Shift for a-protons

Gay and Liang also found large changes in 13C chemical shifts when amines interacted with Bronsted acid sites, and smaller ones when the interaction involved Lewis acid sites. They suggested therefore that fractional coverage of the surface of the amine could be calculated from the magnitude of the 13C chemical shift, based on wo assumptions (1) that chemical shifts are average values from protonated and unprotonated amine molecules and (2) that the shift for a protonated molecule is the same as for the acid solution of the amine. Experiments showed that both assumptions were well founded. It follows that the fraction, /B, of amine molecules bonded to the Bronsted sites can be derived from the equation... 

The chemical shift for a proton is determined, then, by the electronic environ-nent of the proton. In a given molecule, protons with different environments— ion-equivalent protons—have different chemical shifts. Protons with the same 5nvironni t—equivalent protons—have the same chemical shift indeed,/or nmr lurpcses, equivalent protons are defined as those with the same chemical shift. (We lave already seen what the equivalence of protons means in terms of molecular tructure.)... 

Protons are equivalent to one another and have the same chemical shift when they are m equivalent environments Often it is an easy matter to decide simply by mspec tion when protons are equivalent or not In more difficult cases mentally replacing a proton m a molecule by a test group can help We 11 illustrate the procedure for a sim pie case—the protons of propane To see if they have the same chemical shift replace one of the methyl protons at C 1 by chlorine then do the same thing for a proton at C 3 Both replacements give the same molecule 1 chloropropane Therefore the methyl protons at C 1 are equivalent to those at C 3... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton fC. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength Bg of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, Sh for protons, 5c for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 1O. ... 

HETCOR (Section 13.19) A 2D NMR technique that correlates the H chemical shift of a proton to the C chemical shift of the carbon to which it is attached. HETCOR stands for heteronuclear chemical shift correlation. 

Table 3.1 provides a comparison of the carbon chemical shifts for a number of monosubstituted acetonitriles,4 whereas Scheme 3.35 provides proton and carbon data for a couple of more highly substituted systems. 

Figure 19. Correlation of proton and carbon-13 chemical shifts for a number of heterocycles and their anions and cations, o Positions alpha to N other positions.

Figure 23. Correlation of observed and calculated changes in carbon-13 shifts for a number of heterocyclic systems upon protonation. For data and references see Table 12. ° Data from Adam et al. (1969) (Column A) Data from Pugmire and Grant (1968) ignoring changes in A (Column B). x Data from Pugmire and Grant (1968) corrected for changes in A (Column C).

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

Furthermore, in flexible linear peptides the chemical shifts are typical of random structures similar to nonfolded proteins. Deviation from these random shifts sometimes identifies specific conformational preferences. NH-proton chemical shifts depend strongly on external influences (solvent, temperature, concentration, specific sequence). Random coil shifts for these protons correlate less well than chemical shifts of the a-protons or a-carbonsJ19-261 Not only are the shift differences of different heterotopic protons similar, but also those of diastereotopic P-protons. A preferred side-chain conformation is normally only found when there is also a preferred backbone conformation. 

The chemical shifts of a-protons in some conformationally rigid enol ethers, e.g., 1, have been studied. It was found that they depend not only on their relative position with respect to the alkoxy group, but also on the torsional angle between the C —H and the C—C bonds it is claimed that electric-field effects of the alkoxy groups are responsible267. H Chemical shifts have also been used for the stereochemical assignment of eyclohexylidenecyanoacetates 2. 

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

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