title: Toward Understanding the Decomposition of Carbonyl Diazide (N3)2C═O and Formation of Diazirinone cycl-N2CO: Experiment and Computations.
abstract: Carbonyl diazide, (N3)2CO (I), is a highly explosive compound. The isolation of the substance in a neat form was found to provide unique access to two other high-energy molecules, namely, N3-NCO (III) and cycl-N2CO (IV), among the decomposition products of (I). To understand the underlying reaction mechanism, the decomposition reactions including the thermal conversion of two conformers of (I) were revisited, and the potential energy surface (PES) was computationally explored by using the methods of B3LYP/6-311+G(3df) and CBS-QB3. The most stable syn-syn structure (I) readily converts into the syn-anti conformer (ΔHexptl = 1.1 ± 0.5 kcal mol(-1)), which undergoes decomposition in two competing pathways: a concerted path to N3-NCO (III) or a stepwise route to (III) via the nitrene intermediate N3C(O)N (1)(II). The calculated activation barriers (Ea) are almost the same (∼33 kcal mol(-1), B3LYP/6-311+G(3df)). Further decomposition of (III) occurs through a concerted fragmentation into 2 N2 + CO with a moderate Ea of 22 kcal mol(-1), and this process is compared to the isoelectronic species N3-N3 → 3 N2 (Ea = 17 kcal mol(-1)) and OCN-NCO → N2 + 2 CO (61 kcal mol(-1)). No low-energy pathway leading to (IV) was found on the singlet PES. However, the intervention of triplet ground-state (3)(II) from the initially generated (1)(II) through an intersystem crossing (ISC) offers a likely approach to (IV); that is, (3)(II) can decompose in a concerted process (Ea = 30 kcal mol(-1)) by eliminating one N2 to yield the disfavored OCNN (3)(VI). A careful intrinsic reaction coordinate analysis and a combined energy scan of the N-C-N angle reveals a bifurcation point on this triplet PES, which allows a spin crossover to the singlet PES along the reaction coordinate and eventually leads to the formation of the metastable diazirinone (IV).
authors.affiliation: †College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, P. R. China.
authors.forename: Hongmin
authors.initials: H
authors.lastname: Li
authors.affiliation: †College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, P. R. China.
authors.forename: Dingqing
authors.initials: D
authors.lastname: Li
authors.affiliation: †College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, P. R. China.
authors.forename: Xiaoqing
authors.initials: X
authors.lastname: Zeng
authors.affiliation: ¶College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China.
authors.forename: Kun
authors.initials: K
authors.lastname: Liu
authors.affiliation: ‡Institut für Chemie und Biochemie, Freie Universität Berlin, D-14195 Berlin, Germany.
authors.forename: Helmut
authors.initials: H
authors.lastname: Beckers
authors.affiliation: §Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30677, United States.
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer
authors.affiliation: ⊥Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Esselman
authors.affiliation: ⊥Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: McMahon