NMR Spectroscopy

1. NMR Spectroscopy Spectrometer -Hardware http://www.cis.rit.edu/htbooks/nmr/inside.htm

2. Spectrometer

3. NMR Spectrometer

4. Magnet

5. NMR Spectrometer http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm#nmr3

6. Observe Channel

7. Probe

8. 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

9. NMR Sample Preparation The majority of NMR samples are run in solution in NMR tubes http://www.m-ltech.de/nmr-tubes.html

10. 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/ tubes%20and%20spinners.htm http://www.cis.rit.edu/htbooks/nmr/inside.htm

11. 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

12. 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

13. 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

14. 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.

15. Internal Reference – TMS (tetramethylsilane) http://orgchem.colorado.edu/hndbksupport/nmrtheory/NMRtutorial.html

16. 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) 0.00 ppm 3.75 ppm 0.00 ppm 0.00 ppm

17. NMR Solvents *Chemical Shifts in ppm, number in bracket refers to the multiplicity of the peak http://www.chem.ucla.edu/~webspectra/NotesOnSolvents.htm

18. 1H NMR Chemical Shifts for H2O in Solvents http://www.chem.ucla.edu/~webspectra/NotesOnSolvents.htm

19. Concentration Effects on Spectra Quality Too concentrated Too dilute http://www.cis.rit.edu/htbooks/nmr/inside.htm

20. Effect of Number of the Scans (N) on the Signal-to-Noise Ratio (SNR) SNR  N1/2 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

21. Improving Signal/Noise Ratio 8 scans 1 scan 16 scans 80 scans http://www.cis.rit.edu/htbooks/nmr/inside.htm

22. 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 D2O http://www.cis.rit.edu/htbooks/nmr/inside.htm