"Downfield" and "low field" refer to the relative positions of peaks in a nuclear magnetic resonance (NMR) spectrum. Downfield peaks are located at higher frequencies (to the left on an NMR spectrum) and are associated with more deshielded protons. Low field peaks, on the other hand, are located at lower frequencies (to the right on an NMR spectrum) and are associated with more shielded protons.
This phrase means that while downfield peaks may appear at higher frequencies, they are not necessarily associated with lower magnetic fields. In fact, downfield peaks can also appear at higher magnetic fields if the sample contains highly deshielded protons.
Downfield peaks can be identified by their higher frequency (to the left on an NMR spectrum) and by their chemical shift value, which is a measure of proton deshielding. The more deshielded the proton, the further downfield its peak will appear.
The position of downfield peaks can be affected by the chemical environment of the molecule, such as the presence of electronegative atoms or aromatic systems. It can also be influenced by the solvent used in the NMR experiment, as well as the strength of the magnetic field.
Understanding these terms is important because they provide valuable information about the chemical structure and environment of a molecule. By analyzing the position of downfield and low field peaks, scientists can determine the functional groups present in a compound and use this information to identify and characterize unknown substances.