1d nmr spectroscopy
DESCRIPTION
spektrofotometer adalahTRANSCRIPT
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1D NMR Spectroscopy
Chemistry 355
Fall 2010
Note: Most figures are from Silverstein, Webster and Kiemle, 7/e, while others
are from the Web. While sources are not cited for space limitations, the author
claims no credit or copyright for any of these unless specifically noted.
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Principles of Magnetic Resonance
Nuclear spin (angular momentum)
Based on nuclear particles (each has spin )
Must have odd number of protons, neutrons, or both
I = 0; 12C not NMR active
I = ; 1H, 13C, 15N, 31P, 19F, 29Si
I = 1; 2H, 14N
I = 5/2; 17O
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Principles of Magnetic Resonance
Spins orient in a magnetic field
Small proportion
DE = (hg/2p)B0
Radio frequency low energy transition
At 298K and 2.35 T (100 MHz) only 0.00041% of protons in a given sample
are excitable.
Difference is greatest at low temperature
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Principles of Magnetic Resonance
Spins precess about the magnetic field at a
characteristic frequency
Larmor frequency
n1 = (g/2p)B0
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Principles of Magnetic Resonance
(a,b) - By irradiating with Rf energy, we can flip the magnetic moment of a nucleus or nuclei (move it from
vertical)
(c, d) - As the nucleus recovers, it will emit its Larmor frequency as a
damped oscillation called a free
induction decay (FID)
(d)
Rf
pulse
FID
Immediately
After
pulse
Before
pulse
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Magnets Field 1 T = 10,000 gauss Earth 0.3 gauss Permanent magnets
0.1 T (5 MHz) to 2.35 T (100 MHz) Limited by bulk and materials
Electromagnet Normal Superconducting
4.7 T (200 MHz) to 22.3 T (950 MHz)
Cryocooled
Frequency Proportional to field Different for nuclei Spectrometer frequency usually
expressed for 1H
60 MHz at 1.4 T 400 MHz at 9.4 T 950 MHz at 22.3 T 13C frequency that of 1H
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The NMR Spectrometer
Magnet Mini radio station
Transmitter Receiver Amplifier Digitizer (FT) Computer (FT)
transmitter receiver
Sweep
Generator
amplifier
preamplifier
The Magnet and
Spectrometer work
together, but are
independent.
Any magnet can be used with either pulsed FT or continuous wave NMR
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The Basic 1D NMR Experiment Continuous wave
Sweep frequency
Nuclei ring at their resonant frequency
First spectrum 1951
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The Basic 1D NMR Experiment Fourier transform
All frequencies pulsed simultaneously
(Heisenberg)
Listen to all sing back at once - Free Induction
Decay (FID)
Fourier transform
Time to Frequency
Greater sensitivity
Time averaging
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The Basic 1D NMR Experiment
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The Effect of Field
Coupling frequency is a constant (Hz)
Higher frequency = more Hz/ppm
Higher field gives
Higher resolution
Greater sensitivity
0123PPM
0123PPM
0123PPM
0123PPM
0123PPM
60 MHz
100 MHz
200 MHz
400 MHz
800 MHz
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-1012345PPM
The Effect of Field
012345PPM
012345PPM
60 MHz
400 MHz
800 MHz
HO
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Chemical Shift - d Position on frequency scale
Expressed in ppm to make it field independent
Influenced by Electron density about atom
Electronegative atoms deshield
Silicon shields
Organometallics
Anisotropic effects Double bonds deshield
Triple bonds shield
Correlation charts
Best discussion in Carey
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Integration Area under peak is
proportional to relativenumbers of atoms
Accuracy to ~1-5%
Depends strongly on acquisition parameters Relaxation time
Proton integrates
Carbon does not (usually)
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1H-1H Through-bond (J) Coupling Coupled via electron spins
Distance between peaks in Hz First two peaks is always 1st J
Up to five bonds away
2-bond (geminal)
3-bond (vicinal)
4-bond (allylic)
Same both ways (J1,2 = J2,1)
OH, NH solvent dependent, exchangeable
Pascals triangle n + 1 rule
Only holds if all J identical
Pentet (p)
(sxt)
(spt)
(o)
(n)
0123PPM
7 Hz
7 Hz
CH3CH2OH
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1H-1H Through-bond (J) Coupling Magnitudes vary 0-20 Hz
2J>=3J>4J>5J
Free rotation ~7-10 Hz
For hindered rotation J depends on dihedral angle
Karplus relationship Largest at 0o, 180o
Smallest around 80o-90o
Different relationship for 2, 3, and 4 bond
NH2
H
NH2
H
NH2
NH2
NH2
NH2
NH2
NH2
H
NH2
H
NH2
180o
Large J30o
Small J
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1H-1H Through-bond (J) Coupling Nonstandard couplings doublet of doublets (dd)
11 Hz
18 Hz
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1H-1H Through-bond (J) Coupling
Nonstandard couplings doublet of quartets (dq)
5 Hz
8 Hz
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1H-1H Through-bond (J) Coupling
Nonstandard couplings (dqd)
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1H-1H Through-bond (J) Coupling
Look for basic patterns to get couplings
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1H-X Through-bond (J) Coupling Other magnetically active nuclei
13C, 15N, 31P, 19F, 29Si
Metals 6Li, 51V, 195Pt, 199Hg
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13C NMR Same arguments apply as with 1H
Pulse, observe FT for spectrum Larger Sweep width Less sensitivity
1.11% 13C
Chemical shift Electronegative atoms Anisotropic effects
No integration Except quant 13C Long pulse delay
Coupling with protons Decoupling
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13C NMR Decoupling
Irradiate 1H while acquiring 13C
Can be continuous or pulsed
Inverse gated decoupling
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DEPT Editiing Distortionless Enhancement through
Polarization Transfer Irradiate 1H while acquiring 13C
Can be continuous or pulsed
Pulse widths
45o CH, CH2, CH390o CH only
135o CH2 up, CH3, CH down
Normal
Decoupled13C
DEPT 90
DEPT 135
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DEPT Editing Distortionless Enhancement through
Polarization Transfer Irradiate 1H while acquiring 13C
Can be continuous or pulsed
Pulse angle q
45o CH, CH2, CH390o CH only
135o CH2 up, CH3, CH down
Normal
Decoupled13C
DEPT 90
DEPT 135
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Summary of Information from NMR
NMR 1H
Chemical Shift what kind of protons
Integration how many protons and in what groups
Couplings
how many neighbors (n+1)
Which neighbors (J values)
13C How many carbons (normal decoupled)
How many protons (coupled or DEPT)
2D spectra direct connectivity (9/21)
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General Notes on Structure Elucidation of
Synthetic Materials from Spectroscopy Some techniques yield lots of information. Others may not yield as much but may give critical information.
NMR protons, carbons. connectivity IR functional groups MS mol. wt., structural fragments UV-Vis absorbing moieties, esp. aromatics (Characteristic lmax)
Use all the information you can get Reaction info
Compare to starting material to see what has changed Compare to expected data for expected product based on structure Look at possible alternate products based on the reactants/reagents used Be on the lookout for impurities
Starting material and co-reactants Solvents Workup reagents/solvents Cleaning solvents and grease
These are all interactive. You may need to go back and forth a few times to settle on a logical structure
Any structure you propose should agree with all characterization data.
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Spectroscopy Homework
(Silverstein) 1.1-1.3
2.2
3.1, 4.1 No Pople notation
Label equivalent proton groups
Label equivalent carbons
3.10 a Determine splitting pattern and coupling constants
3.4, 4.4 a-d Give structure and assign resonances for proton, carbon, IR and mass spectra to the best of your ability
Due 9/13