Comment on "Computational Study on the Vinyl Azide Decomposition".

It is quite possible that ketenimine is formed, as predicted by the authors, in a concerted reaction in the thermolysis of vinyl azide with a barrier of ca. 38 kcal/mol. However, the theoretical treatment is inappropriate as regards the potential vinylnitrene intermediate or transition state, since singlereference methods such as DFT or MP2 cannot provide a correct description. It is now well-known that the lowest singlet state of vinylnitrene is in fact the open-shell singlet (OSS). It cannot be treated adequately by DFT methods, as it requires at least two determinants to be described correctly. Thus, multiconfigurational methods are required. Several such calculations have been reported for the parent vinylnitrene, and substituted vinylnitrenes, but references to these publications are missing in the work of Duarte et al. Indeed, CASPT2 calculations on the relationship between 2Hazirine 2, acetonitrile 3, ketenimine 4, and vinylnitrene 5, have shown that the thermal ring opening of 2 to OSS vinylnitrene 5 has an activation barrier of ca. 33 kcal/mol (Scheme 1). The OSS vinylnitrene 5 undergoes a very facile 1,2-H-shift to acetonitrile with a barrier of ca. 6.5 kcal/mol, whereas the corresponding rearrangement to ketenimine would require ca. 33 kcal/mol. Although it can be debated whether the OSS vinylnitrene 5 is a transition state for ring closure to 2H-azirine without a barrier, or an intermediate with a small barrier of 5 kcal/mol, the important point is that it lies some 25 kcal/mol below the CSS nitrene. Thus, the OSS nitrene needs to be considered in thermal and photochemical chemistry of the C2H3N ensemble. ■ AUTHOR INFORMATION Corresponding Author *C. Wentrup: e-mail, [email protected]. Notes The authors declare no competing financial interest.