1.5 Proton NMR Spectrum
PULSED EXCITATION
N
n1
BROADBAND RF PULSE
contains a range of frequencies
n2
O CH2 C CH3
(n1 ..... nn)
n3
S
All types of hydrogen are excited simultaneously with the single RF pulse.
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF PROTONS THERE ARE BY INTEGRATION.
Equivalent Hydrogens
magnet and has what is called a magnetic moment
13-3
Nuclear Spin States
The
same effect holds for certain atomic nuclei
• any atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment • the allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus
13-12
The Larmor Equation!!!
DE = kBo = hn can be transformed into
gyromagnetic
frequency of the incoming radiation that will cause a transition
n = n =
gBg 0 2p 2p
Cl C C
CH 3 H Cy cl o h e x e n e (3 s i g n a l s )
(Z )-1 -Ch l o ro p ro p e n e (3 s i g n a l s )
13-27
NMR Spectrometer

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
1/2
1
0
1/2
1
0
5/2
1/2
(I)
Elements with either odd mass or odd atomic number have the property of nuclear “spin”.
Nuclear Spin States
• a nucleus with spin quantum number I has 2I + 1 spin states; if I = 1/2, there are two allowed spin states
ratio g
Bo
strength of the magnetic field
g is a constant which is different for
each atomic nucleus (H, C, N, etc)
Resonance Frequencies of Selected Nuclei
are a powerful magnet, a radiofrequency generator, and a radiofrequency detector
13-19
1.4 Modern Instrumentation: the Fourier-Transform NMR
FT-NMR requires a computer
Chapter 1 Nuclear Magnetic Resonance Spectroscopy
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
- 1/2
DE
degenerate at Bo = 0
= kBo = hn
+ 1/2
Bo
increasing magnetic field strength
Nuclear Magnetic Resonance
• Figure 1.3 the origin of nuclear magnetic “resonance
hn absorption signal
RF Detector Receiver
Recorder
MAGNET
MAGNET
N
S
~ 1.41 Tesla (+/-) a few ppm
Probe
NMR Spectrometer
NMR Spectrometer
Essentials
of an NMR spectrometer
F F C F H Cl
Cl C Cl H Br
Br C Br
Highly shielded protons appear here.
H
Less shielded protons appear here.
SPECTRUM
NMR Spectrum of Phenylacetone
O CH2 C CH3
Equivalent
hydrogens: have the same chemical environment
• a molecule with 1 set of equivalent hydrogens gives 1 NMR signal
O CH3 CCH3 C l CH 2 C H 2 C l H3 C Propanone (Acetone) 1,2-Dichl oroethane Cyclopentane 2,3-Dimethyl2-butene H3 C C C CH3 CH3
An
electron has a spin quantum number of 1/2
with allowed values of +1/2 and -1/2
• this spinning charge creates an associated magnetic
field
• in effect, an electron behaves as if it is a tiny bar
spins and the applied magnetic field is quantized, with
the result that only certain orientations of nuclear magnetic moments are allowed
13-10
The Energy Separation Depends on Bo
13-28
Some Generalizations
• NMR solvent contain deuterium • Tetramethylsilane (TMS) is the reference • Chemical shift in Hz from TMS vary according to frequency of spectrometer!
Highly shielded protons appear way upfield.
1.3 Classical Instrumentation: The Continuous-Wave NMR
typical before 1965; field is scanned
A Simplified 60 MHz NMR Spectrometer
RF (60 MHz) Oscillator Transmitter
• hydrogen atoms using 1H-NMR spectroscopy • carbon atoms using 13C-NMR spectroscopy • phosphorus atoms using 31P-NMR spectroscopy
13-2
Nuclear Spin States
Isotope Abundance Bo (Tesla)
1H
Frequency(MHz)
g(radians/Tesla)
267.53
99.98%
1.00 1.41 2.35 7.05
42.6 60.0 100.0 300.0
13C
1.108%
1.00 2.35 7.05
10.7 25.0 75.0
67.28
13-4
1.1 Spin Quantum Numbers of Some Nuclei
The most abundant isotopes of C and O do not have spin.
Element Nuclear Spin Quantum No