Coupling of Protons to

TABLE 3.4 Classification of Amines by NMR of their Ammonium Salts in Trifluoroacetic Acid. 

Chemical Shifts of several classes of protons attached to 14N are available in Appendix E. 

Sulfhydryl protons usually exchange at a low rate so that at room temperature they are coupled to protons on adjacent carbon atoms (J —8 Hz). They do not exchange rapidly with hydroxyl, carboxylic, or enolic protons on the same or on other molecules thus, separate peaks are seen. However, exchange is rapid 

FIGURE 3.41 NMR spectrum of a-methylene unit of a primary amine at 100 MHz in CF3C02H corresponds to Table 3.4, first line. 

Effect on N—H Singlet, sharp Singlet, broad Singlet, broad 

Deuterium (D or 2H) usually is introduced into a molecule to detect a particular group or to simplify a spectrum. Deuterium has a spin number of 1, a small coupling constant with protons (see Appendix G), and a small electrical quadrupole moment. The ratio of the 7 values for HH to those of HD is about 6.5. 

The spectrum of the undeuterated compound consists of a triplet for the a protons, a quintet for the /3 protons—assuming equal coupling for all protons— and a triplet for the y protons. For the deuterated compound, the a-proton absorption would be absent, the /3-proton absorption would appear, at modest resolution, as a slightly broadened triplet, and the y-proton absorption would be unaffected. Actually, at very high resolution, each peak of the /3-proton triplet would appear as a very closely spaced quintet (7n c c D 1 Hz) since 2nl + I= 2x2xl + 1=5, where n is the number of D nuclei coupled to the /J protons. 

Most deuterated solvents have residual proton im- 

Because of the electrical quadrupole moment of D only broad absorption peaks can be obtained from a spectrum of deuterium nuclei. 

The nucleus 31P has a natural abundance of 100% and a spin number of (therefore no electrical quadrupole moment). The multiplicity rules for proton-phosphorus splitting are the same as those for proton-proton splitting. The coupling constants are large (/H—p 200-700 Hz, and 7HC P is 0.5-20 Hz) (Appendix F) and are observable through at least four bonds. The 31P nucleus can be observed at the appropriate frequency and magnetic field (Chapter 7). 

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Another application of LC-NMR in natural products chemistry concerns biosynthetic studies employing feeding experiments with stable isotope-labelled compounds. H NMR spectra allow the determination of the amount of isotopic label incorporated into metabolites, e.g. by observing signals that arise from /-couplings of protons to 13C-labelled nuclei [41,42],... 

The coupling of protons to their neighboring protons gives very valuable information on how to fit the pieces back together. For the simpler spectra, n number of adjacent equivalent protons will split the resonance into +l lines. Common patterns and peak intensities are shown below. A proton whose peak is a quartet has three adjacent protons a triplet has two adjacent protons a doublet has one adjacent proton and a singlet none. 

About half of the environmental P NMR experiments reported in the literature used proton decoupling to remove scalar coupling of protons to phosphorus. This is done with an additional radio-frequency pulse at the Larmor frequency for protons. 

It must be realized that the effect of spin-spin coupling of protons to other magnetic nuclei may be observed in the proton spectra, and hence some idea of the magnitudes of coupling between protons and some commonly occurring magnetic nuclei may be useful in interpreting proton spectra. 

Coupling of protons to other nuclei are discussed in the sections concerned with those nuclei. 

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