Reference, chemical shift

The study of Lemieux and co-workers (38, 39) which first delineated the stereospecific dependences of the P.M.R. parameters of pyranose derivatives has since been confirmed by many workers (24, 40, 44). In the following section we shall discuss some of the more recently recognized stereospecific dependences, dealing first with the dependences of coupling constants and then with those of chemical shifts. Reference will be made to the P.M.R. parameters of the hexopyranose derivatives which were obtained in experiments outlined in the previous section, together with the parameters of the a-(4) and, / -(5) anomers of 1, 3, 4, 6-tetra-0-acetyl-2-deoxy-D-arafoino-hexopyranose. 

Fig. 6. CP-MAS l3C-NMR spectra of polydimethylsiloxane at 75.47 MHz above and below the melting transition. Chemical shifts refer to TMS = 0 ppm and correspond to the scale at the bottom (Ref.10))...

Fig. 8. Slow exchange-fast exchange transition for the conformational interconversion of crystalline cyclotetraeicosane in CP-MAS l3C-NMR spectra at 75.47 MHz. Chemical shifts refering to TMS = 0 ppm and temperatures in K are indicated at the spectra. (Ref.7 )...

Fig. 15. Splitting pattern with the assignment of the 13C-NMR shifts of meso-4,5-dimethyloctane at 100.6 MHz within the slow exchange regime of the CH—CH bond rotation. Chemical shifts refer to TMS = 0 ppm. (Ref. 30>)...

Spectrum of 7 Chemical Shift (Reference), Coupling with P... 

It is important to note the orientation dependence of the shielding constant, cr, and the fact that shielding is proportional to the applied field, whence the need for chemical shift reference materials such as tetramethylsilane. 

Fig. 8. Oxygen-17 NMR spectra at pH = 1.3, illustrating the formation of the di-nuclear complex at 25°C after (a) 3 min, (b) 1 h and (c) 6 h of mixing [Re02(CN)4]3 and H2170. The total complex concentration [Re] = 0.2 m, /m = 1.0 m (KN03), NMR chemical shift reference is the nitrate ion (<5(N03) = 413 ppm) (5). (Adapted with permission from Roodt, A. Leipoldt, J. G. Helm, L. Merbach, A. E. Inorg. Chem. 1992, 31, 2864-2868. Copyright 1992 American Chemical Society).

Sc, carbon chemical shift, referred to tetramethylsilane (8 = 0) (cf. Sect. I) SCS, substituent-induced chemical shift, or substituent effect difference between S s of a given carbon atom in a monosubstituted and the respective unsubstituted parent molecule (cf. Sect. Ill) NAE, nonadditivity effect nonadditivity of individual SCSs in disubstituted molecules (cf. Sect. IV) ICS, intramolecular-interaction chemical shift = NAE (cf. Sect. IV) A, polarization effect difference in S s of sp2 carbon atoms in a double bond (cf. Sect. IV-C) LEF, linear electric field (cf. Sect. II-B-3) SEF, square electric field (cf. Sect. II-B-3). 

Using a somewhat different approach, Bucci (62) interpreted these substituent influences in terms of a combination of electronegativity and the number m of lone electron pairs at the substituent or at that atom which is directly attached to the a-carbon atom, respectively, and derived the empirical expressions [5] and [6] for, 3C chemical shifts referred to tetramethylsilane (8 = 0) ... 

The scatter plot in Figure 1 gives correlation of the quadrupole splitting and the chemical shift (at 78°K.) for several tin compounds (5, 9, 16) using white tin (beta) for a chemical shift reference. The various sources and their linewidths are also shown. 

J alP Chemical shifts referred to P4Oe have been corrected by +112-5 ppm 19F chemical shifts referred to CF3C02H, CF3C6H5 or F2 have been corrected by — 79 9, — 63 8 and 427 ppm respectively. 

Unless otherwise noted, dilute CF3CO2 was used as chemical shift reference ... 

For quantitative estimation, a sealed reusable capillary tube, with a known quantity of sodium salt of trimethylsilyl propionic acid (TSP) dissolved in 35 pi of D20, is inserted into the NMR tube while obtaining NMR spectra. The internal standard TSP is used as a chemical shift reference as well as a quantitative standard for the estimation of metabolites, and D20 is used as the field-frequency-lock . Spectra are acquired at room temperature. Typical spectra acquired at room temperature of human bile and standard glycine- and taurine-conjugated BAs are shown in Fig. 5.4.16. 

Urine is centrifuged before analysis. A 70- pi volume of TSP in D20 is added to 700 pi urine. This will provide a chemical shift reference as the TSP protons resonate at... 

Some other data on, 5N chemical shifts, referred to internal MeN02, are recorded in Table 25. Comparison of pyrrole with imidazole demonstrates that the deshielding of the pyrrole-type nitrogen is about 15 ppm. Increased nitrogen shielding is observed for 2,1,3-benzothiadiazole ( + 49 ppm) and the parent 1,2,5-thiadiazole ( + 34 ppm). In contrast, the corresponding oxadiazoles were deshielded, as in 2,1,3-benzooxadiazole ( — 36 ppm) and 1,2,5-oxadiazole (— 34 ppm). 

Ito and Fraissard [2] expressed the room temperature l2,Xe NMR chemical shift of Xe adsorbed in porous adsorbent as 8 = 80 + 8s(p = 0) + o,p, where 80= 0 is the chemical shift reference, 8s(p = 0) represents the interaction between a Xe atom and the wall of the adsorbent. The last term represents the contribution arising from binary Xe-Xe interactions which, at moderate loading, is linear with p. In the present study, the observed l2,Xe NMR chemical shifts can be expressed as the function of Xe loading and temperature as ... 

The term 8g(pg, T) can be expressed explicitly according to the relation given by Jameson and co-workers [15], where 80g = 0 is the chemical shift reference. Here, we state without proof that the temperature dependence of the Xe-wall interaction can be explicitly expressed as [10] ... 

Iron species Solvent 15 N chemical shift Reference... 

Srinivasan, P.R. and Lichter, R.L., Nitrogen-15 nuclear magnetic resonance spectroscopy evaluation of chemical shift references, J. Magn. Reson., 28, 227, 1977. 

Chemical shift references for calibration of the spectrum scale may be internal or external. An internal chemical shift reference substance is dissolved... 

To avoid overlap of the resonances of the reference substance and chemicals of interest, and a possible overdose of the reference substance, and also from the point of view of optimum magnet homogeneity, we recommend the external chemical shift reference method with the use of a separate sample tube. 

In principle, preparation of an NMR sample is simple take a suitable amount (1-100 mg) of chemical to be analyzed, dissolve it in a selected solvent, add the chemical shift reference, and then transfer the sample with simultaneous filtration into an NMR... 

NMR spectral parameters, that is, chemical shift (8) and coupling constant (J), may be considerably affected by the sample condition, that is, solvent, pH, sample temperature, concentration, and choice of internal and/or external chemical shift references. Solvent and pH (in water/D20 samples) have the greatest effect. The sample condition should therefore be the same as or comparable to that used for the authentic reference chemical (or library spectrum) and the blank sample. 

If an internal chemical shift reference is to be used, a dilute solution of reference substance (e.g. TSPA-d4) is prepared in the same solvent as used for the test sample (e.g. D20). A few microliters (or a suitable amount) of this reference solution are added to the evaporated sample solution before filtration into the NMR tube. 

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