Reference compound, 58

When preparing a sample it is common practice to add a suitable compound to act as an internal chemical shift reference in the spectrum, and the selection of this must be suitable for the analyte and solvent. In proton and carbon NMR, the reference used in organic solvents is tetramethylsilane (TMS, 0.0 ppm) which has a number of favourable properties it has a sharp 12-proton singlet resonance that falls conveniently to one end of the spectrum, it is volatile so can be readily removed and it is chemically inert. In a few cases this material may be unsuitable such as in the study of silanes or cyclopropanes. For routine 

Nuclide Primary reference S value (MHz) Alternative reference 

An alternative to adding additional reference materials is to use a so-called external reference. Here the spectrum of a separate reference substance is acquired before and/or after the sample of interest and the spectrum reference value carried over. Identical field settings should be used for both which, on some older instruments, requires the same lock solvent, or an additional correction to the spectrum reference frequency must be used to compensate any differences. This restriction does not arise on instruments that use shifting of the lock transmitter frequency to establish the lock condition. 

The referencing of more exotic nuclei is generally less clear-cut than for those in common use and in many cases it is impractical to add reference materials to precious samples, and it is sometimes even difficult to identify what substance is the accepted reference standard. In such cases the S-scale can be used, which does not require use of a specific reference material. Instead this scheme defines the reference frequency for the referenee material of each nuclide at a field strength at whieh the proton signal of TMS resonates at exactly 100.000 MHz. The reference frequencies are scaled appropriately for the magnetic field in use and this then defines the absolute frequency at 0.0 ppm for the nuclide in question. The S values for selected nuclei are also summarised in Table 3.3, whilst more extensive tables are available [19]. 

Newcomers to the world of practical NMR often find the cost of NMR tubes surprisingly high. The prices reflect the need to produce tubes that conform 

5 ppm), which can be removed by lyophilisation. The standard reference materials for some other common nuclides are summarised in Table 3.3. Often these are not added into the solution being studied but are held in an outer, concentric jacket or within a separate axial capillary inside the solution, in which case the reference material may also be in a different solvent to that of the sample. 

Nuclide Primary reference E value/MHz Alternative reference 

It might appear that a magnetic susceptibility correction would be needed if the susceptibilities of sample and reference differ, but this is not the case. With the field/frequency lock established via the deuterated solvent, the applied magnetic field (H0) simply shifts slightly to maintain the magnetic induction (B0) inside the sample tube constant so as to keep the 2H on resonance. If different deuterated solvents are used for sample and reference, a simple correction must be made for the difference in their 2H chemical shifts. 

Tetramethylsilane became the established internal reference compound for H NMR because it has a strong, sharp resonance line from its 12 protons, with a chemical shift at low resonance frequency relative to almost all other H resonances. Thus, addition of TMS usually does not interfere with other resonances. Moreover, TMS is quite volatile, hence may easily be removed if recovery of the sample is required. TMS is soluble in most organic solvents but has very low solubility in water and is not generally used as an internal reference in aqueous solutions. Other substances with references close to that of TMS have been employed, and the methyl proton resonance of 2,2-dimethylsilapentane-5-sulfonic acid (DSS) at low concentration has emerged as the reference recommended by IUPAC for aqueous solutions.55 Careful measurements of the DSS-TMS chemical shift difference when both materials are dissolved at low concentration in the same solvent have shown that for DSS 5 = + 0.0173 ppm in water, and 8 = — 0.0246 ppm in dimethyl sulfoxide. Thus, for most purposes, values of 8 measured with respect to TMS or DSS can be used interchangeably. 

Chemical shifts can be given as B values, where 5 (Xi) is defined as the resonance frequency in a magnetic field in which TMS has a resonance frequency of 

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It is found in lichens and in some algae. It has m.p. 120 C, is very soluble in water and is about twice as sweet as sucrose. It is a reference compound upon which the erythro nomenclature is based. 

Unlike spark-induced combustion engines requiring fuel that resists autoignition, diesel engines require motor fuels, for vhich the reference compound is cetane, that are capable of auto-igniting easily. Additives improving the cetane number will promote the oxidation of paraffins. The only compound used is ethyl-2-hexyl nitrate. 

Table 14 2 repeats some approximate data presented earlier m Table 1 7 for the acid strengths of representative hydrocarbons and reference compounds... 

Table 7.47 Proton Chemical Shifts of Reference Compounds 7.98...

Absolute index values on squalane for reference compounds 653 590 627 652 699 ... 

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Interestingly, if the original comparison of the spectrum from the reference compound with stored m/z data for that compound reveals discrepancies, the stored reference data are updated before the computer goes on to acquire data from the unknown compound. In this mode, the computer is not used simply to acquire and manipulate data but is also used to make decisions... 

Isotope Spin, I Receptivity ratio Reference compound Detection range, ppm V at 7.05 MHz" E... 

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... 

In Vitro Properties. The antibacterial spectmm of most dalbaheptides is known (Table 1). Vancomycin (39) and/or teicoplanin (Table 3) are generally introduced as reference compounds. Direct comparative data for some dalbaheptides tested under the same experimental conditions are given in... 

Owing to the light and air sensitivity of the carotenoids and retinoids, sample handling is a critical issue. It is recommended to conduct extraction of these materials with peroxide-free solvents, to store biological samples at —70° C under argon and in the dark, to perform the analysis under yellow light, and to use reference compounds of high purity (57). 

Comparison of UV data for 3-aminoisothiazoles with those of reference compounds confirms that they exist in the 3-amino form. A more recent investigation of 4-aminoisothiazole (76MI41701) using deuterium exchange experiments of the type described in Section 4.01.5.2, and analysis of the symmetric and antisymmetric NH2 stretching frequencies in its IR spectrum, show that this compound also exists in the 4-amino form. 

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 ... 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Adopting the enantiomers of glyceraldehyde as stereochemical reference compounds originated with proposals made in 1906 by M. A. Rosanoff, a chemist at New York University. 

The reference compound is benzoic acid. The substituent parameter, which is called the Hammett substituent constant cr, is defined by... 

Polarographic techniques have been used by Sturm and Hans to demonstrate that certain amino-thiadiazoles and -benzthiazoles exist in the amino form (cf. also references 62, 63). This method, which involves comparison of the polarographic rdeuction potentials of the potentially tautomeric substance with those of alkylated reference compounds, has not been applied often, but may well prove to be a means to obtain qualitative information quickly. There is a possibility that the method can be modified to yield quantitative data. ... 

What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study both cathodically deposited -Ni(OH)2 and chemically prepared / -Ni(OH)2 [44], Measurements were done at both 77 and 297 K. The results for / -Ni(OH)2 are in agreement with the neutron diffraction data [22]. In the case of -Ni(OH)2 they found a contraction in the first Ni-Ni bond distance in the basal plane. The value was 3.13A for / -Ni(OH)2 and 3.08A for a-Ni(OH)2. The fact that a similar significant contraction of 0.05A was seen at both 77 and 297K when using two reference compounds (NiO and / -Ni(OH)2) led them to conclude that the contraction was a real effect and not an artifact due to structural disorder. They speculate that the contraction may be due to hydrogen bonding of OH groups in the brucite planes with intercalated water molecules. These ex-situ results on a - Ni(OH)2 were compared with in-situ results in I mol L"1 KOH. In the ex-situ experiments the a - Ni(OH)2 was prepared electrochemi-cally, washed with water and dried in vac-... 

Toxicology Many companies are known to use gene expression profiling to assess the potential toxicity of lead compounds. This approach may require a database of reference compounds with known pharmacological and toxicological properties. Lead compounds can be compared to the database to predict compound-related or mechanism-related toxicity [5]. 

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. 

Competition experiments for the partitioning of phenyl radical between a hydrocarbon and the reference compound, carbon tetrachloride, from results given in Ref. 10. The lines show that the product ratio is directly proportional to the ratio of the concentrations of the competing reagents. The plots depict data for toluene (circles.) and cyclohexane (squares). 

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