The Chemical Shift

The plot of signal against magnetic field strength for ethanol in Figure 9-23 shows three principal groups of lines corresponding to the three varieties of hydrogen present: methyl (CH3), methylene (CH3), and hydroxyl (OH). Differences in the field strengths at which signals are obtained for nuclei of the same kind, such as protons, but located in different molecular environments, are called chemical shifts.

Another very important point to notice about Figure 9-23 is that the intensities of the three principal absorptions are in the ratio of 1:2:3, corresponding to the ratio of the number of each kind of proton (OH, CH2, CH3) producing the signal. In general, areas under the peaks of a spectrum such as in Figure 9-23 are proportional to the number of nuclei in the sample that give those peaks. The areas can be measured by electronic integration and the integral often is displayed on the chart, as it is in Figure 9-23, as a stepped line increasing from left to right. The height of each step corresponds to the relative number of nuclei of a particular kind. Unless special precautions are taken, integrals usually should not be considered accurate to better than about 5%.

Why do protons in different molecular environments absorb at different field strengths? The field strength H at a particular nucleus is less than the strength of the external magnetic field Ho. This is because the valence electrons around a particular nucleus and around neighboring nuclei respond to the applied magnetic field so as to shield the nucleus from the applied field. The way this shielding occurs is as follows.

First, when an atom is placed in a magnetic field, its electrons are forced to undergo a rotation about the field axis, as shown in Figure 9-26. Second,

Figure 9-26: Induced magnetic field σH^o at the nucleus as the result of rotation of electrons about the nucleus in an applied magnetic field Ho.

rotation of the electrons around the nucleus is a circulation of charge, and this creates a small magnetic field at the nucleus opposite in the direction to Ho. Third, the magnitude of this diamagnetic  effect is directly proportional to Ho and can be quantified as σHo, in which σ is the proportionality constant. It is important to recognize that σ is not a nuclear property but depends on the chemical environment of the atom. Each chemically different proton will have a different value of σ and hence a different chemical shift.

The actual field H at the nucleus will be Ho−σHo. Because σ acts to reduce the strength of the applied field at the nucleus, it is called the magnetic shielding parameter. The more shielding there is, the stronger the applied field must be to satisfy the resonance condition,

Common usage is: upfield, more shielding; downfield, less shielding; and you should remember that field-sweep spectra always are recorded with the field increasing from left to right.