Measurement of Chemical Shifts

As we have seen, NMR lines in liquids are usually very narrow, and resonance frequencies in the range of tens or hundreds of MHz, can be determined with an 

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Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

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. 

C-4 protonation would be expected to occur preferentially in 3-pyrazolines. It is in fact found that the thermodynamically stable conjugate acid is protonated on C-4 (Aubagnac et al., 1967a and 1969), but a transitory existence of an N-protonated form could also be demonstrated (Aubagnac et al., 1967b). This was identified as N-1 protonated by measurements of chemical shifts caused by protonation of l,2,4-trimethyl-3-phenyl-3-pyrazoline [158]. The signal of... 

The stable free radicals in coal can adversely affect the 13C signal intensity. However, the quantitative reliability of the method can be greatly improved by prior treatment of coal with chemicals such as samarium(II) iodide (Stock et al., 1988) to selectively reduce the organic free radicals. Thus, this potential error that has influenced the quantitative aspects of the method can be reduced by the use of a novel pretreatment technique as well as an appropriate standard for measurement of chemical shifts. 

Computer programs for simulation of proton NMR spectra are available. If accurate measurements of chemical shifts and coupling constants for all of the protons can be obtained, the simulated spectrum will be congruent with the actual spectrum. In many cases, at least some of the spin systems will be first order. If not, reasonable estimates of shifts and coupling constants may be made, and the iterative computer program will adjust the values until the simulation matches the actual spectrum—assuming, of course, that the identification is valid. 

Large line widths (i) prevent the accurate measurement of chemical shifts, (ii) may cause the overlap of resonances when more than one type of sulphur atom is present in a molecule or sample and (iii) have a significant influence on the achievable signal-to-noise ratio (S/N) and, consequently, on the experimental times needed to obtain readable spectra. 

Calculations of the chemical shifts and the CSA of a variety of silylenes are in progress in our laboratory, and we are confident that they will provide valuable information as well as important insights into the electronic structure of these interesting species. We also hope that the calculations will prompt the experimental measurements of chemical shift anisotropies for stable silylenes. 

Low temperature (-50 C) H and F NMR spectra of a- or -substituted ketones, esters and nitriles complexed to boron trifluoride showed that boron coordinates preferentially at the most basic and least-hindered base when more than one coordination site is present in the ligand. Ordinarily a relative measure of chemical shifts for each type of complex would be derived from the spectra of a number of model complexes. For example, F chemical shifts for BF3 complexes of ketones (acetone), esters (methyl acetate), nitriles (acetonitrile) and ethers (diethyl ether) were measured as 149 p.p.m., 150 p.p.m., 144 p.p.m. and 156 p.p.m., respectively. Inspection of the resonance frequency and relative intensities of each peak would then reveal Ae types and ratios of the complexes present in solution. 

The A8 value for pentane-2,4-dione in Table 3 is compatible with a double-minimum potential for the intramolecular hydrogen bond in the enol. This was also the conclusion reached from the microwave and photoelectron spectra. For unsymmetrically substituted diketones, for example 1,1,1-trifluoropentane-2,4-dione, measurement of chemical shifts has been used to estimate values for the equilibrium constant K = [R]/[L] in (38)... 

We are sure that measurements of chemical shifts and associated computational support will play a very important role in the studies of enzyme active sites like in Ref. [28]. 

The possibility of spinning the sample at a speed higher than 20 kHz allows the direct measurement of chemical shifts of protons in HB, leading to an increasing number of studies [64—66]. 

Still, the technique and instrumentation of measurement, the mathematical techniques of analysis, and the available tests for these are such that the competent NMR spectroscopist is very unlikely to make a sizable error in the measurements of chemical shifts and coupling constants. 

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