Cis-trans isomerization process

A molecular-level machine is a particular type of molecular-level device in which the component parts can display changes in their relative positions as a result of some external stimulus.13,9 131 Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g., the photoinduced cis-trans isomerization processes), the term molecular-level machines is only used for systems showing large amplitude movements of molecular components. 

To account for photochemical processes, we adopt a simple model that was proposed by Seidner and Domcke for the description of cis-trans isomerization processes [164]. In addition to the normal-mode expansion above, they introduced a Hamiltonian exhibiting torsional motion. The diabatic matrix elements of the Hamiltonian are given as... 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Scheme 7. The acid cleavage of dinuclear trihydroxo-bridged species is normally followed by a fast cis-trans isomerization process.

Experimental and theoretical studies on the solvent influence on molecular geometries and cis/trans isomerization processes of other so-called push-pull ethenes, R2N CH=CH A R2N+=CH CH=A- (with A = NO2, CHO, CN, etc.), have been collected in references [287, 288], The barriers to isomerization about the C=C bonds of these acceptor-substituted enamines are considerably smaller than those for simple ethenes such as 2-butene Ea = 259 kJ/mol [82]), owing to a significant contribution of the mesomeric zwitterionic structure to the electronic ground state. Increasing solvent polarity increases the contribution of this dipolar mesomeric structure, and hence leads to a decrease in the barrier to C=C isomerization and a simultaneous increase in the barrier to rotation about the C—N bond [288], The calculated sequence of solvent stabilization for such acceptor-substituted enamines is (activated complex for C=C rotation) E form > Z form > (activated complex for C—N rotation). Obviously, the activated complex for isomerization about the C=C bond corresponds to a full zwitterion with maximal solvent stabilization [288]. 

Incorporation of azo-links into polydienes markedly improves the stability of the polymer through a reversible cis-trans isomerization process. Apparently, the more mobile the polymer chains, the more effective the process. Other interesting systems include pyrene, lignin from sugar cane, acidic tar products,aromatic epoxypropylamines, 1,3,5-triphenylpyrazole, and hydroxyethylimides of bicyclic dicarboxylic acids in polyolefins. Studies on PVC stabilization include the effects of phosphites and absorbers. ... 

Fig. 14.11 Cis-trans isomerization processes in the [RuCl2(dppm)2] complex. From Ref. [34]. Reproduced by permission of the American Chemical Society.

The direct or sensitized cis-trans photoisomerization has been described by many authors (58). The Hg( P,) photoisomerization was explained on the basis of vibrationally excited triplet molecules. In the case of the Cd( P,), the photosensitization of unsaturated hydrocarbons mainly results in the cis-trans process. The main difference between mercury and cadmium sensitization resides in the lower energy content delivered in cadmium experiments 366.4 in comparison with 469.4 kJ/mol (59). A very recent study of the zinc photosensitization of 2-butene concludes with the cis-trans isomerization process as the main process (60). 

The first route follows the ground-state N curve all the way. The energy of the N i) orthogonal structure then corresponds to the activation energy of the cis-trans isomerization process. A value of 2.67 eV has been reported for the isomerization of dideuterio-ethy-lene... 

We have studied pMEA, pMAEA and pDRlM in comparison, in order to determine what is the importance of Rau s classification on the photoinduced birefringence. These three polymers have increasing dipole moments, and their comparison clearly indicate that the pseudostilbene -type azobenzenes are the best candidates for photoinduced orientation. Their absorbance in the visible range of the spectrum allows the use of lower power lasers (514 nm), the coincidental absorbances of the cis and trans isomers allows photoexcitation of both trans-cis and cis-trans isomerization processes. Both are necessary for orientation, and the lower the polarity of the azobenzene, the slower the cis-trans thermal isomerization process. The levels, rates and stabilities of the photoinduced birefringence, all are hi er for pDRlM in comparison with the other two, as is the efficiency of the process. Almost all our research is concentrated on the donor-acceptor substituted azobenzenes. 

In Chapter 3, we have seen that aromatic t-systems are planar, stabilized by tt-bonding. Out-of-plane motion of the carbon atoms is therefore an energy consuming process and the ground state is thus stable. In a tt tt excited state, part of the t-bonding disappears, however, and a number of processes become possible. We will be interested particularly in the cis-trans isomerization process. 

This interpretation is further supported by the observation of a decoupling of the cis-trans-isomerization process in bacteriorhodopsin occurring in the dark as a function of hydration as well as by the fact that the photochemical bleaching of the M-412 component is observed at the lowest humidity state. 

Optical "behavior of the isomerizing side chains, specially the cis-trans isomerization process of the azo linkage, was compared to those of nonpolymeric model compounds. The irradiation response and dark relaxation of the polymeric dyes are affected hy the charge and conformation of the copolyelectrolyte. 

The intermediates obtained after the oxidative addition have been usually observed as rra x-[PdL2(R)(X)] complexes [2]. Consequently, it has been proposed that the catalytic cycle goes through this type of trans complex. This does not pose any complication to the transmetalation step, and in fact a number of the calculations presented above use this trans arrangement. However, things are different for oxidative addition and reductive elimination. Both reactions require a cis arrangement of the ligands involved, as discussed in previous sections. Therefore, the isomerization steps shown in Eqs. 11.8 and 11.9 must be postulated in order to close the catalytic cycle in some cases. The cis-trans isomerization process has been studied... 

A number of d square planar complexes undergo a cis-trans isomerization process, 16.39 to 16.40 [34]. Of interest to us in this section is to probe the direct... 

---