Referencing of chemical shifts

Detection of NMR parameters and referencing of chemical shifts NMR Methodology... 

In order to have a reliable measure of chemical shift, we need to have a reference for the value. In proton NMR this is normally referenced to tetramethyl silane (TMS) which is notionally given a chemical shift of zero. Spectrum 1.1 shows what a spectrum of TMS would look like. 

Zhang, H., Neal, S. and Wishart, D. S. (2003). RefDB a database of uniformly referenced protein chemical shifts. J. Biomol. NMR 25,173-195. 

A different method of referencing is used in the E-scale of chemical shifts, sometimes referred to as universal referencing . (We shall reserve the term absolute scale , which is also used in this context24, for a different concept see below.) In this scale the chemical shift is given as the absolute frequency of the signal that would be observed in a magnetic field in which the XH NMR signal of TMS appears exactly at 100 MHz. Thus... 

Tab. 14.1 I nter-relation of chemical shifts in the various referencing schemes. ...

After assignment of the resonances of the protein, secondary-structure information can be retrieved from chemical shifts quite directly. For H , C , Cp, and carbonyl resonances, one observes conformation induced changes of the chemical shifts that can be referenced to chemical shifts measured from the same amino acid in a peptide. As a rule, these structure-induced chemical shifts are low field for H and Cp, and high field for C and carbonyl in a )8-strand. For a-helices, the structure-induced chemical shift is opposite [66] [67]. Thus, the secondary-structure elucidation is possible based on the assignment of the resonances and the determination of the chemical shifts. The result from this analysis for the CaM/C20W complex is shown in Fig. 30... 

Another of the problems of N spectroscopy is the relative lack of chemical shift data it may well be the case that model data have to be generated to answer specific questions. References [78,79, 38], give some very useful shift data and references to other sources. Reference [80] gives an excellent introduction to the area, with a discussion of N referencing, N chemical shift ranges and practical experimental detail, in addition to a survey of long-range correlation experiments. 

As shown in Figure 7.2, an NMR spectrometer consists of a strong magnet, a radio station for sending pulses and a radio frequency receiver to detect the FID, and a computer to control acquisition, record and transform the data. Frequencies for a given nucleus are referenced to a standard, for example the proton peak of tetramethylsilane is assigned 0 frequency and all proton peaks are related to that position in terms of chemical shifts in parts per million (ppm). Chemical shifts place resonances of different types in characteristic regions for example aromatic protons appear at 6-7 ppm, methyl protons at 0.5-1.5 ppm, amide protons at 7.5-9 ppm, etc. The representation of the acquisition of a proton. 

In fact as we must consider both problems of locking the field-frequency ratio and referencing the chemical shifts, a multitude of situations arise from the combination of various sample configurations ... 

If there is more than one set of data found in literature, the data in CDCI3 were preferred, or the data with the most signals assigned. As already mentioned in section 2.1 the C-13 data are in general not substantially dependent on the solvent. Nevertheless, in the aecuracy of chemical shifts a tolerance up to 2-3 ppm should be allowed due to solvent effects, differences in referencing or instrumental reasons. That is why concentration data are not included in the tables. In most cases the C spectra are measured at ambient temperature between 398 and 413 K. In the cases where the spectra were measured at higher or lower temperatute it is indicated in the column Solvent. 

In the accuracy of chemical shifts a tolerance of several ppm should be allowed due to medium dependence and differences in referencing (see above). 

Spectroscopic Analysis. Infrared (IR) spectroscopic analysis was performed on a Beckman Microlab 620 MX computing spectrometer. Samples were cast on a sodium chloride pellet or made into a pellet with potassium bromide. and 13C NMR spectra were obtained using a JEOL HNM-FX 270 MHz Fourier transform NMR spectrometer. Samples were dissolved in deuterium chloroform and chemical shifts were referenced to an internal standard of tetramethylsilane. 

The ZSM-4 sample was prepared following the previously described procedures.[20] The elemental analysis showed that the Si/Al ratio was 3.0. 29Si MAS NMR spectra were recorded at 11.7 T MHz on a Varian InfinityPlus 500 spectrometer on a sample loaded in a 7.5 mm MAS rotor spinning at 4 kHz using a rc/2 rad pulse length and a recycle delay of 360 s. The 29Si chemical shifts are referenced with respect to an external solution of TMS (5Si = 0.0 ppm). 

Proton-decoupled 13C-NMR spectra were recorded on a Varian XL-300 operating at 75.4 MHz. Approximately 250 mg of the sample was dissolved in 3 ml of deuterated chloroform. 13C chemical shifts were referenced internally to CDCL (77 ppm). A delay of 200s was used to ensure relaxation of all the carbon nuclei and 1000 transients were collected to assure a good signal-to-noise ratio. 

Here, ak is the isotropic chemical shift referenced in ppm from the carrier frequency co0, SkSA is the anisotropy and tfk SA the asymmetry of the chemical-shielding tensor, here also expressed in ppm. Note that for heteronuclear cases different reference frequencies co0 are chosen for different nuclei (doubly rotating frame of reference). The two Euler angles ak and pk describe the orientation of the chemical-shielding tensor with respect to the laboratory-fixed frame of reference. The anisotropy dkSA defines the width and the asymmetry t]kSA the shape of the powder line shape (see Fig. 11.1a). 

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