CHNO - Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory.

Fulminic and cyanic acids played a decisive role in the conception of isomerism 200 years ago. Cyanic (HOCN), isocyanic (HNCO), and fulminic (HCNO) acids have been detected in several interstellar sources, but isofulminic acid (HONC) is little known. Here we examine the interrelationships between the four acids and formylnitrene, HC(O)N, at the CASPT2 and three DFT levels. Formylnitrene has a triplet ground state, T0, a closed shell singlet (CSS), S0, and an open-shell singlet (OSS), S1, lying ∼7 and 27 kcal/mol above T0, respectively. The CSS is weakly stabilized by a 12 kcal/mol bond between the N and the O atoms. A conical intersection 12 kcal/mol above T0 permits easy T0-S0 interchange. Formyl azide and formylnitrene (T0 and S0) are isomerized thermally to HNCO. HOCN is best obtained via dissociation of the nitrene (or of HNCO) to H• + NCO• radicals ∼46 kcal/mol above the T0 nitrene. Isofulminic acid, HONC, isomerizes readily to cyanic acid, HOCN, in thermal and photochemical reactions. Fulminic acid, HCNO, can isomerize to HNCO via CSS formylnitrene. Easy tautomerization prevents the preparation of HOCN in quantity. The barrier to isomerization is strongly reduced in small hydrogen-bonded aggregates so that trace amounts of HOCN can exist in equilibrium with HNCO.

Benzotriazoles and Triazoloquinones: Rearrangements to Carbazoles, Benzazirines, Azepinediones, and Fulvenimines.

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.

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes.

Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3-dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3-dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m'-bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m'-bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m'-bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2-phenyl-1-azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol-1 in the singlet state. In the bent triplet states, 1,3-hydrogen shifts interconverting nitrile ylides are potentially possible.

Aryl Nitrile Imines and Diazo Compounds. Formation of Indazole, Pyridine N-Imine, and 2-Pyridyldiazomethane from Tetrazoles.

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. Moreover, FVP of 5-aryltetrazoles is a convenient source of aryldiazo compounds (30/47) and arylcarbenes, including pyridylcarbenes. The factors that determine which path is followed are poorly understood. Calculations at the density functional theory and CASPT2 levels now examine cyclization of N-phenylnitrile imine 13 to indazole 17. A corresponding cyclization of C-phenylnitrile imine 19 can also lead to indazole, but this reaction, which passes through a carbenic nitrile imine, requires a much higher activation energy and is therefore not competitive with the known rearrangements to phenyldiazirines, ring expansion to diazenylcycloheptatetraene, or a new, potential rearrangement to cyanoazepine. C-(2-Pyridyl)nitrile imine 38 is predicted to undergo a new rearrangement to cyanopyridine N-imide 40 with an activation energy of 43 kcal/mol. The experimental observation that 2-pyridyldiazomethane 47 is actually formed requires a reaction with an energy barrier below 43 kcal/mol. This is found in the H-transfer from the tetrazole ring in 5-(2-pyridyl)tetrazole to the pyridine ring with a subsequent formation of 1H-2-(diazomethylene)pyridine and elimination of N2 with a barrier of ca. 26 kcal/mol. This new, facile mechanism has not previously been considered.

Syntheses and structures of benzo-bis(1,3,2-diazaboroles) and acenaphtho-1,3,2-diazaboroles.

Colorless crystalline 2,6-dibromo-4,8-dimethyl-1,3,5,7-tetraphenylbenzobis(diazaborole) 4 resulted from the cyclocondensation of 3,6-dimethyl-1,2,4,5-tetraphenylaminobenzene 3d with two equivalents of boron tribromide in the presence of calcium hydride. Synthesis of the dark-red crystalline 2-bromo-N,N'-bis(diisopropylphenyl)acenaphtho-1,3,2-diazaborole 7 was effected by the cyclocondensation of 1,2-bis(N-2',6'-diisopropylphenylimino)acenaphthene (5) and boron tribromide with subsequent sodium amalgam reduction of the initially formed burgundy red diazaborolium salt 6. Compounds 4, 6 and 7 are characterised by elemental analyses, 1H, 11B and 13C NMR spectroscopy, as well as by single X-ray diffraction studies. The electronic structures of 4, 6 and 7 are subject to DFT calculations.

Rearrangements of Nitrile Imines: Ring Expansion of Benzonitrile Imines to Cycloheptatetraenes and Ring Closure to 3-Phenyl-3 H-diazirines.

Nitrile imines are important intermediates in 1,3-dipolar cycloaddition reactions, and they are also known to undergo efficient, unimolecular rearrangements to carbodiimides via 1 H-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 1 H-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 2 H- and 3 H-indazoles, from which 6-methylenecyclohexadienylidene, phenylcarbene, fulvenallene, and their isomers are potentially obtainable. Moreover, another new rearrangement of benzonitrile imine forms 3-phenyl-3 H-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.

Heteroarylcarbene-Arylnitrene Radical Cation Isomerizations.

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.

Phenylnitrene Radical Cation and Its Isomers from Tetrazoles, Nitrile Imines, Indazole, and Benzimidazole.

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 1 H- and 2 H-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-1 H-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•+.

Phenylnitrene Radical Cation Rearrangements.

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 <35 kcal/mol. The carbene-carbene and carbene-nitrene rearrangements, ring expansions, ring contractions, ring openings (e.g., to cyanopentadienylidene 28.+), and cyclizations taking place in all these radical cations are completely analogous to the thermal and photochemical rearrangements.

Triplet States of Tetrazoles, Nitrenes, and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene.

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.

Trimethylsilylnitrene and Its Surprising Rearrangement to N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide.

Photolysis of trimethylsilyl azide at 254 nm in Ar matrix at 15 K generates the triplet ground state trimethylsilylnitrene 2 aT, observed by ESR spectroscopy (|D/hc|=1.540 cm-1 ; |E/hc|=0.0002 cm-1 ). Calculations at the CASPT2(14,13) level reveal the open-shell singlet nitrene 2 aS(1 A") is a discrete intermediate lying ≈38 kcal mol-1 above the triplet. The normally expected rearrangement of the nitrene 2 aS to dimethylsilanimine 3 a has a high calculated barrier (33 kcal mol-1 ), which explains why this product has never been observed. Instead, the singlet nitrene 2 aS inserts into a methyl C-H bond to yield silaziridine 12 via an activation barrier of only 6 kcal mol-1 . Ring opening of 12 generates a 1-silaazomethine ylide 13, in which a facile 1,2-H shift yields N-(dimethylsilyl)methanimine 5, all with barriers well below the energy of the singlet nitrene.

Direct Observation of an Imidoylnitrene: Photochemical Formation of PhC(=NMe)-N and Me-N from 1-Methyl-5-phenyltetrazole.

The imidoylnitrene 8, N-methyl-C-phenylimidoylnitrene, has been generated by laser photolysis of 1-methyl-5-phenyltetrazole 6 at 5 K and characterized by its ESR spectrum (|D/hc|=0.9602, |E/hc|=0.0144 cm-1 ). In addition, the triplet excited states of 6 and of 2-methyl-5-phenyltetrazole 11 were also observed by ESR spectroscopy in the 5 K matrices (6: |D/hc|=0.123 cm-1 , E/hc=0.0065 cm-1 , 11: |D/hc|=0.126 cm-1 , |E/hc|=0.0056 cm-1 ). The imidoylnitrene 8 is unstable both thermally (disappearing at 80 K) and photochemically (disappearing on continued irradiation at 266 nm). Methyl(phenyl)carbodiimide is the end product of photolysis.

Isoselenocyanates versus Isothiocyanates and Isocyanates.

Alkyl and aryl isoselenocyanates are well known intermediates in the synthesis of various organoselenium compounds, but the knowledge of the physicochemical properties of simple unsaturated derivatives is still fragmentary. Vinyl-, 2-propenyl-, and cyclopropyl isoselenocyanates have been prepared by reaction of selenium in powder with the corresponding isocyanides. The isoselenocyanates of this series, with a variable distance between the N═C═Se group and the unsaturated or pseudounsaturated group, have been studied by UV-photoelectron spectroscopy and quantum-chemical calculations. For each of these three isoselenocyanates, the exploration of conformers and geometrical optimization always converge toward only one local minimum. The vinyl and cyclopropyl derivatives are characterized by similar order of magnitude of interactions between the NCSe group and the substituent, while for allylic compound two noninteracting moieties should be considered. The same conclusions were obtained for vinylic and cyclopropylic sulfur and oxygen derivatives. Thus the type and extent of interactions between the N═C═X (X = O, S, Se) group and an unsaturated (vinyl, allyl, or cyclopropyl) moiety are now clarified.

Theoretical Investigation of the Infrared Spectrum of 5-Bromo-2,4-pentadiynenitrile from a CCSD(T)/B3LYP Anharmonic Potential.

On the grounds of a hybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of a variational and variational-perturbational methods, the IR spectra of 5-bromo-2,4-pentadiynenitrile (BrC5 N) is revisited in the mid-infrared region up to 4500 cm-1 . A position and intensity analysis of our theoretical results allow us to assign the fundamental bands together with their combinations and overtones, in the aforementioned range of frequencies. The main objective of this work is to give an "a priori" complete IR spectrum of BrC5 N, which can be used as a guide for the low-intensity bands in areas not completely studied so far.

Electronic structure of BN-aromatics: Choice of reliable computational tools.

The importance of having reliable calculation tools to interpret and predict the electronic properties of BN-aromatics is directly linked to the growing interest for these very promising new systems in the field of materials science, biomedical research, or energy sustainability. Ionization energy (IE) is one of the most important parameters to approach the electronic structure of molecules. It can be theoretically estimated, but in order to evaluate their persistence and propose the most reliable tools for the evaluation of different electronic properties of existent or only imagined BN-containing compounds, we took as reference experimental values of ionization energies provided by ultra-violet photoelectron spectroscopy (UV-PES) in gas phase-the only technique giving access to the energy levels of filled molecular orbitals. Thus, a set of 21 aromatic molecules containing B-N bonds and B-N-B patterns has been merged for a comparison between experimental IEs obtained by UV-PES and various theoretical approaches for their estimation. Time-Dependent Density Functional Theory (TD-DFT) methods using B3LYP and long-range corrected CAM-B3LYP functionals are used, combined with the ΔSCF approach, and compared with electron propagator theory such as outer valence Green's function (OVGF, P3) and symmetry adapted cluster-configuration interaction ab initio methods. Direct Kohn-Sham estimation and "corrected" Kohn-Sham estimation are also given. The deviation between experimental and theoretical values is computed for each molecule, and a statistical study is performed over the average and the root mean square for the whole set and sub-sets of molecules. It is shown that (i) ΔSCF+TDDFT(CAM-B3LYP), OVGF, and P3 are the most efficient way for a good agreement with UV-PES values, (ii) a CAM-B3LYP range-separated hybrid functional is significantly better than B3LYP for the purpose, especially for extended conjugated systems, and (iii) the "corrected" Kohn-Sham result is a fast and simple way to predict IEs.

Imidoylnitrenes R'C(═NR)-N, Nitrile Imines, 1H-Diazirines, and Carbodiimides: Interconversions and Rearrangements, Structures, and Energies at DFT and CASPT2 Levels of Theory.

The structures, energies, and rearrangements of imidoylnitrenes H-C(═NH)-N, H2N-C(═NH)-N, Ph-C(═NH)-N, H-C(═NPh)-N, and MeO-C(═NCN)-N (10a-e) are investigated at DFT and CASPT2 levels of theory. Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6'), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol-1 above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol-1 higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2-12 kcal mol-1), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37-48 kcal mol-1. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37-60 kcal mol-1. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12-20 kcal mol-1. Calculated vibrational data, UV-vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9-1 cm-1 and nonzero |E/hc| values are predicted.

Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero-Wolff Rearrangement of Benzotriazoles to Cyanocyclopentadienes and 1H-Benzo[b]azirines.

The thermal rearrangements of benzotriazole 1 to fulvenimine 4 and 1H-benzazirine 7 are investigated at DFT and CASPT2 levels of theory. Ring opening of benzotriazole 1 to 2-diazo-cyclohexadienimine 2 followed by N2 elimination affords Z- and E-2-iminocyclohexadienylidenes 3, which have triplet ground states (3A″). The open-shell singlet (OSS) (1A″) and closed-shell singlet (CSS) (1A') of 3 lie ∼15 and 40 kcal/mol higher in free energy, respectively. The OSS 3 (1A″) is best described as a 1,3-diradical, whereas the CSS (1A') has the character of a carbene. A hetero-Wolff rearrangement of OSS 3 yields fulvenimine 4, which is a precursor of cyanocyclopentadiene 5, with a calculated activation barrier of 38 kcal/mol at the CASPT2(8,8) level, whereby there is a surface crossing from the OSS to the CSS near the transition state. The barrier for cyclization to 1H-benzo[b]azirine 7 is only ∼13 kcal/mol. Therefore, reaction paths involving the singlet iminocyclohexadienylidene diradicals 3 will necessarily cause equilibration with 1H-benzazirine 7 prior to ring contraction to iminofulvene 4 and cyanocyclopentadiene 5, in agreement with experimental observations based on 13C labeling. The thermolysis of 1-acetylbenzotriazole 7 leads to the analogous N-acetyl-diazocyclohexadienimines 8, N-acetyliminocyclohexadienylidene diradicals 9, and N-acetylfulvenimine 10. The E-N-acetyliminocyclohexadienylidene E9 ring closes to the N-acetyl-1H-benzazirine 11 prior to ring contraction to N-acetylfulvenimine 10, and the Z-N-acetyl-2-diazocyclohexadienimine Z8 ring closes to 2-methylbenzoxazole 12. 1H-benzazirines are predicted to be spectroscopically observable species.

The Least Stable Isomer of BN Naphthalene: Toward Predictive Trends for the Optoelectronic Properties of BN Acenes.

The least stable isomer of the parental BN naphthalene series has been synthesized in a simple four-step sequence. Its experimental electronic structure characterization via UV-PES, cyclic voltammetry, and UV-vis spectroscopy in direct comparison with three other known BN naphthalene isomers has established two guiding principles for predicting the electronic structures of BN acene compounds: (1) Orientational BN isomers have similar HOMO-LUMO gaps. (2) For each pair of orientational BN isomers, the more thermodynamically stable compound has the lower HOMO energy. Furthermore, we demonstrate that BN/CC isosterism in the context of BN-9,1-Naph can impact crystal packing to favor a cofacial π-stack motif.

Ab Initio Modeling of the IR Spectra of Dicyanoacetylene in the Region 100-4800 cm(-1).

On the grounds of a hybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of a variational-perturbational method, the IR spectrum of dicyanoacetylene is revisited in the region 100-4800 cm(-1), comparing our results with previous experimental data. A position and intensity analysis of our theoretical results allows us to assign fundamentals, combinations, and overtone bands in the aforementioned range of frequencies. Moreover, the outcomes show a good agreement with the most reliable experimental values and predict several unobserved or unassigned overtones and combinations in the mid-infrared region.

The Electronic Structure of Some Cyanohydrins-A†Spectroscopically Under-Investigated Family of Compounds.

Cyanohydrins are usually formed by addition of hydrogen cyanide to aldehydes or ketones while the elimination of HCN from cyanohydrins is easily observed upon heating. The low thermal stability of these highly boiling compounds leads to difficult studies in the gas phase where partial or complete decomposition is usually observed. Consequently, the reported physicochemical properties of such compounds in the gas phase are still scarce. Keeping with this, four simple cyanohydrins, the glycolonitrile and methyl, vinyl and ethynyl derivatives, have been selected. These are possible candidates for the Interstellar Medium, where the corresponding aldehydes and HCN have been detected, and could have played an important role in prebiotic chemistry, as already proposed for some of them. Three well-suited spectroscopic techniques, namely, UV photoelectron, infrared, and Raman spectroscopies, in tandem with quantum calculations, have been chosen for the structure analysis. Photoelectron spectroscopy, successfully performed with gaseous compounds, provides the first comparative study on cyanohydrins in the gas phase.