Comment on “Computational Study on the Vinyl Azide Decomposition

Comment pubs.acs.org/JPCA

Comment on “Computational Study on the Vinyl Azide Decomposition” Curt Wentrup,*,† Cláudio M. Nunes,‡ and Igor Reva‡ †

School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld 4072, Australia Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

‡

J. Phys. Chem. A 2014, 118. DOI: 10.1021/jp500140j

I

Scheme 1. CASPT2 Calculated Energies Based on CASSCF(6,5) 2σ + 3π Geometries for Ground and Transition States in Thermal Reactions of 2H-azirine 2 and OSS Vinylnitrene 5 (Energies in kcal/mol Relative to 2)7

n a recently accepted paper by Duarte, Miranda, and Esteves da Silva,1 the authors report a DFT study of vinyl azide decomposition2 with calculations of intrinsic reaction coordinates at the B3LYP/6-311++G(d,p) level, essentially concluding that the decomposition of vinyl azide 1 is conformationally dependent. The s-trans conformer 1t reacts in two steps via 2H-azirine 2: s‐trans‐H 2CCHN3 (1t) → cyc ‐H 2CNCH (2H ‐azirine, 2) + N2 → H3CCN (3) + N2

without a nitrene intermediate, but via a transition state resembling the closed-shell singlet (CSS) nitrene and lying about 50 kcal/mol above the 2H-azirine (or 69 kcal/mol according to an MP2/6-311++G(d,p) single-point calculation). The s-cis conformer of the azide (1c) leads to the formation of ketenimine in a single-step, again without a nitrene intermediate:

CSS nitrene. Thus, the OSS nitrene needs to be considered in thermal and photochemical chemistry of the C2H3N ensemble.

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Corresponding Author

*C. Wentrup: e-mail, [email protected].

s‐cis‐H 2CCHN3 (1c) → H 2CCNH (4) + N2

Notes

The authors declare no competing financial interest.

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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.1 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).3−7 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,4−7 and substituted vinylnitrenes,8 but references to these publications are missing in the work of Duarte et al.1 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).7,9 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,4 or an intermediate with a small barrier of 5 kcal/mol,7 the important point is that it lies some 25 kcal/mol below the © 2014 American Chemical Society

AUTHOR INFORMATION

ACKNOWLEDGMENTS This research was partially supported by the Australian Research Council, the National Computing Infrastructure facility (MAS grant g01) sustained by the Australian Government, and the Portuguese “Fundaçaõ para a Ciência e a Tecnologia”, research project PTDC/QUI-QUI/118078/2010 and Postdoctoral Grant No. SFRH/BPD/86021/2012 (CMN).

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REFERENCES

(1) Duarte, D. J. R.; Miranda, M. S.; Esteves da Silva, J. C. G. A computational Study on the Vinyl Azide Decomposition. J. Phys. Chem. A 2014, DOI: 10.1021/jp500140j. (2) Experimental thermal and photochemical reactivities of vinyl azide H2CCHN3 have been described previously. On flash vacuum pyrolysis, vinyl azide 1 gives rise primarily to the 2H-azirine 2 accompanied by some acetonitrile 3, which was confirmed to be a secondary product resulting from the thermal isomerization of 2Hazirine 2: (a) Guillemin, J.-C.; Denis, J.-M.; Lasne, M.-C.; Ripoll, J.-L. Metastable Isomers of Acetonitrile: Syntheses of Vinylideneamine and Δ1-Azirine. J. Chem. Soc., Chem. Commun. 1983, 238. (b) Bock, H.; Dammel, R.; Aygen, S. Pyrolysis of Vinyl Azide. J. Am. Chem. Soc. 1983, 105, 7681. Photolysis of vinyl azide 1 at λ = 254 nm under low-

Received: April 23, 2014 Published: June 18, 2014 5122

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The Journal of Physical Chemistry A

Comment

temperature matrix isolation conditions gives 2H-azirine 2 and ketenimine 4 as primary photoproducts concomitantly with acetonitrile 3 and traces of the corresponding isonitrile, both secondary products derived from 2H-azirine 2: (c) Maier, G.; Schmidt, C.; Reisenauer, H. P.; Endlein, E.; Becker, D.; Eckwert, J.; Hess, B. A.; Schaad, L. J. Hydrogen Cyanide N-Methylide: Preparation, Spectroscopic Identification, and Its Relationship to Other C2H3N Isomers. Chem. Ber. 1993, 126, 2337. (3) Parasuk, V.; Cramer, C. J. Multireference configuration interaction and second-order perturbation theory calculations for the 1 3A″, 1 1A″, and 1 1A′ electronic states of vinylnitrene and vinylphosphinidene. Chem. Phys. Lett. 1996, 260, 7. (4) Bornemann, C.; Klessinger, M. Conical intersections and photoreactions of 2H-azirines. Chem. Phys. 2000, 259, 263. (5) Klessinger, M.; Bornemann, C. Theoretical study of the ringopening reactions of 2H-azirines - a classification of substituent effects. J. Phys. Org. Chem. 2002, 15, 514. (6) Karney, W. L.; Borden, W. T. Ab Initio Study of the Ring Expansion of Phenylnitrene and Comparison with the Ring Expansion of Phenylcarbene. J. Am. Chem. Soc. 1997, 119, 1378. (7) Bégué, D.; Dargelos, A.; Berstermann, H. M.; Netsch, K. P.; Bednarek, P.; Wentrup, C. Nitrile Imines and Nitrile Ylides: Rearrangements of Benzonitrile N-Methylimine and Benzonitrile Dimethylmethylide to Azabutadienes, Carbodiimides, and Ketenimines. Chemical Activation in Thermolysis of Azirenes, Tetrazoles, Oxazolones, Isoxazolones, and Oxadiazolones. J. Org. Chem. 2014, 79, 1247. (8) Nunes, C. M.; Reva, I.; Pinho e Melo, T. M. V. D; Fausto, R.; Šolomek, T.; Bally, T. The Pyrolysis of Isoxazole Revisited: A New Primary Product and the Pivotal Role of the Vinylnitrene. A LowTemperature Matrix Isolation and Computational Study. J. Am. Chem. Soc. 2011, 133, 18911. (9) The alternative ring opening of 2 to formonitrile ylide, H2CN(+)CH(−), with a barrier of 48 kcal/mol was also calculated7 but is less relevant in the present context.

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