1. Nuclear Magnetic Resonance Spectrometry Chap 19

2. Classical Description of NMR
Absorption Process
Relaxation Processes (to thermal equil.)
Spin-Lattice
Spin-Spin

3. Behavior of Magnetic Moments of Nuclei
Fig. 19-6
Circularly-polarized
radio frequency mag.
field B1 is applied:
When applied rf frequency
coincides with νLarmor
magnetic vector begins to
rotate around B1
Component absorbed (d or l)
is same as direction of
precession Bo
Clockwise
rotation

4. Spin-Lattice (Longitudinal) Relaxation
Precessional cones representing
spin ½ angular momenta:
number β spins > number α spins
After time T1 :
Populations return to
Boltzmann distribution
Momenta become random
T1 ≡ spin-lattice relaxation time
Tends to broaden NMR lines

5. Spin-Spin (Transverse) Relaxation
Occurs between 2 nuclei having
same precessional frequency
Loss of “phase coherence”
Orderly spins to disorderly spins
T2 ≡ spin-spin relaxation time
No net change in populations
Result is broadening

6. Fourier Transform NMR Nuclei placed in strong magnetic field, Bo
Nuclei precess around z-axis with momenta, M
Intense brief rf pulse (with B1) applied at 90° to M
Magnetic vector, M, rotates 90° into xy-plane
M relaxes back to z-axis: called free-induction decay
FID emits signal in time domain

7. Fig 19-6 Behavior of Mag Moments with 90° Pulse
Free Inductive Decay

8. Fig 19-7 Two Nuclear Relaxation Processes
Spin-Lattice
(Longitudinal)
Spin-Spin
(Transverse) M Mz

9. FID emits signal in time domain
As relaxation proceeds, the FID decreases exponentially θ

10. Behavior of Mag Moments with 90° Pulse
Brief rf pulse applied
FID is detected at νLarmor
Not necessary to know νLarmor ; short pulse is analog of a
hammer striking a bell exciting a range of frequencies.

11. Simple FID of a sample of spins with a single frequency
Fourier Transform
NMR Spectrum

12. Simple FID of species with two frequencies

13. Fig 19-8 13C FID Signal for Dioxane
νRF = νLarmor
Fourier transform of (a)

14. Fig 19-9 13C FID Signal for Dioxane
νRF ≠ νLarmor
Fourier transform of (a)

15. Fig 19-10 13C FID Signal for Cyclohexane

16. Fig 19-11 Wide-Line NMR Spectrum of H2O
In Glass Tube
νRF = 5 MHz
= 10-4 T

17. Fig 19-12
Proton NMR Spectrum
of Ethanol at 60 MHz
Remainder of Chapter:
High Resolution
NMR Spectra

18. Bo = Bapplied – σBapplied
Environmental Effects
Chemical Shift
Nearby electrons and nuclei generate small B fields which tends to oppose Bapplied:
Bo = Bapplied – σBapplied
where σ ≡ screening constant
It is the local field Bo that interacts with magnetic moments!
Now, resonance condition:
Common to hold νRF constant (e.g., 100 MHz) and sweep Bo

19. Abscissa Scales for NMR Spectra
In terms of chemical shift, δ
Almost impossible to measure absolute Bo
Measure change in Bo relative to internal standard: Tetramethylsilane (TMS)

20. High Resolution NMR Spectrum of Ethanol
Fig
High field
High shield
Low field
Low shield
in ppm Bo

21. Fig 19-13 Effect of Field Strength on Chemical Shift