Sandwich conformation

The results obtained have suggested a sandwich conformation of the host-guest complex in which DABCO was protected from the solvent and other molecules. The cross-peaks between coordinated DABCO and complex protons in the 2D ROESY spectra have indicated the accommodation of the DABCO molecule in the host cavity and confirmed the sandwich binding mode. 

Sanders and coworkers have prepared an interesting molecular assembly 32 in which the central porphyrin moiety is held in a sandwich conformation between the two pyromellitimide acceptors by coordination of its pyridyl groups with the zinc ions of the two peripheral porphyrin moieties [86]. Steady state fluorescence studies revealed that the fluorescence of the central porphyrin was quenched by at least 90% relative to the unbound material. This was interpreted in terms of electron transfer to the pyromellitimide moieties. Of course, energy transfer from the metallated porphyrins to the central free base porphyrin might also be expected in this molecule. 

The nuances in which details of nomenclature become important occur in those molecules in which not all of the atoms or bonds of a ring are connected to the "spiro" metal atom. For example, returning the focus to Figure 9, Margl, Schwarz and Bloechl [11] advise "Experimental studies find that beryllocene prefers a so-called "slip sandwich" conformation of Cs symmetry in the crystal as well as the gas phase i.e., Be has one t - and one -coordinated ring."... 

Steric effects have also been shown to be important in intramolecular exciplex formation (Pragst et al., 1978). It was found that the intramolecular exciplex formed by 3-(N-methyl-N-p-tolyl) amino-l-(9-anthraceno)propane had a dipole moment of 12.2 D whereas 3-(N-p-tolyl)amino-l-(9-anthraceno)-propane gave an exciplex having a dipole moment of 15 D. It was suggested that in the former compound the exciplex deviates from the sandwich conformation and consequently has less charge-transfer character. 

Conformational requirements for fluorescent exciplex formation It was stated earlier that the fluorescent exciplex of [36 = 2] has a sandwich conformation. This statement was based on earlier studies of exciplex formation of naphthylalkylamines (Chandross and Thomas, 1971 Brimage and Davidson, 1971). It was found that when a dimethylene and trimethylene chain was interposed between the amino and hydrocarbon groups that... 

Electronic excitation of either the donor or acceptor of the dyad generates two sets of locally excited states. In principle, the population of the folded set of excited states may be increased but in practice, this likelihood becomes negligible for long chain lengths n > 6) since competing ET and decay processes occur more rapidly than the rate of production of a sandwich conformation from an extended one, which would require rotation about several C-C bonds to achieve. 

X-ray analysis of the crystalline 2 2 complex of beauvericin with Ba++ has revealed the interesting structure shown in Fig. 32118 The two Ba++ ions in the center of the complex are held together by three picrate anions which contribute to the coordination. Each of the two beauvericin molecules contributes three chelating amide oxygens. The resulting structure has much in common with the sandwich conformations shown in Fig. 30. 

It seems reasonable to assume that the dimeric benzene ion has a sandwich conformation. A similar structure has been invoked to explain excimer spectra from neutral benzene molecules in solution where stable dimers are observed (27). Also Howarth and Fraenkel (II) have suggested that the two hydrocarbon moieties of the dimeric ion of naphthalene lie in parallel planes. Ishitani and Nagakura (12) have suggested a sandwich structure for paracyclophane anions but they also consider the possibility of a diphenyl type of structure. 

Oscilloscope traces of photocurrent transients recorded with a 120 ym thick PTS crystal in sandwich conformation. The crystal had been prepared by one side solvent etching of a native TS-6 monomer crystal and subsequent thermal polymerization. Photocurrents were excited through a semitransparent Ag-contact by 1.06 ym pulses of a Nd-laser at a dose of about 10 photons cm 2. Carrier generation has... 

X-ray determination of the structure of the latter confirmed the monomeric formulation, the bent sandwich conformation and the presence of a o-bonded alkyl group. The Th-C O-bond distance is 2.54(1)A. An agostic interaction Th-H of 2.71(9)A is also present (fig. 44). The reaction of the alkyl complex with H2 in alkane solvent produces an oligomeric hydride [COTCp ThH]. ... 

Due to the steric requirement of these substituents the formation of a columnar structure with infinite M M interactions is inhibited, and only the association of pairs of molecular units is allowed. The Ni Ni distance is 3.21 A [164]. If the same compound is crystallized in the presence of benzimidazole, the [Ni(dmg-BF2)2]2 dimer units are sandwiched between sheets of benzimidazole molecules due to n-n interactions resulting in an increased Ni Ni separation of 3.358 A [165]. With anthracene the n-n interactions seem to be stronger, because in this case the parent dimer molecule is cleaved. Each monomer now has a conformation of type B (Fig. 32) and is sandwiched by anthracene molecules [166]. Compound 121 has the same configuration [163d]. 

The molecular orbital interactions are almost independent of the conformation of the sandwich complex with respect to rotation of the Cp rings. Both the staggered (Dsd) as well as the eclipsed conformation (Dsh) possess similar binding energies... 

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

P2j Z = 2 DX = 1.43 R = 0.067 for 1269 intensities. The uracil residue is in the anti (63.4°) disposition. The conformation of the D-ribosyl group is 2T3 (176.8°, 37.5°). The orientation about the exocyclic, C-4 -C-5 bond is t (—174.2°). The phenyl and uracil ringsofthe same molecule lie in almost parallel planes, 120 pm apart. The phenyl group is disordered. The uracil ring is sandwiched by the phenyl rings, and vice versa. The 0-1 and N-a atoms of the peptide backbone are hydrogen-bonded to 0-4 and N-3 of atranslationally related uracil to form cyclic dimers. Such interactions serve as models for nucleic acid-protein interactions. [Coordinate errors H(02 ) x should be —1574, instead of —1474 H(Na)2 z should be —145 instead of— 645.]... 

This model of the receptor-bound conformation of TIP is characterized by a clustered configuration of the three aromatic moieties with the Phe3 aromatic ring sandwiched between the Tyr1 and Tic2 aromatic rings. 

In the Group XIV compounds the two rings are not parallel, due to the influence of the metal lone pair, giving rise to a bent-sandwich molecular structure142 (C2V symmetry), different from that in other stable cyclopentadienyl derivatives, such as Fe(Cp)2, where the two rings are parallel (D5h symmetry in the eclipsed conformation and D5d symmetry in the staggered one). 

OLED on an Al-PET substrate. The device performance does not deteriorate after repeated bending, suggesting that there is no significant stress-induced change in the characteristics of the OLEDs fabricated on PET foil [71,82]. The results demonstrate the feasibility of fabricating flexible displays using a variety of plastic substrates including metal-laminated plastic foils, or a metal film sandwiched between two plastic foils. The flexible device structures enable a display to conform, bend, or roll into any shape and thus make possible other product concepts. 

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