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Lecture Date: February 13th, 2008
Nuclear Magnetic Resonance 2
NMR Experiments
NMR experiments fall into some basic categories:
– Basic pulse methods
Single pulse
Selective pulse or selective decoupling
Solvent suppression
– 2D and multi-dimensional experiments
unravel complex spectra by separation of overlapping signals,
control of “mixing” between signals (to obtain more data)
– Multiple resonance (heteronuclear techniques)
Are often 2D or nD sequences
– Diffusion, dynamics and relaxation experiments
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Common Solution-state NMR Experiments for
Organic Structural Analysis
Experiment Acronym Information
Provided
GASPE
DEPT
Gated-spin echo
Distortionless editing by
polarization transfer
13C multiplicity
(C, CH, CH2, CH3)
COSY correlated spectroscopy 1H-1H covalent
bonding, 2-4
bonds
HMQC heteronuclear multiple
quantum coherence
1H-13C covalent
bonding, 1 bond
HMBC heteronuclear multiple
bond correlation
1H-13C covalent
bonding, 2-4
bonds
NOE difference,
NOESY,
ROESY
nuclear Overhauser
effect spectroscopy
1H-1H proximity in
space, 1.8-4.5 A
Pulse Sequences
Modern NMR involves flexible spectrometers that can
implement pulse sequences, which are designed to
extract and simplify relevant information for the
spectroscopist
Designed to harness a property or properties of the
nuclear spin Hamiltonians
– J-coupling
– Chemical shift
– Quadrupolar coupling
– Dipolar coupling
Or, are designed to measure a bulk effect
– Relaxation
– Diffusion
– Chemical exchange or dynamics
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Basic Pulse Sequences
A single pulse and acquire
An Example of 1D NMR
Top – 1H spectrum
Middle – Selective pulse
Bottom – homonuclear decoupling
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Multi-dimensional NMR
The general scheme of 2D and multi-dimensional NMR:
Evolution (t1) Detection (t2)Preparation Mixing (tm)
Experiment Time
2D NMR data has two frequency dimensions:
Can include NOE or J-
coupling mixing
FT(t1) FT(t2)
A Simple 2D NMR Spectrum
Diagonal Peak
Cross peak
(“correlation”)
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2
3
4
5
12345
F1(ppm)
F2(ppm)
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An Example of 2D NMR – the COSY Experiment
Correlations are
observed between
J-coupled protons!
(Example is a sample
of sucrose in D2O)
Applications of NMR
Structural analysis
Quantitative analysis
Stereochemical and conformational analysis
Solid-state analysis
