oxygen atom. These atoms pull electron density away from the surrounding atoms, thus
deshielding the proton from the magnetic field. The more the proton’s electron density is pulled
away, the less it can shield itself from the applied magnetic field, resulting in a reading further
downfield. With this same reasoning, we know that if we had an electron-donating group, such as
the silicon atom in TMS, it would help shield the ^1 H nuclei and give it a position further upfield. This
is why tetramethylsilane is used as the reference or calibration peak; everything else in proton NMR
will be more deshielded than it.
KEY CONCEPT
Now, let’s make it a little more interesting. Consider a compound containing protons that are within
three bonds of each other: in other words, a compound in which there are hydrogens on two
adjacent atoms. When we have two protons in such close proximity to each other that are not
magnetically identical, spin–spin coupling (splitting) occurs. Let’s use the molecule in Figure 11.5
to demonstrate this concept.
Figure 11.5. 1,1-Dibromo-2,2-dichloroethaneMNEMONIC
When dealing with ^1 H–NMR on the MCAT, think of a proton as being surrounded by a shield
of electrons. As we add electronegative atoms or have resonance structures that pull
electrons away from the proton, we Deshield and move Downfield.Each peak or group of peaks that are part of a multiplet represents a single group of
equivalent protons
The relative area of each peak reflects the ratio of the protons producing each peak
The position of the peak (upfield or downfield) is due to shielding or deshielding effects
and reflects the chemical environment of the protons