Radical reaction C 3 H+NO: A mechanistic study

Although a number of hydrocarbon radicals including the heavier C 3 ‐radicals C 3 H 3 and C 3 H 5 have been experimentally shown to deplete NO effectively, no theoretical or experimental attempts have been made on the reactivity of the simplest C 3 ‐radical towards NO. In this article, we report our detailed mechanistic study on the C 3 H+NO reaction at the Gussian‐3//B3LYP/6‐31G(d) level by constructing the singlet and triplet electronic state [H,C 3 ,N,O] potential energy surfaces (PESs). The l ‐C 3 H+NO reaction is shown to barrierlessly form the entrance isomer HCCCNO followed by the direct O‐elimination leading to HCCCN+ 3 O on triplet PES, or by successive O‐transfer, N‐insertion, and CN bond‐rupture to generate the product 1 HCCN+CO on singlet PES. The possible singlet–triplet intersystem crossings are also discussed. Thus, the novel reaction l ‐C 3 H+NO can proceed effectively even at low temperatures and is expected to play an important role in both combustion and interstellar processes. For the c ‐C 3 H+NO reaction, the initially formed HcCCCNO can most favorably isomerize to HCCCNO, and further evolution follows that of the l ‐C 3 H+NO reaction. Quantitatively, the c ‐C 3 H+NO reaction can take place barrierlessly on singlet PES, yet it faces a small barrier 2.7 kcal/mol on triplet PES. The results will enrich our understanding of the chemistry of the simplest C 3 ‐radical in both combustion and interstellar processes, which to date have received little attention despite their importance and available abundant studies on its structural and spectroscopic properties. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 641–660, 2006