in frequency (Hz) relative to a standard, the universally accepted reference compound being
tetramethylsilane, (CH 3 ) 4 Si (TMS). The desirable properties of this compound are that it has twelve
highly shielded protons in identical chemical and magnetic environments thereby producing a single
sharp resonance peak at a higher field than most other organic protons. Furthermore, it is chemically
inert, soluble in organic solvents and boils at 27°C. For aqueous samples, sodium 2,2'-dimethyl- 2 -
silapentane- 5 - sulphonate, (CH 3 ) 3 SiCH 2 CH 2 CH 2 SO 3 Na (DSS), can be used. In practice chemical shifts
are expressed in dimensionless units δ by dividing them by the operating frequency of the instrument,
thus facilitating comparisons between spectra run on different instruments
By including a factor of 10^6 , δ values fall within the range 0–15 for most organic protons, the values
then being expressed in parts per million (ppm). An alternative system, not now officially recognized, is
the τ scale in which the TMS peak is assigned the value 10, and τ is defined as 10 – δ.
The approximate chemical shift positions for organic protons in different chemical environments are
shown in Figure 9.30. It will be noted that alkene protons absorb at a lower field (> δ4.5) than alkyne
protons (~ δ2.7), although the latter are more acidic and therefore experience less diamagnetic
shielding. This, and other anomalous effects in unsaturated molecules, can be explained in terms of
shielding and deshielding zones in space caused by the circulation of π-electrons. Specific examples of
the effect, known as diamagnetic anisotropy, are illustrated in Figure 9.31.
For ethyne, which is a linear molecule, the most strongly induced circulation of π-electrons occurs
when the molecular axis is parallel to the applied field. The resulting field due to electron circulation
can be represented by a cone within which there is a net shielding and outside of which there is a net
deshielding (Figure 9.31(a)). As the ethyne protons lie within the shielded zones, they absorb at a
relatively high value of the applied field. For alkenes and aldehydes, the greatest effect is when the
double bond is orientated perpendicular to the applied field. For example, all the protons of ethene and
the aldehydic proton of ethanal lie in deshielded zones (Figure 9.31(b) and (c)) and hence are absorbed
at a relatively low value of the applied field. The most pronounced effect occurs in aromatic compounds
(δ 6 – 9) where π-electrons can circulate around the ring producing a so-called ring current. Aromatic
protons lie in deshielded zones (Figure 9.31(d)) and absorb at comparatively low values of the applied
field.
Lastly, the effect of hydrogen-bonding should be noted. Protons which are hydrogen-bonded are
deshielded relative to the non-bonded situation and their chemical shifts can vary over a wide range.
The effect is observed in alcohols, phenols, amines and carboxylic acids and, as in the case of infrared
spectra it is temperature, concentration and solvent dependent (p. 383).