Twist structure

Among the possible CC bond conformers for 2,2 -dichlorobiphenyl (DCBP) are syn and anti planar structures and two twist structures. 

In summary we think that, on a superficial basis, a comparison of the effects of different nucleophilic species added covalently at the (3-nitrogen atom of an arenedi-azonium ion yields results that are almost trivial. Of more interest are unexpected results such as those of Exner and Lakomy for the substituent -N = CHC6H5. A possible explanation for the latter results emerged when the twisted structure of the substituent became known. We emphasize, however, that definitive explanations on the basis of Hammett or related substituent constants are not found very frequently. 

In polysulfide solutions there exist chainlike dianions with practically arbitrary chain lengths n. Berghof, 8ommerfeld, and Cederbaum have investigated the onset of stabihty in the 8 (n=2-8) series of dianions based on 8CF calculations with the DZPD basis set [89]. The isolated chainlike 8 isomers were found to exhibit twisted structures, and the onset of electronic stabihty (with respect to electron auto detachment) was predicted to occur at 71=7. Branched isomers were found to be electronically more stable than the... 

Stacked together along the a-axis. The long molecular axes are tilted by 13° within the layers. By Raman spectral measurements Tashiro et al. found that the biphenyl group of the compound has a twisted structure in the liquid crystalline state as well as in the a-form. 

The picosecond TR experiments described above for BR reveal that a hot unrelaxed J intermediate with a highly twisted structure forms and then vibrationally cools and conformationally relaxes within 3ps to form the K intermediate. Subsequently, an isomerization induced protein conformational change takes place during 20-100 ps to produce the KL inermediate. ... 

A gas-phase electron diffraction study95 has shown that cis -hexatriene has a slightly twisted structure (torsional angle of 10° around the central C=C bond) and the lengths of the terminal C=C, central C=C and C—C bonds are 1.336, 1.362 and... 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

On the other hand, cellular membranes are composed of chiral molecules such as phospholipids and cholesterol, but the homochirality of these constituents is not obviously manifested in the membrane s structure. However, in certain cases biological membranes exhibit a distinct helical or twisted structure, which is a very conspicuous sign of the chirality of the supramolec-ular aggregate. These chiral supramolecular aggregates are the subject of this chapter. 

The reason for the lower liquid crystalline (LC) temperature of the BB model compound seems to be that the biphenyl unit of this compound was adopting a twisted structure in the LC state [26], Therefore, we prepared various polyarylates containing the BB unit and determined their thermal properties and the moduli of the as-spun fibers, as shown in Table 19.3. 

The interest in twisted double bonds has also generated a considerable activity in the theoretical field, and interesting stable twisted structures have been proposed, which, however, still await experimental confirmation. 

A remarkable temperature dependence of the UV-vis spectra was observed for tetrasilyldigermene 68 in solution (red at room temperature, yellow at 77 K)61 Mechanistic studies have shown that this phenomenon is due to an equilibrium between two different conformations around the Ge=Ge bond, for example, the bent structure and a twisted structure. 

Note 4 The director precession in a chiral nematic mesophase is spontaneous and should be distinguished from an induced twisted structure produced by a mechanical twist of a nematic mesophase between confining surfaces. 

Figure 1.5. View of the Mobius stabilized annulene. Crystallographic data from Ajami, 2003. The tetradehydrodianthracene and twisted structures are shown left and right of the figure, respectively. C and H are represented by black and light grey balls, respectively.

Structure for this compound. However, H-NMR experiments carried out at various temperatures and NOE measurements show that this picture is too simple the cavity of [Pd(24)]Cl2 is unstable and collapses via a twisting motion of the arms and the imidazolyl groups (Fig. 14). At room temperature a fast equilibrium exists between a left-handed and a right-handed twisted complex. At lower temperatures the dynamic equilibrium slows down, and at — 95 °C two separate imidazolyl proton signals can be observed in the H-NMR spectrum due to non-equivalence of the imidazolyl groups in a helically twisted structure. 

Ni(II) complex (Fig. 18) reveals that the metalloclip has a twisted structure. Two of the methoxy groups are pointing into the cavity, in this way blocking the access of a guest molecule. As a consequence, resorcinol is not bound in the cleft of 31a. 

Quantum mechanical calculations show that the silyl cation (19) has a twisted structure, and that the a-C02 group provides substantial electrostatic stabilization.58 Isotope effects for its formation reaction are also reported.58 Evidence is provided for the stabilization of incipient oxocarbenium ions by axial electronegative substituents, as in (20) the presence of the most electronegative substituent results in the fastest reaction.59 Lewis acid-promoted cleavage of spirocyclic dioxanes such as (21) involves oxonium ions, and high axial vs equatorial product selectivities are possible with the correct choice of Lewis acid and nucleophile.60 Reactions which lead to 1,3-dioxenium salts have been reviewed.61... 

There are two different kinds of sources of molecular chirality central chirality and axial chirality (Fig. 1). Central chirality is due to the existence of chiral carbon, whereas axial chirality originates from twisted structures of molecules, between which a sufficiently high energy barrier exists, preventing the chiral conformational interconversion in ambient conditions. Surprisingly, however, the introduction of nonchiral molecules to chiral liquid crystalline environments sometimes enhances the chirality of the systems [3-5]. This means that inherently nonchiral molecules act as chiral molecules in chiral environments. This occurs in the following way. Molecules with axial chirality behave as nonchiral molecules when the potential barrier is low enough for chiral conformational interconversion. But when such... 

Ylanne, J., Scheffzek, K., Young, P., and Saraste, M. (2001a). Crystal structure of the alpha-actinin rod reveals an extensive torsional twist. Structure 9, 597-604. 

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