Chemical shift external reference

Chemical shift is referred to CS2 as external standard. A shift towards higher frequencies is defined as positive. 

The observed stability constants were calculated from the phase solubility diagrams [7, 8], which were determined in water at 25 °C. H-NMR spectroscopy was carried out in 0.02M NaOD solution at 24.5 0.5 C using a JEOL FX-lOO spectrometer. H chemical shifts were referred to tetramethyl silane external standard. 

Estimated precision in the chemical shifts is 0.05 p.p.m. The chemical shifts are given relative to external 1,4-dioxane, which was introduced into some samples only to obtain chemical shifts. Spectra obtained at 258 for — 10% solutions. Spectra of compounds were obtained at 22.5 MHz see Ref. 20. Spectra of compounds were obtained at 22.5 MHz see Ref. 24.J Spectrum obtained at 100.6 MHz see Ref. 24. Data taken from Ref. 61. Chemical shifts for GalNAc only are given. The data given in the parentheses for compounds 51 and 32 refer to the carbon count. 

P chemical shift (8) relative to external 85% H3P04. b The designations (b) and (t) refer to bridging and terminal phosphido atoms. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

Fig. 16.— 13C-N.m.r. Spectra of Polysaccharides (in DsO) Containing (1— 3)- and (1—>4)-Linked /3-D-Mannopyranosyl Residues, Obtained from [2H]-Labelled D-Glu-coses. (Temperature, 70° chemical shifts expressed as 8C, relative to external tetra-methylsilane the number marked with a superscript asterisk refers to the position of labelling in each D-glucose derivative.)...

Chemical shift values upheld (from HCONH2 as external reference) in 1.7 M Me2CO-d solutions. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

The chemical shift is measured relative to a reference compound, for example LiBr in THF or H2O. Due to the common dynamic processes observed between organohthium complexes, an external reference is often used. One problem is that there is not a single reference compound in general use, but different external references are employed. Thus, chemical shifts reported in different investigations must be compared with caution. 

The NMR spectra of adsorbed xenon were obtained on a Bruker MSL-300 spectrometer operating at 83.0 MHZ and 295K. Typically, 2000-40000 signal acquisitions were accumulated for each spectrum with a recycle delay of 0.3s between 90 pulses. The Xe NMR chemical shifts were referenced to that of external xenon gas extrapolated to zero pressure using Jameson s equation [11]. All resonance signals of xenon adsorbed in zeolites were shifted downfield from the reference but were taken to be positive in this report. 

All chemical shifts are in ppm, and liquid ammonia was the external reference used. 

Here, a denotes the shielding constant caused by the anisotropic shielding of the external magnetic field by the electron shell around the resonating nuclei. The so-called isotropic chemical shift, 5, is defined as <5 = cr ref—<7, where cr ref is the shielding constant of the nuclei in a reference material, and <5 is defined by the following ... 

For line shift measurements with the eight-pulse cycle between 180°K and room temperature, the reference was acetyl chloride. Its frequency was measured relative to a spherical tetramethylsilane (TMS) sample at room temperature, and all results are reported relative to this TMS on the r scale, (r = a + 10 ppm, where a is the signed chemical shift used in solid state NMR.) At lower temperatures, the reference was a single crystal of Ca(OH)2, oriented in the magnetic field such that the major axis of its proton chemical shift tensor was parallel to the external field (19). Thus, it is assumed that the proton chemical snift of the Ca(OH)2 remained unchanged as the temperature was varied. 

Figure 4. The eight-pulse line shape and the peak locations of the Th4Hi5 (LP) powder sample as a function of temperature using a Ca(OH)2 single crystal as reference. The reference is oriented such that the major principal axis of the proton chemical shift tensor is parallel to the external magnetic field. A shift to the left signifies an increase in the value of

FIGURE 43. 29Si NMR MAS spectrum of Si50(PC>4)6. The chemical shift is referenced to the M signal of the Q8M8 external standard (+11.5 ppm). Delay between 30 ° pulses is 20 s, number of scans is 2242 and the frequency is 39.74 MHz. Reproduced by permission of Elsevier Science from Reference 147... 

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