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The mechanism(s) of thermal rearrangement of azulenes have been enigmatic for several decades. Herein, we have employed density functional theory (DFT) calculations at the M06-2X/6-311+G(d,p) level together with single-point calculations at the CCSD(T) level to assess possible mechanisms of the experimentally observed azulene and naphthalene automerizations. Of the two mechanisms proposed for naphthalene automerization, it is found that the benzofulvene (BF) route is favored over the naphthvalene mechanism by ∼6 kcal/mol and is energetically lower than the norcaradiene-vinylidene mechanism (NVM) for the azulene-naphthalene rearrangement (

The thermal rearrangement of azulene to naphthalene has been the subject of several experimental and computational studies. Here, we reexamine the proposed mechanisms at the DFT level. The use of different functionals showed that the HF-exchange contribution significantly affects reaction energies and barrier heights. Accordingly, all proposed pathways were investigated with the optimal method, M06-2X/6-311+G(d,p), which confirms the norcaradiene-vinylidene mechanism (A) as the dominant unimolecular route (

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200 years ago Ørsted laid the foundation of electromagnetism in his famous experiment in which a magnetic needle is deflected in the electrical field of a platinum wire. For this he used his own Cu-Zn trough battery, which was among the best then available, but 21 years later it was surpassed by the coal-zinc battery invented by Bunsen, which became highly successful and acclaimed. That year, 1841, Bunsen made his first direct contact with Scandinavia when he visited Berzelius in Stockholm, Palmstedt in Gothenburg, and Ørsted, Scharling, and Zeise in Copenhagen. Like almost everybody in continental Europe, they adopted Bunsen's battery, and Ørsted used it for his experiments with a very large electromagnet. The paths of Ørsted's and Bunsen's research crossed again much later through the synthesis of elemental aluminum, which was first achieved by Ørsted in 1825 (although it was probably not obtained as the pure metal) and performed quite differently by Bunsen, by electrolysis using his coal-zinc battery, in 1854.

Both photolysis and flash vacuum pyrolysis (FVP) of tetrazoles (1/5) are known to generate nitrile imines (13, 19, and 38), which rearrange to 1H-diazirines, imidoylnitrenes, and carbodiimides. Mor...

Zeises Salz, KPt(C2H4)Cl3, war die erste charakterisierte metallorganische Verbindung; es war auch der erste Olefin‐π‐Komplex. Es wurde 1825–1830 publiziert, inmitten eines Streites zwischen Dumas auf der einen Seite und Berzelius und Liebig auf der anderen, die jeweils die Etherin‐(Ethylen‐)Theorie bzw. die Radikaltheorie verteidigten. Obwohl Zeises Auffassung, dass die Verbindung Ethylen enthalt, bestatigt wurde, wurde der Streit viele Jahre fortgesetzt. Dies geschah zu einer Zeit, als die Theorien der organischen Chemie in rasanter Entwicklung waren, lange bevor ein klares Verstandnis der Natur der Molekule, Bindung und Struktur vorhanden war. Zeise sah die Struktur seines Salzes als das Produkt einer Addition von PtCl2 an Ethylen. Jensen vermutete eine zentrale Bindung an Ethylen, aber benotigte theoretische Hilfe, um sie zu erklaren. Sein Versuch, eine theoretische Erlauterung von Huckel zu bekommen, schlug fehl, und es war schlieslich Dewar, der 1951 die Natur der π‐Komplexe im Sinne der Molekulorbitaltheorie deutlich machte.

Zeise's salt, KPt(C2 H4 )Cl3 , was the first characterized organometallic compound; it was also the first olefin π-complex. It was published in 1825-1830 in the middle of a fight between Dumas on the one hand and Berzelius and Liebig on the other, who defended the etherin (ethylene) and radical theories, respectively. Although Zeise's formulation as a compound containing ethylene was vindicated, the fight went on for many years. This was a time when the theories of organic chemistry were being developed, before any clear understanding of the nature of molecules, bonding, and structure. Zeise thought of the structure of his salt as a product of the addition of PtCl2 to ethylene. Jensen assumed a central bonding to ethylene but needed theoretical assistance to explain it. His attempt to obtain such an explanation from Huckel failed, and it was Dewar who explained the nature of π-complexes in molecular orbital terms in 1951.

The denitrogenative rearrangements of several types of benzotriazoles were investigated by DFT (B3LYP/6-311G(d,p)) and CASPT2(10,10)sp/6-311G(d,p) calculations. The Graebe-Ullmann synthesis of carbazoles 18 by pyrolysis or photolysis of 1-arylbenzotriazoles 14 proceeds without the involvement of benzazirines and without Wolff-type ring contraction to fulvenimines. However, 1-aryltetrahydrobenzotriazoles undergo both cyclization to tetrahydrocarbazole and ring contraction. Triazoloquinones like 34 undergo predominant ring contraction to aminofulvenediones like 38 and also ring expansion to azepinediones like 40 and cyclization to N-arylbenzaziridinediones 39, whereas carbazolediones are not formed. Denitrogenation of 1-methylbenzotriazole 64 results in a facile 1,2-H shift with formation of N-phenylmethanimine 67. 1-Cyanobenzotriazole 71 undergoes destructive pyrolysis with charring, and the calculations predict the occurrence of several low-activation energy reaction pathways.

In the Dimroth rearrangement of heterocycles, often pyrimidines, an exocyclic and a ring substituent are interchanged. However, the term Dimroth rearrangement is frequently used even when there is no knowledge of the reaction mechanism and alternatives are likely. Here, we have employed density functional theory (DFT) calculations at the M06-2X/6-311+G(d,p) level to determine the most plausible rearrangement pathways of 3-aminothiocarbonylquinazoline 5, tetrahydrofuranylpyrimidine 21, and 5-allyltriazocine 30. For the rearrangement of quinazoline 5 to 9, the [1,3]-sigmatropic shift of the thioamido group with an activation barrier of 26.7 kcal/mol is much preferred over the Dimroth rearrangement (∼46 kcal/mol). An even lower barrier of 21.6 kcal/mol applies to a stepwise [1,3]-shift. The migration of the tetrahydrofuranyl unit in pyrimidines like 21 → 23 can take place by means of a [1,3]-sigmatropic shift with a low barrier (≤17.5 kcal/mol) rather than a Dimroth rearrangement under acidic conditions and most likely also under neutral conditions (∼30 kcal/mol). In the rearrangement of 5-allyl-6-iminotriazocine 30 to 32, the [3,3]-sigmatropic shift (aza-Cope rearrangement) is preferred over the Dimroth mechanism under neutral conditions, but in the presence of acid, the azonia-Cope rearrangement of an allyl group and the true Dimroth rearrangement have comparable activation energies.

Photolysis of 1-phenyltetrazole 2b at either 193 or 266 nm in a Ne matrix as well as flash vacuum pyrolysis (FVP) of the same compound with isolation of the products in Ne matrix generate phenylcarbodiimide PhN=C=NH 6 very efficiently as the principal product in an almost pure state. Minor amounts of phenyl azide 8 and 1-azacyclohepta-1,2,4,6-tetraene 10, the product of ring expansion of phenylnitrene, are also formed, and the amounts of both substances increase as a function of photolysis time. The photolysis of 2-phenyltetrazole 1b in a Ne matrix at 266 nm for 0.5 min generates the allenic N-phenylnitrile imine PhN=N(+)=CH(-) 3b, absorbing strongly and cleanly at 2021 cm(-1) in the IR spectrum. At longer photolysis times peaks ascribed to phenyl azide, phenylnitrene, 1-azacycloheptatetraene, and phenylcarbodiimide 6 become prominent. FVP of 2-phenyltetrazole yields indazole 15 as the major product together with a small amount of phenylcarbodiimide 6. Indazole formation is ascribed to cyclization of the nitrile imine in the ground state. It is not formed on FVP of 1-phenyltetrazole in agreement with the computational result that the thermodynamically most favourable path for rearrangement of the putative N-phenylimidoylnitrene 5b yields phenylcarbodiimide 6 rather than phenylnitrile imine 3b. Further pyrolysis of indazole affords a minor yield of fulvenallene 18.

Photolysis of 2,3-diazidonaphthalene-1,4-dione (1) in methyltetrahydrofuran matrices forms 2-(λ1-azaneyl)-3-azidonaphthalene-1,4-dione (vinylnitrene 32), as confirmed by electron paramagnetic resonance spectroscopy. The zero-field splitting (zfs) parameters for 32 (D/hc = 0.5338 cm–1, and E/hc = 0.0038 cm–1) reveal significant 1,3-biradical character. Irradiating 32 yields 2-(λ1-azaneyl)-1,3-dioxo-2,3-dihydro-1H-indene-2-carbonitrile (alkylnitrene 33), which has zfs parameters typical of a cycloalkylnitrene (D/hc = 1.57 cm–1, and E/hc = 0.00071 cm–1). Photolysis of 1 in argon matrices verifies that 32 forms 33.

International audience; Nitrile imines are important intermediates in 1,3-dipolar cycloaddition reactions, and they are also known to undergo efficient, unimolecular rearrangements to carbodiimides via 1H-diazirines and imidoylnitrenes under both thermal and photochemical reaction conditions. We now report a competing rearrangement, revealed by CASPT2(14,12) and B3LYP calculations, in which C-phenylnitrile imines 8 undergo ring expansion to 1-diazenyl-1,2,4,6-cycloheptatetraenes 12 akin to the phenylcarbene—cycloheptatetraene rearrangement. Amino-, hydroxy-, and thiol-groups in the meta positions of C-phenylnitrile imine lower the activation energies for this rearrangement so that it becomes potentially competitive with the cyclization to 1H-diazirines and hence rearrange to carbodiimides. The diazenylcycloheptatetraenes 12 thus formed can evolve further to cycloheptatetraene 30 and 2-diazenyl-phenylcarbene 16 over modest activation barriers, and the latter carbenes cyclize very easily to 2H- and 3H-indazoles, from which 6-methylenecyclohexadienylidene, phenylcarbene, fulvenallene, and their isomers are potentially obtainable. Moreover, another new rearrangement of benzonitrile imine forms 3-phenyl-3H-diazirine, which is a precursor of phenyldiazomethane and hence phenylcarbene. This reaction is competitive with the ring expansion. The new rearrangements predicted here should be experimentally observable, for example, under matrix photolysis or flash vacuum pyrolysis conditions.

Recently, nicotinoyl nitrene (2) has been generated from the photodecomposition of nicotinoyl azide (1) and used as the key intermediate in probing nucleobase solvent accessibility inside cells. Following the 266 nm laser photolysis of nicotinoyl azide (1) and isonicotinoyl azide (5) in solid N

International audience; Phenylnitrene radical cations m/z 91, C6H5N, 8a•+ are observed in the mass spectra of 1-, 2-, and 5-phenyltetrazoles, even though no C–N bond is present in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate that initial formation of the C-phenylimidoylnitrene 13•+ and/or benzonitrile imine radical cation 19•+ from 1H- and 2H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13•+ to the phenylcyanamide ion 15•+ over a low barrier. A cyclization of imidoylnitrene ion 13•+ onto the benzene ring offers alternate, very facile routes to the phenylnitrene ion 8a•+ and the phenylcarbodiimide ion 14•+ via the azabicyclooctadienimine 16•+. Eliminations of HNC or HCN from 14•+ and 15•+ again yield the phenylnitrene radical cation 8a•+. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14•+ to the phenylcyanamide ion 15•+ requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine −3-phenyl-1H-diazirine–phenylimidoylnitrene–phenylcarbodiimide/phenylcyanamide rearrangement has parallels in thermal and photochemical processes, but the facile cyclization of imidoylnitrene 13•+ to azabicyclooctadienimine 16•+ is facilitated by the positive charge making the nitrene more electrophilic. Furthermore, the benzonitrile imine radical cation 19•+ can cyclize to indazole 24•+, and a series of intramolecular rearrangements via hydrogen shifts, ring-openings and ring closures allow the interconversion of numerous ions of composition C7H6N2•+, including 19•+, 24•+, the benzimidazole ion 38•+ and o-aminobenzonitrile ion 40•+, all of which can eliminate either HCN or HNC to yield the C6H5N•+ ions of phenylnitrene, 8a•+, and/or iminocyclohexadienylidene, 34•+. Moreover, benzonitrile imine 19•+ can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45•+. The N-phenylnitrile imine ion 2d•+ derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a•+ but may also cyclize to the indazole ion 24•+. The N-phenylimidoylnitrene 59•+ derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15•+ and hence phenylnitrene radical cation 8a•+.

Following the eruption of Hekla in 1845-1846 Bunsen was invited by King Christian VIII of Denmark and Iceland to participate in a geochemical expedition to Iceland together with the geologist Sartorius von Waltershausen and the physiologist Carl Bergmann from Gottingen. The French mineralogist Des Cloizeaux went to Iceland separately and joined the expedition. The eudiometer invented by Bunsen was crucial for his accurate characterization of volcanic gases and determination of the composition of mixtures. His analyses of the chemical compositions of numerous rocks and minerals led him to the classification of two fundamental rock types, the more silica rich (nowadays called felsic) and the less silica rich, more basic (mafic) in agreement with Des Cloizeaux and the Danish scientist J. C. Schythe. Bunsen also formulated the correct mechanism of geyser action and helped disband the theory of connections between geysers, volcanoes and the sea. He disagreed vehemently with Waltershausen over the mechanisms of formation of sal ammoniac and of volcanic rocks and their chemical compositions. He revealed himself as an ardent experimentalist vigorously opposed to hypotheses of any kind, which also made him dismiss the new chemical theories, for example, those of Dumas and Kekule.

International audience; The electronic structure and the rearrangements of the phenylnitrene radical cation C6H5N.+2.+ have been investigated at DFT and CASPT2(7,9) levels of theory. The 2B2 state has the lowest energy of five identified electronic states, and it can undergo ring expansion to the 1-azacycloheptetetraene radical cation 4.+ with an activation energy of ca. 28 kcal/mol. Ring opening and recyclization provide a route to 5-cyanocyclopentadiene radical cation 8.+, which may undergo facile 1,5-hydrogen shifts. The 2-, 3-, and 4-pyridylcarbene radical cations 31.+, 35.+ , and 39.+ interconvert with the phenylnitrene radical cation via azacycloheptatetraenes with activation barriers

Carbene und Nitrene konnen sowohl in Singulett‐ als auch Triplett‐Zustanden existieren, die zuweilen gleich stabil sind und entweder thermisch oder photochemisch interkonvertiert werden konnen. Viele Carben‐ und Nitren‐Reaktionen verlaufen uber Tunnelprozesse bei tiefen Temperaturen. Zahlreiche Singulett‐ und Triplett‐Zustande wurden spektroskopisch charakterisiert, und ein detailliertes Verstandnis der chemischen und physikalischen Eigenschaften von Carbenen und Nitrenen ist im Entstehen. In den letzten Jahren gab es signifikante Fortschritte auf dem Gebiet der direkten Beobachtung von Carbenen, Nitrenen und vielen anderen reaktiven Zwischenstufen mittels Matrix‐Photolyse und Vakuum‐Blitzpyrolyse (flash vacuum pyrolysis, FVP) gekoppelt mit Matrixisolation bei kryogenen Temperauren. Unser Verstandnis von Singulett‐ und Triplett‐Zustanden hat sich durch das Zusammenspiel von Spektroskopie und theoretischen Studien weiterentwickelt. Die Existenz bistabiler Carbene und Nitrene sowie viele Beispiele von Tunnelprozessen wurden demonstriert, ebenso wie die Korrelation zwischen den magnetischen Nullfeldparametern D und Spindichten in Nitrenen und Carbenen. Dieser Kurzaufsatz gibt einen Uberblick uber einige diese Entwicklungen.

Dipivaloylketene (2) is obtained by flash vacuum pyrolysis of furan-2,3-dione 6 and dimerizes to 1,3-dioxin-4-one 3, which is a stable but reactive ketene. The transannular addition and rearrangement of enols formed by the addition of nucleophiles to the ketene function in 3 generates axially chiral 2,6,9-trioxabicyclo[3.3.1]nonadienes (bisdioxines) 4. When arylamines are used as the nucleophiles under neutral conditions, decarboxylation occurs during the formation of bisdioxines 8. However, when water or alcohols are added to 3 under acidic conditions, bisdioxine-carboxylic acids and esters 10 and 11 are obtained. Acid hydrolysis of the bisdioxines proceeds through the addition of water to a C=C double bond and results in a second transannular oxa-Michael-type reaction and generation of tetraoxaadamantanes 5. This reaction is decarboxylative when free carboxylic acid functions are present in the bisdioxines, thus forming 21 and 22, but carboxylic acid derivatives are preserved to yield compounds 20, 23, 25, 28, and 29. A hydrogenolysis of the dibenzyl ester 23 yields the free dicarboxylic acid 24. The tetraoxaadamantanes are formed in high yields (65–95%) in most cases, but the addition of water to the concave inside of the bisdioxines becomes severely hindered in cyclic derivatives, so that the 38-membered ring compound 32 requires microwave heating at 170 °C to form tetraoxaadamantane 33, and the catenated compound 36 and calix[6]arene derivative 37 did not form tetraoxaadamantanes. The reaction mechanisms of bisdioxine and tetraoxaadamantane formation are discussed.

The flash vacuum pyrolysis (FVP) of 1 H -benzotriazoles is an important and high-yielding source of cyanocyclopentadienes (cyclopentadienecarbonitriles). Early research on the pyrolysis of 1-acylbenzotriazoles only revealed formation of benzoxazoles, while the important formation of cyanocyclopentadienes was missed. Similarly, previous studies of the pyrolysis of 1-alkoxycarbonylbenzotriazoles revealed the formation of 2-alkoxybenzoxazoles, 1- and 2-alkylbenzotriazoles and a variety of decomposition products, but again formation of cyanocyclopentadienes was not reported. We now report the formation of methylcyanocyclopentadienes 7–9 and unsubstituted 1-cyanocyclopentadiene 4 as well as the benzoxazoles 33 and 24 in FVP reactions of 1-acetylbenzotriazole 32 and 1-methoxycarbonylbenzotriazole 21a , respectively. Similarly, 1-benzoylbenzotriazole 18 affords 2-phenylbenzoxazole 20 and phenylcyanocyclopentadienes 27 . The reactions are interpreted in terms of Wolff-type ring contraction to N -acylfulvenimines, which may take place from either 2-diazocyclohexadienimine valence isomers of the benzotriazoles or from singlet iminocyclohexadienylidene diradicals/carbenes. Homolysis of the N CO bond in the N -acylfulvenimines generates free radicals, which can recombine to form the methylcyanocyclopentadienes 7–9 , undergo hydrogen abstraction to afford the unsubstituted 4 , and in the case of 1-benzoylbenzotriazole 18 dimerize to form biphenyl. The FVP of 1-ethoxycarbonylbenzotriazole 21b yields ethylcyanocyclopentadienes 45 as well as the unsubstituted cyanocyclopentadiene 4 , which results from elimination of ethene. FVP of 1-methylbenzotriazole 22a does not lead to methylcyanocyclopentadienes 7–9 ; instead a 1,4-hydrogen shift in the putative N -methyliminocyclohexadienylidene diradical 47 yields N -phenylmethanimine 48 . The proposed reaction mechanisms are supported by DFT calculations.

Flash vacuum pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser pyrolysis, pulsed pyrolysis with supersonic jet expansion, very low pressure pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.

Pyrolysis of hydrazines and their derivatives often results in homolytic cleavage of the N N bonds, but molecular rearrangements and eliminations are also observed in many cases. Thermal reactions in solution, in the solid state, and in the gas phase under static, flow, and flash vacuum pyrolysis conditions are considered in this review. Arrhenius or Eyring activation parameters are reported whenever available, but the original literature should always be consulted, as these parameters are often derived under widely different experimental conditions. The compound classes investigated include open-chain and cyclic hydrazine derivatives and their relationship with azomethine imines, N-amino-heterocycles, N-isocyanoamines and N-isocyanatoamines, N-aminocarbodiimides, azines, hydrazones, semicarbazides and semicarbazones, hydrazides, aminimides, amidrazones, azimines, triazenes, tetrazadienes, pentazadienes and hexazatrienes, and triazines. Understanding of reaction mechanisms is emphasized whenever possible. Reactive intermediates involved in these pyrolysis reactions include ketenes, ketenimines and other cumulenes, free radicals and diradicals, zwitterions, carbenes, nitrenes, nitrile ylides, nitrile imines, arynes and azetes.

This Essay deals with the fascinating and highly explosive compounds fulminating gold and fulminating silver, which are easily made by treatment of gold dissolved in aqua regia with ammonia, and by reaction of silver oxide or silver salts with ammonia, respectively. Fulminating gold in particular captivated the alchemists in the 16th to 18th centuries. Numerous preparations were described, as well as numerous attempts to make volatile, sublimable or distillable gold, and to use the products so obtained (which were most likely gold chlorides) to make the sought-after tincture, which would "heal" the "impure" metals and transform them into gold, and equally be a panacea to cure all human illnesses.

International audience; 5-Phenyltetrazole 1e is an important source of phenylnitrene or the phenylnitrene radical cation (m/z 91) under thermal, photochemical, and electron impact conditions. Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c yield pyridylnitrene radical cations 9a•+ (m/z 92) upon electron impact. In contrast, 2-(5-tetrazolyl)pyridine 12a•+ generates 2-pyridyldiazomethane 24•+ and 2-pyridylcarbene 26•+ radical cations (m/z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene–nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyl)pyridine, 12a•+ → 21•+, taking place in the radical cations. Subsequent losses of N2 generate the pyridinium diazomethyl radical 22•+ or pyridinium-2-carbyne 23•+. These two ions can isomerize to 2-pyridyldiazomethane 24•+ and 2-pyridylcarbene 26•+, the latter rearranging to the phenylnitrene radical cations 9a•+. 13C-labeling of the tetrazole rings confirmed that 2-(5-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the 13C label, but 4-(5-tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c•+, which has lost the 13C label. 2-Pyridylcarbene/phenylnitrene radical cations (m/z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5-a]pyridine 34. Similarly, 4-pyridylnitrene radical cation 18c•+ or its isomers (m/z 92) is obtained from 1,2,3-triazolo[1,5-a]pyrazine 36. Several other α-heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by 13C-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-13C. In general, 5-aryltetrazoles generate arylnitrene radical cations under electron impact, but α-heteroaryltetrazoles generate α-heteroarylcarbene radical cations

Variable-temperature NMR and ESR spectroscopic studies reveal that bis(dibenzo[a,i]fluorenylidene) 1 possesses a singlet ground state, 1(S0), while the 90° twisted triplet 1(T1) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1(T1) at temperatures between 300 and 500 K yields the exchange interaction Jex/h c=3351 cm−1 and a singlet–triplet energy splitting of 9.6 kcal mol−1, which is in excellent agreement with calculations (9.3 kcal mol−1 at the UKS BP86/B3LYP/revPBE level of theory). In contrast, the zero-field splitting parameter D is very small (calculated value −0.018 cm−1) and unmeasurable.

Extremely low rotational barriers for ethylenic C=C double bonds in the range 5-17 kcal/mol have been measured by low temperature H-1 and C-13 NMR spectroscopy and confirmed by calculations at the M06-2X level for 5-methylene-1,3-dioxane-4,6-diones (Meldrum's acid derivatives) and 4-methyleneisoxazol-5(4H)-ones. The barriers are ascribed to the push-pull (donor-acceptor) nature of the alkenes. A correlation between the calculated C=C bond lengths and the ratios of electron occupancies in the ethylenic pi and pi* orbitals (the pi/pi* quotient) constitutes a useful measure of the push-pull character.

Contrary to a previous report based on pyrolysis-mass spectrometry, it is shown that 1-vinylbenzotriazole 4 undergoes flash vacuum pyrolysis (FVP) to N -phenylketenimine (PhN C CH 2 ) 7 and indole 5 in parallel, not consecutive, reactions. N- Phenylketenimine does not rearrange to indole to any significant extent, but on FVP at 950 °C it decomposes in a free radical reaction to yield phenylacetonitrile, acetonitrile, benzene, biphenyl and o -tolunitrile. Phenylacetonitrile itself at 1000 °C yields essentially the same products. 1-(2-Methylpropeny)benzotriazole 22 pyrolyzes to C,C -dimethyl- N- phenylketenimine 23 , and at higher temperature to methacrylonitrile 25 and benzene in a free radical reaction. 2-Vinylbenzotriazole 21 rearranges to 1-vinylbenzotriazole, which then affords N -phenylketenimine, phenylacetonitrile and indole at 900 °C. 2-Phenyl-1-azirine 10 , phenylacetonitrile and a trace of N -phenylketenimine are formed by FVP of α-styryl azide 8 . 2-Phenyl-1-azirine itself pyrolyzes to phenylacetonitrile, benzonitrile and a trace of indole at 900 °C. 3-Phenyl-1-azirine 12 , phenylacetonitrile and indole are formed on pyrolysis of β-styryl azide 11 , and matrix photolysis of 11 yields 3-phenyl-1-azirine and 1-phenylketenimine (PhCH C NH) 13 . The paper emphasizes the dangers of relying solely on mass spectrometry in investigations of mechanisms of pyrolysis reaction. Observed mass spectra may on occasion correspond to very low yields of products that are not characteristic of the main reaction paths. Moreover, there are cases where major FVP reaction products have been nearly impossible to observe by online mass spectrometry.

Flash vacuum pyrolysis (FVP) of benz[a]azulene yields phenanthrene and 2-ethynylbiphenyl. FVP of cyclohepta[b]indole similarly yields phenanthridine and 2-cyanobiphenyl. The reversibility of the reactions is demonstrated by FVP of 2-ethynylbiphenyl and 2-isocyanobiphenyl. All the observed reactions are in accord with the norcaradiene-vinylidene mechanism of the azulene-naphthalene rearrangement, whereas other proposed mechanisms are ruled out.

N-15-Labeling demonstrates that the two nitrogen atoms in the 2-pyridylnitrene radical cation 2(+) become equivalent prior to fragmentation in the mass spectrometer. Furthermore, the mass spectra of 6- and 7-tetrazolo[1,4-a]pyridine are identical, as are those of 5- and 8-tetrazolo[1,5-a]pyridine, thereby again demonstrating interconversion of the nitrogen atoms in 2-pyridylnitrenes. These rearrangements parallel the reactions established under thermal (flash vacuum pyrolysis) and photochemical condition. Calculations of the energies of ground and transition states at the CASPT2(7,8) level support the notion that 2-pyridylnitrenes undergo very easy and exothermic ring expansion to 1,3-diazacycloheptatetraene 3, both in the neutrals and the radical cations. In addition, the ring opening to 4-cyanobutadienylnitrene 4 can take place in both the neutrals and the radical cations with modest activation barriers.

A reinterpretation of the spectroscopic data for the products of flash vacuum pyrolysis (FVP) of 8- and 9-membered polymethylenetetrazoles 1, 3, 5, and 7 with annelated benzene rings indicates that these are not cyanamides 2, 4, 6, and 8, but 9- and 10-membered cyclic carbodiimides 9, 10, 11, and 12. FVP of the corresponding tetrazolo[1,5-a]azocine and tetrazolo[1,5-a]azonine derivatives 13 and 17 was investigated by isolation of the products in cryogenic matrices and by matrix photolysis and found to yield 9- and 10-membered cyclic carbodiimides 14 and 18 as transient intermediates en route to indolo[2,3-b]quinoline 15 and dibenzo[b,f]naphthyridine 19.

2-Diazomethylpyridines 1D and 6D, the valence isomers of [1,2,3]triazolo[1,5-a]pyridines 1T and 6T, have been observed directly at ∼2080 cm(-1) by a combination of mild flash vacuum pyrolysis (FVP) at 200-600 °C with low temperature IR spectroscopy. Calculations confirm a ca. 17 kcal/mol barrier for the formation of 2-diazomethylpyridine 1D from [1,2,3]triazolo[1,5-a]pyridine 1T, the diazo compound lying ca. 5 kcal/mol above the triazole. In the higher temperature range (400-600 °C) 2-diazomethylpyridine 1D eliminates N2 with formation of 2-pyridylcarbene 2 and rearrangement to 1-cyanocyclopentadiene 4. 2-Diazomethylpyridine 1D undergoes 1,3-dipolar cycloaddition with tetracyanoethylene (TCNE) at 20-90 °C to yield 3-(2-pyridyl)cyclopropanetetracarbonitrile 11 and 3-(tricyanovinyl)-[1,2,3]triazolo[1,5-a]pyridine 13T via unobserved pyrazolines 10 and 12. FVP of triazole 13T affords an IR absorption at 2080 cm(-1) ascribed to the corresponding diazo compound 13D.

Photolysis of 1- and 5-aryltetrazoles at 5-10 K using a 266 nm laser immediately generates their triplet excited states, which are characterized by their electron spin resonance (ESR) spectra with zero-field splitting parameters D = 0.12-0.13 cm-1 and E = 0.002-0.008 cm-1. Further photolysis of all of the aryltetrazoles affords arylnitrenes ( D ≅ 1 cm-1), and in the case of 5-aryltetrazoles also arylcarbenes ( D ≅ 0.5 cm-1). The formation of arylnitrenes from 5-aryltetrazoles, where no aryl-N bond is present, is explained by the photochemical rearrangement of initially formed nitrile imines ArCN+N-R to carbodiimides. The monosubstituted carbodiimide PhN═C═NH isomerizes to phenylcyanamide, PhNH-CN, and photolysis of the latter causes rapid elimination of HCN and formation of phenylnitrene. When N-methyl groups are present in the tetrazoles, methylnitrene, CH3-N, is formed too. In the case of 5-phenyltetrazole, additional hydrogen shift and fragmentation afford cyano- and isocyanonitrenes, NCN and CNN.