Department of Chemistry, University of Alberta         September 2005
NMR News 2005-03
 News and tips from the NMR support group for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca         http://nmr.chem.ualberta.ca

There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.

Other content of this NMR News is no longer meaningful and has been removed May 2010.

Contents 

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solvents

 

Solvents
Especially new NMR users have occasionally difficulties understanding what they see in their spectrum due to the solvent. Residual solvent signals are unavoidable because 100% deuteration simply does not exist. In addition, most NMR solvents are hygroscopic to various degrees. Once a solvent bottle or ampoule is opened, some humidity is absorbed from the air. This is obviously only a concern in H1 and not in C13 NMR. Over time the water peak can easily be larger than the solvent peak. The best way to keep the moisture content in check is through the use of molecular sieves. The store room has them and they come in different types depending on the pore diameter:
 
type type and pore diameter use for
3A potassium ion, 3 A acetone, methanol, DMSO
4A sodium ion, 4 A CDCl3, CD2Cl2, C6D6, toluene-D8

Representative spectra of seven solvents are shown below: CDCl3, C6D6, CD2Cl2, D2O, CD3OD, DMSO, Acetone-D6. The samples used to measure the spectra contained only the solvent (plus impurities). The solvents were stored with molecular sieves in the bottle for at least one night. Recorded at 400 MHz (100MHz C13), except where indicated. A systematic overview of the coupling patterns is shown here.
 

CDCl3:  7.26 ppm (H1)

The H2O impurity is at 1.527 ppm. 1J(CH) = 209.0 Hz, the lines close to the solvent peak are
spinning side-bands (ssb).
 

CDCl3:  77.0 ppm (13C)

The 1J(CD) coupling constant is 32.0 Hz. Intensity is 1:1:1 because D is a spin=1 nucleus and each carbon is coupled to one D.

 C6D6:  7.15 ppm (H1)

The H2O impurity is at 0.388 ppm. 1J(CH) = 158.0 Hz.
 
 C6D6:  128.0 ppm (13C)

The 1J(CD) coupling constant is 24.3 Hz. Intensity is 1:1:1 because D is a spin=1 nucleus and
each carbon is coupled to one D.
 
 CD2Cl2:  5.32 ppm (H1)

The H2O impurity is at 1.513 ppm. 1J(CH) = 178.0 Hz (not visible here). 2J(HD) = 1.1 Hz.
Intensity is 1:1:1 because D is a spin=1 nucleus and the proton is coupled to one D in CHDCl2.
 

 CD2Cl2:   53.8 ppm (13C)

The 1J(CD) coupling constant is 27.3 Hz. Intensity is 1:2:3:2:1 because D is a spin=1 nucleus and each carbon is coupled to two D.
 
 D2O:  4.75 ppm (H1)

Typically free of impurities but the residual HDO peak is quite intense.

The C13 spectrum is nothing but baseline, of course!
 
CD3OD:  3.30 ppm (H1)

The CD3-OH and H2O resonance are basically identical at 4.8 ppm. 2J(HD) = 1.65 Hz.
Intensity is 1:2:3:2:1 because D is a spin=1 nucleus and the H in CHD2-OD is coupled to two D.
See also just below.

Same spectrum as right above but 40 times enlarged vertically. 1J(CH) = 140.0 Hz. Interesting is the small signal at 3.32 ppm. It arises from CH2D-OD and the 2J(HD) is also 1.65 Hz. Intensity is 1:1:1 because only one D is present to split the two protons! The 0.02 ppm difference in chemical shift is a so-called isotope shift. The rest is due to spinning side bands (ssb).
 

CD3OD:  49.0 ppm (13C)

The 1J(CD) coupling constant is 21.4 Hz. Intensity is 1:3:6:7:6:3:1 (at least in theory) because D is a spin=1 nucleus and the carbon is coupled to three D.
The intensities are slightly distorted likely due to insufficient relaxation. A better example for intensities can be seen for DMSO.
DMSO:  2.49 ppm (H1)

The H2O impurity is at 3.3 ppm (and quite a lot of it which is typical for DMSO, even when stored
over a 3A  molecular sieve). 1J(CH) = 137.0 Hz (not visible here). 2J(HD) = 1.85 Hz. Intensity is 1:2:3:2:1 because D is a spin=1 nucleus and the H in CHD2-R is coupled to two D.
 

DMSO:  39.5 ppm (13C)

The 1J(CD) coupling constant is 21.0 Hz. Intensity is 1:3:6:7:6:3:1 because D is a spin=1 nucleus and each carbon is coupled to three D.
 
Acetone-D6: 2.04 ppm (H1)

The H2O impurity is at 2.79 ppm in this case but the chemical shift is strongly concentration-dependent.

A closer look shows that there are two water peaks: the singlet at 2.79 ppm is from H2O, the
 1:1:1 signal at 2.756 ppm is from HDO with a 2J(HD) of 2.15 Hz. The isotope shift is 0.034 ppm.

2J(HD) = 2.2 Hz. Intensity is 1:2:3:2:1 because D is a spin=1 nucleus and the H in CHD2-CO-CD3
is coupled to two D. See also just below.

Same spectrum as right above but 20 times enlarged vertically and recorded at 600 MHz. 1J(CH) = 126.0 Hz. 2J(HD) = 2.2 Hz. Interesting are the small signals at ca. 2.06 and 2.07 ppm. 2.06 ppm is from CH2D-CO-CD3 and its 2J(HD) is also 2.2 Hz. Intensity is 1:1:1 because only one D is present to split the two protons!
2.07 ppm is from CH3-CO-CD3, i.e. no D at all "on one side". The 0.017 ppm difference in chemical shift is a so-called isotope shift which can be observed twice. At 400 MHz partial overlap of the main signal at 2.04 and the one at 2.06 ppm make it hard to see both signals hence the 600 MHz spectum.
 

Acetone-D6:  29.8 and 206.1 ppm (13C)

The 1J(CD) coupling constant is 19.4 Hz. Intensity should be 1:3:6:7:6:3:1 because D is a spin=1 nucleus and each carbon is coupled to three D. There is some distortion present in the intensities. In addition a strong singlet at 206.3 ppm is observed from the carbonyl group.
 

 

The Table below provides an overview of the coupling patterns based on one spin 1/2 nucleus (for example H1, 13C and many more) and zero, one, two and three deuterium atoms coupled to the spin 1/2 nucleus by the same J(HD) or J(CD) coupling constant. Intensities of the observed signals are shown with color coding to indicate the origin of each line's intensity. Dashed signals are only shown to indicate the origin of the coupling pattern but do not contribute to the final intensity directly (but indirect through the subsequent lines). Coupled nuclei are shown in bold purple color.
 
H1 or C13
spin=1/2		 D
spin=1		 coupling patterns

examples
1 0

CHCl3 in CDCl3
1 1

C13-D:
CDCl3
C6D6

H1-D:
CH2D in CD3OD
CHDCl2 in CD2Cl2
HDO in Acetone-D6

1

2

C13-D:
 CD2Cl2

 

H1-D:

CHD2 in:
CD3OD
DMSO
Acetone-D6
 
1 3

C13-D:

CD3 in:
CD3OD
DMSO
Acetone-D6
 
H1 or C13
spin=1/2		 D
spin=1		 coupling patterns

examples

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