Nuclear Magnetic Resonance Spectroscopy (NMR) Proton NMR
A hydrogen nucleus can be viewed as a proton, which can be viewed as a spinning charge. As with any spinning
charge, an intrinsic magnetic field is set up
On the left, a toy top precesses about its vertical axis. The hydrogen, atom on the right, precesses about a magnetic field. Because it has only one proton, a single mass with a positive charge, it has a large magnetic moment (red arrow).
A spinning proton produces a magnetic field similar to a bar magnet.
The Hydrogen Atom Is a Magnet
• If HH
is aligned with Ho
, this will be a lower energy arrangement than when HH
is opposed to Ho
. This creates an energy gap (ΔE) and if energy equal to ΔE is applied to the proton, it "flips spin states."
• This means that when the proton absorbs the energy, the proton magnetic field changes from aligned to opposed (low energy to high energy) - it flips its spin state.
• In order to absorb ΔE, a particular magnetic field must be applied; when this occurs they are said to be in resonance. If the magnetic field changes, ΔE changes.
• The sample is immersed in a very strong magnetic field and this aligns the nuclei that have spin, like a compass needle aligning with the Earth's field. The alignment takes a number of patterns depending on the total spin. Each alignment has a different energy.
• The nucleus is “rattled” with a pulsed radio wave. When the correct
frequency is applied during this pulse, the nucleus jumps from one alignment (energy ) to another alignment. This is a "resonance" similar to when a person pushes a
swing a the same rate as the swing's period - the swing gets higher.
External Magnetic Field Ho
HH aligned opposite Ho HIGHER ENERGY
HH aligned with Ho LOWER ENERGY the nucleus precesses around its axis with a precessional frequency, !prec
requires smaller Ho
requires larger Ho small "E large "E
The proton spins "off-axis," and it precesses around the axis at a certain frequency, the precessional frequency. This information becomes important if we place the proton, which is now a small magnet, into the field of a large external magnet represented by Ho. As with
any two magnets, the small magnet will be influenced by the large magnet and the field of the smaller magnet will orient itself relative to the larger magnet (Ho). There are two possible orientations, the proton magnetic field HH can be aligned with Ho or opposed to Ho
This is an energy gap, represented by ΔE.
• Moving charge creates magnetic fields, spinning as moving. • Moving charge creates magnetic fields, spinning as moving. Neutrons Neutrons are made of 3 quarks and so the charge associated with the neutron,
are made of 3 quarks and so the charge associated with the neutron, while totaling zero, is not
while totaling zero, is not symmetric symmetric . One side can be thought of as . One side can be thought of as slightly positive while the other slightly negative. Spinning generates slightly positive while the other slightly negative. Spinning generates the magnetic field.
the magnetic field.
• ”Spin" is strictly a "property" of the subatomic particles, a quantum
number. Protons and neutrons each have a spin of size "1/2". To get the
total spin of a nucleus, we must add the spins of all the nuclear members of an isotope vectorially. This in turn gives the nuclear magnetic moment of the
• If the total spin of a nucleus is 0 then NMR cannot detect the nucleus. O-16 and C-12 are examples of spin 0 so cannot be detected.
• The spin quantum number is I. Here, if I = 0, there is no NMR.
• Nuclei with spin of 1/2 include H-1, the proton and C-13. Commonly used nuclei with Spin of 1 include H-2 and N-14.
Spin Quantum Number: I
Nucleus Number of protons Number of neutrons Spin (I)
H 1 0 1/2
H 1 1 1
C 6 6 0
C 6 7 1/2
O 8 8 0
O 9 9 1
F 10 9 1/2
N 7 8 1/2
Common nuclei and their spin quantum numbers.
Proton NMR is common since the proton is a common nuclei
All the other isotopes are low abundance, so Fourier Transform techniques are required, and Longer acquisition times, to obtain good NMR
Nuclei and Spin
I = 2I+1 = 2
2 orientations 1 signal
I = 1 2I+1 = 3
3 orientations 3 signals
1 H 2 H
There are 2 orientations for spin = 1/2, so there is one
transition (one signal) upon absorption. For spin = 1, there are 3 orientations, and 3 possible transitions (3 signals) per nucleus. Transition = Signal
For spin = 1/2, one signal per nucleus (1
C) For spin = 1, 3 signals per nucleus (2
Spin and Orientation: Signals per Nucleus
a short blast of radio waves is delivered to the sample and then re-emitted radiation by the sample is monitored over time. Because frequency and time are related by the Heisenberg Uncertainty Principle (like energy and position are), if we know the duration of the radiation pulse precisely we will have many different frequencies present at the same time.
All of the nuclei are excited by the pulse, but then they began to "relax" and emit radio waves of an energy that matches their ΔE. The result is a free induction decay (FID). The complex FID pattern contains information on all the nuclei that were excited by our radiation pulse, and we can convert theses oscillations in time back to each nucleus' frequency by using a mathematical process called a Fourier transform.
http://chemlab.truman.edu/CHEM121Labs/Electronegativity.htm ww.chemie.uni-erlangen.de/ bauer/music3.htm
FT-NMR Time and Frequency Domains: Fourier Transform
High Field Low Field
Typical proton NMR spectrum
The change will be measured in Hertz (Hz) while ΔE is measured in megahertz (MHz). This means that the signal in Hz is in millionths relative to ΔE.
• As ΔE is changed incrementally, each proton will resonate (absorb ΔE) for its particular value of Ho
and we generate a series of absorption peaks for each different proton. This means that each peak will represent a signal for a different type of proton.
• We must establish a zero point. We use tetramethylsilane [(Me3
Si; TMS] as an internal standard. This molecule gives rise to one peak for the methyl groups and we measure everything relative to it.
Low Field, High Field and Zero