Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance 2
Lecture Date: February 13th, 2008
Selected Applications of NMR
 Structural analysis
 Stereochemical and conformational analysis
 Quantitative analysis
 Solid-state analysis
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
Common Solution-state NMR Experiments for
Organic Structural Analysis
Information
Provided
Experiment
Acronym
GASPE
Gated-spin echo
DEPT
Distortionless editing by
polarization transfer
COSY
correlated spectroscopy
1H-1H
HMQC
heteronuclear multiple
quantum coherence
1H-13C
covalent
bonding, 1 bond
HMBC
heteronuclear multiple
bond correlation
1H-13C
NOE difference,
NOESY,
ROESY
nuclear Overhauser
effect spectroscopy
13C
multiplicity
(C, CH, CH2, CH3)
covalent
bonding, 2-4
bonds
covalent
bonding, 2-4
bonds
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
An Example of 1D NMR
Top – 1H spectrum
Middle – Selective pulse
Bottom – homonuclear decoupling
Structural Analysis – 13C NMR and Editing
13C
spectra of
cholesteryl acetate:
(a) continuous 1H
decopling
(b) 1H during
acquisition (no
NOE)
(c) GASPE (APT)
(d) DEPT-135
Multi-dimensional NMR

The general scheme of 2D and multi-dimensional NMR:
Can include NOE or Jcoupling mixing
Preparation
Evolution (t1)
Mixing (tm)
Experiment Time

2D NMR data has two frequency dimensions:
FT(t1)
FT(t2)
Detection (t2)
A Simple 2D NMR Spectrum
1
Cross peak
(“correlation”)
2
3
4
Diagonal Peak
5
5
4
3
F2 (ppm)
2
1
F1 (ppm)
An Example of 2D NMR – the COSY Experiment
Correlations are
observed between
J-coupled protons!
(Example is a sample
of sucrose in D2O)
Structural Analysis: 1H –13C Correlation
The 1H-13C HSQC
analysis of
clarithromycin:
Structural Analysis: Long-range 1H –13C Correlation
The 1H-13C HMBC
analysis of
carvedilol:
Structural Analysis: 1H –15N Correlation
The 1H-15N longrange HMQC
analysis of
telithromycin:
Determination of Relative Stereochemistry
NOE difference spectroscopy
Determination of Absolute Stereochemistry
Chemical Shielding around the Benzene Ring
12
10
Absolute Isotropic Shielding (ppm)
Remember the ring
current effect?
8
Above Ring
In Ring Plane
6
4
2
0
-2
0.0
2.0
4.0
6.0
8.0
Distance from Ring Center (A)
Chemical Shielding around the Benzene Ring (Expanded View)
1
Absolute Isotropic Shielding (ppm)
0.8
0.6
shielding (opposes field)
Above Ring
0.4
In Ring Plane
0.2
0
-0.2
-0.4
-0.6
deshielding (aligned with field)
-0.8
-1
4.0
5.0
6.0
Distance from Ring Center (A)
J. A. Dale and H. S. Mosher, J. Am. Chem. Soc., 95, 512-519 (1973).
C. E. Johnson and F. A. Bovey, J. Chem. Phys., 29, 1012 (1958).
7.0
8.0
Determination of Absolute Stereochemistry by
Mosher-Dale Method

Procedure: Derivatize a chiral alcohol with MPTA, -methoxy-(trifluoromethyl)phenyl acetic acid

Because a phenyl group’s deshielding effects drop off more rapidly with
distance than its shielding effects, protons close to a phenyl should be more
shielded!

Example: 5-nitro-2-pentanol
1 NO2
2
3.51q
7.4-7.5m
9
8
H3CO
Ph
F3C
3
4
7
6
O
(S)-MPTA-Cl
=> (R)-MPTA ester
2
4.45t
2.02m
1.69m
O
3.55q
7.4-7.5m
9
8
H3CO
Ph
1.26d
CH3
5
10
1 NO
F3C
11
H 5.15m
(R)-alcohol
|5JH9,F10| = 1.2 Hz
|3JH11,H5| = 6.2 Hz
|3JH2,H3| = 6.9 Hz
|4JH2,H4| = 0 Hz
7
2
1.35d
6
3
11
H3C
O
4
1.83m
1.62m
5
10
O
(S)-MPTA-Cl
=> (R)-MPTA ester
4.34dt
H 5.15m
(S)-alcohol
|5JH9,F10| = 1.1 Hz
|3JH11,H5| = 6.3 Hz
|3JH2,H3| = 6.8 Hz
|4JH2,H4| = 2.2 Hz
J. A. Dale and H. S. Mosher, J. Am. Chem. Soc., 95, 512-519 (1973).
A. Guarna, E. O. Occhiato, L. M. Spinetti, M. E. Vallecchi, and D. Scarpi, Tetrahedron, 51, 1775-1788 (1995).
19F
Quantitative Analysis: TFA Salt Stoichiometry
Solid-state Nuclear Magnetic Resonance



NMR in solids, like solution-state, relies
on the behavior of nuclear spin energy
levels in a magnetic field. However, the
interactions that affect NMR spectra act
differently.
In liquids, molecules reorient and
diffuse quickly, leading to narrow
isotropic resonances.
In solids, the fixed orientation of
individual crystallites leads to a range
of resonance frequencies for
anisotropic interactions.
No field
Field = B0
m=-1/2
E=(h/2)B0
m=+1/2
E
Solid-state NMR: Magic-Angle Spinning

The following anisotropic interactions
are dependent on their orientation
with respect to the large magnetic
field (B0):
–
–
–


broadening 
P2 cos   3 cos2   1
dipolar (homo- and heteronuclear)
coupling
1st-order quadrupolar coupling
anisotropic chemical shift
These can be averaged away over
time by spinning at a root of the
scaling factor:
The result of magic angle spinning
(often combined with dipolar
decoupling):
E. R. Andrew, A. Bradbury, and R. G. Eades, Nature, 183, 1802 (1959).
I. J. Lowe. Phys. Rev. Lett. 2, 285 (1959).
Cross-Polarization
 Cross-polarization is an example of a double resonance experiment

– Two resonances, typically two different nuclei, are excited in a
single experiment.
Cross-Polarization combined with MAS (CP-MAS):
– Enhancement of signal from “sparse” spins via transfer of
polarization from “abundant” spins
– The “Hartmann-Hahn condition” allows for efficient energy
transfer between the two spins, usually via dipolar interactions
– The basic CP pulse sequence for 1H to 13C experiments:
90
CP
CW Decoupling
1H
13C
CP
E. O. Stejskal and J. D. Memory. “High Resolution NMR in the Solid State,” Oxford University Press, New York (1994).
A. Pines, M. G. Gibby and J. S. Waugh. J. Chem. Phys., 59, 569 (1973).
An Example: Polymorphism in Carvedilol

13C
CP-TOSS spectra of the polymorphs of SKF105517 free base
4
5
6
3
7
2
O
O
8
H3C
1
9
10 NH
11
12
OH
15
13
O
16
14
17
19
18
20 NH
21
22
26
23
25
 Amorphous forms generally give broadened spectra
24
An Example: Polymorphism in Carvedilol

15N
SSNMR spectroscopy also shows similar effects.
4
5
6
3
7
2
O
O
8
H3C
1
9
10 NH
11
12
OH
15
13
O
16
14
17
19
18
20 NH
21
22
26
23
25
24
 Advantages: simple and easy-to-interpret spectra, valuable information

about the nitrogen chemical environment
Disadvantage: much lower sensitivity
LC-SPE-NMR for Impurity Analysis
LC separation and solid-phase extraction (SPE) concentration
Magnetic Resonance Imaging
• The basic idea: a linear magnetic field gradient imposes a
linear spread of Larmor frequencies on a sample.
0   B0
Gradient
0   B0
Figure from S. W. Homans, A Dictionary of Concepts in NMR, Oxford, 1989.
For more details, see P. G. Morris, NMR Imaging in Medicine and Biology, Oxford University Press, 1986.
Magnetic Resonance Force Microscopy
 A “combination” of
AFM and
EPR/NMR
 Uses a nano-scale
cantilever to detect
spin motion
induced by RF via
in an magnetic field
Rugar, D.; et al. Nature 2004, 430, 329–332.
R. Mukhopadhyay, Anal. Chem. 2005, 449A-452A.
Nuclear Spin Optical Rotation (NSOR)
 Measures NMR signals by detecting phase shifts
induced in a laser beam as a the beam passes
through a liquid
 Gives excellent spatial resolution
 Currently lacks sensitivity
 Developed by Romalis group at Princeton
Nature 2006, 442, 1021
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