Chemical Shift Benchmark Values

We will now begin exploring the three characteristics of every signal in an NMR spectrum. The first characteristic is the location of the signal, called its chemical shift (8), which is defined relative to the frequency of absorption of a reference compound, tetramethylsUane (TMS)  

The left side of an NMR spectrum is described as downfield, and the right side of the spectrum is described as upfield  

But keep in mind that these terms are used in a relative way. For example, we would say that the signal at 2.5 ppm (in the spectrum above) is downfield from the signal at 1.2 ppm. Similarly, the signal at 6.8 ppm is upfield from the signal at 7.1 ppm. 

The protons of alkanes typically produce signals between 1 and 2 ppm. We will now explore some of the effects that can push a signal downfield (relative to the protons of an alkane). Recall that electronegative atoms, such as halogens, withdraw electron density from neighboring atoms  

Fluorine is the most electronegative element, and therefore produces the strongest effect. When multiple halogens are present, the effect is generally additive, as can be seen when comparing the following compounds  

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Jacobs et al. [1] used two-dimensional NMR spectroscopy to assign the H and 13C chemical shifts of cucurbitacin B, establishing these values as a benchmark for analyzing the spectra of other cucurbitacins isolated from the same source. In fact, the characteristic groups present in all cucurbitacins are easily detectable by means of NMR and typically include the presence of carbonyls at C-ll and C-22, which appear between 212-213 ppm, as well as hydroxyls at C-16(a) and C-20( 3), which appear at approximately 71-73 ppm and 77-79 ppm, respectively. Other characteristic patterns of substitution such as the presence of acetoxyl groups at C-2 and C-25 have also been observed. In this case, the acetylation of the hydroxyl at C-2 modifies both the C-l and C-3 values. Indeed, when a carbonyl is present, this latter change involves a major shift, namely from 213 to 205 ppm. In the case of a second double... 

We begin with a benchmark study of relative NMR chemical shifts in which calculated values are compared to experimental gas-phase results from the work of Jameson and Jameson [166]. The set of molecules here has been used in a number of studies [47, 52, 60, 116, 167] in order to calibrate the accuracy of different theoretical approaches. Figure 6.11... 

The calculation of NMR parameter has been studied extensively see [3, 73] for general overviews. In 2001, Sebastian and Parrinello implemented the NMR chemical shift calculation in the plane wave AIMD code CPMD [74]. From this implementation it was possible to treat extended systems within periodic boundary conditions, i.e., the method was applicable to crystalline and amorphous insulators as well as to liquids. The problem of the position operator was solved by the use of maximally localized Wannier functions. Several benchmark calculations showed good agreement with experimental values. 

By committing a few numbers to memory, it is possible to predict the chemical shifts for the protons in a wide variety of compounds, including alcohols, ethers, ketones, esters, and carboxylic acids. The following numbers are used as benchmark values ... 

The singlet at 3.9 ppm (with an integration of 3) represents a methyl group. The chemical shift is downfield from the expected benchmark value of 0.9 ppm for a methyl group, indicating that it is likely next to an oxygen atom ... 

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