Twist mechanisms

Pt and 119Sn NMR data show Pt(SnCl3)5 to be non-rigid (on the NMR timescale) down to 183 K, owing to an intramolecular process, possibly a Berry twist mechanism [132],... 

FIGURE 20.11 The Bailar top) and Ray-Dutt (bottom) trigonal twist mechanisms. 

If forming the transition state by a twist mechanism leads to some expansion of the lattice, it would be expected that a higher lattice energy would hinder racemization. In a very detailed study (Kutal and Bailar, 1972) of the racemization of ( + )-[Co(en)3]X3-nH20 (where X = Cl-, Br, I-, or SCN ), it was found that the rate varied with the nature of the anion. The rate was found to decrease in the order... 

Bailar, J. C. Jr. (1958). /. Inorg. Nucl. Chem. 8, 165. The paper describing trigonal twist mechanisms for racemization. 

Fig. 7. Structures of five-coordinate Cu2+ from first principles molecular dynamics. A Berry twist mechanism for the interconversion of the two structures is shown (from left to right) the reorientation of the main axis of a square pyramidal configuration by pseudo-rotations via a trigonal bipyramidal configuration. The grey atoms in the plane of the trigonal bipyramid are all candidates for becoming apical atoms in a square pyramid.

It is useful to note that the same issue does not arise in complexes that have a horizontal symmetry plane. There, a wedge geometry allows L and hj to attain equivalent positions above and below that plane. These conceptual issues have been addressed in the case of oxorhenium(V) complexes by two experimental studies, each of which supports intervention of intermediate(s) that undergo turnstile or trigonal twist mechanisms. In so doing, L and L7 attain equivalent or at least interchangeable positions. These studies are the subjects of the next two sections. 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

It has not been easy to use AV values to support a racemization mechanism, nor indeed to rationalize AK in terms of known mechanisms. For racemization, AV° is zero and to a first approximation AV might also be expected to be near zero for a twist mechanism. For both Cr(phen)3 and Nifphenlj however AV values are near zero, although the... 

Linkage isomerization cannot occur by a twist mechanism. 

Mechanisms leading to geometrical isomerization in complexes of this type resemble those already discussed with M(AA,)3 and M(Li)(L2), Secs. 7.6.2 and 7.6.3. As well as twist mechanisms, dissociation of either the unidentate ligand or one-ended dissociation of the... 

The photoprocess involves intramolecular twist mechanism without bond cleavage. The reactive intermediate is a triplet state of pseudo-tetrahedral geometry. The transformation is photochemically allowed but thermally disallowed from symmetry considerations. 

The Cotton effects in mixed amino acidate/acetylacetonate complexes [Cr(acac)2L] (L = l-alanine, L-valine or L-phenylalanine) have been studied 774 absolute configurations were assigned by reference to the parent tris(acetylacetonate) complexes. Synthesis was achieved by the photolysis of mixtures of the amino acid and [Cr(acac)3]. The partial photoresolution of both cis- and irans-(l,l,l-trifluoro-2,4-pentanedionato)chromium has been accomplished by irradiation with circularly polarized light (5461 A) in chlorobenzene solution.775 The results indicated that both bond rupture and twist mechanisms were important. A number of other jS-diketonates have also been investigated.776... 

Figure 74 (a) The Berry-twist mechanism connecting a trigonal bipyramidal and square pyramidal geometry (b) e modes of vibration (c) correlation of one-electron energy levels... 

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