NMR reference compounds

Proton nmr. In the simplest experiment, the sample and a small amount of a reference compound such as tetramethylsilane [75-76-3] (TMS), are placed in a tube, usually of 5-mm diameter. Typical samples may be a neat Hquid or a solution containing as Htde solute as 0.01 mg/cm. The... 

The reference compound methyloxirane gives the H NMR spectrum 11a shown with expanded multiplets. What information regarding its relative configuration can be deduced from the expanded H multiplets of monordene displayed in 11b ... 

However, we have to criticize more specifically the paper by Lown et al. (1984), who characterized alkanediazonium ions, as well as (E)- and (Z)-alkanediazoate ions, by 15N NMR spectroscopy. They also report NMR data on the (E)- and (Z)-benzenediazohydroxides as reference compounds, describing the way they obtained these compounds in only three lines. Obviously the authors are not familiar with the work on the complex system of acid-base equilibria which led 30 years earlier to the conclusion that the maximum equilibrium concentration of benzenediazohydroxide is less than 1 % of the stoichiometric concentration in water (see Ch. 5). The method of Lown et al. consists in adding 10% (v/v) water to a mixture of benzenediazonium chloride and KOH in dimethylsulfoxide. In the opinion of the present author it is unlikely that this procedure yields the (Z)- and CE>benzenediazohydroxides. Such a claim needs more detailed experimental evidence. 

The difference in resonance NMR frequency of a chemically shielded nucleus measured relative to that of a suitable reference compound is termed chemical shift [164,165], and is a measure of the immediate electromagnetic environment of a nucleus. While the chemical shift depends on the Bo field, J does not. Chemical shifts, which cover a range of about 10 ppm for protons (i.e. 600 Hz in case of a 14.1 kG magnetic field) and 250 ppm for 13C, are the substance of NMR. 

The crude 2-aminothiazoles were dissolved in DMSO-d6 (2 mL). A reference solution of p-dimcthoxybcnzcne in DMSO-d6 (2M, 50 pL) was added to each of the samples, and proton NMR spectra were recorded. A 5 s delay was added between scans. The amount of 2-aminothiazole present was determined by a comparison of integral peak heights of the 2-aminothiazole and the reference compound. 

Reference Compound For H NMR i.e., Proton-NMR, tetramethyl silane (TMS), (CH3)4Si, is employed mostly as the reference compound, because of the fact that its protons resonate at higher field strength than most other protons. 

It is technically possible, but very difficult, to measure the exact frequency of a radio signal, and in practice the frequency of the energy absorbed by a test compound (usually called the resonance frequency) is measured relative to that of a reference compound. This reference may be mixed with the sample (direct referencing), or if contamination of the sample is undesirable it may be placed in a separate container within the sample tube (external referencing). In proton and 13C NMR, the reference compound usually used is TMS (tetra-methyl silane) or its water-soluble derivative DSS (2,2-dimethylsilapentane 5-sulphonic acid). These compounds give a sharp proton peak at the right-hand side of a typical NMR spectrum (Figure 2.39). 

Figure 2.39 NMR spectrum of ethanol (CH3 CH2 OH). The three peaks are due to the three groups in the compound, which can be split into multiple peaks using a higher resolution. The peak at zero chemical shift is due to the reference compound, DSS.

Second, considerable effort has been expended to create 13C NMR reference data systems that will enable spectroscopists to extract structural information from their experimental results by comparing them with documented spectra of related compounds data banks stored in computers have been compiled and are accessible for substructure searches (510-519). The advent of these facilities means that SCS evaluations have been-at least in principle-outstripped in an important field of their application, namely in their use for establishing molecular constitution and assigning signals. In cases of doubt one should rely... 

Tetramethylsilane, (CH3)4Si, (abbreviated commonly as TMS) is the usual reference compound chosen for both and NMR and it is normally added directly to the solution of the substance to be examined. TMS has the following advantages as a reference compound ... 

Conformational equilibria still fast on the NMR timescale. From reference compounds (diastereomeric analogs, e.g., 4-/err-butyl or 3,5-cis-dimethyl substitution operating as a kind of conformational anchor), the NMR parameters of the purely equatorial and axial conformers (Pcq and Pax, respectively) can be determined. The equilibrium constant K [Eq. (3)] from these parameters and that of the population-weighted observed NMR parameter, P, can be estimated from the fast equilibrium [59CI(L)568] ... 

J. C. C. Freitas, A. Wong and M. E. Smith, Solid-state natural abundance Mg NMR studies of Na2MgEDTA-4H20—a possible new reference compound for Mg NMR spectroscopy. Magn. Reson. Chem., 2008,47,9-15. 

NMR instruments use either constant magnetic fields and variable radiofrequencies or constant frequencies and variable magnetic fields, but the abscissa of NMR spectra is always measured in units of frequency (cps) in relation to an added reference compound such as tetramethyl-silane (TMS) and divided by the radiofrequency of the instrument to yield dimensionless numbers on either the 8-scale, which sets the reference signal equal to zero, or on the T-scale, which sets it equal to 10 thus 8 -f- t = 10. The spectra in this report use the 8 scale and show the reference signal of TMS at 8 = 0. 

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