Comment pubs.acs.org/JPCA
Comment on “Computational Study on the Vinyl Azide Decomposition” Minh Tho Nguyen,*,†,‡,§ Tran Dieu Hang,§ Nina P. Gritsan,∥,⊥ and Vitaly G. Kiselev*,∥,⊥ †
Computational Chemistry Group and ‡Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam Department of Chemistry, KU Leuven, B-3001 Leuven, Belgium ∥ Novosibirsk State University, 630090 Novosibirsk, Russia ⊥ Institute of Chemical Kinetics and Combustion SB RAS, 630090 Novosibirsk, Russia §
J. Phys. Chem. A 2014, 118 (27), 5038−5045. DOI: 10.1021/jp500140j J. Phys. Chem. A 2014, 118 (27), 5122−5123. DOI: 10.1021/jp503958z
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uarte et al.1 recently reported a computational study on the decomposition mechanism of vinyl azide (VA, H 2 CCHN 3 ) using both density functional theory (B3LYP) and ab initio (MP2) methods. The mechanism of N2 elimination was found to be dependent on the conformation involved; viz., N2-loss from syn-VA was calculated to occur in a single step via a Curtius-like rearrangement through a transition state TS2 (Figure 1) giving rise to ketenimine (H2CC
the triplet and closed-shell singlet vinyl nitrene (VN), and of the minimum-energy crossing points (MECP) between the singlet and triplet PES manifolds in the mechanism of VA decomposition. Wentrup et al.2 subsequently criticized the paper of Duarte et 1 al. and considered their theoretical treatment to partly be inappropriate. These authors2 pointed out that, in contrast to a concerted isomerization 2H-azirine → acetonitrile proposed by Duarte et al.,1 the most favorable pathway is a two-step reaction occurring through an open-shell singlet vinyl nitrene (osVN).2,3 Moreover, the authors2 indicated that multiconfigurational methods, instead of the single-reference B3LYP and MP2 employed, could be required for a correct description of the VA decomposition. Accordingly, a question of great interest is whether singlereference quantum chemical methods, employed without any benchmarking, can correctly probe the mechanism of VA decomposition. On the one hand, the moderate size of the VA molecule, composed of five carbon and nitrogen atoms, allows a plethora of reliable and feasible methods to be applied. In the present case, the approach used by Duarte et al.1 does not meet the state-of-the-art quantitative computations. On the other hand, both authors1,2 appeared to omit an important pathway of VA reactions. Therefore, we point out here the shortcomings of both previous papers focusing mainly on the primary reactions of VA decomposition using appropriate methods. More specifically, our comments concern the following points: (a) To account for a possible multiconfigurational nature of the wave functions of transition structures for N2-loss from VA, noted as TS1 and TS2 in Figure 1, we performed geometry optimizations using CASSCF(12e,10o) wave functions with subsequent energy refinement by the NEVPT2 single-point calculations (Figure 1, blue values). We were unable to locate transition structures corresponding to nonconcerted VA decomposition, which lead to formation of the open-shell vinyl nitrene (os-VN, Figure 1). Thus, the N2 elimination from VA most likely occurs in a concerted way without intermediacy of VN. At the same time, the CASSCF wave functions of both TS1 and TS2 turn out to have predominant contributions of
Figure 1. Relative enthalpies at 0 K (Δ(Δ0H0K)) of the stationary points on the PES corresponding to thermolysis of vinyl azide (VA). syn-VA was chosen as a reference for calculations of relative thermodynamics. Single-point CCSD(T)-F12b/cc-pVTZ-F12 energies (marked in red) were calculated using the CCSD(T)-F12b/cc-pVDZF12 optimized geometry. Single-point NEVPT2/TZVP energies (marked in blue) were calculated using the CASSCF(12,10)/TZVP optimized geometries. Zero-point energies in both cases were computed at the corresponding levels of theory. B3LYP/6-311+G(d,p) relative electronic energies (ΔE) from ref 1 are given in parentheses. All values are given in kcal/mol.
NH). In turn, the N2-loss from anti-VA was also found to be a concerted process yielding cyclic 2H-azirine via TS1 (Figure 1).1 The 2H-azirine further undergoes unimolecular rearrangement yielding acetonitrile (CH3CN), which is the most stable isomer of the [C2H3N] system. An important conclusion1 is that it is adequate to consider just the closed-shell singlet state for the thermal decomposition of both VA isomers. In spite of a significant difference of 8−10 kcal/mol on the energy barriers between the methods employed (B3LYP and MP2), the authors1 neither commented on their accuracy, nor attempted to improve these key energetic values. In addition, the paper1 contains some unsound speculations on the role of © XXXX American Chemical Society
Received: October 25, 2015 Revised: November 26, 2015
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DOI: 10.1021/acs.jpca.5b10449 J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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ACKNOWLEDGMENTS The authors are indebted to KU Leuven (GOA and IRO programs) and NSU (5-Top-100 development program) for continuing support. M.T.N. thanks Ton Duc Thang University for supporting his stays in Vietnam. Prof. Jose Elguero from CISC Madrid is acknowledged for helpful information.
closed-shell Hartree−Fock configurations (∼86 and 88%, respectively). This implies that the single reference (SR) methods are appropriate for treating the potential energy surface of VA in its singlet ground state. However, to provide quantitative data, the accuracy of SR results need to be carefully benchmarked. Therefore, having confirmed the predominantly closed-shell electronic structure of TS1 and TS2, we reoptimized the geometries of VA and both TSs at a high level of theory, viz. the explicitly correlated coupled-cluster theory CCSD(T)-F12 with the cc-pVDZ-F12 basis set, with subsequent energy refinement using single-point computations at the CCSD(T)-F12/cc-VTZ-F12 level. Let us note that the T1 values of CCSD wave functions were found to be moderate, being 0.027 for TS1 and 0.021 for TS2. The energy barrier of the pathway via TS1 is calculated to be 31.3 kcal/mol, which is 1.4 kcal/mol lower than that via TS2 (Figure 1, red values). Both barriers are lower and lie closer to each other than their counterparts4 computed at the B3LYP and MP2 levels.1 Figure 1 also points out that the multireference perturbative methods underestimate the energy barriers for N2 elimination of VA by 5−6 kcal/mol (with respect to the CCSD(T)-F12 values).5 (b) We revealed that another multistep pathway, which commences with a 1,5-dipolar cyclization of syn-VA yielding 4H-1,2,3-triazole (Figure 1), is plausible and competitive with a direct N2-loss. The corresponding energy barrier involving the transition structure TS3 was in fact found to be lower than those of direct N2 loss via both TS1 and TS2 (Figure 1). This ring−chain tautomerism was discussed in earlier experimental6 and theoretical7 studies but was entirely missing in both refs 1 and 2. The triazole ring can in turn undergo further rearrangements and cyclo-reversions.8 Therefore, this cyclization channel should be considered in more detail in future studies of VA decomposition. (c) The minimum energy crossing points (MECP) between singlet and triplet potential energy surfaces (PES) of VA have not correctly been identified by Duarte et al.1 The crossing points in Figure 5 of ref 1 actually refer to profoundly different regions of the full-dimensional PES of VA. Although only the N−N bond lengths coincide for these points, all other degrees of freedom are different. Proper procedures for a MECP localization are described in detail elsewhere.9 In summary, single-reference quantum chemical methods were found to be suitable for study of primary thermolysis reactions of VA. However, the DFT (B3LYP functional) and especially MP2 methods tend to substantially overestimate the energy barriers for N2 elimination. Moreover, the authors of both previous papers1,2 omitted the 1,5-cyclization reaction, which is an energetically low-lying entrance channel of VA, and this channel should carefully be scrutinized in forthcoming studies. In addition, speculations1 on the role of MECP and of the closed-shell singlet and triplet vinyl nitrene in decomposition mechanism of VA are irrelevant.
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Comment
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REFERENCES
(1) Duarte, D. J. R.; Miranda, M. S.; Esteves da Silva, J. A Computational Study on the Vinyl Azide Decomposition. J. Phys. Chem. A 2014, 118, 5038−5045. (2) Wentrup, C.; Nunes, C. M.; Reva, I. Comment on ‘Computational Study on the Vinyl Azide Decomposition’. J. Phys. Chem. A 2014, 118, 5122−5123. (3) 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, 1247−1253. (4) The values given in ref 1 are actually the “bare” electronic energy (ΔE) differences. The conventional definition of the energy barrier is an enthalpy difference at 0 K (Δ0H0K) with correction for zero-point vibrational energies (ZPE). (5) The simple kinetic estimations using a NEVPT2 energy barrier of 25 kcal/mol indicate that the lifetime of vinylazide at room temperature would be a few hours. (6) (a) L’abbe, G.; Mathys, G. On the Mechanism of the Thermal Decomposition of Vinyl Azides. J. Org. Chem. 1974, 39, 1778−1780. (b) Bock, H.; Dammel, R.; Aygen, S. Gas-Phase Reactions. 36. Pyrolysis of Vinyl Azide. J. Am. Chem. Soc. 1983, 105, 7681−7690. (7) Burke, L. A.; Leroy, G.; Nguyen, M. T.; Sana, M. Theoretical Study of the Vinyl Azide - v-Triazole Isomerization. J. Am. Chem. Soc. 1978, 100, 3668−3674. (8) Preliminary computations pointed out that several channels leading to decomposition of 4H-1,2,3-triazole have relative energy barriers below 30 kcal/mol (with respect to syn-VA). (9) Abate, B. A.; Peralta, J. E. The Performance of Density Functional Approximations for the Structures and Relative Energies of Minimum Energy Crossing Points. Chem. Phys. Lett. 2013, 590, 227−230 and references therein.
AUTHOR INFORMATION
Corresponding Authors
*M. T. Nguyen. E-mail:
[email protected]. *V. G. Kiselev. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. B
DOI: 10.1021/acs.jpca.5b10449 J. Phys. Chem. A XXXX, XXX, XXX−XXX
