NMR Spectroscopy

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NMR Spectroscopy

• Spectrometer -Hardware

http//www.cis.rit.edu/htbooks/nmr/inside.htm
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Spectrometer
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NMR Spectrometer
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Magnet
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NMR Spectrometer
http//www.cem.msu.edu/reusch/VirtualText/Spectrp
y/nmr/nmr1.htmnmr3
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Observe Channel
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Probe
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Probe

• Requirement for probe
• small enough and symmetrically placed in magnet
  to keep field homogeneiety
• provide means of locking
• able to handle large RF voltages as well as
  receive and process weak FID sinals

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NMR Sample Preparation
The majority of NMR samples are run in solution
in NMR tubes
http//www.m-ltech.de/nmr-tubes.html
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NMR Sample Tube
An NMR sample typically requires about 0.75 mL of
solution
3 cm
Spinner
5 mm
http//www.soton.ac.uk/nmr/ tubes20and20spinner
s.htm
http//www.cis.rit.edu/htbooks/nmr/inside.htm
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5 mm and 10 mm NMR tubes
5 mm tube for 1H NMR
10 mm tube for 13C NMR
http//www.soton.ac.uk/nmr/tubes20and20spinners
.htm
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Spinning the NMR Sample Tube
Homogeneous Inhomogeneous
Spinning averages out Bo magnetic
Bo magnetic the
inhomogeneities in field
field
magnetic field
http//www.cis.rit.edu/htbooks/nmr/inside.htm
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NMR Solvents

• Most NMR spectra are recorded for compounds
  dissolved in a solvent. Therefore, signals will
  be observed for the solvent and this must be
  accounted for in solving spectral problems.
• To avoid spectra dominated by the solvent signal,
  most 1H NMR spectra are recorded in a deuterated
  solvent. However, deuteration is not "100", so
  signals for the residual protons are observed.
  For chloroform as a solvent (CDCl3), the residual
  signal is due to CHCl3, so a singlet signal is
  observed at 7.26 ppm.

http//www.chem.ucla.edu/webspectra/NotesOnSolven
ts.htm
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NMR Solvents

• It used to be common practice to add Me4Si (TMS),
  or related compounds, as an internal reference
  standard for 1H and 13C NMR spectra with the
  proton signal occurring at 0.00 ppm and the
  carbon signal occurring at 0.00 ppm in the 13C
  NMR spectrum. However, modern spectrometers can
  "lock" on solvent signals, so addition of
  internal reference standards is not usually
  required.

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Internal Reference TMS (tetramethylsilane)
http//orgchem.colorado.edu/hndbksupport/nmrtheory
/NMRtutorial.html
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Reference Compounds for NMR Spectroscopy
Tetramethylsilane (TMS) Dioxane 3-(Trimethylsil
yl)- propionic acid-d4, sodium salt (TSP) (for
use in D2O) 2,2-dimethyl-2-silapentane-
5-sulfonate sodium salt (DSS) (for use in D2O)
0.00 ppm
3.75 ppm
0.00 ppm
0.00 ppm
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NMR Solvents
Solvent 1H NMR Chemical Shift 13C NMR Chemical Shift
Acetic Acid 11.65 (1) , 2.04 (5) 179.0 (1) , 20.0 (7)
Acetone 2.05 (5) 206.7 (13) , 29.9 (7)
Acetonitrile 1.94 (5) 118.7 (1) , 1.39 (7)
Benzene 7.16 (1) 128.4 (3)
Chloroform 7.26 (1) 77.2 (3)
Dimethyl Sulfoxide 2.50 (5) 39.5 (7)
Methanol 4.87 (1) , 3.31 (5) 49.1 (7)
Methylene Chloride 5.32 (3) 54.00 (5)
Pyridine 8.74 (1) , 7.58 (1) , 7.22 (1) 150.3 (1) , 135.9 (3) , 123.9 (5)
Water (D2O) 4.8
Chemical Shifts in ppm, number in bracket refers
to the multiplicity of the peak
http//www.chem.ucla.edu/webspectra/NotesOnSolven
ts.htm
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1H NMR Chemical Shifts for H2O in Solvents
Solvent Chemical Shift of H2O (or HOD)
Acetone 2.8
Acetonitrile 2.1
Benzene 0.4
Chloroform 1.6
Dimethyl Sulfoxide 3.3
Methanol 4.8
Methylene Chloride 1.5
Pyridine 4.9
Water (D2O) 4.8
http//www.chem.ucla.edu/webspectra/NotesOnSolven
ts.htm
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Concentration Effects on Spectra Quality
Too concentrated
Too dilute
http//www.cis.rit.edu/htbooks/nmr/inside.htm
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Effect of Number of the Scans (N) on the
Signal-to-Noise Ratio (SNR)
SNR ? N1/2
N N1/2
1 1.00
8 2.83
16 4.00
80 8.94
800 28.28
Methyl Ethyl Ketone (MEK)
Often spectroscopists approximate this quantity
as the average peak height divided by the
amplitude of the noise in the baseline
http//www.cis.rit.edu/htbooks/nmr/inside.htm
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Improving Signal/Noise Ratio
8 scans
1 scan
16 scans
80 scans
http//www.cis.rit.edu/htbooks/nmr/inside.htm
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Solvent Effects
Protons in certain chemical environments may be
found over a wide range of chemical shifts as a
result of interactions with solvent
molecules. The proton on a hydroxyl group, for
example, may hydrogen bond with solvents such as
D2O, resulting in a change in the resonance
frequency
You wouldnt actually see this peak due to H-D
exchange with D2O
http//www.cis.rit.edu/htbooks/nmr/inside.htm