NMR: PRACTICAL ASPECTS
Transcript of NMR: PRACTICAL ASPECTS
NMR: PRACTICAL ASPECTSPedro M. Aguiar
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Sample Preparation• Well prepared sample can yield high quality spectra • Poorly prepared sample typically yields low quality spectra
• Tubes of appropriate quality • Higher fields require higher quality tubes • Contact local facility manager for specifics
• Sample conc. at typical field instruments (MWt. < 1000) • 1H NMR: >= 10 mM
• 13C NMR: > = 40 mM
• Filter sample to remove insoluble material • No signals • Hampers ability to detect signal of soluble components
• Dry using vacuum and store in ovens no warmer than 80 °C
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Sample PreparationAssumed detection region for shimming
ideal
reality
Good 35-45 mm
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Sample PreparationAssumed detection region for shimming
ideal
reality
• Poor shimming • Weak signal (not in
detector)
Short < 35 mm
Good 35-45 mm
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Sample PreparationAssumed detection region for shimming
ideal
reality
• Poor shimming • Weak signal (not in
detector)
• May impact shimming • Weak signal (dilute)
Short < 35 mm
Good 35-45 mm
Long > 45 mm
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Sample Volume: 5 mm NMR tube
600 µL volume Well-shimmed Good lineshape
300 µL volume poorly-shimmed Poor lineshape
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Limited Sample volume• In cases of Limited sample volume partially filled 5 mm tube
yields poor data
• Alternatives
• Smaller NMR tubes • 3mm tubes (180-200 µL volume)
• Coaxial Inserts • 50-200 µL volumes
• Susceptibility-matched (Shigemi) tubes • 80-300 µL
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Relative Chemical Shifts
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Relative Chemical Shifts
Downfield
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Relative Chemical Shifts
UpfieldDownfield
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Relative Chemical Shifts
UpfieldDownfield
✗
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Relative Chemical Shifts
UpfieldDownfield
Lower frequencyHigher frequency
✗
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Relative Chemical Shifts
UpfieldDownfield
Lower frequencyHigher frequency
✗
✓
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Relative Chemical Shifts
Shielded or More shielded
De-shielded or Less shielded
UpfieldDownfield
Lower frequencyHigher frequency
✗
✓
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Relative Chemical Shifts
Shielded or More shielded
De-shielded or Less shielded
UpfieldDownfield
Lower frequencyHigher frequency
✗
✓✓6-8
Apodisation and Processing
FT
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Apodisation and Processing
Exponential Apodisation (line broadening) is most common for 1D
Optimum is to use “LB” of 0.5-1 times FWHM of peaks
€
e−πLBt
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Apodisation and Processing
FT
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Quantitative NMR?• The integrated signal intensity in NMR can be reflective of the
number of nuclei of a given type
1.01.52.02.53.03.5 ppm
3.0
2.0
2.0
2.0
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Quantitative NMR?• The integrated signal intensity in NMR can be reflective of the
number of nuclei of a given type
• Caveats • Signal intensities not enhanced artificially (e.g., nOe, INEPT/DEPT, pH2)
• Often 13C, 31P, 11B and 29Si on walk-up instruments not quantitative
• All nuclei must be allowed to reach equilibrium before spectrum acquired • Pre-experiment/recycle/relaxation delay must be long enough • 3-5 times T1
• 1H (1-10s), 19F (1-20 s), 13C (10-600 s), 29Si (5-1000 s)
rd
rd
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Quantitative spectraNucleus Experiment Quantitative Comments1H Single-pulse ✓ Recycle delay long enough (5-10s)
19F Single-pulse ✓ Recycle delay long enough (1-60s) Background interference
13C Single-pulse w/ 1H decoupling ✗ nOe enhances signals of sites with
hydrogens
Alternative is to use Inverse-gated decoupling Allow long relaxation time before each scan (10-60s)
31P Single-pulse w/ 1H decoupling ✗ nOe enhances signals of sites with
hydrogens
Alternative is to use Inverse-gated decoupling Allow long relaxation time before each scan (5-30s)
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Baseline Correction• Accurate intgration requires good definition of zero intensity (i.e.,
baseline) • Automatic Routines (polynomial or spline) work well for most small
distortions • In some cases may need other methods
Portion of signal below zero contributes a negative value to sum (integral)
After BC
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Baselines due to ‘stuff’ in probe• NMR probes are made of stuff • Borosilicate glass and quartz (10/11B and 29Si) • Fluoropolymers (19F and sometimes 13C) • Metals in the detection coil (63/65Cu, 195Pt)
• Result in obtrusive background signals/baselines
19F
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Broad signals, short FIDs
Solution is to ‘remove’ offending points Then, use backwards linear prediction
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Linear Prediction• Time-domain (FID) fitting routines to replace missing data • Existing points used as a basis set • Backwards linear prediction generates point at beginning • Useful if long delay before acquisition (alternative to very large ph1)
x
time
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19F
As-collected
Removal of initial points & Backwards Linear Prediction
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Are integrals Accurate?• Integrals in most software packages
done numerically • Works well when signal-to-noise is high
• Alternative is lineshape fitting • Works will even if signal-to-noise is low
Numerical Integration
Lineshape Fitting
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Real Example lineshape Fits: Pt-195 NMR
Samples Courtesy: Imelda Silalahi and Duncan Bruce, University of York
High signal-to-noise signal Integration and lineshape similar result
Low signal-to-noise signal Integration yields erratic result Lineshape fit yields consistent result
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Bandwidth and Integrals• Probe and Pulses utilised have intrinsic bandwidths • Peaks very far apart may be affected in different ways • Can be issue for 19F if very large shifts are present • E.g., Ar-F (130 ppm) vs. M-F (-350 ppm)
667 ppm
400 ppm
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Coupling to Quadrupolar Nuclei• Quadrupolar nuclei can often have T1s comparable to 1/J
[(R)2P)2Co-H
Magnetically equivalent phospines P-31 (N.A. 100%, I-1/2) Cobalt Co-59 (N..A. 100%, I-7/2)
Expect octet of triplets Observe a blob?
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Coupling to Quadrupolar Nuclei• Quadrupolar nuclei can often have T1s comparable to 1/J
1/J is a measure of time it takes a nucleus to ‘tell’ another nucleus what state its in.
If T1 comparable to 1/J, then the spin-state can change before a nucleus has time to ‘tell’ another nucleus what state it is in
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Coupling to Quadrupolar Nuclei• Quadrupolar nuclei can often have T1s comparable to 1/J
[(R)2P)2Co-H
Magnetically equivalent phospines P-31 (N.A. 100%, I-1/2) Cobalt Co-59 (N..A. 100%, I-7/2)
1H spectrum with 59Co decoupling results in triplet (due to 31P J-coupling)
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DEPT-135 vs 13C 1D
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Correlation Spectroscopy (i.e., 2D NMR)• Collect a series of 1D spectra where some period of additional
evolution occurs • Evolution of states during t1 (often linked via J-coupling)
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Correlation Spectroscopy (i.e., 2D NMR)• Collect a series of 1D spectra where some period of additional
evolution occurs • Evolution of states during t1 (often linked via J-coupling)
something something else1st
evo
l.
2nd evolution
25-2
Correlation Spectroscopy (i.e., 2D NMR)• Collect a series of 1D spectra where some period of additional
evolution occurs • Evolution of states during t1 (often linked via J-coupling)
something something else1st
evo
l.
2nd evolution
something something else1st
evo
l.
2nd evolution
25-3
Correlation Spectroscopy (i.e., 2D NMR)• Collect a series of 1D spectra where some period of additional
evolution occurs • Evolution of states during t1 (often linked via J-coupling)
something something else1st
evo
l.
2nd evolution
something something else1st
evo
l.
2nd evolution
something something else
2nd evolution1st evolution
…
25-4
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
Connectivity limited to 4 or 5 bonds
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
Connectivity limited to 4 or 5 bonds
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
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Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
29-2
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
29-3
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
29-4
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
30-1
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
30-2
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
30-3
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
rd
90
t2
90
t1
30-4
Correlation Spectroscopy (i.e., 2D NMR)• COrrelation SpectroscopY (COSY) • One of most basic 2D experiments • Information about connectivity
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
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rd
90
t2
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t1
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Total Correlation Spectroscopy (TOCSY)• Provides ‘all’ correlations irrespective of magnitude • Allows distinction of coupling networks • Does not permit identification of ‘neighbouring’ spins
rd
90
t2
90
t1
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
0
90
mix
• Homonuclear Hartman-Hahn (HOHAHA) mixing • Mixing times • 10-30 ms: 1-5 bonds • 40-100 ms: 1-10 bonds
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TOCSY vs COSY
COSY TOCSY
Correlations between individual coupled spins
Correlations between all spin in a spin-system
Limited to 4 bonds Beyond 4 bonds Correlations limited by presence of non-H bearing atoms
Follow coupled spins sequentially to understand connectivity
Allow identification of groups of spins from specific parts of molecule e.g., sugar rings in polysaccharide Sidechains in peptide/protein
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Spin or chemical Exchange : NOESY/EXSY• Nuclear Overhauser effect • Dipolar cross-relaxation • ‘COSY-type’ signals may also be present
rd
90
t2
90
t1
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mix
• No RF applied during mixing • Mixing times • 50-800 ms
• Negative correlations = nOe • Positive correlations = Xchng
ppm
5 4 3 2 1 0 ppm
5
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3
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1
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Spin or chemical Exchange : NOESY/EXSY• Nuclear Overhauser effect • Dipolar cross-relaxation • ‘COSY-type’ signals may also be present
rd
90
t2
90
t1
90
mix
• No RF applied during mixing • Mixing times • 50-800 ms
• Negative correlations = nOe • Positive correlations = Xchng
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
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Spin or chemical Exchange : NOESY/EXSY• Nuclear Overhauser effect • Dipolar cross-relaxation • ‘COSY-type’ signals may also be present
rd
90
t2
90
t1
90
mix
• No RF applied during mixing • Mixing times • 50-800 ms
• Negative correlations = nOe • Positive correlations = Xchng
ppm
5 4 3 2 1 0 ppm
5
4
3
2
1
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Distance Information from nOe• nOe inensity is related to the distance between the two nuclei
• Acquisition of spectra at different mixing times can allow extraction of distances
Figure from: P. Brocca, P. Berthault, S. Sonnino; Biophysical Journal (1998), 74(1), 309
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When NOESY doesn’t work…• Nuclear Overhauser effect near zero for molecules of MWt.
800-2000 (depending on solvent)
rd t2
90
t1 mix
• Use rOe (rotating frame Overhauser effect)
• Mixing times • 5-100 ms
• 2D spectrum similar to NOESY
• TOCSY-like peaks may be present
Small molecules
large molecules
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Heteronuclear Correlation• Used for probing connectivity between two different types of
nuclei
rd1/2J
t1
1H
X
1/2Jt2
rd1/4J
t1
X
t2
1H
1/4J 1/4J 1/4J
HMQC
HSQC
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Some Common H-X J-coupling ranges• 13C • One-bond: 120-170 Hz • Two-bond: <30 • Three-bond: <15
• 31P • One-bond: 30-1000 • Two-bond: <50 • Three-bond: <40
• 15N • One-bond: 65-100 Hz
• Two-bond: < 11Hz • Three-bond: < 11 Hz
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HSQC vs HMQC• HSQC • Heteronuclear Single-Quantum Correlation • 1H-detected 1H-X correlation spectrum
• Typically used to probe ‘one-bond’ correlations • Can be higher resolution
• Variants with multiplicity (XH, XH2, XH3 etc.) selection
• HMQC • Heteronuclear Multiple-Quantum Correlation
• 1H-detected 1H-X correlation spectrum
• Typically used to probe ‘one-bond’ correlations • can be higher sensitivity
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HSQC vs. HMQC
ppm
3.54.04.55.05.5 ppm
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3.54.04.55.05.5 ppm
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• HSQC capable of intrinsically higher resolution than HMQC
1H{13C} HSQC 1H{13C} HMQC
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Edited HSQC• Adds a ‘DEPT-like’ selection • XH and XH3 positive
• XH2 negative
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Weak signals in HSQC• Weak signals in HSQC can arise from various sources • Minor impurities/byproducts • Multiple-bond correlations
ppm
3.54.04.55.05.5 ppm
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3.54.04.55.05.5 ppm
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x4
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Multiple-Bond Correlation
• Heteronuclear Multiple-bond correlation experiment
• Multiple-bond correlations have smaller J-couplings
• HMBC effectively an HMQC with 1/2J set to optimize for smaller J-coupling • Often have an additional evolution to minimize signals from correlations
between sites with large J-couplings
• Use together with HSQC/HMQC to determine C-C connectivity
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HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
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3.54.04.55.05.5 ppm
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HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
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ppm
3.54.04.55.05.5 ppm
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Single-bond correlation
43-2
HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
60
70
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90
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ppm
3.54.04.55.05.5 ppm
60
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Single-bond correlation
43-3
HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
60
70
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ppm
3.54.04.55.05.5 ppm
60
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Single-bond correlation
43-4
HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
60
70
80
90
100
ppm
3.54.04.55.05.5 ppm
60
70
80
90
100
Single-bond correlation
43-5
HMQC/HSQC plus HMBC• Use HMQC/HSQC together with HMBC to determine
connectivity
ppm
3.54.04.55.05.5 ppm
60
70
80
90
100
ppm
3.54.04.55.05.5 ppm
60
70
80
90
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Single-bond correlation
?
43-6
NO Forward Linear Prediction
With Forward Linear Prediction
Linear Prediction: indirect dimension17 min
17 min
4 min
4 min
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Relevant LiteratureBooks • “Modern NM Techniques For Chemistry Research”, A.E. Derome • “High-Resolution NMR Techniques in Organic Chemistry”,
T.D.W. Claridge • “Nuclear Magnetic Resonance” , P.J. Hore (Oxford Primer) • “Spin Dynamics”, M.H. Levitt
Journals/Book Series • Concepts in Magnetic Resonance • Annual Reports in NMR (Chemistry Library) • Encyclopedia of NMR (DH Coffee room)
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