NMR Spectroscopy Spectrometer -Hardware

Spectrometer

NMR Spectrometer

Magnet

NMR Spectrometer

Observe Channel

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

NMR Sample Tube 5 mm “Spinner” 3 cm An NMR sample typically requires about 0.75 mL of solution tubes%20and%20spinners.htm

5 mm and 10 mm NMR tubes 5 mm tube for 1 H NMR 10 mm tube for 13 C NMR

Spinning the NMR Sample Tube Homogeneous Inhomogeneous Spinning averages out B o magnetic B o magnetic the inhomogeneities in field field magnetic field

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 1 H 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 (CDCl 3 ), the residual signal is due to CHCl 3, so a singlet signal is observed at 7.26 ppm.

NMR Solvents It used to be common practice to add Me 4 Si (TMS), or related compounds, as an internal reference standard for 1 H and 13 C NMR spectra with the proton signal occurring at 0.00 ppm and the carbon signal occurring at 0.00 ppm in the 13 C NMR spectrum. However, modern spectrometers can "lock" on solvent signals, so addition of internal reference standards is not usually required.

Internal Reference – TMS (tetramethylsilane)

Reference Compounds for NMR Spectroscopy Tetramethylsilane (TMS) Dioxane 3-(Trimethylsilyl)- propionic acid-d 4, sodium salt (TSP) (for use in D 2 O) 2,2-dimethyl-2- silapentane- 5-sulfonate sodium salt (DSS) (for use in D 2 O) 3.75 ppm 0.00 ppm

NMR Solvents Solvent 1 H NMR Chemical Shift* 13 C NMR Chemical Shift* Acetic Acid11.65 (1), 2.04 (5)179.0 (1), 20.0 (7) Acetone2.05 (5)206.7 (13), 29.9 (7) Acetonitrile1.94 (5)118.7 (1), 1.39 (7) Benzene7.16 (1)128.4 (3) Chloroform7.26 (1)77.2 (3) Dimethyl Sulfoxide2.50 (5)39.5 (7) Methanol4.87 (1), 3.31 (5)49.1 (7) Methylene Chloride5.32 (3)54.00 (5) Pyridine8.74 (1), 7.58 (1), 7.22 (1)150.3 (1), (3), (5) Water (D 2 O)4.8 *Chemical Shifts in ppm, number in bracket refers to the multiplicity of the peak

1 H NMR Chemical Shifts for H 2 O in Solvents SolventChemical Shift of H 2 O (or HOD) Acetone2.8 Acetonitrile2.1 Benzene0.4 Chloroform1.6 Dimethyl Sulfoxide3.3 Methanol4.8 Methylene Chloride1.5 Pyridine4.9 Water (D 2 O)4.8

Concentration Effects on Spectra Quality Too concentrated Too dilute

Effect of Number of the Scans (N) on the Signal-to-Noise Ratio (SNR) NN 1/ SNR  N 1/2 Often spectroscopists approximate this quantity as the average peak height divided by the amplitude of the noise in the baseline Methyl Ethyl Ketone (MEK)

Improving Signal/Noise Ratio 1 scan 8 scans 16 scans80 scans

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 D 2 O, resulting in a change in the resonance frequency You wouldn’t actually see this peak due to H-D exchange with D 2 O