Block rotation

Mesophase with a helicoidal supramolecular structure of blocks of molecules with a local smectic C structure. The layer normal to the blocks rotates on a cone to create a helix-like director in the smectic C. The blocks are separated by plane boundaries perpendicular to the helical axis. At the boundary, the smectic order disappears but the nematic order is maintained. In the blocks the director rotates from one boundary to the other to allow the rotation of the blocks without any discontinuity in the thermomolecular orientation. 

Alternatively, four-circle diffractometer can have a k (kappa) geometry, in which the / circle is replaced by two arcs, one of which ( > block ) rotates around the vertical > axis and the other ( k block ) around the k shaft, mounted on the (o block (Figure 12). The k and co axes always form the magic angle a = 54°. Note that in either geometry, every... 

Normally, an H2 ligand rotates too fast about the M-(H2) bond for separate rotamers to be observable. In d complexes, however, where only one d orbital is available, back donation is only possible in one plane but not at right angles to that plane. This means that the H2 loses all the back donation stabilization when it has rotated 90° from its preferred conformation. This in turn provides a significant rotation barrier leading to the phenomenon of blocked rotation, detected by Chaudret and Sabo-Etiemie in compounds of the type [Cp2TaH2L]+ (3), where barriers of up to 11 kcalmoR were found. 

Goscombe B. D. and Everard J.-L. (2001) Tectonic evolution of Macquarie Island extensional structures and block rotations in oceanic crust. J. Struct. Geol. 23, 639—673. 

Whereas three or more or//io-chlorines block rotation about the ring-connecting bond, congeners with none, or only one, or//io-chlorine can relatively easily achieve a planar configuration (colloquially referred to as coplanars ) and may behave as isosteres (compounds with similar shape, functionality, and polarity) to 2,3,7,8 — tetrachlorodibenzodioxin (TCDD). These bind significantly to the dioxin receptor, and meas-... 

A unique case of QEC involving only a H2 ligand is known. The complexes [CpiM(H2)(L)]+ (M = Nb, ) show blocked rotation oftheH2 ligand in the NMR... 

The rigid blocks rotate in a field that was described by Markis Roussus (2000). The panorama of the city of Alger after earthquake shocks in May 2003 had shown that conserved houses attached to blocks had random residual inclination and this approved the rotation wave action at a ground surface. 

The relative block rotation releases radon from mineral lattices at fresh wear of solid surfaces. Radon is observed in surface waters and plays the role of a tectonic events indicator, see Nikolaevskiy (19%, 1998). Subsequent radon appearance at large distances corresponds to a wave with the velocity 10 km/day measured at ground surface. 

Deronzier and Latour ° have extended their study of autoxidation of Ti porphyrins to include the influence of axially coordinated tdt. In Ti (tdt)(tpp), where ttp = tetraphenylporphyrin, the dithiolene ligand experiences the strong porphyrin ring currents as indicated by the upfield shift of tdt ortho and meta protons to 5.14 and 5.83 p.p.m., respectively. Lowering the temperature below 200 K was sufficient in a peroxo complex, Ti (02)(tpp), to block rotation of the axial ligand. However, rotation of tdt around the C2 axis (ignoring the Me on tdt) was still too rapid on the NMR timescale even at 180 K. Although the Ti d y orbital can interact with the tdt S atoms in a manner similar to the peroxo complex, the orbital overlap is much weaker and the resultant barrier to rotation is lower. 

More than 60 examples of diversely substituted atropisomeric scaffolds were fully described. When presented a fixed chirality, the blocked rotation around the N-aryl bond yielded diastereomers. Table 5 reports selected examples of all the combinations of blocking substituents in R, R, and R. ... 

In phosphorus-stabilized carbanions, interaction of the n orbital on the car-banionic center with the o orbital may increase the barrier to rotation about the C -P bond [37],but this interaction has been demonstrated to block rotation insufficiently in hthiated thiophosphonamides, even at low temperature (Fig. 12). It was also observed that HMPA stabilizes the carbanion in lithiated thiophosphonamides by dissociation of the lithium ion, decreasing the rate of rotation about the C-P bond [38]. 

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In the gaseous state, coupling of the vibrational degrees of freedom with the rotational ones produce rotational-vibrational bands, characteristic of the shape of the molecule and of its symmetry. If the molecule is made of n atoms, the number of dynamical (vibrational or mechanical) degrees of freedom is equal to 3n. Among them, 3n — 6 may be optically active because the three block translations and the three block rotations of the molecule do not contribute to the vibrational or rotational energy of the system. 

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