Instant Notes: Analytical Chemistry

● The illustrations show that alkene (~5.3 d/ppm) and aromatic protons

(~7.3 d/ppm), and those close to a carbonyl double bond (~9.5 d/ppm) all lie

in deshielding zones (-) where the fields associated with the circulating

electrons augment the applied field, Bo. These proton resonances therefore

appear further downfield than would be expected. The effect is particularly

pronounced for the aromatic ring due to the generation of a strong ring

current by the circulating p-electrons. An alkyne triple bond shows the

greatest effect when it is aligned parallel to the applied field, the proton lying

in a shielded zone and therefore appearing further upfield (1.5 d/ppm) than

would be expected.

● Hydrogen-bonding, which leads to the deshielding of protons. This reduces

the electron density around the proton involved, thereby decreasing the

degree of diamagnetic shielding. The effect is observed in the proton spectra

of alcohols, phenols, carboxylic acids and amines where both inter- and

intramolecularH-bonding (X-H◊◊◊Y where X and Y = O, N or S) can occur.

The OH, NH, NH 2 or SH proton resonances show variable downfield shifts

due to additional deshielding by the Y atom, the effect being concentration-

dependent for intermolecular bonds but not for intramolecular bonds.

Choice of solvent also has an influence if it can bond to the protons.

Carboxylic acid dimers and intramolecularly bonded structures, such as the

enolic form of b-diketones and 1,2-substituted aromatic rings with appro-

priate groups, form particularly strong H-bonds, resonances often appearing

between 12 and 16 d/ppm. Figure 6 illustrates four spectra of ethanol

recorded at different concentrations, where the OH proton resonance moves

progressively upfield as the degree of hydrogen bonding diminishes with

dilution in tetrachloromethane.

● Paramagnetic deshielding, which augments Bo. The degree of deshielding

(negative s) is variable and more complex in origin. It is mainly of

importance in structures with unpaired electrons, such as transition metal

complexes.

Charts and tables of chemical shifts are used to aid the interpretation of

proton and carbon-13 spectra, and examples of these are given in Topic E13.

E12 – Nuclear magnetic resonance spectrometry: principles and instrumentation 255

OH CH 2 CH 3 [EtOH]

1 M

0.1 M

0.01 M

0.001 M

6420

d(^1 H)/ppm

Fig. 6. Hydrogen-bonded OH resonance shifts in ethanol as a result of dilution in

tetrachloromethane.