Reference compounds in NMR

The most generally used reference compound in NMR is tetramethylsilane (TMS). This substance absorbs at higher field than almost all organic compounds for both H (used in proton NMR) or for (used in NMR). In a relative NMR scale, TMS is set at 0 Hz at the right edge of the scale. A chemical shift parameter can be calculated using this reference. Applying rel. (2) to the reference and to a sample, it results ... 

Chemical shifts are therefore usually reported relative to a common reference compound. In NMR studies of chlorine, bromine and iodine compounds chemical shifts are mostly measured relative to the corresponding halide ion in aqueous solution. Since the ion shifts themselves are dependent on the nature of the counter-ion, salt concentration and temperature they are not ideal references. 

As discussed earlier, the chemical shift of a hydrogen signal—that is, the field required for the hydrogen to be in resonance—varies slightly with the chemical environment of the hydrogen. To measure chemical shifts, a small amount of a reference compound, usually tetramethylsilane (TMS), is added to the sample. The separation, in hertz, between the peak of interest and the peak due to TMS is measured. TMS is chosen as the reference because it has only one NMR peak and this peak occurs in a region of the spectrum where it does not usually overlap with other absorptions. Figure 14.4 illustrates the use of TMS as a reference compound in the spectrum of acetone. 

In some cases, notably 3lP NMR, the chosen reference compound is not inert toward most samples. One way around this problem is to put the reference compound in a small capillary tube, which itself is carefully centered within the sample tube. Such a reference compound is referred to as external, that is, not dissolved in the sample solution. 

It is obvious from this example that only relative magnetic shielding for a nucleus is obtainable from an NMR spectrum. One therefore uses reference compounds in order to be able to make comparisons between different compounds for a given nucleus. The compound (CH3)4Si (tetramethylsilane) is often used as the reference for proton shielding. 

Chemical shifts in NMR spectra are usually reported relative to the peak position of tetramethylsilane (TMS) added to the sample as an internal reference. If TMS is not sufficiently soluble, a capillary with TMS may be used as external reference. In this case, owing to the different volume susceptibilities, the local magnetic fields in solvent and reference differ, and the peak position of the reference must be corrected. For a D2O solution in a cylindrical sample and neat TMS in a capillary, the correction amounts to +0.68 and -0.34 ppm for superconducting and electromagnets, respectively. These values must be subtracted from the chemical shifts relative to external TMS if its position is set to 0.00 ppm. Alternatively, secondary references with (CH3)3SiCH2 groups may be used. The following spectra of two such secondary reference compounds in D2O were measured at 500 MHz with TMS as external reference. Chemical shifts are reported in ppm relative to TMS upon correction for the difference in the volume susceptibilities of D2O and TMS. As a result, the peak for the external TMS appears at 0.68 ppm. 

Ruthenium has seven stable isotopes, of which Ru and ° Ru are NMR active. Although their natural abundances and resonance frequencies are quite similar the smaller quadrupole moment of ruthenium-99 makes the former more favourable from a NMR spectroscopic point of view. The Ru NMR chemical shift range is enormous (> 18 000 ppm). The shifts of ruthenium compounds and complexes follow roughly the same trends as those of iron and osmium. The reference compound in Ru NMR is the (Ru(CN)6] anion (6 = 0 ppm). Some characteristic Ru NMR chemical shifts are collected in Table 7. 

Fig. 1 Ligand-based NMR methods for fragment library screening, (a) 1D H NMR controi spectrum of reference compound, (b) STD NMR spectrum of reference compound in the presence of the target protein. Only resonances of atoms which contact the protein are present in the STD spectrum (c) c-STD of reference compound and known competitor in the presence of the target protein. The STD signal of reference compound decreases because the known competitor with higher affinity displaces it from the binding site...

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 physical data index summarizes the quantitative data given for specific compounds in the text, tables and figures in Volumes 1-7. It does not give any actual data but includes references both to the appropriate text page and to the original literature. The structural and spectroscopic methods covered include UV, IR, Raman, microwave, MS, PES, NMR, ORD, CD, X-ray, neutron and electron diffraction, together with such quantities as dipole moment, pX a, rate constant and activation energy, and equilibrium constant. 

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. 

---