Nuclei will absorb radiation of slightly different frequency depending upon their local magnetic environments. Protons are shielded by valence electrons surrounding them which circulate in an applied magnetic field producing a local diamagnetic current in the opposite direction. This diamagnetic shielding will affect the frequency of radiation necessary to cause a nucleus to spin flip (the resonance frequency).
Figure 3. Diamagnetic shielding of a nucleus by valence electrons.
The differences in resonance frequencies are very small. For instance the difference in resonance frequency for the protons in chloromethane and fluoromethane is 72 Hz. Since the incident radiation had a frequency of 60 MHz, this difference is about 1 part per million. This cannot be measured accurately, therefore, we will only measure the difference between the resonant frequency of a reference compound and the substance to be analyzed. The most common reference is tetramethylsilane (CH3)4Si, also called TMS. Thus, when a compound is analyzed, the resonances of its protons are reported in terms of how far (in Hz) the are shifted from those of TMS.
The shift from TMS for a given proton depends upon the strength of the applied magnetic field. In a field of 1.41 Tesla the resonance of a proton is about 60 MHz, whereas in a field of 2.35 Tesla the resonance appears at about 100 MHz. Hence, for a given proton, the shift (in Hz) from TMS is 5/3 larger for the 100 MHz machine as for the 60 MHz machine. To avoid this confusion, resonance frequencies for protons in a compound are reported in terms of chemical shift (d). The chemical shift is determined as below:
d = (shift in Hz) / (spectrometer frequency in MHz)
For instance, the protons in bromomethane, CH3Br have the following resonance frequencies:
Strength of Field: |
B0 = 1.41 Tesla | B0 = 2.35 Tesla |
Operating Frequency: |
60 MHz | 100 MHz |
Shift From TMS: |
162 Hz | 270 Hz |
d value: |
2.70 ppm | 2.70 ppm |
The d scale (or ppm scale) is independent of the instrument used to obtain the spectrum, and these are the values used to report proton resonances in the literature.
The protons in tetramethylsilane (TMS) resonate at 0 ppm. Most protons in organic compounds will resonate at higher frequencies. As we shall see, the position of the absorbance will tell us a great deal about the environment around a particular proton, and will lead to structural information about the compound in question.
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