Isomers, nuclear magnetic resonance

Both vapor-phase chromatography and high performance Hquid chromatography, along with nuclear magnetic resonance spectroscopy, have been used for isomer and composition analysis. 

Several 3-azido[l,2,4]triazines 1013 were prepared (76JOC2860 77JHC1221) by treating the corresponding 3-hydrazino derivatives with nitrous acid. Azidotriazines 1013 were spontaneously cyclized to tet-razolo[l,5-/j][l,2,4]triazines 1014 rather than their isomers 1015. These transformations were studied using nuclear magnetic resonance and infra-... 

Strohschein, S., Pursch, M., and Albert, K., Hyphenation of high performance liquid chromatography with nuclear magnetic resonance spectroscopy for the characterization of (3-carotene isomers employing a Cjq stationary phase, J. Pharm. Biom. Anal., 21, 669, 1999. 

Hentschel, P. et ah. Structure elucidation of deoxylutein 11 isomers by on-line capillary high performance liquid chromatography- H nuclear magnetic resonance spectroscopy, J. Chromatogr. A, 1112, 285, 2006. 

Because carotenoids are light- and oxygen-sensitive, a closed-loop hyphenated technique such as the on-line coupling of high performance liquid chromatography (HPLC) together with nuclear magnetic resonance (NMR) spectroscopy can be used for the artifact-free structural determination of the different isomers. 

Electron spin resonance, nuclear magnetic resonance, and neutron diffraction methods allow a quantitative determination of the degree of covalence. The reasonance methods utilize the hyperfine interaction between the spin of the transferred electrons and the nuclear spin of the ligands (Stevens, 1953), whereas the neutron diffraction methods use the reduction of spin of the metallic ion as well as the expansion of the form factor [Hubbard and Marshall, 1965). The Mossbauer isomer shift which depends on the total electron density of the nucleus (Walker et al., 1961 Danon, 1966) can be used in the case of Fe. It will be particularly influenced by transfer to the empty 4 s orbitals, but transfer to 3 d orbitals will indirectly influence the 1 s, 2 s, and 3 s electron density at the nucleus. 

Three different isomers are formed consecutively on reacting [RuCl2(PPh3)3] with 4,6 -bis(pyrazol-l-yl)pyrimidine, bpzpm the final product is cis,trons-[RuCl2(PPh3)2(bpzpm)] (160). Nuclear magnetic resonance spectroscopy (NMR) shows facile cis < - trans interconversion for [H(dtbp)Ru(p-Cl)2Ru(dtbp)H], where dtbp = bis[di(t-butyl)phosphano-methane or -ethane. Crossover experiments implicate mononuclear intermediates in these isomerizations (161). 

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

G. Sun, Fullerenes identihcation of isomers based on nuclear magnetic resonance spectra, in Dekker Encyclopedia of Nanoscience and Nanotechnology, (eds J. L. Atwood and J. W. Steed), Marcel Dekker, New York, 2004, p. 1223. 

Until recently it was thought that mono-substituted hydrazines invariably reacted with 5-methylisothiouronium salts to give compounds of type (XVII) [129, 192, 193] however, examples have now come to light in which the alternative isomer (XVI) has been isolated from this reaction [21, 131, 194, 195). This formulation is based on (a) the failure of the products to react with benzaldehyde, [21, 129, 131, 194], (b) the unambiguous synthesis of the two possible alternative structures [21, 129, 194], and (c) an examination of the nuclear magnetic resonance spectra of the mono- and di-protonated forms of some... 

The most stable isomer agrees with experimental nuclear magnetic resonance and X-ray data. (Adapted from Laurencin et ah, 2006)... 

The spectroscopic properties of meso-ionic compounds have been discussed in detail elsewhere and the reviewers do not feel that it would be useful to include a comprehensive account here. Ultraviolet, infrared, and nuclear magnetic resonance spectra of meso-ionic heterocycles provide general support for the conjugative interaction that would be expected for aromatic heterocycles, " but detailed interpretation of their spectra is not justifiable. Mass spectrometry has been shown to be particularly useful for distinguishing between pairs of meso-ionic isomers... 

Table 1.1 lists minimum lifetimes for observation of separate species and the appropriate spectroscopic methods. The time scale of nuclear magnetic resonance (NMR) experiments is particularly long, and many conformational isomers and some constitutional isomers (see below) interconvert rapidly within the time of observation and appear to be more symmetric than simple bonding considerations would imply. We will expand on these ideas after the next two sections. 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

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