1. Nuclear Magnetic Resonance
Unit 16

2. Introduction
NMR is the most powerful tool available for organic structure determination.
It is used to study a wide variety of nuclei: 1H
13C
15N
19F
31P

3. Nuclear Spin
A nucleus with an odd atomic number or an odd mass number has a nuclear spin.
The spinning charged nucleus generates a magnetic field.

4. External Magnetic Field
An external magnetic field (B0) applies a force to a small bar magnet, twisting the bar magnet to align it with the external field.
The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field.

5. Magnetic Shielding
If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.
But protons are surrounded by electrons that shield them from the external field.
Circulating electrons create an induced magnetic field that opposes the external magnetic field.

6. NMR Signals
The number of signals shows how many different kinds of protons are present.
The location of the signals shows how shielded or deshielded the proton is.
The intensity of the signal shows the number of protons of that type.
Signal splitting shows the number of protons on adjacent atoms.

7. Electromagnetic spectrum

8. Molecular Spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of
hydrogen atoms using 1H-NMR spectroscopy
carbon atoms using 13C-NMR spectroscopy
phosphorus atoms using 31P-NMR spectroscopy

9. Nuclear Magnetic Resonance
The origin of nuclear magnetic “resonance”

10. Nuclear Magnetic Resonance
Resonance: in NMR spectroscopy, resonance is the absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state
The instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal
signal: a recording in an NMR spectrum of a nuclear magnetic resonance

11. Nuclear Magnetic Resonance
if we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei
but hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field
the circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding

12. Nuclear Magnetic Resonance
the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small
the difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency

13. Nuclear Magnetic Resonance
signals are measured relative to the signal of the reference compound tetramethylsilane (TMS)
for a 1H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS
for a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS
Chemical shift (): the shift in ppm of an NMR signal from the signal of TMS

14. NMR Spectrometer

15. NMR Instrument Figure: 13_07-02UN.jpg Title: NMR Laboratory Caption:
Photo of a modern 300-MHz NMR spectrometer. The metal container at the right contains the superconducting magnet, cooled by a liquid helium bath.
Notes:

16. NMR Spectrometer
Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio- frequency detector
The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning
Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds

17. NMR Spectrum 1H-NMR spectrum of methyl acetate
Downfield: the shift of an NMR signal to the left on the chart paper
Upfield: the shift of an NMR signal to the right on the chart paper

18. Signal Areas
Relative areas of signals are proportional to the number of H giving rise to each signal
Modern NMR spectrometers electronically integrate and record the relative area of each signal

19. Magnetic Resonance Imaging (MRI)
Nuclear Magnetic Resonance Imaging is a noninvasive diagnostic tool.
“Nuclear” is omitted because of public’s fear that it would be radioactive.
Computer puts together “slices” to get 3-D images.
Tumors are readily detected.

20. MRI Scan of a Human Brain
MRI scan of a human brain showing a metastatic tumor in one hemisphere.

21. MRI Image of the Pelvic Region