Non-planar Hydrocarbons Research Articles

Atomic pseudopotentials for reproducing π‐orbital electron behavior in sp2 carbon atoms

AbstractA pseudopotential system for an sp2 carbon atom is built and tested as a building block for various pseudohydrocarbon polyenes and polycyclic aromatic hydrocarbons. This pseudosystem has a central charge of Z = 1; it contains only one electron. It is employed in ab‐initio calculations in which several physical characteristics including the orbital energies and first ionization energy, as well as first excitation energy and UV spectra, are found to be well‐reproduced by the pseudosystem. Remarkably, not only are the π excitation energies in good agreement with the reference calculations, but also transition densities and intensities, confirming that the virtual space obtained with the pseudopotentials is of excellent quality. Finally, this approach is capable of reproducing the π electron systems of small or large, planar or nonplanar hydrocarbons at low computational cost.

Centrohexaindane: six benzene rings mutually fixed in three dimensions - solid-state structure and six-fold nitration.

The solid-state molecular structure of centrohexaindane (), a unique hydrocarbon comprising six benzene rings clamped to each other in three dimensions around a neopentane core, and the molecular packing in crystals of ·CHCl3 are reported. The molecular Td-symmetry and the Cartesian orientation of the six indane wings of in the solid state have been confirmed. The course and limitation of electrophilic aromatic substitution of are demonstrated for the case of nitration. Based on nitration experiments of a lower congener of , tribenzotriquinacene , the six-fold nitrofunctionalisation of has been achieved in excellent yield, giving four constitutional isomers, two nonsymmetrical ( and ) and two C3-symmetrical ones ( and ), all of which contain one single nitro group in each of the six benzene rings. The relative yields of the four isomers (∼3 : 1 : 1 : 3) point to a random electrophilic attack of the electrophiles at the twelve formally equivalent outer positions of the aromatic periphery of , suggesting electronic independence of its six aromatic π-electron systems. In turn, the pronounced conformational rigidity of the centrohexacyclic framework of enables the unequivocal structural identification of the isomeric hexanitrocentrohexaindanes by (1)H NMR spectroscopy.

Functionalized aromatics aligned with the three Cartesian axes: Extension of centropolyindane chemistry

The unique geometrical features and structural potential of the centropolyindanes, a complete family of novel, 3D polycyclic aromatic hydrocarbons, are discussed with respect to the inherent orthogonality of their arene units. Thus, the largest member of the family, centrohexaindane, a topologically nonplanar hydrocarbon, is presented as a "Cartesian hexabenzene", because each of its six benzene units is stretched into one of the six directions of the Cartesian space. This feature is discussed on the basis of the X-ray crystal structures of centrohexaindane and two lower members of the centropolyindane family, viz. the parent tribenzotriquinacenes. Recent progress in multiple functionalization and extension of the indane wings of selected centropolyindanes is reported, including several highly efficient six- and eight-fold C-C cross-coupling reactions. Some particular centropolyindane derivatives are presented, such as the first twelve-fold functionalized centrohexaindane and a tribenzotriquinacene bearing three mutually orthogonal phenanthroline groupings at its molecular periphery. Challenges to further extend the arene peripheries of the tribenzotriquinacenes and fenestrindanes to give, eventually, graphite cuttings bearing a central bowl- or saddle-shaped center are outlined, as is the hypothetical generation of a "giant" nanocube consisting of eight covalently bound tribenzotriquinacene units. Along these lines, our recent discovery of a related, solid-state supramolecular cube, containing eight molecules of a particular tribromotrinitrotribenzotriquinacene of the same absolute configuration, is presented for the first time.

Polycyclic Aromatic Hydrocarbons: Chemistry of DNA Adduct Formation

Polycyclic aromatic hydrocarbon carcinogens are usually activated for DNA binding by the metabolic formation of bay region dihydrodiol epoxides with R,S,S,R stereochemistry. Such metabolites from planar hydrocarbons reacted preferentially with the amino groups of deoxyguanosine residues, whereas those from nonplanar hydrocarbons were more efficiently trapped by DNA and reacted preferentially with the amino groups of deoxyadenosine residues, in some cases. Molecular modeling and NMR measurements indicated that the conformations of DNA adducts depend upon the hydrocarbon involved and the cis or trans opening of the epoxide ring during adduct formation. The structural characterization of carcinogen-DNA adducts and investigations of relationships between specific adducts and biological effects represent an important background that can be valuable in molecular epidemiological approaches.

On the nature of the “cycloheptatrienyl trianion”: The decisive importance of counterion stabilization in polymetallated “carbanion” systems

Like all other “polycarbanions”, the planar (D 7h) cycloheptatrienyl “trianion” is much too high in energy to exist as a free entity; instead, stabilization by three lithium counterions results in a non-planar hydrocarbon skeleton and a favorable energy.

ChemInform Abstract: PHOTOELECTRON SPECTRA OF POLYNUCLEAR AROMATICS. V. CORRELATIONS WITH ULTRAVIOLET ABSORPTION SPECTRA IN THE CATACONDENSED SERIES

IP’s derived from the high‐resolution PE spectra of 61 catacondensed hydrocarbons are presented, most of them for the first time, and correlated with the energies of the four main singlet–singlet transitions. For the p and β′ bands (1La and 1Ba) a linear correlation with IP1 and IP2, respectively, is established. The α and β band energies (1Lb and 1Bb) correlate linearly with the mean of IP1 and IP2. The regressions encompass planar as well as nonplanar hydrocarbons of the helicene type; standard deviations range from 0.070 to 0.126 eV. Comparison with Huckel and PPP theory shows the empirical correlations to be superior; their potential in disentangling diffuse uv spectra is demonstrated. Many β′ bands are assigned for the first time, and estimates are given for the position of the α band in anthracene, tetracene, and pentacene. From the slopes and intercepts of the regressions, important conclusions regarding the wavefunctions of the excited states and the role of electron repulsion and CI can be drawn....

Photoelectron spectra of polynuclear aromatics. V. Correlations with ultraviolet absorption spectra in the catacondensed series

IP’s derived from the high-resolution PE spectra of 61 catacondensed hydrocarbons are presented, most of them for the first time, and correlated with the energies of the four main singlet–singlet transitions. For the p and β′ bands (1La and 1Ba) a linear correlation with IP1 and IP2, respectively, is established. The α and β band energies (1Lb and 1Bb) correlate linearly with the mean of IP1 and IP2. The regressions encompass planar as well as nonplanar hydrocarbons of the helicene type; standard deviations range from 0.070 to 0.126 eV. Comparison with Hückel and PPP theory shows the empirical correlations to be superior; their potential in disentangling diffuse uv spectra is demonstrated. Many β′ bands are assigned for the first time, and estimates are given for the position of the α band in anthracene, tetracene, and pentacene. From the slopes and intercepts of the regressions, important conclusions regarding the wavefunctions of the excited states and the role of electron repulsion and CI can be drawn. In some hydrocarbons the energies of the four uv transitions are shown to correlate with IP1 and IP3 rather than with IP1 and IP2. An analysis of this finding in terms of perimeter orbitals is given, and a novel classification of PE bands based on vibrational fine structure and correlation with perimeter states is presented. Correlations between the IP’s and chemical reactivity are pointed out.

Photoelectron spectra of polynuclear aromatics. III. The effect of nonplanarity in sterically overcrowded aromatic hydrocarbons

The high-resolution photoelectron spectra of 65 polycyclic aromatic hydrocarbons containing up to 11 condensed benzene rings have been obtained in the gas phase at temperatures between 20 and 450°C. Vibrational analysis of the spectra coupled with semiempirical MO calculations permitted positive identification of the π ionizations in the 6–11 eV range. Irrespective of size and symmetry of the hydrocarbons, two-thirds of all π bands are found to precede the onset of σ ionization, which occurs at 10.6±0.2 eV in the molecules beyond naphthalene. The possibility of Jahn-Teller forces operating in the degenerate ionic ground states of D3h and D6h molecules is discussed, and comments are made on the origin of the 1350 cm−1 vibrational progression that prevails in the first band of all planar hydrocarbons. In sterically overcrowded systems, excitation of low-frequency twisting and out-of-plane bending modes results in a blurring of the vibrational structure. In cases where reliable x-ray data are unavailable (e.g., dibiphenylene-ethylene), this observation allows a distinction to be made between planar and nonplanar hydrocarbons. The present data are of sufficiently high quality to permit the question as to the planarity of phenanthrene to be settled with some certainty.