Isomers theoretical

The importance of the theory was further demonstrated by the discovery of the existence of optically active inorganic compounds, and the isolation of the exact number of optical isomers theoretically possible for the spatial arrangement of the atoms.1 Friend 2 and others criticised the theory on the grounds that in simple compounds, such as sodium chloride or cobaltous chloride, the chlorine is ionised and yet is attached to sodium or cobalt atom directly, whereas in the ammino-coinpounds the acid capable of ionisation is that which is not directly attached to metal. For instance, in chloro-pentammino-cobaltic chloride, [CoCI(NH3)5]C12, it is the chlorine outside the first zone which is ionised in solution. Also, the dissociable acidic groups are not attached to any point within the complex, but simply hover round the central complex in an indefinite manner. Thus a definite valency for ionisable... 

Rhodotorulic acid (RA), a dihydroxamate siderophore, forms dimeric complexes with iron, aluminium and chromium of the stoichiometry M2(RA)3 at neutral pH 36 188). The coordination chemistry of this siderophore is probably the most complicated of the siderophores. The combination of cis-trans, A and A configurations of two iron miters, connected by three RA molecules, makes 42 non-redundant isomers theoretically possible each can be simulated by molecular models. Recently three different isomers or mixtures of isomers of Cr2RA3 were separated by reversed phase HPLC-chromatography177). The visible spectrum of the most abundant fraction corresponds to the cis isomer the two other fractions are very similar to the visible spectrum of the trans Cr(men)3 isomer. The CD spectra, in comparison with the Cr(men)3 model complex, show two different optical isomers, assigned as A -trans and A -trans. The A isomer preparation seems also to contain a certain amount of the A configuration. This is the first time that two different, kinetically stable optical isomers have been isolated from the metal complexes of a siderophore 177). 

Structures and Other Properties of Polyynes and their Isomers Theoretical and Experimental Results.371... 

Most of the isomers theoretically possible by the condensation of two glucose units have been isolated 26a). (See also under Isomaltose.)... 

Only racemate II contains the insecticidally active S-S -isomer. Theoretically it is possible, that a peculiar stereoselectivity predominates, leading to the preferential formation of the uninteresting racemate I. In case of the epimerizable cyanohydrine esters 373 and others the kinetically controlled primary isomeric ratio can be changed afterwards by thermodynamic equilibration. 

The only biologically active form of vitamin Bs is D-(- -)-biotin, a unique steroisomer foimd in nature among the eight isomers theoretically possible [1,40]. In animal and plant tissues, most biotin is covalently bound to a lysine residue, free (D-biocytin) or belonging to biotin-dependent enzymes through an amide attachment. Biotin is also the form used for food enrichment [4]. 

The reaction between K2Fe(CO)4 and a tetrachlorotetrasilane resulted in the formation of an iron complex with a disilene ligand (Equation (33)). Both the (E)- and (Z)-isomers were formed. The reaction, when monitored by H NMR spectroscopy, indicated the initial formation of the (Z)-isomer with isomerization to the more stable ( )-isomer over time. Kinetic studies were also performed, and suggested that the ( )-isomer is >3kcalmol more stable than the (Z)-isomer. Theoretical calculations for similar molecules with varying R groups indicated that the stability of the molecule is dependent on the bulkiness of the R group. 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

Theoretical treatment of the reaction as a conrotatory process proceeding through the very unstable Z,Z, -isomer of benzene satisfactorily accounts for the observed activation barrier. ... 

Furthermore, the relatively high reactivity of 2-chloropyridine i -oxide as compared to that of the 4-isomer and the detailed inconsistency with theoretical parameters have also been explained in terms of built-in solvation via either direct electrostatic interaction or hydrogen bonding (structures 15 and 16, respectively). 

In agreement with the previously reported theoretical study, the results of semi-empirical calculations showed that the formation of the Dewar isomer is favored [99H(50)1115]. Probably, the observed formation of the azirine derives from a thermal isomerization of the first photoproduct, in line with that described in the case of furan and thiophene derivatives (Fig. 11). 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

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