Chemical shifts reference standards

Although originally the chemical shift reference standards were powdered CuCl, or K3[Cu(CN)4] in D20, all Cu chemical shifts are now routinely... 

Additional substances are sometimes added to the sample solution in order to facilitate measurements or to provide an internal reference for chemical shift or quantitation. Certain experiments benefit from the use of a small amount of a paramagnetic relaxation agent such as chromium(III) acetylacetonate (Secs. II.D.4 and II.G). A chemical-shift reference standard may be added if exact shifts are required. Some quantitative analyses employ an internal spin-counting intensity reference such a standard must not interfere with the sample spectrum. If the reference material is inert, it can be added directly to the sample. If it might react with the sample, the... 

The resonance frequency in a magnetic field strength that gives the resonance of SiMe4 at exactly 100 MHz. Commonly accepted chemical shift reference standard material. 

PULSE SEQUENCE= has been added as a new LDR. The allowed abscissa units in XUNITS= have been modified as have the YUNITS= for time domain and 2D NMR experiments. To cover concentration effects the LDR SAM-PLE DESCRIPTION= should be used where relevant. In addition, a new LDR. SOLVENTNAME= should contain a description of the solvent including such information as pH, ionic strength, etc., for aqueous solutions. A new LDR. STANDARDNAME= has also been proposed which will not only include the chemical shift reference standard name but also whether it is internal or external. 

Table 1. Chemical Shift Reference Standards for Post-Transition Metals...

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. 

The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). X-ray powder diffraction patterns were recorded after synthesis and template removal on a Siemens D5000 diffractometer using CuKa radiation. After calcination, nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2010 sorption analyzer. Al MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer using single pulse excitation with standard 4 mm rotors The resonance frequency was (Oo/271 = 104.31 MHz for Al using a 7t/20 pulse and a 0.5 s recycle delay. A 0.1 M solution of aluminum nitrate in water was employed as the chemical shift reference... 

Theoretically, any external standard may be used, provided that it is clearly defined, and that precise data are available which make possible the accurate recalculation of nitrogen chemical shifts referred to other standards. Practically, there are limitations concerning the concentration of nitrogen nuclei in the reference compound, the sensitivity of its nitrogen chemical shift to concentration and solvent effects in the case of solutions, effects due to possible impurities, nuclear Overhauser effects on the resonance upon proton decoupling etc. 

As mentioned in Section 2.3.1, the almost universally adopted primary reference for chemical shifts in NMR spectra is the compound tetramethylsilane (TMS). Incidentally, TMS is also employed as the primary chemical shift reference in H and Si NMR [18]. Some old reports give chemical shifts relative to benzene or carbon disulfide, but the use of the TMS resonance as the zero chemical shift mark is now commonplace. Unfortunately, TMS is highly volatile, toxic, and flammable, which makes problematic its use as a direct reference sample. A simple alternative is to use another sample with a known spectrum as an external and secondary reference. In the case of solid-state NMR there are many known secondary standards, such as adamantane (higher-frequency peak at 38.6 ppm), hexamethylbenzene (higher-frequency peak at 132.2 ppm), and glycine (higher-frequency peak at 176 ppm) [4,15,98]. 

In F NMR experiments, a problem exists in the choice of chemical shift reference since the reactivity of some fluorine compounds has often precluded their use as internal standard. However, the standard... 

A number of analytical methods have been developed for the determination of chlorotoluene mixtures by gas chromatography. These are used for determinations in environments such as air near industry (62) and soil (63). Liquid crystal stationary columns are more effective in separating m- and chlorotoluene than conventional columns (64). Prepacked columns are commercially available. ZeoHtes have been examined extensively as a means to separate chlorotoluene mixtures (see Molecularsieves). For example, a Y-type 2eohte containing sodium and copper has been used to separate y -chlorotoluene from its isomers by selective absorption (65). The presence of ben2ylic impurities in chlorotoluenes is determined by standard methods for hydroly2able chlorine. Proton (66) and carbon-13 chemical shifts, characteristic in absorption bands, and principal mass spectral peaks are available along with sources of reference spectra (67). 

It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton fC. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength Bg of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, Sh for protons, 5c for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 1O. ... 

Entry Compound" Chemical shift, 6 Solvent standard Reference... 

IUPAC recommends CD3N02 (90% in CDC13) as the chemical shift standard for both 14N and 15N). However, some spectroscopists reference nitrogen spectra to liquid NH3 (not a very convenient standard ) and IUPAC recommend this as an alternative for 15N. The chemical shift of CH3N02 with respect to liquid ammonia is 380.5 ppm. 

In NMR spectroscopy, however, the chemical shift measurement we make takes place in an environment of our making that is both entirely artificial and arbitrary (i.e., the magnet ). For this reason, it is essential to reference our measurements to a known standard so that we can all speak the same language, no matter what make or frequency of spectrometer we use. 

There seems to be no universal reference standard in 19F NMR as there is in proton NMR and this can cause confusion. Chemical shifts may be quoted relative to CFCI3 or to CF3COOH and there maybe a... 

Chemical shift Position of resonance in an NMR spectrum for any signal relative to a reference standard. Chiral centre An atom in a molecule (usually but not exclusively carbon) which is bound to four different atoms or groups such that the mirror image of the whole molecule is not super-imposable on the molecule itself. A chiral centre in a molecule implies the possibility of the isolation of two distinct forms of the compound which are known as enantiomers. 

NMR is an analytical technique that has been applied to the studies of chemical reactions with promising results. The chemical shift parameter may be used as a measure of the relative proportions of different species that are present in solution. These shifts are measured relative to a standard reference sample. 

By comparing the chemical shifts and peak heights of an unknown with standards or known reference materials, some predictions of the unknown structure can be made. In some exceptional but important cases, such as the photodetachment of negative ions, the wavelength of light causing photoionization is in the visible or even the near infrared, allowing extremely precise structure determinations. 

The carbon-13 NMR spectrum of griseofulvin (Figure 3) was obtained at ambient temperature in DMSO-d containing TMS as internal reference utilizing Varian Associates XL-100-15 spectrometer equipped with Fourier accessories The system was locked to the deuterium resonance frequency of the solvent, and operated at a frequency of 25.2 MHz for carbon-13. The chemical shifts are reported ( c, ppm.) from the Internal standard TMS. 

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