Trans state, definition

According to the spectral features and isomerization behavior, the aromatic azo compounds have been classified as azobenzene type, aminoazobenzene type, and pseudostilbene type (Rau, 1990). For the azobenzene-type molecules, the 71-71 transition band (320 nm) appears at shorter wavelength than n-n transition band (430 nm) (Kumar and Neckers, 1989). The cis-to-trans isomerization is relatively slow at room temperature, and the existence of cis isomers can be easily identified by the spectroscopic method. For aminoazobenzene-type and pseudos-tilbene-type molecules, the ti-ti and n-n transition bands are overlapped. The cis state of the molecules is unstable, which relaxes back to the trans state quickly. According to the definition, the azo polymers given in Figs. 5.1 and 5.2 can be assigned to contain pseudostilbene-type and azobenzene-type chromophores, respectively. 

It is convenient to express the energy of a given single bond relative to the energy of the trans state. The energy of a pair of bonds is defined relative to the state where the bond i is in the trans state, and all subsequent bmids j > i are also in the trans states. From this definition follows... 

Comparing the potentials across each row, we can test the idea of additivity of ortAo-substituent effects for 2-fluoro, 6-chloro, and then 2-fluoro-6-chloro substitution. The definition of a = 0 changes across each row to permit easy visual addition of the potentials in the physically appropriate manner to assess the degree of additivity of ort/io-chlorine and ort/io-fluorine substituent effects. For o-fluorotoluene and 2-fluoro-6-chlorotoluene, a = 0 denotes the (minimum-energy) pseudo-trans conformation for o-chlorotoluene, a=0 denotes the pseudo-cis conformation. The notion of additivity has considerable merit in all three electronic states. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

A transdermal is one such drug. What does the term transdermal mean Trans means through and dermal means skin, therefore a transdermal drug is one absorbed through the skin. The United States Pharmacopeia CUSP) 24 [1], offers the following definition/discussion of a transdermal delivery system ... 

Azo compounds can exist in either the cis or trans form. It is reasonable to assume that the azoalkanes in Table 5-8 exhibit the trans configuration. Contrary to the small solvent effects obtained in the decomposition of trans -azoalkanes, the thermolysis of definite cu-azoalkanes reveals a significant solvent influence on rate. Thermolysis of ah-phatic symmetrical cw-tert-azoalkanes can lead either to the corresponding trans-tert-azoalkanes, presumably via an inversion mechanism, or to tert-alkyl radicals and nitrogen by decomposition via a free-radical transition state [192]. An example of the first type of reaction is the Z)I E) isomerization of [1,1 jazonorbornane. Its rate is virtually solvent-independent, which is consistent with a simple inversion mechanism [565, 566], The second reaction type is represented by the thermal decomposition of cis-2,2 -dimethyl-[2,2 ]azopropane, for which a substantial decrease in rate with increasing solvent polarity has been found [193] cf Eq. (5-60). 

It has been suggested that cis-bis(2,2 -bipyridyl) complexes show greater complexity in the region 700-800 cm" than do iraras-bis(2,2 -bipyridyl) complexes (500). One complex, [Rh(III)(bipy)2Cl2], which is now known to be definitely cis and which gives a rich spectrum between 700 and 800 cm, has also been said to be the trans isomer (287, 452). Since the complex spectra are generally recorded for the solid state, it is clear that spectra-structure correlations of the above type must be used with caution. 

A particularly well studied system which undergoes photochemical cis -> trans isomerization is Pt(gly)2- The work on these compounds through 1969 has already been discussed in detail elsewhere (7), and thus only the major conclusions thereof will be mentioned here. A definitive study by F. Scandola et al. (93) showed that the photochemical cis - trans isomerization of this species occurs with an intramolecular "twisting" mechanism (Fig. 7). The thermally relaxed reactive excited state was proposed to have a pseudo-tetrahedral geometry as shown in Fig. 7. 

With the object, therefore, of definitely establishing the configuration of tropine and of pseudo-tropine, we have conducted experiments on the resolution of these bases, and some of their derivatives, by fractionally crystallising their salts with certain optically active acids. It may at once be stated that the results of these experiments point to the conclusion that both the bases in question are internally compensated compounds. The relation between them must, therefore, be of the nature of a cis-trans-isomerism, as concluded by Willstatter. (loc. cit.). Attempts were made to racemise tropine by heating the latter at high temperatures with hydrochloric acid, but these were unsuccessful. Source Barrowcliff 1909... 

---