Nuclear magnetic resonance carboxylic acids

In order to co clarify the role of complex formation, the new data on stability constants should be accumulated, being collected at strictly similar conditions. It should be also mentioned that any analysis of equilibrium in solutions involving anions of polybasic hydroxy carboxylic acids requires the data on the deprotonation constants of the acid in question. This information would be crucial for conclusions regarding the presence and stability of mixed complexes in the system. Valuable knowledge about the structure of complex compounds present in solutions (and in precursors as well, see later) may be gained by means of vibrational spectroscopy (IR and Raman spectra) and nuclear magnetic resonance. 

Fulvic and humic acids have been investigated with carbon-13 and proton nuclear magnetic resonance spectrometry, GC/MS, and IR spectroscopy. The fulvic and humic acids were found to be predominantly carboxylic and aromatic with a high proportion of 0- and Ji-substituted carbon atoms, although aliphatic ones were also observed. 

In the nuclear magnetic resonance spectra of carboxylic acids, the carboxyl proton is seen to absorb at unusually low magnetic fields. This is illustrated in Figure 18-3 by the spectra of phenylethanoic acid (C6H5CH2C02H)... 

The presence of a carboxylic acid group is indicated by strong infrared absorption in the region of 1720 cm-1 (C=0 str.) and broad absorption between 3400 cm-1 and 2500 cm-1 (OH str.) in the nuclear magnetic resonance spectrum the acidic hydrogen (replaceable by D20) will appear at very low field (3 10-13). 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

Certain of the p-chlorophenyl-substituted carboxylic acids are related products in a multistep synthesis. A schematic outline of synthetic procedures is presented to clarify these relationships. Properties of seven new p-chlorophenyl-substituted intermediates and six new p-chlorophenyl-substituted carboxylic acids are reported. All of the 13 new compounds have been characterized by conventional criteria. Both gas-liquid chromatography and nuclear magnetic resonance are used to assess the purity of the carboxylic acids and their intermediates. 

Le-Thanh, H. and Vocelle, D., Nuclear magnetic resonance study of complexes formed between conjugated Schijf bases and carboxylic acids. Can. J. Chem. 72, 2220-2224 (1994). 

The incorporation of an aspartic acid residue with a atom at the side-chain carboxyl function at position 25 into aspartyl protease has made this catalytically essential group visible for nuclear magnetic resonance (NMR) spectroscopy (Fig. 2)... 

The rate constant (36) and (37) as a function of temperature correlated well with the experimental data obtained for the carboxylic acid protons of crystalline perprotobenzoic acid and ring-deuterobenzoic acids by nuclear magnetic resonance 7) [75] and inelastic neutron scattering (for an analysis of the experiment see Refs. 76 and 77). It should be noted that some of the major parameters of the model (for instance,. /) allowed the direct determination by fluorescence line narrowing technique. 

The trifluoroethyl (CF3CH2—) derivative of organic acids and alcohols has been found to be useful for distinguishing carboxyl from primary hydroxyl from secondary hydroxyl functional groups by F nuclear magnetic resonance spectroscopy (Roller and Dorn, 1982). This derivative is formed under conditions shown in reaction (4) ... 

Nuclear magnetic resonance studies revealed that normal prothrombin contains y-carboxyglutamate, a formerly unknown residue that evaded detection because its second carboxyl group is lost on acid hydrolysis during amino acid analysis. The abnormal prothrombin formed subsequent to the administration of anticoagulants lacks this modified amino acid. In fact, the first 10 glutamate residues in the amino-terminal region of prothrombin... 

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