Aromatic molecules, crystals

It is this resonance energy that would be in the main responsible for the difference in energy of the crystal and the gas of diatomic molecules Li2. But the heat of formation of Li2 molecules from atoms is only 6-6 kcal./g.-atom, whereas that of the metal is 39kcal./g.-atom. It seems unlikely, by comparison for example with the analogous case of Kekule-like resonance in aromatic molecules, that the great difference, 32-4 kcal./g.-atom, could result from the synchronized resonance, of type f Li—Li Li Li)... 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

Most well-studied peroxidases are designed to oxidize small aromatic molecules, with the exception of cytochrome c peroxidase. It generally is thought that such aromatic molecules bind near the heme edge where an electron can transfer directly to the heme edge (44), which is supported by both crystal structures (45, 46) and NMR studies (47). However, recent work suggests that some physiologically important substrates may utilize other sites on the enzyme surface (48, 49). 

The adduct formed by two lithium atoms with polycondensed aromatic hydrocarbons crystallizes with two solvating molecules of TMEDA. The structure of the crystals derived from naphthalene (73) and anthracene (74) was elucidated by XRD. This arrangement of the unsolvated lithium atoms, in 7 -coordination fashion on the opposite sides of two contiguous rings, was found by MNDO calculations to be the most favorable one for naphthalene, anthracene and phenanthrene (75) . 

It is interesting that benzene and naphthalene form monolayer surface structures on the Pt(l 11) crystal face at 300 K and higher temperatures while monolayer surface structures form only at low temperatures ( 200 K) on the Ag(l 11) crystal face While these aromatic molecules are held by strong chemical bonds to the platinum, their heats of adsorption must not be greater than the heats of sublimation... 

The organic single crystals considered here for hole injection are composed of aromatic molecules with the following structural formulas ... 

The only purine-aromatic complex crystal structure published thus far is the tetramethyluric acid-pyrene structure (6). The orientation of the molecules in this complex is shown below in VIII. 

The distances between saturated hydrocarbon molecules in crystals can be calculated by the use of these radii, with consideration also of the possibility of molecular or group rotation. Another factor must be introduced for aromatic molecules.64 The double bonds in these molecules project above and below the plane of the ring in such a way as to give to the ring an effective thickness of about 3.4 A, as observed in anthracene, durene, hexamethylbenzene, benzbisantkrene, and many other aromatic hydrocarbons. The same value is also found between the layer of graphite. 

Some diamagnetic crystals (graphite, bismuth, naphthalene and other aromatic substances) show prohounced diamagnetic anisotropy. The observed anisotropy of crystals of benzene derivatives correspond to the molar diamagnetic susceptibility —54 X 10 with the field direction perpendicular to the plane of the benzene ring and —37 X lO"6 with it in the plane. This molecular anisotropy has been found to be of some use in determining the orientation of the planes of aromatic molecules in crystals.1... 

Consider, now, radiationless transitions in pure molecular crystals of aromatic molecules. At the very outset we must realize that crystal field effects may lead to the inversion of the order of the triplet and singlet exciton levels relative to the ordering of the corresponding molecular states.12 The Davydov tight binding formulation of exciton theory leads to the following representation for the manifold of optically accessible (k = 0) energy levels in a pure molecular crystal 138... 

The compounds M(NH3)2Ni(CN)4 (M = Zn or Cd), which consist of two-dimensional polymeric sheets of tetracyanonickelate ions bridged by coordinating diamminemetal(II) cations, function as host lattices for clathration of small aromatic molecules such as thiophene, furan, pyrrole or pyridine IR studies indicate the presence of hydrogen bonding between the host lattice ammonia and the aromatic guest molecules.132,133 A crystal structure determination of the related clathrate Cd(en)Ni(CN)4(pyrrole)2 has been reported.134 Similarly, the complex Cd(py)2Ni(CN)4 consists of polymeric [Cd—Ni(CN)4] layers held together by Cd-bound pyridine.135... 

A review of recent research, as well as new results, are presented on transition metal oxide clusters, surfaces, and crystals. Quantum-chemical calculations of clusters of first row transition metal oxides have been made to evaluate the accuracy of ab initio and density functional calculations. Adsorbates on metal oxide surfaces have been studied with both ab initio and semi-empirical methods, and results are presented for the bonding and electronic interactions of large organic adsorbates, e.g. aromatic molecules, on Ti02 and ZnO. Defects and intercalation, notably of H, Li, and Na in TiC>2 have been investigated theoretically. Comparisons with experiments are made throughout to validate the calculations. Finally, the role of quantum-chemical calculations in the study of metal oxide based photoelectrochemical devices, such as dye-sensitized solar cells and electrochromic displays, is discussed. 

Compound 311 formed stoichiometric co-crystals with perfuorinated aromatic molecules, hexafluorobenzene (BzF) and octafluoronaphthalene (NpF), by intermolecular Ar-ArF interactions, and 311 showed photochromic reactivity in the co-crystals as well as in the one-component crystal of 311. 

In the X-ray analysis of a crystal structure the first step is the determination of the space group and the number of molecules in the unit cell. Occasionally it may be immediately apparent from such data that the molecule itself possesses certain elements of symmetry, and these may define or at least limit the possible molecular conformations. The elements most commonly found in aromatic molecules are centres of symmetry and twofold rotation axes. It might have been expected that the plane of symmetry would manifest itself in aromatic systems, but this is disappointingly rare. Indeed, amongst the structures reviewed here the only cases where a crystallographic symmetry plane coincides with that of a planar molecule occur in s-triazine and pyrazine (see Section V, A, 5). 

A fluorescent cationic tetranuclear gold(I) rectangle, [(/v-Ph2PAnPPh2)Au2 (//-4,4,-bpy)2Au2(/i.-Ph2PAnr,Ph2)]X4 (X = PF6, NO3), was assembled using 9,10-bis(diphenylphosphino)anthracene and 4,4/-bipyridyl [124]. The molecular rectangle has a cavity of 7.921(3) x 16.76(3) A as reflected from its crystal structure, and its complexation behavior towards various aromatic molecules at the cavity was demonstrated. 

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