Absorption optical isomerism

Molecules which contain a chiral cobalt as well as an asymmetric nitrogen exist in four possible optical isomeric forms. These are represented for Co(sar)(hbg) +, hbg = NH2C( = NH)NHC(=NH)NH2 in Fig. 7.12. All four optically-active isomers have been isolated and characterized by cd, nmr and vis/uv absorption spectroscopy. The kinetics of... 

The absorption spectra of optically active (d or /), racemic and inactive isomers are generally found to be similar. Optical, isomerism has no effect on Aauuc or However, when a compound has more than one asymmetric... 

IV Optical isomerism and pharmacokinetic effects A Isomer effects on absorption and... 

A special case of ligand isomerism also arises when the ligands are optical isomers—enantiomorphs—of each other. One interesting problem is the extent to which electron absorption bands, which, as a first approximation, are supposed to be localized on a transition metal ion, acquire optical activity because of the activity of a coordinated ligand. An example of this is provided by the ligand mentioned above, propylenediamine (pn) which exists in optically isomeric forms. 

Infrared, nuclear magnetic resonance, ultraviolet, optical rotary dispersion and circular dichroism measurements have been used for the spectral analysis of thiiranes. A few steroidal thiiranes have been reported to possess infrared absorption in the range from 580 to 700 cm The intermediate thiocyanate derivatives (RSCN) have a strong sharp peak at 2130-2160 cm the isomeric isothiocyanate (RNCS) shows a much stronger but broad band at 2040-2180 cm. ... 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

The optical absorption, fluorescence and photochemical properties of Ag, provide clear evidence for the occurrence of two distinct forms of Ag in each of the three rare gas matrices (4,8,9). This observation shows that the matrix environment can act to stabilize distinct isomeric forms of this species. It appears that a common form of Ag° is produced during matrix formation in Ar, Kr and Xe and a structurally different yet common form of Ag" is produced by photoinduced aggregation of Ag° atoms in Ar matrices and photoisomerization of Ag, in Kr and Xe matrices. The results available for Ag ,Ag° in rare gas matrices are summarized below ... 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

Infrared absorption peaks [Table], 234 Inhibitors, 39 Initiation, 34 Intermediates, 31, 41 Inorganic esters, 262, 272, 276 Inversion, 124, 139 Ion-dipole attraction, 23 Ionic bonds, 6 Isobutyl group, 146 Isolated bonds, 146 Isomerism, 2 alkyl halides, 118 cis-trans, 88 geometric, 88 optical, 70 Isomerization, 202 Isomers of butane, 50 of heptane, 66 of 2-hexene, 11 of pentane, 50 Isoniazide, 457 Isoprene rule, 181 Isopropyl group, 54 Isoquinoline, 458 Isotope effect, 130 lUPAC, 56... 

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

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