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Reply to Comment on “The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence for Benzemethanimine Formation” Rui Montenegro Pinto, Mauro Guerra, Maria Lourdes Costa, and António Alberto Dias J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 03 Jul 2015 Downloaded from http://pubs.acs.org on July 3, 2015
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Reply to Comment on “The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence for Benzemethanimine Formation”
R. M. Pinto, M. Guerra, M. L. Costa, and A. A. Dias LIBPhys – Laboratório de Instrumentação, Engenharia Biomédica e Física da Radiação Departamento de Física, Faculdade de Ciências e Tecnologia, FCT, Portugal. E-mail:
[email protected];
[email protected] J. Phys. Chem. A 2015. DOI: 10.1021/acs.jpca.5b02453.1 In a comment to our recent work, 1 C. Wentrup questions whether or not benzonitrile (PhCN) is formed and detected at a later stage of the gas-phase pyrolysis of benzyl azide (BA), from the thermal decomposition of the product benzemethanimine (PhCH=NH). Furthermore, the author raises some additional questions that we address below.
1. UV-PE and matrix-IR spectra of PhCH=NH Part of the UV-PE spectrum of PhCH=NH presented in Ref. 2 was obtained from flashvacuum thermolysis (FVT) of N-benzylidene-tert-butylamine at 620 oC, and confirmed as a minor (5%) byproduct of the full FVT of the parent compound at 800 oC (by indirect NMR detection of benzaldehyde). We note that (i) the experimental conditions (e.g. pressure) for FVT are very different from ours, (ii) the first PE band of PhCH2=NH in Ref. 2 is largely overlapped with that of isobutene, (iii) there is an unassigned band at 7.15 eV, and (iv) no mechanism and transition structures (TSs) were proposed for the formation of PhCH2=NH from N-benzylidene-tert-butylamine. A clean, unoverlapped photoelectron spectrum of PhCH=NH was not obtained in Ref. 2, since the first 3 bands of PhCH=NH are overlapped with the first band of isobutene in such work. However, the derived first 3 vertical IEs of 8.8, 9.5 and 9.8 eV compare reasonably well with our computed values of 9.03, 9.36 and 10.06 eV (Table S2, in SI of Ref. 1). In any case, in our work the first 3 bands of PhCH=NH are not resolved experimentally and hence any arguments concerning the mechanism of decomposition based on the evidence in Figure 6 of our paper will be unaffected with the extra information provided by Vu et al.2 Also, in contrast to our 1 ACS Paragon Plus Environment
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analysis, the author does not consider in Ref. 2 the formation of the isomer Nmethyleneaniline (PhNCH2), which was characterized by DiStefano et al.3 and can also overlap with the spectral profile assigned to PhCH=NH in Ref. 2. Overall, both our work1 and Vu et al. analysis2 offer interesting and remarkable experimental data that should be enhanced in the future when better experimental resolution and control of the pyrolysis process becomes available. Regarding the IR spectrum of PhCH=NH isolated in Ar matrix (10 K), 4 we were not aware of it at the time of writing. In our work, however, we present a more complete assignment of the matrix-isolation IR spectrum of PhCH=NH supported by calculations, which is not performed in Ref. 4.
2. Nitrene formation The sentence that appears on page 1 of our paper “So far, no nitrenes have been detected from thermal decomposition of organic azides” is given within a context and is meant to apply to the study of the type of organic azides we have studied previously in our group. i.e. aliphatic azides. Therefore, it is not meant to be a categorical statement covering the detection of nitrenes from all organic azides’ pyrolysis. Dianxun and co-workers5 reported the UV-PE spectrum of CH3N obtained from the pyrolysis of methylazide, although under very particular conditions (on a heated molecular sieve and likely to be a surface process) and in the presence of NO. On the other hand, Kuzaj et al.6 report on the ESR spectrum of matrix-isolated phenyl nitrene, obtained from gas-phase pyrolysis (at 500 oC) of phenyl azide. While these two examples support the formation of nitrenes from azides’ pyrolysis, they were obtained under very different conditions (NO presence, molecular sieve) and/or using specific methods (ESR), which are not comparable to our experimental setup and analysis, and refer to azides which are different from the type of azides we have studied previously. In no case did we neglect the importance of nitrenes as high-energy intermediates in the pyrolysis of organic azides, as precursors for the chemical activation of some decomposition channels. We mention them in the manuscript (cf. Introduction), as well as in previous papers7. All of the computational chemistry presented in our work was performed with the goal of confirming or disproving our experimental results. Thus, facing no evidence from matrix-isolation IR and UVPES monitoring of BA pyrolysis that supports 2 ACS Paragon Plus Environment
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the formation of a nitrene species, we took the simplest computational approach to explain the formation of the observed products. Note that open-shell singlet (OSS) calculations require high-level multi-configuration methods (e.g., CASSCF) which are computationally very expensive and often rely on a good choice of the active space to produce a meaningful output.
3. Benzonitrile pathway We have also sought the imine → benzonitrile + H2 pathway to support our results, but that does not explain the formation of HCN directly from PhCN. Metcalfe et al.8 have observed the formation of HCN from PhCN by a free radical mechanism involving C6H5●, but his experiments were carried out at atmospheric pressure which is not comparable to the conditions found in our setup. A viable explanation could be the formation of PhCN from the decomposition of PhNCH2 which also yields HCN.9 However, we have not found unambiguous evidence supporting the formation of PhNCH2. The assignment of the band at ca. 9.7-9.8 eV to PhCN can also be repeated for benzene and a band with maximum at ca. 9.25 eV because the first band is very broad with overlapping structure (Fig. 6b). They are both un-confirmed hypothesis, and so we prefer to rely on the small C6H6 peak observed in the matrix-isolation IR spectrum (inset of Fig. 7c). We have plotted the stick spectrum of PhCN (list format in Ref. 4) overlapped with ours, and indeed the peaks at 2235 and 558 cm-1 match those of benzonitrile. However, other peaks overlap considerably with those of the imine, and so we prefer to present a definite assignment perhaps in future work, with a more sturdy analysis involving the pyrolysis of 2-, 3- and 4-MeBA which are more prone to the type of rearrangements C. Wentrup suggests. 4. HNC → HCN reaction When HCN is detected in the PE spectrum, there is at least 438 kJ/mol (~ 105 kcal/mol) of thermal energy available to activate PhCH=NH decomposition into HCN and benzene (from TS5). Therefore, we say that HNC evolves to HCN “barrierless” because the 30 kcal/mol barrier can be easily surpassed; it is an overstatement, easily understood by others researchers working on this subject. 3 ACS Paragon Plus Environment
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5. Cyclic intermediate The “cyclic intermediate” term was defined in our manuscript (cf. Introduction) as a longlived TS or stable species that could explain the observed products from BA pyrolysis without involving imine formation (in the context of a Type 2 mechanism). It could be a possible state between BA-G and benzazetine (which is obviously more stable). However, since no evidence was found for its formation in the spectra, we presented it as a possible decomposition route sought via computational chemistry. Given our previous work, this route was explored computationally, for comparison. We note that generation and detection of intermediates depends on pyrolysis conditions/method and probing technique. As such, more sensitive techniques could say how realistic such pathway is likely to be.
6. PES for the pyrolysis of BA Finally, since we did not find unambiguous evidence in the spectra for the benzonitrile pathway, nor a transition structure for the reaction PhCH=NH → PhCN + H2, the benzonitrile route was not considered with respect to other more likely mechanisms. However, a more detailed picture of BA pyrolysis could be obtained using high-level computational methods (e.g., CASSCF) , which would complement the PES presented in Fig. 9 of our original work.1
References (1) Pinto, R. M.; Guerra, M.; Copeland, G.; Olariu, R. I.; Rodrigues, P.; Barros, M. T.; Costa, M. L.; Dias, A. A. The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence for Benzenemethanimine Formation J. Phys. Chem. A 2015, DOI: 10.1021/acs.jpca.5b02453. (2) Vu, T. Y; Chrostowska, A.; Huynh, T. K. H.; Khayar, S.; Dargelos, A.; Justyna, K.; Pasternak, B.; Lesniak, S.; Wentrup, C. New Reactions of N-tert-Butylimines. Formation of N-Heterocycles on FVT. Chem. Eur. J. 2013, 19, 14983-14988.
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(3) Distefano, G.; Giumanini, A. G.; Modelli, A.; Poggi, G. Reinvestigation of the Formaldehyde-Aniline Condensation. Part 4. Ultraviolet Photoelectron and Electron Transmission Spectra of N-Methyleneaniline and its Symmetric Dimethyl RingSubstituted Homologues and Semiempirical Theoretical Evaluations. J. Chem. Soc., Perkin Trans. 2 1985, 0, 1623−1627. (4) Morawitz, J.; Sander, W.; Traeubel, M. Intramolecular Hydrogen Transfer in (2Aminopheny1) carbene and 2-Tolylnitrene. Matrix Isolation of 6-Methylene-2,4cyclohexadienimine. J. Org. Chem. 1995, 60, 6368-6378. (5) Jing, W.; Zheng, S.; Xinjiang, Z.; Xiaojun, Y.; Maofa, G.; Dianxun, W. The CH3N Diradical: Experimental and Theoretical Determinations of the Ionization Energies. Angew. Chem. Int. Ed. 2001, 40, 3055-3057. (6) Kuzaj, M.; Lüerssen, H.; Wentrup, C. ESR Observation of Thermally Produced Triplet Nitrenes, Angew. Chem., Int. Ed. Engl. 1986, 25, 480-82. (7) Pinto, R. M.; Dias, A. A.; Levita, G.; Rodrigues, P.; Barros, M. T.; Dyke, J. M.; Costa, M. L. Pyrolysis of 3-Azidopropionitrile Studied by UV Photoelectron and Matrix-Isolation IR Spectroscopies: Formation of Ketenimine H2CCNH. J. Mol. Struct. 2012, 1025, 151−159. (8) Rad, S.T.E.; Metcalfe, E. The Pyrolysis of Benzonitrile Fire Mat. 1993, 17, 33-37. (9) Patterson, J. M.; Shiue, C.-Y.; Smith Jr., W. T. Benzonitrile Formation in the Pyrolysis of Aromatic Nitrogen Compounds J. Org. Chem. 1973, 38, 2447-2450.
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