Quantitative NMR spectroscopy

A plethora of pure drugs, their respective combinations and their dosage forms have been assayed by NMR-spectroscopy quantitatively by various researchers and the result(s) thus obtained were duly verified and compared with the standard methods prescribed in various official compendia. A few typical examples of such drugs shall be described briefly here ... 

The properties of a copolymer depend on its composition, monomer sequence and stereochemical structure. Although compositional analysis can be achieved by several methods other than NMR spectroscopy, quantitative data on monomer sequence distribution can only be obtained from NMR spectroscopy. I3C NMR chemical shifts of C=0 carbons of PMMA are sensitive to pentad to heptad stereochemical sequences. The C=0 carbon signals for the copolymers of methacrylates are also sensitive to triad comonomer sequence. Thus it should be difficult to assign both tactic and comonomer sequence signals, especially in the case of copolymers with low stereoregularity. 

S units by quantitative NMR spectroscopy (Quantitative C NMR of Lignins section) or from 2D HSQC spectra (Quantification in 2D NMR section). Other differences between GS lignins are frequently the presence of unique entities, such as incorporated hydroxycinnamates (Ferulates in Lignins Cell Wall Cross-Linking in Grasses by Ferulates section), and y-acylated units (p-Hydroxybenzoates in Various Flardwoods section) [309]. 

Van Hecke P and Van Huffel S (eds.) (2001) NMR spectroscopy quantitation, special issue NMR in Biomedicine... 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

In addition to modem spectroscopic methods ( H nmr spectroscopy, ftir spectroscopy) and chromatographic methods (gc, hplc), HBr titration (29) is suitable for the quantitative analysis of ethyleneimine samples which contain relatively large amounts of ethyleneimine. In this titration, the ethyleneimine ring is opened with excess HBr in glacial acetic acid, and unconsumed HBr is back-titrated against silver nitrate. 

Measuring Protein Sta.bihty, Protein stabihty is usually measured quantitatively as the difference in free energy between the folded and unfolded states of the protein. These states are most commonly measured using spectroscopic techniques, such as circular dichroic spectroscopy, fluorescence (generally tryptophan fluorescence) spectroscopy, nmr spectroscopy, and absorbance spectroscopy (10). For most monomeric proteins, the two-state model of protein folding can be invoked. This model states that under equihbrium conditions, the vast majority of the protein molecules in a solution exist in either the folded (native) or unfolded (denatured) state. Any kinetic intermediates that might exist on the pathway between folded and unfolded states do not accumulate to any significant extent under equihbrium conditions (39). In other words, under any set of solution conditions, at equihbrium the entire population of protein molecules can be accounted for by the mole fraction of denatured protein, and the mole fraction of native protein,, ie. 

J3 4 = 3.45-4.35 J2-4 = 1.25-1.7 and J2-5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. C chemical shifts for thiophene, from tetramethylsilane (TMS), are 127.6, C 125.9, C 125.9, and C 127.6 ppm. 

It would be pertinent to point out (25,27) that the trisubstituted isomer of the enamine of 2-aIkylcyclohexanone reacts in a quantitative manner with ethyl azodicarboxylate to give the addition product (35). This reaction in Conjunction with NMR spectroscopy can thus be employed for the determination of the amount of the trisubstituted isomer. According to the authors, hydrolysis of 35 furnishes the corresponding cw-2,6-disubstituted cyclohexanone (36) this seems unlikely since it would involve the stereo-electronically unfavored equatorial protonation of the enamine. 

The problem has been overcome most rehably by using - or N NMR techniques, which provide a wider range of chemical shifts and, thus, allow one to quantitatively characterize many dynamic processes which are too fast to be measured by methods of NMR spectroscopy. 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

The synthesis of 1 -benzothiepin 1 -oxide (23) can be achieved via complex formation with tricarbonyl iron, and quantitative oxidation of the coordination compound 22 with 3-chloroperoxy-benzoic acid. Subsequent irradiation at — 50 C provides 23, which crystallized as yellow needles after low-temperature (-40 C) chromatography, and was characterized by 1H NMR spectroscopy at — 30 C23 before loosing sulfur within one hour at 13°C to give naphthalene. 

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

H-NMR spectroscopy can be used to determine alkenesulfonates in mixtures [115]. Under normal conditions, 1-alkenesulfonate shows a signal separated from the other positional isomers [122]. Moreover, the utilization of a lanthanide shift reagent makes possible even the separation of the signals of isomeric alkenesulfonic acids and hydroxyalkanesulfonic acids as their methyl esters [124]. 13C-NMR spectroscopy, which is not as quantitative, simply gives the cis/trans ratio of the main positional isomer. 

Unfortunately, in most of the previous examples, the extent of the asymmetry-induction was determined by chiroptical measurements (ORD, CD) that gave qualitative and not quantitative information. The NMR chiral shift efficiency of TRISPHAT 8 and other hexacoordinated phosphate anions was therefore considered as an excellent analytical tool to provide accurate measurement of the induced selectivity by NMR spectroscopy. 

The reactions depicted in Eq. (1) are suitable for calorimetric investigations since they proceed rapidly and quantitatively as monitored by NMR. spectroscopy. The. solution calorimetric protocol has been described elsewhere." The enthalpy values were determined by anaerobic solution calorimetry in THF at 30 C by reacting 4 equivalents of each carbene with one equivalent of tetramer. The results of this study are presented in Table I. 

The conversion of 70 to the final PPV 60 is then carried out thermally at relatively low processing temperatures (about 100-150 °C) with elimination of (unstable) alkylsulfinic acid. TGA-mass spectroscopy, FT-IR, UV/Vis and CP/MAS NMR spectroscopy are all consistent with quantitative elimination and formation of PPV 60. 

As relatively few standard compounds are available from commercial or other sources, identification of flavonol glycosides has to be achieved by alternative means, for example UV-, H- and C-NMR spectroscopy. Therefore hydrolysing all glycosides to aglycones followed by HPLC determination offers a practical method for the quantitative determination of flavonoids in tea (Hertog et al, 1993a Wang and Helliwell, 2001). 

An early example of surface-mediated synthesis led to the conversion of Os3(CO)i2 into [Os5C(CO)i4] [13]. Quantitative characterization of the formation of this cluster on MgO by C NMR spectroscopy [15] showed that the synthesis gave [OssCCCOIh] " in a yield of about 65% other products included triosmium and tetraosmiimi carbonyl clusters. A schematic representation of the surface chemistry is shown in Fig. 1. 

Spectrophotometric and spectrofluorimetric methods provide a wealth of information concerning structural determinations (identification, purity and precise measurement of concentration) and chemical changes in alkaloids. These techniques yield both quantitative and qualitative data on the effect of solvents, pH and other physiological conditions [141-143]. X-ray crystallography, H and NMR spectroscopy, infrared spectroscopy (IR) and circular dichroic spectroscopy were also used to study the physical properties... 

Dithionite reduction of 13C-labeled WV-15 (structure in Scheme 7.6) was carried out in anaerobic D20 and the resulting products evaluated by quantitative 13C-NMR spectroscopy immediately after the reduction (Fig. 7.8).45 The spectrum indicates that both the sulfite ester and the sulfonate are formed in a 60 40 ratio. Also, no long-lived methide species are observed in this reaction. 

M. Maiwald, H. H. Fischer, Y.-K. Kim, K. Albert, H. Hasse 2004, (Quantitative high-resolution on-line NMR spectroscopy in reaction process monitoring), /. Magn. Reson. 166, 135. 

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