Spectroscopic Methods in Organic Chemistry-Chapter 3: NMR
Tuesday, September 10, 2013
Some atomic nuclei have a nuclear spin (I), and the presence of a spin makes these nuclei behave
rather like bar magnets.
In the presence of an applied magnetic field, the nuclear magnets can orient themselves in 2I+1
In the application of NMR spectroscopy, H and C are the most important, and both have spins of
1/2. These nuclei, therefore, can take up only one of two orientations;
A low energy orientation aligned with the applied field.
A high energy orientation opposed to the applied field.
ΔE = hγB /oπ Difference in energy
γ is the magnetogyric ratio (a proportionality constant, differing for each nucleus, which essentially
measures the strength of the applied magnetic field) The number of nuclei in the low energy state (N ) andαthe number of nuclei in the high energy state
(Nβ) will differ by an amount determined by the Boltzmann distribution:
N /N = exp(-ΔE/kT)
When a radio frequency signal is applied to the system, this distribution is changed if the radio
frequency matches the frequency at which the nuclear magnets naturally precess in the magnetic
Some of the N nuαlei are promoted from the low energy state to the high energy state
The frequency in Hz (the resonance frequency) is