Chemical shift standards

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. 

Fig. 2. Simultaneous detection of multiple fluorinated molecules in vivo. To explore the hypothesis that uptake of the anticancer drug 5FU by tumors is pH dependant, we infused 5FU (0.4 ml (50 mg/ml) IV), the extracellular pH reporter CF3POL (400 mg/kg IP), and the chemical shift standard NaTFA (200 mg/kg IP) into an anesthetized rat (1% isoflurane) with a subcutaneous 13762NF breast tumor (1.4 x 1.5 x 1.1 cm). Thirty minutes after administration, all four molecules were detectable simultaneously in 17 min. At this stage, no metabolites of 5FU were detected.

Unless otherwise noted, CFCI3 was used as a chemical shift standard, solvent CH3CN. 6-Fluorotryptophan was used as a chemical shift standard. 

Mohan et al. [8] used an NMR spectroscopic method to characterize impurity D in samples of clopidogrel bisulfate. The method entailed XH NMR (at 400.13 MHz) and 13C NMR (at 100.62 MHz), with a sample concentration of 1 mg/ml in DMSO-t/f, (this solvent also served as an internal chemical shift standard). It was found that the 1H NMR spectrum of impurity D exhibited one hydrogen band than did that of clopidogrel, while the 13C NMR and DEPT135 NMR spectra indicated the presence of one methyl carbon, two methylene carbons, eight methine carbons, and five quaternary carbons. This corresponded to a similar structure as for clopidogrel, but with one less methylene carbon and one more methine carbon. 

T 16 soM-state CPMAS and solution spectra of this porphyrin are shown below. Give possible explanations for die difference in appearance. C is CDCI3 in the solution spectrum P is a crystalline polyethylene chemical shift standard added to the solid. 

Wide scans (0 to 1000 eV) were performed for surface elemental analyses. The wide scans were carefully inspected for trace element contamination. Detailed 20 eV scans of the C-ls (275 to 295 eV), 0-ls (520 -540 eV) and Al-2s (105 to 125 eV) regions for the pyrolytic carbon and of the C-ls and 0-ls for the polystyrene were run to determine both elemental stoichiometry and chemical shifts. Standards were available to give accurate chemical shift data for various carbon-oxygen functional groups. These included poly(ethylene terephthalate), poly(ethylene oxide) and anthraquinone (17 ). The latter was run at -50°C in order to minimize volatility under our high vacuum conditions. Table I sunmarizes these results. All spectra were charge - referenced to a C-ls line for an alkyl-like carbon at 284.0 eV. The Scofield theoretical XPS photoelectric cross sections ( ) were used for elemental quantitation. 

Instrument frequency, solvent and chemical shift standard should be given, e.g. 5h (300 MHz, CDCI3) - the standard can be in a preamble. 

Compounds with high internal mobility, such as adamantane, can have line widths as narrow as 0.01 ppm. This is why in solid-state NMR, adamantane is used not only as a chemical shift standard, but also for optimizing experimental variables and for shimming probes. A typical line width encountered in crystalline organic compounds is of the order of 0.5 ppm, but the exact value will be strongly dependent on the mobility of the compound. It is not unusual to resolve the majority of carbon resonances in compounds containing 40 or more carbons. 

The solid-state MSi NMR spectrum of talc displays a single resonance at -98.5 ppm [24], Upon, VA1 substitution, the wSi chemical shift value is shielded (more positive ppm value), whereas further distortion of the tetrahedral sheet is reflected in a decreased chemical shift value (deshielded, increased negative ppm value). The authors of this report suggest that solid-state wSi NMR may be used to determine the compositional nature of phyllosilicate minerals. In each of these studies, the spectra were obtained at a frequency of 71.5 MHz on a Fourier transform spectrometer interfaced to a 8.45-T magnet. Tetramethylsilane (TMS) was used as an external chemical-shift standard. 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

There is still no unique standard for the calibration of15N NMR spectra. Six solid standards as well as liquid ammonia are frequently used as external references for the calibration of nitrogen shieldings in the solid state. Although chemical shifts in solids do not depend on the temperature and concentration (as the chemical shifts of liquid standards) there are discrepancies in the literature data on the exact chemical shifts for particular standards. In order to avoid errors created by conversion of literature data from different chemical shift scales, precise resonance positions of chemical shift standards are required. Since the difference in the resonance frequency between proton decoupled spectra... 

Substituents Acid system Temp. °C Chemical shifts Standard Ref. 

Fig. 7.4 S olution-state H NMR spectrum of 1-chloroethene, plotted on the chemical shift scale. Here, the zero, 0 ppm, is set by Vreference the frequency at which hydrogens of a chemical shift standard are resonant at...

In some cases, it may be advisable, or necessary, to include a chemical shift standard with the sample. 

Chemical shifts are most often reported in reference to the absorption of the protons of TMS (tetramethylsilane), which is defined as zero on the 8 scale. A small amount of TMS is either included as an internal standard in the solvent for a sample, or the NMR spectrometer itself is calibrated electronically to a chemical shift standard. 

Table 2 Recommended chemical shift standards for state NMR SSNMR secondary lUPAC primary standard standard Br, and 1 solid- Secondary chemical shift (ppm) ...

The compositions and sequence distributions of P(3HB-co-3HH) copolyesters were analyzed by NMR technique. The copolyesters were dissolved in CDCI3 at a concentration of 10 mg/mL and their H and C NMR spectra were recorded on a JEOL ALPHA-400 spectrometer. Tetramethylsilane (Me4Si) was used as internal chemical shift standard. 

Determination of PHA polymer composition by nuclear magnetic resonance (NMR). Twenty mg of each polymer was dissolved in 1 ml of CDCI3 and subjected to both H and C NMR analysis. H NMR spectra were recorded using a JEOL a-400 spectrometer with a 5.0 ps pulse width (45 pulse angle), 3-s pulse repetition, 7500-Hz spectra width, and 16K data points. For C NMR analysis, ta were collected using a JEOL ECP-500 spectrometer with a 7.0-ps pulse width (45° pulse angle), 5-s pulse repetition, 25000-Hz spectra width, and 64K data points. Tetramethylsilane (Me4Si) was used as an internal chemical shift standard. 

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