Chem 325 NMR Intro 1 Physical properties, chemical properties, formulas Shedding real light on molecular structure: Frequency ν Wavelength Wavelength λ Frequency ν Velocity c = 2.998 × 10 8 m⋅s -1 λ×ν = c Energy of a photon : E = hν = hc/λ h = Planck’s constant = 6.626 × 10 -34 J⋅s The Electromagnetic Spectrum
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Chem 325 NMR Intro - StFX · Chem 325 NMR Intro 1 Physical properties, chemical properties, formulas Shedding real light on molecular structure: Frequency νννν Wavelength Wavelength
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Chem 325 NMR Intro
1
Physical properties, chemical properties, formulas
Shedding real light on molecular structure:
Frequency νννν
Wavelength
Wavelength λλλλ
Frequency νννν
Velocity c = 2.998 ×××× 108 m⋅⋅⋅⋅s-1
λλλλ ×××× νννν = c
Energy of a photon: E = hνννν = hc/λλλλ
h = Planck’s constant = 6.626 ×××× 10-34 J⋅⋅⋅⋅s
The Electromagnetic Spectrum
Chem 325 NMR Intro
2
Emission and Absorption Spectroscopy
Nuclear Magnetic Resonance
Spectroscopy
Let’s go for a spin!
Electron Spin: A Fourth Quantum Number
The Stern-Gerlach Experiment
Chem 325 NMR Intro
3
Nucleus: nucleons: protons, neutrons
-these also have ‘spin’ properties: up, down
- spins add together in a complicated way to give total nuclear spin I, characteristic of a given type of nucleus
- some general guidelines….
oddoddevenodd
1H, 13C, 19F, 31P
(I = ½)
35Cl, 37Cl (I = 3/2)
127I (I = 5/2)
½, 3/2, 5/2, …
evenoddoddeven
2H, 14N (I = 1)
10B (I = 2)1, 2, 3, …oddevenoddodd
4He, 12C, 16O, 32S0eveneveneveneven
ExamplesTotal Spin
IZA#p#n
The net nuclear spin gives rise to a number of spin states
#spin states = 2I +1
Spin states characterized by mI or Izvalues:
For a given I value, mI = Iz = +I, I-1, I-2,..,-I+1, -I.
e.g. for I = 0, mI = 0 (only!)
for I = ½, ml = +½, -½
for I = 1, ml = +1, 0, -1
Normally all nuclear spin states are degenerate
→ same energy
-degeneracy can be removed by application of an external magnetic field
Chem 325 NMR Intro
4
Amount of splitting of the nuclear spin states, ∆∆∆∆E,
is directly proportional to the applied magnetic
field strength B0
is directly proportional to magnetogyric ratio γγγγ of
the particular nucleus type
νγπ
γ hBBh
)2
( E 00 ===∆ h
)2
( 0Bπ
γν =
νννν is the frequency of the EM radiation required for
the transition from the lower to the upper spin states
40.01.00251.719F
75.07.05
50.04.70
10.71.0067.2813C
6.51.0041.12H
300.7.05
200.4.70
42.61.00267.531H
Frequency νννν
(MHz)
Field
strength B0
(Tesla)
γγγγ
(106 rad/Tesla ×××× sec)
Nucleus
• Increasing B0 increases ∆∆∆∆E
• Increasing B0 results in a higher frequency νννν of EM radiation required to produce the transition
• For a given B0, different types of nuclei have different ∆∆∆∆E, thus different νννν values
• EM radiation with νννν values in MHz range are radio waves
• So far: allows us to identify which types of atoms our molecule has by monitoring which EM frequencies are absorbed at a given applied magnetic field strength (but limited to those nuclei with I ≠≠≠≠ 0!)
Precession
Interaction of the nuclear magnetic moment and the
applied field causes the rotational axis to precess about the
field axis (z-axis) (like a toy top)
H B0
ω
Precessional frequency or Larmor frequency ωωωω
For a given field strength B0, nuclei of
different types precess at different
Larmor frequencies according to their
magnetogyric ratio γγγγ values:
ωωωω = γγγγB0
Chem 325 NMR Intro
5
Mechanism of Absorption
When a photon of, say, νννν = 60 MHz encounters this spinning
charged system the two can couple and change the spin state