Isomers, classification substitution

An alternative method that enables the classification of geometrical and optical isomers is accomplished by substituting the Fe + ion with kinetically more inert d Cr + or... 

Taubers review article 64), which provided the first basis for classifying inert and labile complexes, made use of all available observations of rates of formation or substitution reactions. Most of the observations were qualitative, but for the purpose of the classification it was sufficient to know whether the reaction was complete within a couple of minutes or whether a complex was sufficiently inert for geometrical or optical isomers to be isolated. Important qualitative observations came from attempts to resolve complexes, and much of the quantitative data came from studies of rates of racemization or isomerization. 

Absolute electron affinities can be obtained by classification of biological molecules to establish different values of AAG. This is illustrated for riboflavin, vitamin K, vitamin A, polyazines, and hydroxyprimidines. The Ea of these compounds are also predicted by substitution and replacement rules. Those for the dia-zines range from 0.2 eV to 0.4 eV. The values for 1,2,4 triazine and 1,2,4,5 tetrazine are 0.9 eV and 1.7 eV. The replacement of an additional CH by N increases the Ea by 0.6 eV. Therefore, the predicted values for pentazine and hexazine are 2.2 eV and 2.9 eV. The CURES-EC method gives better approximations to these Ea and can differentiate between isomers. 

When substitution takes place, the relation between the amounts of the isomers formed is influenced by (1) the group present, (2) the group introduced, (3) the temperature, and (4) the catalyst, if one is used. (1) In the classification given above all the groups in the first division send an entering element or radical to the para and ortho positions, but the groups differ among themselves in their effect on the relative proportion of the two isomers formed. For example, when fluorobenzene and chlorobenzene are nitrated at 0 , the mixture of ortho and para nitro-derivatives obtained contains in the first case 12.4 per cent and m the second, 30.1 per cent of the ortho compound. (2) While the... 

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. 

Classification Aromatic hydrocarbon Definition One of several aromatic compds. contg. two benzene nuclei with two or more substituted chlorine atoms 209 isomers possible Properties Mobily oily liq. to wh. cryst. solid to hard noncryst. resin (colorless cryst. in pure form) sol. in oils, org. soivs. pract. insol. in water m.w. 291.98-360.86 sp.gr. 1.4-1.5 (30 C) b.p. 340-375 C flash pt. (OC) 195C... 

Although chiral calix[4]arenes can be generated by simply attaching chiral residues at the upper [40] or lower [41-43] rim of the calixarene skeleton, recent interest has been focused on the possibility of synthesizing inherently chiral calix[4]arenes, which are build up of nonchiral subunits and consequently owe their chirality to the fact that the calixarene molecule is nonplanar. Molecular asymmetry can arise from the substitution pattern at the lower rim and/or conformation. In this respect, Shinkai has recently reported a systematic classification of all possible chiral isomers derivable from calix[4]arene, and delineated some basic concepts for the design and synthesis of chiral derivatives [44]. 

---