Isomers identification

Correlation of the single-crystal structure results with the powder diffraction pattern, establishing x-ray powder diffraction as a convenient, powerful means of isomer identification and... 

Mistry, N., Roberts, A.D., Tranter, G.E., Francis, P., Barylski, I., Ismail, I.M., Nicholson, J.K., and Lindon, J.C., Directly coupled chiral HPLC-NMR and HPLC-CP spectroscopy as complementary methods for structural and enantiomeric isomer identification application to atracurium besylate, Anal. Chem., 71, 2838, 1999. 

Increasing reliance on mass spectrometry as the universal detector for GC has not solved all the problems of additive identification. Isomer identification is impossible (except for REMPI technology), but is hardly an issue in additive analysis. 

A dry, thin lycopene layer heated at 50°C, 100°C, and 150°C showed first-order kinetic decay (Lee and Chen 2002). At 50°C, isomerization dominated in the first 9h however, degradation was favored afterward. On the other hand, at 100°C and 150°C degradation proceeded faster than isomerization. Although cis isomer identification was not confirmed by standards, the mono-civ lycopene isomers, 5-cis-, 9-cis-, 13-cis-, and 15-m-, degraded at the same rate as did all-trans-lycopene,... 

Sun, G., Kertesz, M. (2001) Isomer Identification for Fullerene C84 by 13C NMR Spectrum A Density-Functional Theory Study, J Phys Chem A 105, 5212-5220. 

Guidance in the assignment of structure to isomeric species Cr(OH2)6-n(OS(CH3)2)n3+ for n = 2, 3, and 4 is obtained by comparing the relative stabilities of the two species having each of these compositions with the statistically expected values. The observed relative concentrations of the more easily eluted A isomers and the less easily eluted B isomers leads to the conclusion that the more easily eluted isomer is the cis isomer for n = 2 and 4 and the fac isomer for n = 3. This is summarized in Table I. Although it cannot be asserted that the isomeric species in this system should have exactly the statistically expected relative concentrations, the assignment proposed on this basis is more rational than the opposite one in which the observed relative concentration would differ from the statistically expected ones by factors of 22, 2.8, and 10, respectively, for n = 2, 3, and 4. These isomer identifications for n = 2 and 3 are supported by observations of the amounts of the isomeric species with n = 3 formed from each of the species with n = 2 (11),... 

To determine the effect of a group on orientation is, in principle, quite simple the compound containing this group attached to benzene is allowed to undergo substitution and the product is analyzed for the proportions of the three isomers. Identification of each isomer as ortho, meta, or para generally involves comparison with an authentic sample of that isomer prepared by some other method from a compound whose structure is known. In the last analysis, of course, all these identifications go back to absolute determinations of the Korner type (Problem 10.8, p. 332). 

Sovocool, G.W., R.K. Mitchum, and J.R. Donnelly. 1987. Use of the "ortho effect" for chlorinated biphenyl and brominated biphenyl isomer identification. Biomed. Environ. Mass Spectrom. 14 579-582. 

Another spectroscopic technique which can be coupled to GC is infrared spectroscopy. It is particularly valuable when isomer identification is required. On its own, however, infra red (IR) spectroscopy is not very useful because it cannot differentiate homologues without additional molecular weight information. Besides, the gas phase spectra differ from the widely available condensed phase spectra, which means that new libraries would have to be built to enable analyte identification based on library searches. Thus, IR spectroscopy found its niche application in GC in combination with mass spectrometry. 

J. Grainger, RC. McClure, A. Liu, et al.. Isomer identification of chlorinated dibenzo-p-dioxins by orthogonal spectroscopic and chromatographic techniques, Chemosphere, 32, 13-23, 1996. 

The relative retention time (relative to dy-nitronaphthalene) and mass spectrum of each standard compound were used to assign specific isomer identification to a few of the nitro-aromatics detected in the extracts. The other nitro-aromatic isomers detected were identified by appropriate mass spectra and retention times. The availability of other specific nitro-aromatic isomers may allow assignments to be made of those compounds. 

Not including isomers-identification by relative retention time br = branched. 

I, II) different isomers ( ) identification confirmed by comparison with authentic standards. 

Sun, G. Y, 8c Kertesz, M. (2001b). Isomer identification for fullerene C84 by C NMR spectrum A density-functional theory study. Journal of Physical Chemistry A, 105(21), 5212-5220. 

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