The rearrangements of naphthylnitrenes: UV/Vis and IR spectra of azirines, cyclic ketenimines, and cyclic nitrile ylides

Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet naphthyl nitrenes, (1)()1 and (1)()2. Relaxation to the corresponding lower energy, persistent triplet nitrenes (3)()1 and (3)()2 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines 15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously, but this work provides the first experimental observation of such species. The factors which are responsible for the special stability of the ylidic species 17 and 20 are discussed.     

Carbenic Behaviour of Isocyanoamines generated by Flash Pyrolysis of 4-Arylhydrazono-isoxazol-5-ones. Rearrangement to Cyanamides and Indazoles. Preliminary communication

Flash pyrolysis of the 4-arylhydrazono-3-methyl-isoxazol-5-one ( 1 ) yields carbon dioxide, acetonitrile, and isocyanoarylamines 2 which under the reaction conditions rearrange to either arylcyanamides 3 or indazoles 4 . It is shown by deuterium labelling that only the isocyanoamines not the isomeric nitrile imines are intermediates in the formation of indazoles.     

Dissociation of the diphenylnitrile imine radical cation into benzonitrile and [phenylnitrene]+˙

The collision induced dissociation/mass analysed ion kinetic energy mass spectra of 2,5-diphenyltetrazole demonstrate the decay sequence [diphenyltetrazole]^+˙→ [diphenylnitrile imine]^+˙→ m/z 91. The m/z 91 ion was shown to be identical to the ion formed by loss of N₂ from the phenyl azide radical cation, thus suggesting the phenylnitrene structure for the m/z 91 ion.     

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C(7)H(6) Energy Surface

Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C(7)H(6). Fulvenallene (6) is the most stable C(7)H(6) isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol(-)(1), respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet ((3)A") ground state, 16 kJ mol(-)(1) more stable than the singlet state ((1)A'). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol(-)(1)) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the (3)A(2) and (3)B(1) triplet states of cycloheptatrienylidene ((3)3) are predicted to lie very close in energy. The singlet "aromatic" cycloheptatrienylidene ((1)3, (1)A(1)) is found to be a transition structure interconverting two chiral cyclic allenes (4) and it lies approximately 25 kJ mol(-)(1) below the triplet states. Bicyclo[3.2.0]hepta-1,3,6-triene (5) is predicted to be a stable equilibrium structure, lying in a significant energy well. Rearrangement of 4 to 5 constitutes the rate-determining step for the rearrangement of phenylcarbene to fulvenallene (6), the ethynylcyclopentadienes (9), and spiro[2.4]heptatriene (7). Rearrangement of 9A to 6, via a 1,4-H shift, requires a large barrier of 325 kJ mol(-)(1). Rearrangement of benzocyclopropene (10) to 6 involves a methylenecyclohexadienylidene intermediate (27) and is associated with an energy barrier of 223 kJ mol(-)(1). The calculated mechanisms and energetics for the interconversions of various C(7)H(6) isomers are in good accord with experimental results to date.     

A Thermally Populated,Perpendicularly Twisted Alkene Triplet Diradical

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 J_ex/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.     

Azulene–Naphthalene‐Type Rearrangements in Benz[a]azulene and Cyclohepta[b]indole

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.     

An Organometallic Compound which Exhibits a DNA Topology‐Dependent One‐Stranded Intercalation Mode

Understanding how small molecules interact with DNA is essential since it underlies a multitude of pathological conditions and therapeutic interventions. Many different intercalator compounds have been studied because of their activity as mutagens or drugs, but little is known regarding their interaction with nucleosomes, the protein‐packaged form of DNA in cells. Here, using crystallographic methods and molecular dynamics simulations, we discovered that adducts formed by [(η⁶‐THA)Ru(ethylenediamine)Cl][PF₆] (THA=5,8,9,10‐tetrahydroanthracene; RAED‐THA‐Cl[PF₆]) in the nucleosome comprise a novel one‐stranded intercalation and DNA distortion mode. Conversely, the THA group in fact remains solvent exposed and does not disrupt base stacking in RAED‐THA adducts on B‐form DNA. This newly observed DNA binding mode and topology dependence may actually be prevalent and should be considered when studying covalently binding intercalating compounds.     

Kekulé’s Oscillating D3h Cyclohexatriene Structure of Benzene

Kekulé postulated that neighbouring carbon atoms in benzene undergo incessant collision with each other, thereby leading to the interchange of double and single bonds, which amounts to an oscillation between two cyclohexatriene structures in dynamic equilibrium. It has been claimed that Kekulé arrived at a fully symmetric D_6h structure of benzene and that the oscillation hypothesis should not be attributed to him. However, Clausius’ collision theory, which was known at the time, implies that, when the absolute temperature approaches zero, the collision frequency will tend toward zero too, i.e. collisions will stop, and a static, D_3h cyclohexatriene obtains. The classical collision theory did not allow Kekulé to construct the desired D_6h structure as the energy minimum. The theory of harmonic oscillators would have allowed it, but that was not attempted at Kekulé’s time.     

Analysis of triazines and associated metabolites with electrospray ionization field-asymmetric ion mobility spectrometry/mass spectrometry

Triazines comprise an important pollutant class owing to continued use in certain countries, and owing to strong environmental persistence that leads to problems even in countries like Sweden where the use of triazines has been prohibited for some years. We investigated mass-selective detection for analysis of triazines. More specifically, we studied the background reduction and sensitivity enhancement that result from the use of a new interface technique, field-asymmetric ion mobility spectrometry (FAIMS), in conjunction with electrospray ionization ion-trap mass spectrometry. This technique allows for ion sorting and discrimination against the considerable "chemical noise", nonspecific cluster and fragment ions, which are typically generated in electrospray ionization. This paper presents results of a pilot study of triazines and some metabolites in ideal solvents. Our long-range goal is automated analysis with mass-selective detection coupled to membrane-based sample cleanup and enrichment for additional enhancement in sensitivity.     

Demonstration of molecular assembly on Si (100) for CMOS-compatible molecule-based electronic devices

In this work, we establish the potential of a UV-promoted direct attachment of alkanes with alcohol and thiol linkers to the silicon (100) surfaces for use in molecular electronic devices with increased potential for integration with existing CMOS technologies. Characterization of the self-assembled monolayers via Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and X-ray photoemission spectroscopy shows that the films assembled on the Si (100) are comparable in quality, aliphatic monolayer coverage, and extent of substrate oxidation to those assembled on the more extensively studied Si (111) crystal face. Simple Si (100)-based electronic devices fabricated with the monolayers exhibited molecule-dependent electrical characteristics. These data highlight the effectiveness of the assembly on Si (100), the ability to fabricate enclosed Si (100)-based molecular devices, and the potential for the future integration of these devices with more conventional technologies.