Ab initio study of molecular properties and decomposition products of 1-azidoalkynes-a challenge for experimentalists

In this publication, a characterization of different azidoalkyne compounds using high-level ab initio quantum chemical methods is presented. For this purpose, the molecular structures and the 13C NMR chemical shifts have been calculated at the MP2 and CCSD(T) level of theory and the influence of zero-point vibration as well as the solvent on the chemical shifts are discussed. Furthermore, a comparison of the energy barriers of the decomposition under N2 separation for a set of 1-azidoalkynes with different functional groups has been carried out. The molecular structures and properties of the resultant decomposition products have been investigated. It is remarkable that large deviations of the NMR chemical shifts of ethylthioethynyl azide occur in comparison to the experiment. These deviations are far outside of the error bars. Electron correlation effects are of high importance if an accurate description of the chemical shifts of 1-azidoalkynes shall be obtained. A comparison of the energy barriers of the decomposition under N2 separation of 1-azidoalkynes with different functional groups indicates that the stability of 1-azidoalkynes is not increased by typical donor or acceptor groups but rather by silyl or phenyl substituents. The molecular geometries of the decomposition products indicate that the equilibrium structures are of carbene character. Some of the results presented here are in contradiction to previous experimental publications and cast a new light on some open challenges for experimentalists.