NMR active nuclei are seldom found in isolation and even if they were to exist in isolation the NMR information revealed would be of little value to the NMR spectroscopist. The shielding and deshielding influence of electrons in the neighbourhood results in differences in resonance frequencies.
The effective magnetic field becomes less than the applied magnetic field due to the shielding by electrons in the neighbourhood so the applied field requires an increase to bring about resonance. The converse is true when neighbouring electrons shield the nucleus. In effect shielding and deshielding result from different chemical environments and resonance frequencies can be different on account of the surrounding electronic environment of the nuclei.
It is clear that NMR spectrum cannot be obtained on isolated nuclei. It therefore is necessary to use a suitable standard which can define the degree of shielding or deshielding of nuclei in different chemical environments. Tetramethylsilane \((CH_3)_4Si\) is an ideal reference for reporting chemical shifts due to different groups.
Chemical shift is a dimensionless quantity but its magnitude is extremely small in comparison to the applied magnetic field or frequency. Therefore the observed value is multiplied by \(10^6\) and reported in parts per million (ppm). Conventionally the chemical shift scale ranges from 0 to 12 ppm. TMS is conventionally assigned 0 ppm and the values increase to the left along the x-axis.
Influencing factors on chemical shifts
Electronegative atoms present in molecules tend to draw the electron density towards themselves and deshield the nucleus. An increase in electronegativity of the surrounding groups will result in decrease of the electron density and lead to an increase in chemical shift value due to the shielding of the nucleus.
Anisotropy refers to the property of the molecule where a part of the molecule opposes the applied field and the other part reinforces the applied field. Chemical shifts are dependent on the orientation of neighbouring bonds in particular the π bonds. Examples of nucleus showing chemical shifts due to π bonds are aromatics, alkenes and alkynes. Such anisotropic shifts are useful in characterizing the presence of aromatics or other conjugated structures in molecules.
Hydrogen bonding results from the presence of electronegative atoms in neighbourhood of protons .The resulting deshielding leads to higher values of chemical shifts. This confirms the presence of hydrogen bonding in the molecules.
Chemical shifts of NMR active protons and other nuclei serve to provide a wealth of structural information on molecules.