Nuclear Magnetic Resonance (NMR) Spectroscopy
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Nuclear Magnetic Resonance (NMR) Spectroscopy. Dr. Vincent J. Storhaug. Deuterated Solvent Signals in 13 C NMR Spectra. Why do you see this as a triplet at 77 ppm? 1 J CD = 45 Hz n = number of nuclei I = spin quantum number

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Deuterated Solvent Signals in 13C NMR Spectra

Why do you see this as a triplet at 77 ppm?

1JCD= 45 Hz

n = number of nuclei

I = spin quantum number

What do you expect the multiplicity to be for Acetone-d6 (C3D6O)?


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90º Pulse Calibration: Off Resonance

Resonance Line

Transmitter Frequency


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Advantages to Using Higher Applied Fields

60 MHz – propyl bromide

300 MHz – propyl bromide


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Advanced NMR Experiments

Advanced NMR Experiments

Two Dimensional

Homonuclear

1H-1H

Correlation

Heteronuclear

13C-1H, 15N-1H, 13C-15N

Correlation

One Dimensional

1-Channel

2-Channel

Solvent Suppression

HOMODEC

NOEDIF

DEPT

INEPT






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Solvent Suppression Advantage

Three commonly used techniques:

Using the Decoupler to provide Presaturation (Good)

Using a Series of Pulses to provide Presaturation (Better)

Using Pulse Sequences involving the Gradients (Best)

Set up the experiment the same as a homodec experiment – homonuclear decoupling.

*Changes the experiment from a one-pulse experiment to a two-pulse experiment.


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One-Channel Two-Pulse Sequence Advantage

n

y

n

d1

delay 1

at

acquisition time

pw1

d2

delay 2

pw2

time

0 s (more if needed)

2 s


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Homonuclear Decoupling Experiment Advantage

n

n

y

d1

delay 1

at

acquisition time

pw1

d2

delay 2

pw2

time

5*T1

0 s








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HOMODEC Experiment – Ethyl Crotonate Advantage

Expanded 1H spectrum for ethyl crotonate.

(a) Control spectrum. (b) Spectrum with 4-Me group irradiated. (c) Spectrum with H-2 irradiated.


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NOEDIF Experiment Advantage

The Nuclear Overhauser Effect is a net change of the signal intensity of one spin due to the relaxation of a saturated spin that is dipole-dipole coupled to the first spin.

An NOE may be positive OR negative (the latter is more common for large molecules)

NOE’s develop through space rather than through bonds (i.e. – through dipole-dipole

interactions and not through J coupling)

T1 is the key! Fluctuating fields are good, tumbling and molecules are bad.


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2D NMR Acronymns Advantage

NOESY NOE Spectroscopy

ROESY Rotating-Frame NOE Spectroscopy

COSY Correlation Spectroscopy

TOCSY Total Correlation Spectroscopy

HETCOR Heteronuclear Correlation

Long Range HETCOR Long Range Heteronuclear Correlation

COLOC Correlation through Long-Range Coupling

HSQC Heteronuclear Single Quantum Correlation

HMBC Heteronuclear Multiple-Bond Correlation

EXSY Exchange Spectroscopy

APT Attached Proton Test

Solvent Suppression Methods

WATERGATE Water Suppression Through Gradient Tailored Excitation

SECSY Spin Echo Correlated Spectroscopy

DANTE Delay Alternating with Nutation for Tailored Excitation

CHESS Chemical Shift Selective Imaging Sequence

INADEQUATE Incredible Natural Abundance Double Quantum Transfer Experiment

DUMBO Decoupling Using Mind Boggling Optimization

GROPE Generalized Compensation for Resonance Offset and Pulse Length Errors

DOUBTFUL Double Quantum Transitions for Finding Unresolved Lines


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pw180 Advantage

pw180

13C NMR - DEPT(Distortionless Enhancement of Polarization Transfer)

mult*pw90

pw90

pw90

Dec pw90

pw90

pw90

pw90

mult is an arrayed parameter (0.5, 1, 1, 1.5) that leads to a value for the θ pulse of mult*pp: mult=0.5 (θ = 45°) gives approximately equal excitation of all protonated carbons

mult=1.0 (θ = 90°) excites CH’s only (or mainly)

mult=1.5 (θ = 135°) gives CH’s, CH3’s up, CH2’s down.

dm (decoupler modulation) is set to 'nny'.


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13 AdvantageC NMR - DEPT(Distortionless Enhancement of Polarization Transfer)

In a DEPT135 experiment:

-CH3 and CH peaks appear as normal

-CH2- peaks appear inverted

Quaternary C are not usually seen

This way the number of H attached to C can usually be deduced.

Standard 13C NMR Spectrum

of ethylbenzene


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13 AdvantageC NMR - DEPT(Distortionless Enhancement of Polarization Transfer)

In a DEPT135 experiment:

-CH3 and CH peaks appear as normal

-CH2- peaks appear inverted

Quaternary C are not usually seen

This way the number of H attached to C can usually be deduced.

Standard 13C NMR Spectrum

of ethylbenzene


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DEPT 135 Advantage


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DEPT 90 Advantage

DEPT 135


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