All-trans rule

Figure 8. (a) The all-trans rule of TB coupling The magnitude of the interaction, A (it), is maximized for an all-trans alignment of sigma bonds and becomes progressively smaller with increasing number of clsold conformations. 

Fig. 11 The all-trans rule of through-bond °f decreases with...

In summary, orientation effects on ET rates are important, amounting to modulation of rates by factors as large as 20. They are certainly as important as bridge configuration effects (i.e., the all-trans rule) and orbital symmetry effects in influ-... 

The importance of specific orbital alignments in the TB propagation of remote stereoelectronic influences can be assessed directly by structure-sensitive spectroscopic techniques. The results of many such studies have been summarized as various effects or rules that express the dependence on geometrical factors, such as the all-trans zig-zag or W pattern of skeletal bridge bonds. Such a W-effect 105... 

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

It was mentioned above (see Sect. 2) that for centrosymmetic molecules, such as all-trans polyenes and a,c<>-disubstituted frans-polyenes, there is a complementarity between the selection rules for IP- and 2P-allowed transitions. Transitions from the groimd state (which is of gerade type) to a B state are visible in the IPA spectriun, while transitions from the ground state to an Ag state appear in the 2PA spectrum. For this reason, 2P spectroscopy has... 

The case of a hypothetical linear aromatic chain (m = oo) is also shown in Figure 6.6(f). Delocalization of o-a-m is ruled out. However, the alternative all-trans zig-zag arrangement of planar ethylene units, connected by... 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

Since so (trans) < 0.005], it is possible to rule out a photosensitized isomerization mechanism involving the establishment of thermal equilibrium between 11-cis and all-trans forms during the lifetime of the triplet state. Also excluded is the quantitative population of a common minimum along the C q-C 2 torsional coordinate. This conclusion is consistent with recent studies (169,175) which, in variance with the previous investigation (171), have reported different T-T spectra upon excitation of all-trans and 11-cis retinal. 

The cations [cis-HjRh(dppp)2] and [cis-HjRh[(-l-)-diop]2] are similar to the known [cis-H2Rh[P(CHj)3] ] and show a u(Rh-H) stretch in the IR, a pair of multi-plets in the high-field nuclear magnetic resonance (NMR) at 25°C and two resonances in the P NMR. The lower field resonance for each system is attributed to the mutually trans-P atoms, and that at higher field to the P atoms trans to the hydrides. The presence of two P resonances at all temperatures rules out a cis-trans rearrangement. Both [cis-H2Rh(dppp)2] and [cis-HjRh[(-l-)-diop]2] lose on dissolving in solution under Ar. 

Both patients had clear anatomical causes for blindness, and unilateral (rather than bilateral) blindness suggested a limited role for systemic arsenic toxicity. Nevertheless, a weak contribution of ocular arsenic toxicity should not be ruled out. Both arsenic trioxide and all-trans retinoic acid can increase intracranial pressure, resulting in pseudotumor cerebri and a secondary increase in intraocular pressure, which may augment retinal injury. Also, arsenic trioxide can cause vasoconstriction and worsen retinal artery occlusion. Finally, elemental arsenic was detected in the eyes at 30-50% of the plasma concentration, a ratio comparable to that in cerebrospinal fluid. This may have direct retinal toxicity, especially with the high peak concentrations associated with intravenous arsenic trioxide. Full ophthalmic evaluation is recommended in patients receiving longterm or intravenous arsenic trioxide. 

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