Geometrical isomerisation

The cis-trans or isomerisation is equally reported in both thermal 

Irradiation of a pure cis- or irons- isomer or a cis- trans mixture will lead to an equilibrium between these two geometrical isomers, which on further irradiation does not change. This equilibrium state is termed as photostationary state. 

The ratio of cis- and trans- isomers in photostationary state will depends on the absorption maximum and molar excitation coefficient (Emax) lli  

Formation of Perpendicular State [p-state] (Common Intermediate) 

The direct irradiation of geometrical isomer gives rise to vertical excited state, i.e., from cis Sp to cis and trans Sq to Trans Sj. After passing life time of 

---

No geometrical isomerisation of tiglic acid to angelic acid, or vice versa, accompanied hydrogenation... 

Loss of activity of solutions of some dmgs such as the tetracyclines can occur because of epimerisation of the dmg molecule, while others such as vitamin A lose activity because of geometrical isomerisation. 

It was observed that electrochemical oxidation of all-trans P-carotene (1) and canthaxanthin (16) in CH2CI2 leads to significant trans-cis isomerisation [105]. It was suggested that the isomerisation mechanism involved cation radicals and/or dications which could easily undergo geometrical isomerisation. This proposal was supported by AMI molecular orbital calculations, which showed that the energy barrier from trans to cis is much lower in the cation radical and dication species than in the neutral carotenoid [105]. 

Geometrical isomerisations involving the azobenzene chromophore continue to attract attention. Low temperature flash photolysis studies of 4-aminoazobenzene and 4-(N,N-dimethylamino)azobenzene in an ethanol glass on the microsecond timescale indicate that for both compounds the photoexcited Z-isomer produces two species. One of these is the JF-isomer the other is a short-lived intermediate, tentatively identified as a zwitterionic species, which decays to form the E-isomer. The donor-acceptor substituted azobenzene sodium 4-(4 -aminophenyl-azo)benzoate has been reported to complete the E- to Z-photoisomerisation... 

The values of Ti are about 6 x 10 Torr s" (as before) for the isomerisation to propylene, and 4 x 10 Torr" s for the interchange between the cis and trans modifications. The difference between these two figures is sufficiently large that it cannot be caused by an error in the experiment for both reactions to yield the same value of r for the geometric isomerisation would have to be near 30 instead of around 5 to 6 Torr ... 

Trost was importantly able to extend this catalyst system to DAAA of acyclic enolate precursors to form a number of a-tertiary ketones using the same catalytic conditions [40]. More significant was the observation that the geometry of the enolate precursor affected both the rate of the reaction and the absolute configuration of the product suggesting that there is no significant geometric isomerisation of the enol carbonate or the Pd-enolate complex. 

In parallel with saturation of the double bonds during the catalytic hydrogenation of oils, geometrical isomerisation of fatty acids via intermediate free radicals also occurs. Isomerisation using a nickel catalyst proceeds to equilibrium therefore, partially hydrogenated fats can contain 50-60% of trans isomers of fatty acids, especially of octadecenoic acids. The main isomer is usually elaidic acid. Similarly, trans isomers of octadecenoic acid are formed by biohydrogenation in the composite stomach of ruminants where the main trans isomer is vaccenic acid. The process, in this case, is far from equilibrium so the trans isomers content in the milk fat of ruminants is about 3 6%. 

The extent of geometric isomerisation during hydrogenation can be somewhat reduced by using catalysts based on precious metals. The resulting products are usually liquid and contain lower amounts of wa s-monoenoic fatty acid and of stearic acid. The trans isomers content still remains at 1-10%. 

---