Rotation phenyl rings

A variable-temperature NMR spectroscopic study of the titanium(IV) complex 43 also indicated free rotation of the five-membered rings, but, as in the ferrocene derivative 38 allowed the determination of the activation barrier for the phenyl ring rotation (AG (-90 °C) = 9.8 0.5 kcal mol1). 

Figure 2. Ground state and excited state energy profiles for phenyl ring rotation (energy vs. dihedral angle, 9) in a) biphenyl, b) 2,2, 6,6 -tetrasubstituted biphenyl (33).

The lowest barrier rotation pathways for the phenyl ring rotation occurs for the synchronous motion of the two phenyl rings. Two energy barriers are encountered at rotations of 90° and 270° relative to the minimum-energy conformation, corresponding to an energy value of 42 kj mol-1. It is worth noting that this value is very close to that found in DPP, 42 kj mol-1. 

Cooperativity of phenyl ring rotation with carbonate conformation change. 

Intermolecular cooperativity associated with the phenyl ring rotation, which concerns groups as far as 7 A from the moving phenyl ring, as well as a rearrangement of all the units within a volume of about 1 nm. ... 

Phenyl rings rotated 45° about CoHs—C bond. 

Fig. 14. ZINDO-derived /3EFish for [Ru(CsCC6H4-4-N02)(PMe3)2(7 5-CsH5)] showing (a) the effect of Ru-C bond length variation and (b) the effect of acetylide phenyl ring rotation. (Adapted with permission from I. R. Whittall, M. G. Humphrey, D. C. R. Hockless, B. W. Skelton, and A. H. White, Organometallics 1995,14,3978. Copyright 1995 American Chemical Society.)...

It was suggested by Gegiou et al. that the photoisomerization of azobenzene probably proceeds via a mechanism different than the cis-trans isomerization of stilbene. Such an isomerization process in azobenzene could involve a pyramidal inversion of a nitrogen atom, in contrast to stilbene and its derivatives, where rotation about the central double bond is required. Based on the data, the AT->AC isomerization mechanism may be similar to that of the triphenylmethane dyes, whose ground state structure is known to resemble a three-dimensional, propeller-shaped D3 structure with the phenyl rings rotated 32 degrees from the central plane. 

This effect, however, is dynamic. For example, as the phenyl ring rotates out of conjugation with the amide the torsional barrier about the amide bond returns to full strength. Thus the effective barrier to rotation about a conjugating bond is dependent on ... 

At room temperature, the resonances of the ortho phenyl protons are broad for Ga(TPP)Cl, and split into a doublet for Ga(TPP)F. This reflects the well-known phenomenon of the restricted rotation of phenyl rings [12-17]. Even if the rate of phenyl ring rotation is not very well established, it is clear that the chemical shift difference is higher, when the axial ligand is the more electronegative halide (X = F). It thus appears tiiat the metal is not coplanar with the porphyrin. The above assumptions were confirmed by the x-ray crystal structure of Ga(TPP)Cl [7]. 

A typical example of a partial motional averaging of the chemical shift anisotropy is that of an unprotonated aromatic carbon belonging to a para-substituted phenyl ring rotating about its local symmetry axis. For such a... 

In-phase motions of the phenyl rings attached to the same isopropylidene unit, observed in both isolated molecule (DPP) and bulk BPA-PC. Cooperativity of phenyl ring rotation with carbonate conformation change. 

Note that, while the free hydrazine has a planar structure with E-configuration with respect to the N-N bond, the coordinated ligand adopts the Z-conliguration with the phenyl ring rotated by 90° with respect to the hydrazone fragment ... 

K, which was assigned to rotational motion of phenyl rings aroimd the bond connected to the backbone chain. This temperature for the y-relaxation is very close to the onset temperature of the fast motion in PS. It can, therefore, be considered that the change of the spectra from inelastic-like to quasielastic-like far below Tg is caused by the relaxation motion of the phenyl ring rotation. This idea has been directly confirmed in an inelastic neutron scattering experiment on PS with deuterated phenyl group [94]. 

Internal and overall motions that take place on a dynamic scale from 10 to 10 s can affect the line shape of NMR powder spectra. This is often the case for phenyl ring rotations or chain isomeriza-tions in polymeric liquid crystals and for rotational diffusion in discotic mesophases. Line shape analysis allows different types of motions to be discriminated and the relative kinetic parameters to be obtained. Moreover multidimensional exchange NMR can be applied to characterize molecular details of dynamical processes when the correlation times range between 10 and 10 s. 

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