Contrasting Behavior in Azide Pyrolyses: An Investigation of the Thermal Decompositions of Methyl Azidoformate, Ethyl Azidoformate and 2-Azido-N,N-dimethylacetamide by Ultraviolet Photoelectron Spectroscopy and Matrix Isolation Infrared Spectroscopy

Chemical Dynamics Simulations of Curtius Reaction of Acetyl- and Fluorocarbonyl Azides

Curtius rearrangement is the elimination of N₂ from carbonyl azides RC(O)N₃ to form isocyanates RNCO. Two mechanisms, viz., stepwise and concerted have been proposed in the literature for this reaction. The stepwise mechanism involves the formation of a nitrene RC(O)N by elimination of N₂ followed by an intramolecular rearrangement of the nitrene to form the isocyanate. The concerted mechanism is a single-step pathway forming the N₂ + RNCO products directly. Previous experimental and theoretical studies have indicated that the mechanism is usually concerted for thermal reactions and both stepwise and concerted are preferred under photochemical conditions. In the present work, we investigated the mechanism of Curtius rearrangement of two carbonyl azides with different substituents (R = CH₃ and F). Atomic level reaction mechanisms were studied using chemical dynamics simulations under thermal reaction conditions. Classical trajectories were generated on-the-fly at the density functional B3LYP/6-31+G* level of electronic structure theory with similar initial conditions for both the molecules. Simulation results showed a dominant concerted mechanism for CH₃C(O)N₃ and the operation of both the mechanisms for FC(O)N₃. The fluorocarbonyl nitrene FC(O)N had an appreciable lifetime before undergoing intramolecular rearrangement to form the isocyanate. In a small number of trajectories, the product isocyanate produced via the concerted dissociation of FC(O)N₃, isomerized back to the nitrene form.

Insights into the Photodecomposition of Azidomethyl Methyl Sulfide: A S2/S1 Conical Intersection on Nitrene Potential Energy Surfaces Leading to the Formation of S-Methyl-N-sulfenylmethanimine

UV photodecomposition of azidomethyl methyl sulfide (AMMS) yields a transient S-methylthiaziridine which rapidly evolves to S-methyl-N-sulfenylmethanimine at 10 K. This species was detected by infrared matrix isolation spectroscopy. The mechanism of the photoreaction of AMMS has been investigated by a combined approach, using low-temperature matrix isolation FTIR spectroscopy in conjunction with two theoretical methods, namely, complete active space self-consistent field and multiconfigurational second-order perturbation. The key step of the reaction is governed by a S₂/S₁ conical intersection localized in the neighborhood of the singlet nitrene minimum which is formed in the first reaction step of the photolysis, that is, N₂ elimination from AMMS. Full assignment of the observed infrared spectra of AMMS has been carried out based on comparison with density functional theory and second-order perturbation Møller-Plesset methods.

Direct observation of methoxycarbonylnitrene

Methoxycarbonylnitrene CH₃OC(O)N, the missing link in the frequently studied Curtius-rearrangement of CH₃OC(O)N has been directly observed by using matrix-isolation IR and EPR spectroscopy.

The decomposition of benzenesulfonyl azide: a matrix isolation and computational study

The stepwise Curtius-rearrangement of benzenesulfonyl azide, PhS(O)₂N₃, has been proven experimentally by the direct observation of the novel intermediates sulfonyl nitrene PhS(O)₂N and N-sulfonyl imine PhNSO₂ and theoretically by quantum chemical calculations.

Conformational composition, molecular structure and decomposition of difluorophosphoryl azide in the gas phase

Difluorophosphoryl azide F₂P(O)N₃ has one dominating conformer in the gas phase at ambient temperature. It decomposes upon flash vacuum pyrolysis to the hitherto unknown nitrene F₂P(O)N.

Complexation of transition metals by 3-azidopropionitrile. An electrospray ionization mass spectrometry study

Most complexes of azides and transition metals involve the N(3)(-) azide anion as a ligand other than an organic azide. Complexes of organic azides with metals are involved in biological applications and in the deposition of nitrenes on metal surfaces, producing nitride layers for semi-conductors preparation; this makes the study of these interactions an important issue. This work describes a study of the complexation of nickel and cobalt by 3-azidopropionitrile by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of NiCl(2) and CoCl(2) in methanol/water. In the case of nickel, other NiX(2) salts were investigated (where X = Br or NO(3)) and other solvents were also studied (notably ethanol/water). All complexes detected were single positively charged, with various stoichiometries, some resulted from the fragmentation of the ligand, the loss of N(2), and HCN being quite common. The most abundant cations observed were [Ni(II)AzAzX](+), where X = Cl, Br, NO(3). Some of the complexes showed solvation with methanol/ethanol/water. Metal reduction was observed in complexes where a radical was lost, resulting from the homolytic cleavage of a metal-nitrogen bond. Collision induced dissociation (CID) experiments followed by tandem mass spectrometry (MS-MS) analysis were not absolutely conclusive about the coordination site. However, terminal ions observed from the fragmentation routes were explained by a proposed gas-phase mechanism. Density functional theory calculations were carried out and provided structures for some complexes, pointing to the possibility of 3-azidopropionitrile acting as a mono- or a bidentate ligand.

The Characterisation of Molecular Alkali-Metal Azides

Matrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkali-metal azides MN3 (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN3, RbN3 and CsN3 are present as stable high-temperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN3 in particular, by the detection of the isotopomers Cs(14N15N14N) and Cs(15N14N14N). The IR spectra are assigned to a "side-on" (C(2v)) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN3, RbN3 and CsN3 isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2 cm(-1), respectively, and correspond to the N3 asymmetric stretch. The N3 bending mode in CsN3 is identified at 629 cm(-1). An additional feature routinely observed in these experiments occurred at approximately 2323 cm(-1) and is assigned to molecular N2, perturbed by the close proximity of an alkali-metal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition.