Trans-isomers symmetry properties, 64

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

Stereoisomerism in 1,4-poly-1,3-dienes results in significant differences in the properties of the cis and trans polymers. The trans isomer crystallizes to a greater extent as a result of higher molecular symmetry and has higher Tm and Tg values (Table 8-1). These trends are... 

Since they generally have more symmetry than cis isomers, trans isomers in most cases have higher melting points and lower solubilities in inert solvents. The cis isomer usually has a higher heat of combustion, which indicates a lower thermochemical stability. Other noticeably different properties are densities, acid strengths, boiling points, and various types of spectra, but the differences are too involved to be discussed here. 

Dynamic NMR studies indicate that the barrier to alkyne rotation in dithiocarbamate bisalkyne complexes is near 15 kcal/mol (87). For unsym-metrical alkyne ligands, as in Mo(PhC=CH)2(S2CNMe2)2, several isomers are possible with like substituents either adjacent ( cis ) or opposite ( trans ). Analysis of the NMR properties follows the logic presented by Faller and Murray for CpMo(RC=CR)2Cl (94). The C2 molecular symmetry dictated by the chelates can produce two different trans isomers with the two alkyne protons of PhC=CH in each isomer equivalent by C2 rotation (Fig. 26). Only one unique cis isomer is possible, but the two... 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

Substitution at one or more metal sites will generally break the symmetry of the cluster core, and can greatly influence its electronic properties and reactivity. Consider, for example, the possible substitutions of a metal M into an octahedral core of composition MfiX v (x = 8, 12). The first substitution will afford an MsM X core, for which the symmetry has been lowered to C4v. A second substitution generates an M4M 2Xx core with two possible isomers One in which the M atoms are positioned at trans vertices (D4/,) and another where they are positioned at cis vertices (C2v). With a third substitution to give an M3M 3Xx core, fac and mer isomers become possible, while further substitutions simply repeat the pattern with M and M interchanged. Here again, the substitutions can be anticipated to alter the basic electronic properties of the cluster. Moreover, the outer-ligand substitution chemistry could potentially be quite different... 

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