Interpretation of Proton Spectra

NMR absorption spectra are characterized by the chemical shift of peaks and spin-spin splitting of peaks. Recall that the chemical shift is caused by the drifting, not orbiting or spinning, of nearby electrons under the influence of the applied magnetic field. It is therefore a constant depending on the applied field (i.e., if the field is constant, the chemical shift is constant). The chemical shift therefore identifies the functional group, such as methyl, methylene, aldehydic H, aromatics, and so on (see Table 3.3). All proton spectra shown have TMS as the reference, with the TMS absorbance set at 0.0 ppm. The student should note that all of the 300 MHz proton NMR spectra provided by Aldrich Chemical Company, Inc., also include the 75 MHz C spectrum at the top. C NMR spectra are discussed in Section 3.6.4. 

Another piece of information is obtained from the relative area of the absorption peaks in the spectrum, which tells us the relative number of protons in each group. So, a proton NMR spectrum should be examined for (1) the number of proton resonances, which tells you how many different types of protons are in the molecule (2) the chemical shift of the resonances, which identifies the type of proton (3) the multiplicity of the resonances, which identifies the adjacent equivalent protons and (4) the intensity (area) of the resonances, which tells you the relative number of each type of proton. Some examples of common classes of organic compounds are discussed. Not every type of compound is covered in this brief overview. Students needing more detailed spectral interpretation should consult the references by Silverstein and Webster, Pavia et al., or similar texts. 

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As you are no doubt aware, integrals are one of the key parameters in the interpretation of proton spectra and are pivotal in quantification. They measure the area under a peak and this is directly proportional to the number of protons (in the case of proton NMR) in that environment. Most software will automatically try to identify the peaks in your spectrum and integrate them for you. If you need to do it yourself, then it is a fairly trivial matter of defining the start and end point of the integrals of interest. The only complication is that you may need to tweak the slope and bias of the integral. This should be unnecessary if you have got the phase and baseline of your spectrum correct. If you find that you need to adjust slope and bias, we suggest that you go back and try to sort out baseline and phase a bit better. 

The interpretation of proton spectra depends on three features chemical shifts, multiplicities of resonances and integrated peak areas. These are exemplified in the spectrum of ethanol, CjHjOH, shown in Figure 2,... 

At first glance splitting may seem to complicate the interpretation of NMR spectra In fact It makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal With practice we learn to pick out characteristic patterns of peaks associating them with particular structural types One of the most common of these patterns is that of the ethyl group represented m the NMR spectrum of ethyl bromide m Figure 13 15... 

As we ve mentioned before, the interpretation of NMR spectra is often made complex by the sheer quantity of information that you are confronted with. This is every bit as true for carbon NMR as it is for proton and when you combine the two, that huge pile of information just gets bigger... More important still then that you approach the pile in a logical, methodical manner. 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

The proton NMR spectrum of 2,2 -bipyridine has been obtained and analyzed in a variety of solvents by several authors. ° The full interpretation of the spectra is in accord with the transoid conformation I in solution. The behavior of the chemical shifts of protons at positions 3 and 3 indicates the existence of a strong deshielding effect exerted by the nitrogen atoms of the adjacent rings. Interestingly, the proton NMR spectra of 2,2 -bipyridine taken in various solvents indicate self-association and stacking of the molecules in some cases. The spectra of some substituted 2,2 -bipyridines, - - 2,3 -bipyridine, ° 2,4 -bipyridine, ° 3,3 -bipyri-dine, and 4,4 -bipyridine - - ° have been investigated in detail. It was... 

The theoretical work of Haigh and Mallion 108, m) has given a sound base for the interpretation of the spectra. The authors argued that the chemical shift of a strongly overcrowded proton like H(l) of hexahelicene depends on three different effects ... 

Low-molecular weight stars were prepared to facilitate the interpretation of NMR spectra. Figure 3 shows the NMR spectrum of a virgin sample indicating resonances at 6=1.95 and 6=1.65 ppm, characteristic of protons of the terminal -CH2-C(CH3)2-C1 group [65]. The absence of resonances at 6 4.6 and 6-4.8 ppm, characteristic of terminal unsaturation, also suggests that the arms carry tert-Cl end groups. 

The proton coupled spectrum reveals a larger one-bond coupling constant (. /SiH) of about 215 Hz. The coupling pattern derived from the two- and three-bond coupling is complex but the pattern might serve as a starting point in the interpretation of 29Si spectra of reaction products. 

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