NMR Solvents & Referencing
Solvents
For 1H experiments protio-solvent should be avoided, if possible, as the strong solvent signals will interfere with the spectrum required . The solvent should normally contain a strong deuterium signal (e.g. CDCl3) so that the field can be "locked" for the duration of the experiment. This is especially important for 1H measurements where good resolution is required.All of the spectrometers have an option for running samples unlocked (i.e. without d-solvent) but this usually only appropriate with certain 31P or 11B measurements. Solvent suppression experiments can sometimes be used to reduce residual solvent signals in 1H spectra (see 1D experiments guide).
Solvent Supplies
CDCl3 (100g) is available from stores as a proactis stores item (order using a white form with code CHEM000408).Other solvents can be borrowed from S100 or in signed out to a group if there is a specific need.
Solvent Signals
Please see Cambridge Isotope
Laboratories chart for Chemical Shifts, boiling points etc.
of NMR solvents (courtesy of Cambridge Isotope Laboratories). Solvent costs
- Currently we spend well over £11,000 on deuterated solvents per annum and it is vital that they are used sensibly. They are brought in bulk each year and are available without charge from the NMR lab (S100). THF is not provided for free, but can be purchased via the NMR service.
| Solvent (10g unless stated) |
Cost per bottle (2011)
|
Cost per sample(2011) |
| d-Chloroform
(100g) |
£
6.11 |
£
0.04 |
| Deuterium
Oxide (100g) |
£
28.18 |
£
0.20 |
| d-Dimethylsulfoxide
|
£
7.12 |
£
0.51 |
| d-Acetone |
£
10.87 |
£
0.78 |
| d-Benzene |
£
9.32 |
£
0.67 |
| d-Acetonitrile |
£
10.62 |
£
0.76 |
| d-Methanol |
£
8.11 |
£
0.58 |
| d-Toluene |
£ 18.47 |
£ 1.32 |
| d-Pyridine |
£ 25.20 |
£ 1.80 |
| d-Dichloromethane | £ 19.74 |
£ 1.41 |
| d-Tetrahydrofuran (0.75ml) |
- |
£ 9.50 |
- Deuterochloroform, CDCl3, is the cheapest and purest
deuterated solvent. It should be used whenever possible
(<10 pence/sample).
- Tetramethylsilane (TMS)
is also available and may be added if required (the vapour
from the TMS bottle put into the solution with a dropping
pipette is enough for 1H experiments).
- Always use the least expensive solvent compatible with your sample. In particular please try to avoid using the solvents in bold above.
- Alternatives with similar solubility properties are: Benzene instead of Toluene. Acetone instead of Acetonitrile/Methanol/Dichloromethane.
- Routine 1H spectra are unlikely to justify the use of an expensive solvent. 1H measurements are so sensitive, that using a more dilute solution because of limited solubility is often wiser than using an expensive solvent.
- Mixtures of protio- and deutero-solvents may be used to reduce costs (except where 1H spectra are required!). e.g. A solution of 20% CD2Cl2 in CH2Cl2 is suitable for use on all instruments. However, do not use CDCl3/CHCl3 mixtures as CHCl3 contains ethanol and CDCl3 is cheap.
- Before any solvents are taken out of S100 they must be
signed out in the appropriate book.
Referencing
All NMR spectra run by the service are calibrated in the ppm
scale. For 1H, 2H and 13C internal TMS is the accepted reference
(0ppm), and this is used whenever its signal can be observed in
the sample. For all other nuclei (or samples without TMS), spectra are referenced via the deuterated solvent against the magnet field and hence known reference compounds. The reference substances (0ppm) for the more common nuclei are as follows:
11B Boron trifluoride etherate (BF3.OEt2)
15N Nitrate ion (NO3-) for JEOL & NO2 for Varian.
19F Fluorotrichloromethane (Cl3F)
31P 85% Phosphoric acid (H3PO4)
195Pt 21.4MHz for TMS = 100MHz
199Hg Dimethylmercury (HgMe2)
(these should be quoted as 'external references')