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