Discrimination of isomers

Nevertheless, developments over the last years have led to the situation that almost every type of LC has been and can be effectively coupled to FTIR. Furthermore, there always will be particular applications (e.g., discrimination of isomers or polymer analysis) where IR data on separated compounds can be highly valuable. Furthermore, for the solution of complex analytical problems, the possible integration of the information on molecular structure provided by FTIR, MS, and/or nuclear magnetic resonance (NMR) would be highly advantageous. Illustrative for this statement is the recent development of hyphenated systems employing multiple interfacing of the same LC system to several spectrometric detectors (UV absorbance, MS, NMR, and FTIR). It is the complementary nature of the data provided by each spectrometric technique that leads to an enormous information content of the total system. 

As mentioned above, GC/FTIR is very useful for structural analysis it may provide essential stmctural information for analytes, but the technique lacks the sensitivity needed to become competitive with GC/MS. In terms of data interpretation, however, IR spectra are more discriminative and provide greater confidence in identification than mass spectra. These properties make GC/FTIR a powerful tool for the discrimination of isomers and the identification of compounds with closely related structures, particularly in combination with GC/MS. 

GC/FTIR is very useful for identification analysis of volatile compounds. This technique produces excellent spectroscopic information and is best suited for discrimination of isomers and the identification of compounds with closely related structures. The GC/FTIR identification... 

Shen, C.Y, Li, J.Q., Wang, Y.J., Wang, H.M., Han, H.Y, Chu, YN. (2010) Discrimination of isomers and isobars by varying the reduced-field across drift tube in proton-transfer-reaction mass spectrometry (PTR-MS). Inter-... 

Based on the modest ability of the (+)-isomers of MDMA and MBDB to inhibit the reuptake of norepinephrine (NE) into hypothalamic synaptosomes (Steele et al. 1987). it seemed possible that noradrenergic pathways might be involved in the eue. In ano er series of drug discrimination experiments designed to test this hypothesis, the specific NE uptake inhibitor (-)-tomoxctine was tested for stimulus transfer in doses up to 10 mg/kg in MDMA-trained rats. At 5 mg/kg, 67 percent of the animals responded on, the drug lever. However, pretreatment with tomoxetine in six rats trained to discriminate MDMA from saline had no effect on the discrimination of a subsequent dose of MDMA. 

With isocyanates R"N=C=0 as unsymmetrical heterocumulenes the reactivity to yield bis(insertion)products is markedly higher and there are many more possibilities (2) for isomerism. In fact, we find now that the product selection is more varied than reported in a previous article [3], and that e. g. the use of 1 R,R = Me, and phenyl- or isopropylisocyanate results in mixtures of isomers, even at low temperatures. Sterically more discriminating derivatives of 1 with R = Me, R = rm-Bu and R = wo-Pr, R = H yield one major stable isomer with PhN=C=0 which could be identified as the (cis O, N ) form by NMR spectroscopy. 

Three-dimensional (3-D) descriptors of molecules quantify their shape, size, and other structural characteristics which arise out of the 3-D disposition and orientation of atoms and functional groups of molecules in space. A special class of 3-D indices is quantitative descriptors of chirality. If a molecule has one or more chiral centers, the spatial disposition of atoms can produce enantiomers, many of which will have the same magnitude of calculated and experimental physicochemical properties having, at the same time, distinct bioactivity profiles. Basak and coworkers [22] have developed quantitative chirality indices to discriminate such isomers according to their structural invariants which are based on the Cahn-Ingold-Prelog (CIP) rules. 

Jones FN, Elliot D, Individual and substance differences in the discriminability of optical isomers, Chem Senses and Flavor 1 317—321, 1975. 

Tembreull R, Lubman DM. 1984. Use of resonant two-photon ionization with supersonic beam mass spectrometry in the discrimination of cresol isomers. Anal Chem 56(11) 1962-1967. 

The stereoselective cyclopropanation of chiral alkenes can be divided into two classes cyclic and acyclic alkenes. Furthermore, within each class, a subdivision exists involving those that contain a proximal basic group that can direct the cyclopropanation reaction of zinc carbenoids and the others that do not. The discrimination of reactivity between alkenes that possess a proximal basic group and those that do not was first highhghted early on when Simmons and Smith noticed that the cyclopropanation of l-(o-methoxyphenyl)-l-propene was more efficient than that of the related meta and para isomers (equation 46). ... 

Xu, L., H.-Y. Wang, and Q. Su, A Newly Proposed Molecular Topological Index for the Discrimination of Cis/Trans Isomers and for the Studies of QSAR /QSPR. Comput. Chem., 1992 16, 187-194. 

Alternatively, it is possible that the very short life-time of the car-banion is the product-determining factor. Since less rotation is required for it, retention would always be preferred, and inversion would be observed only if the conformer obtained after 60° rotation is by-passed either as a result of the stability of the carbanion, or if the rotation and the elimination rates are comparable. We will call this route in which the importance of retention, inversion and isomerization decrease in this order, the preferred retention mechanism . Discrimination between conformers due to steric interactions leading to them does not play a role in determining the product ratio, and the difference in the degree of retention for each member of the pair of isomers cannot be predicted. 

In the case of analytes with identical elemental formulas ( true isobars ) as positional or geometrical isomers (e.g. 11-hydroxycortisol and 21-hydroxycortisol or testosterone and epi-testosterone)—a discrimination of analyte and interfering compound is not possible with any mass analyzer—even if enabling highest mass resolution. Potentially, the disintegration pattern of isomers may be different, allowing analytical discrimination but in most cases chromatographic baseline separation of analyte and isomer prior to their MS/MS detection is required for unequivocal quantitative measurement. 

Three other isomers (85, 86, and 87) were also prepared in a similar manner. Discrimination of the isomers by spectroscopic analysis was found difficult because H and 13C NMR spectra of four... 

The 13C NMR shift of the tetrazole C(5) atom is particularly sensitive to the substitution pattern of the ring and allows discrimination between isomers. Two crystal forms of the angiotensin II antagonist irbesartan, which is a tetrazole derivative, have been identified as desmotropes using solid-state 13C (and 1SN) NMR spectroscopy. Substantial differences between the spectra of these forms were observed and one crystal form was deduced to be the 1H-... 

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