NMR Spectroscopy
Spectrometer -Hardware
http://www.cis.rit.edu/htbooks/nmr/inside.htm
Spectrometer
NMR Spectrometer
Magnet
NMR Spectrometer
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm#nmr3
Observe Channel
Probe
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

NMR Sample Preparation
The majority of NMR samples are run in solution in NMR tubes
http://www.m-ltech.de/nmr-tubes.html
NMR Sample Tube
An NMR sample
typically requires
about 0.75 mL of
solution
3 cm
“Spinner”
5 mm
http://www.cis.rit.edu/htbooks/nmr/inside.htm
http://www.soton.ac.uk/~nmr/
tubes%20and%20spinners.htm
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/tubes%20and%20spinners.htm
Spinning the NMR Sample Tube
Homogeneous
Bo magnetic
field
Inhomogeneous
Bo magnetic
field
Spinning averages out
the inhomogeneities in
magnetic field
http://www.cis.rit.edu/htbooks/nmr/inside.htm
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/NotesOnSolvents.htm
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.
Internal Reference – TMS (tetramethylsilane)
http://orgchem.colorado.edu/hndbksupport/nmrtheory/NMRtutorial.html
Reference Compounds for NMR Spectroscopy
Tetramethylsilane (TMS)
0.00 ppm
Dioxane
3.75 ppm
3-(Trimethylsilyl)propionic acid-d4,
sodium salt (TSP)
(for use in D2O)
2,2-dimethyl-2silapentane- 5-sulfonate
sodium salt (DSS)
(for use in D2O)
0.00 ppm
0.00 ppm
NMR Solvents
1H NMR Chemical Shift*
13C NMR Chemical Shift*
Solvent
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/NotesOnSolvents.htm
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/NotesOnSolvents.htm
Concentration Effects on Spectra Quality
Too concentrated
Too dilute
http://www.cis.rit.edu/htbooks/nmr/inside.htm
Effect of Number of the Scans (N) on the
Signal-to-Noise Ratio (SNR)
SNR  N1/2
H3C
C
H2C
CH3
Methyl Ethyl
Ketone (MEK)
O N
N1/2
1
1.00
8
2.83
16
4.00
80
8.94
800 28.28
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
Improving Signal/Noise Ratio
1 scan
16 scans
8 scans
80 scans
http://www.cis.rit.edu/htbooks/nmr/inside.htm
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.
You wouldn’t actually
see this peak due to
H-D exchange with
D2O
The proton on a hydroxyl
group, for example, may
hydrogen
bond
with
solvents such as D2O,
resulting in a change in
the resonance frequency
http://www.cis.rit.edu/htbooks/nmr/inside.htm