NMR Spectroscopy Spectrometer -Hardware .
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Transcript of NMR Spectroscopy Spectrometer -Hardware .
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 recei
ve 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
5 mm
“Spinner”3 cm
An NMR sample typically requires about 0.75 mL of solution
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 13
C NMR
http://www.soton.ac.uk/~nmr/tubes%20and%20spinners.htm
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
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)
Dioxane
3-(Trimethylsilyl)- propionic acid-d4, sodium salt (TSP)(for use in D2O)
2,2-dimethyl-2-silapentane- 5-sulfonate sodium salt (DSS)(for use in D2O)
3.75 ppm
0.00 ppm
0.00 ppm
0.00 ppm
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
http://www.chem.ucla.edu/~webspectra/NotesOnSolvents.htm
*Chemical Shifts in ppm, number in bracket refers to the multiplicity of the peak
1H NMR Chemical Shifts for H2O in SolventsSolvent 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)
N N1/2
1 1.00
8 2.83
16 4.00
80 8.94
800 28.28
SNR N1/2
Often spectroscopists approximate this quantity as the average peak height divided by the amplitude of the noise in the baseline
H3C
C
H2C
CH3
O
Methyl Ethyl Ketone (MEK)
http://www.cis.rit.edu/htbooks/nmr/inside.htm
Improving Signal/Noise Ratio
1 scan 8 scans
16 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.
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 wouldn’t actually see this peak due to H-D exchange with D2
O
http://www.cis.rit.edu/htbooks/nmr/inside.htm