Notes on NMR Solvents 

I. Introduction

 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. In chloroform solvent (CDCl3), this corresponds to CHCl3, so a singlet signal is observed at 7.26 ppm. For methanol solvent, this corresponds to CHD2OD, so a 1:2:3:2:1 pentet signal is observed at 3.31 ppm. (Recall that deuterium has a spin quantum number (I) of 1, so n deuterium atoms will split a proton signal into 2In+1 lines.) The same solvents are used for 13C NMR spectra, so the same rules about splitting patterns apply here also.

 It used to be common practice to add Me4Si, or related compounds, as an internal reference standard for 1H and 13C NMR spectra with the proton signal occurring at 0.0 ppm and the carbon signal occurring at 0.0 ppm in the 13C NMR spectrum. However, modern spectrometers can "lock" on solvent signals, so addition of internal reference standards is not usually required.

II. NMR Solvent Signals

 The chemical shifts (d) of solvent signals observed for 1H NMR and 13C NMR spectra are listed in the following table. The multiplicity is shown in parentheses as 1 for singlet, 2 for doublet, 3 for triplet, etc.
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

 III. NMR Water Signals

 Signals for water occur at different frequencies in 1H NMR spectra depending on the solvent used. Listed below are the chemical shift positions of the water signal in several common solvents. Note that H2O is seen in aprotic solvents, while HOD is seen in protic solvents due to exchange with the solvent deuteriums.
Solvent 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

C. A. Merlic 1997

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