KEY CONCEPT
TMS provides a reference peak. The signal for its ^1 H atoms is assigned δ = 0.Nuclear magnetic resonance is most commonly used to study ^1 H nuclei (protons), although any
atom possessing a nuclear spin (with an odd atomic number, odd mass number, or both) can be
studied, such as ^13 C, ^19 F, ^17 O, ^31 P, and ^59 Co. The MCAT, however, only tests knowledge of ^1 H–NMR.
BRIDGE
Nuclei with odd mass numbers, odd atomic numbers, or both, will have a magnetic moment
when placed in a magnetic field. Not all nuclei have magnetic moments (^12 C, for example).
Atomic numbers and mass numbers are discussed in more detail in Chapter 1 of MCAT
General Chemistry Review.PROTON NMR (^1 H–NMR)
Most hydrogen (^1 H) nuclei come into resonance 0 to 10 ppm downfield from TMS. Each distinct set
of nuclei gives rise to a separate peak. This means that if multiple protons are chemically
equivalent, having the same magnetic environment, they will lead to the same peak. For example,
Figure 11.4 depicts the ^1 H–NMR of dichloromethyl methyl ether, which has two distinct sets of ^1 H
nuclei. The single proton attached to the dichloromethyl group (Ha) is in a different magnetic
environment from the three protons on the methyl group (Hb), so the two classes will resonate at
different frequencies. The three protons on the methyl group are chemically equivalent and
resonate at the same frequency because this group rotates freely, and on average, each proton sees
an identical magnetic environment.