-butadiene isomer represents the dominant reaction product; possible minor contributions of less than a few percent from the aromatic pyridine molecule might be feasible. Low-temperature kinetics studies demonstrate that the overall reaction is very fast from room temperature down to 23 K with rate coefficients close to the gas kinetic limit. This finding, combined with theoretical calculations, indicates that the reaction proceeds on an entrance barrier-less potential energy surface (PES). This combined experimental and theoretical approach represents an important step toward a systematic understanding of the formation of complex, nitrogen-bearing molecules-here on the C 5 H 6 N PES-in low-temperature extraterrestrial environments. These results are compared to the reaction dynamics of D1-ethynyl radicals (C 2 D; X 2 Σ +) with 1,3-butadiene accessing the isoelectronic C 6 H 7 surface as tackled earlier in our laboratories.

Formation of fulvene in the reaction of C2H with 1,3-butadiene

“…The reaction of the cyano radical (CN), which is isoelectronic with C 2 H, with 1,3-butadiene has also been studied [27]. Previous studies of the reactions of the isoelectronic C 2 H and CN radicals with unsaturated hydrocarbons have shown that they react similarly, both in terms of reaction rate and major products [28][29][30][31].…”

“…CN + 1,3-butadiene proceeds via addition and H-elimination to yield isomers of C 5 H 5 N. Crossed-molecular beams experiments suggest a maximum of 3-6% pyridine and 94-97% 1-cyano-1,3-butadiene is formed under single collision conditions. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations predict only 0.02% pyridine formation at 0 eV collision energy, with a maximum of 6% at the limits of the errors of the calculations [27]. If C 2 H + 1,3-butadiene were analogous to CN + 1,3-butadiene, one might expect a small branching fraction for benzene, with the remainder of the products being 1,3-hexadien-5-yne.…”

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

“…For all these reasons, CN reactions with small unsaturated hydrocarbons have been widely investigated in kinetics [1,[16][17][18][19][20][21][22][23] and reaction dynamics [24][25][26][27][28][29][30][31][32] experiments. More specifically, the reaction CN + C 2 H 4 was found to be very fast, approaching the gas kinetics limit, at very low and high temperatures [16,23], confirming its role in cold and hot environments.…”

Gas phase reactive collisions, experimental approach

“…Present experimental work has shown that reactions with carbon containing radicals usually lead to products that are carbon enriched with respect to the reagent [18] and then these processes can be important routes to the construction of ring species many of which are of biological importance. For instance, a very recent work concerning the reaction of the cyanide radical CN with the 1,3-butadiene molecule, H 2 CCHCHCH 2 , provided some evidence that the aromatic pyridine C 5 H 5 N could be produced although in small quantity [113]. Therefore reactions of simple stable C 2 , C 3 and C 4 -based hydrocarbons with radicals should also be completed.…”