L3C-NMR spectroscopy

Problem 20,16 How could you distinguish between the isomers cyclopentanecarboxylie acid and 4-hydroxycyclohexanone by and l3C NMR spectroscopy (See Problem 20.14.)... 

Naphtho analogues, naphtho[2,l-e]tetrazolo[l,5-6][l,2,4]triazine, naph-tho[l,2-e]tetrazolo[l,5-b][l,2,4]triazine, and naphtho[2,3-e]tetrazolo[l,5-Zj][1, 2,4]triazine, were prepared (82JOC3168 84JOC3199) by cyclization of the respective hydrazine with sodium nitrite in acetic acid or by azide displacement of a leaving group. Elucidation of the site of annulation of the tetrazole ring was accomplished by X-ray analysis and l3C-NMR spectroscopy (Scheme 189). 

The stability of reumycin, fervenulin, and xanthothricin in acid-base media was studied by H- and l3C-NMR spectroscopy (85MI1). Infrared and absorption spectra of reumycin 311 were studied at low temperature (87MI2). The experimental data corresponded to the quantum chemical calculations of electron transmission. The pKa of 311 in aqueous medium... 

In another legume, Afzelia bella Harms (Caesalpinoidene), the same authors have isolated the new rrfree amino acids. The structure of the diacid 54 is based on mass, H, and l3C-NMR spectroscopy (95) the proposed absolute configuration of 54 was also demonstrated by X-ray diffraction (96). Compound 54 is probably identical with the diacid independently isolated from the red alga Chondria coerulescens (Crouan) Falk. (Rhodomelaceae) (97). 

The a-phenylthioether 28 was oxidized in the absence of a nucleophile by low temperature electrolysis (Scheme 15). The corresponding alkoxycarbenium ion pool 26 was formed, which exhibited a single set of signals in H and l3C NMR spectroscopy. The chemical shifts were quite similar to those obtained by the oxidative C-Si bond dissociation described in the previous section. Subsequently, the cation pool was allowed to react with allyltrimethylsilane to obtain the allylated product 27. 

Nevertheless, I. l3C NMR spectroscopy has not yet reached the importance and ubiquity of 3H NMR, and detailed discussions of 13C spectral parameters are... 

Effects of various substituents on 13C chemical shifts can be found in textbooks (2-5,11) or reviews concerned with applications of l3C NMR spectroscopy to specific classes of chemical compounds (16-24,142). Therefore, this section is limited to recent information about effects of less common substituents and to reference data for various carbo- and heterocyclic molecules. It is not the author s intention to present a comprehensive survey rather, a number of typical examples have been selected. The reader may use this section as an entry into the original papers and the references cited therein. 

Tertiary carbanions have been isolated, but secondary and primary structures do not exhibit sufficient kinetic stability. The lifetimes of the former is highly dependent on structure, counterion, solvent and other factors. One of the simpler tertiary carbanions is present in (perfluoro-fer/-butyl)cesium obtained as a solid from the reaction of perfluoro(2-methyl-propene) and cesium fluoride in tetraglyme.36 The structure of the salt was analyzed by 19F and l3C NMR spectroscopy. 

The structural assignments of 40, as based on H-NMR spectra, have been confirmed by l3C-NMR spectroscopy.90 In the formation of adducts the carbon atoms undergo upfield shifts in the order C6 C5 > C4 > C2. In accordance with this behavior, indicating a change in hybridization of C-6 from sp2 to sp3, the 7(C6-H) value decreases from 180 Hz of the initial substrate to 150 Hz of the adduct. In decoupling experiments, selective irradiation at vH6 and vH5 causes the C-6 and C-5 doublets, respectively, to collapse into singlets. 

Despite the potential for geometrical isomers, in almost all cases, a single set of signals is observed by both H and l3C NMR spectroscopy for silyl nitronates. This... 

The concern that the solid state structures do not represent those in the solution phase in which the H NMR data were obtained was addressed by comparing the NMR spectra in both phases. Thus it was demonstrated for 17 and 18 through the use of solid state CPMAS and solution l3C NMR spectroscopy that there were no fundamental differences in the structure or charge distribution of the cation in solution or the solid state91 92. 

Modern analytical techniques have been developed for complete characterization and evaluation of a wide variety of sulfonic acids and sulfonates. Titration is the most straightforward method of evaluating sulfonic acids. Spectroscopic methods for sulfonic acid analysis include ultraviolet spectroscopy, infrared spectroscopy, and lH and l3C nmr spectroscopy. Modem separation techniques of sulfonates include liquid chromatography and ion chromatography. See also Chromatography. 

Conformational analysis of 4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidine-3-acetates and -3-carboxylates 30 (R = H) and their mono-methylated (R = Me, R1 = H) and 6,9-, 7,9-, and 8,9-dimethylated derivatives were carried out by H and l3C NMR spectroscopy (86JOC394). At ambient temperature the 6-methyl derivatives predominantly adopt the energetically most favorable half-chair conformation with a pseudoaxial methyl group. In the other half-chair conformation a serious 1-3 allylic strain exists between the pseudoequatorial methyl group and the adjacent carbonyl group. At the 7- and 8-methyl derivatives the half-chair conformations with equatorial methyl group occur almost exclusively, but the 9-... 

Addition of bromine in dichloromethane or of thioacetic acid in ethanol onto the methylene group of 9-arylidene and 9-carboxymethylene moieties of 6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidines 546 stereoselectively gave 9-substituted 6,7,8,9-tetrahydro derivatives 547 and 548, respectively, at ambient temperature (Scheme 34) (90JHC247). Addition was also stereoselective with respect to the C(9) and C(9)—C centers, giving the erythro diastereomers as the primary products, which may then undergo epimerization to the threo isomers. The structure and epimerization of the products were studied by H and l3C NMR spectroscopy and by molecular mechanics calculations. 

Wilson, M. A., Barron, P. F., and Gillam, A. H. (1981a). The structure of fresh-water humic substances as revealed by l3C NMR spectroscopy. Geochim. Cosmochim. Acta 45, 1743-1750. 

Proton magnetic spectroscopy is discussed first. After a brief discussion of the theory behind this technique, the use of H-NMR spectroscopy to determine the structure of the hydrocarbon part of the compound is described. Next the use of l3C-NMR spectroscopy to gain information about the carbons in the compounds is presented. Finally, examples of the use of various combinations of these techniques to identify unknown organic compounds are discussed. 

Click Coached Tutorial Problems for more practice counting carbons in l3C-NMR Spectroscopy. 

As for l3C-NMR spectroscopy where the diagnostic carbon atoms were most frequently those a to the nitrogen atom, so too in 1 H-NMR spectroscopy the diagnostic protons are most frequently those at carbons a to the nitrogen, principally H-6e and H-6 e. Also often helpful were the signals from protons attached to carbons of the spirotetrahydrothiophene ring, i.e., H-17A, H-17B, H-17 A, and H-17 B. 

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