Trans conversion

The frequency job on the middle structure produces one imaginary frequency, indicating that this conformation is a transition structure and not a minimum. But what two minima does it connect Is it the transition structure for the cis-to-trans conversion reaction (i.e. rotation about the C=C bond) ... 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

Detection of an Intermediate. In many cases, an intermediate cannot be isolated but can be detected by IR, NMR, or other spectra. The detection by Raman spectra of NOj was regarded as strong evidence that this is an intermediate in the nitration of benzene (see 11-2). Free radical and triplet intermediates can often be detected by ESR and by CIDNP (see Chapter 5). Free radicals [as well as radical ions and EDA complexes] can also be detected by a method that does not rely on spectra. In this method, a doublebond compound is added to the reaction mixture, and its fate traced. One possible result is cis-trans conversion. For example, cis-stilbene is isomerized to the trans isomer in the presence of RS- radicals, by this mechanism ... 

Another interesting example of aurophilic interactions influencing electronic excited state properties is the unusual photochemical cis to trans conversion observed in dinudear goldhalide bis(diphenylphosphino)ethylene complexes, Au2X2(dppee)... 

Schwerdtfeger, P., Bruce, A.E. and Bruce, M.R.M. (1998) Theoretical Studies on the Photochemistry of the cis to trans Conversion in Dinuclear Goldhalide Bis (diphenylphosphino) ethylene Complexes. Journal of the American Chemical Society, 120, 6587-6597. 

Fig. 7. Selected geometric parameters (A) of the optimized rotational transition-state structure for antilsynA-trans bis(syn),A-trans conversion via the n3-1sTn,V(C3),A-tr

This means that 4-nitrostilbene is a more effective electron acceptor than nitrobenzene. This theoretical conclusion is verified by experiments. The charge-transfer complexes formed by nitrobenzene or 4-nitrostilbene with Af,Af-dimethylaniline have stability constants of 0.085 L mol or 0.296 L mol respectively. Moreover, the formation of the charge-transfer complex between cis-4-nitrostilbene and A/,Af-dimethylaniline indeed results in cis-to-trans conversion (Dyusengaliev et al. 1995). This conversion proceeds slowly in the charge-transfer complex, but runs rapidly after one-electron transfer leading to the nitrostilbene anion-radical (Todres 1992). The cis trans conversion of ion-radicals will be considered in detail later, (see sections 3.2.5.1, 6.4, and 8.2.1). 

On one-electron reduction followed by one-electron oxidation, cis isomers of 2- and 4-nitrostilbenes turn into the neutral nitrostilbene molecules, but in the trans forms. On oxidation of the naked anion-radicals, the neutral trans forms are the only products (cis trans conversion degrees were 100%). In the case of the coordination complexes, the trans isomers are formed only up to 40% (Todres 1992). Scheme 3.45 describes these transformations. 

The radical >C(Ar)-C < is oxidized by ligand transfer as Jenkins and Kochi (1972) indicated. If the cation-radical [>C=C<]+ obtained as a result of the initial electron transfer is not fully consumed in the reaction, it is reduced by Cu(I) and returns in the form of its geometrical isomer. In the olefin cation-radical state, cis—>trans conversion has to take place, and it indeed takes place in the systems considered (Obushak et al. 2002). 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

Unexpectedly, the analysis of the MOs of the cation-radical from (5-cyano vinylferrocene reveals the possibility for cis trans conversion if more than one-electron oxidation takes place. Namely, the cation-radical has an MO, which is four levels higher in energy than the one occupied by the single electron, that is centered on the cyanoethylene fragment (Todres et al. 1992). 

The hardening effect of fatty oil isomerization processes is limited by the fact that cis-trans conversion of the double bond of mono-unsaturated fatty acids is an equilibrium reaction in the case of oleic and elaidic acid esters the equilibrium mixture consists of 67% elaidic acid and 33% oleic acid this equilibrium ratio is practically independent of the isomerization temperature77. 

Formation of charge-transfer complexes was observed in homogeneous solutions. Because of the limited solubility of chloranil, the reaction was performed in a boiling solvent. In hexane and methylene chloride (b.p. s are 69 and 40°C, respectively), the degree of the cis-to-trans conversion was 50 and 30%, respectively. No conversion was observed in the absence of chloranil when the r/.v-dimethoxystilbene was kept in those solvents at the temperatures noted. In the case of benzene (b.p. 80°C), no conversion was observed. This... 

After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the -rr-electron cloud with the Fe+ center. As a result, cis —> trans conversion never occurs (Todres et al. 1980 Todres 2001). 

Analysis of the BPA-PC resulting structure was in very good agreement with the experimental and calculated results obtained on DPP and DPC. In the latter case, it is found that there is no tram to cis or cis to trans conversions in the glassy state, the tram, tram conformation being more favoured than the trans, cis (or cis, tram) one. 

The conclusion that the ac electrolysis of Re rans-SP CCCOjCl proceeds via excited states is also supported by the direction of isomerization. In thermal reactions of stilbene derivatives and radicals cis to trans conversions are generally observed (61). Contrary to this behavior the photolysis and ac electrolysis lead to energetically uphill trans to cis isomerization. 

Such a behavior is evidence for the lability of the electronic structure of the silicon atom, because it undergoes rearrangement under the action of both chemical and geometric factors. These factors are comparable in their effect the substitution of the F atom for H brings about almost the same changes in the characteristics of the silicon atom as does the cis-trans conversion of the substituent (OH group). 

Figure 3-2. Potential energy relative to H + C02 for selected points on the HOCO PES from left to right (i) the transition state for HOCO <-> H + COz, (ii) tnms-HOCO (ds-HOCO and the small cis-trans conversion barrier are not shown), and (iii) the transition state for HOCO <-> OH + CO from the low temperature rate measurements, this transition state is known to be very loose (Frost et al. 1991a, 1993).

K4[Mn2(CN)4(NO)4] has been isolated in cis and trans forms.14 Evidence for a dimeric structure with Mn—Mn bonds derives from the diamagnetism of the compounds, the IR spectra and cis-trans conversions. Yellow crystals of the cis are obtained from the reaction in liquid... 

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