Chloroform is one of the most commonly used solvents in the NMR Facility and many users use the residual solvent peak to reference their spectra. Unfortunately, I have seen a range of values for the chemical shift of chloroform. Its possible that chloroform's chemical shift is temperature dependent and the different values were obtained at different temperatures. To test this, and to determine the correct chemical shift to use, I decided to measure the temperature dependence of the residual chloroform peak in deuterochloroform.
I used a sample of 99.8% deuterochloroform spiked with 0.05% (v/v) tetramethylsilane (TMS). The 1H peak shown by this solvent is due to the 0.2% of chloroform that is not deuterated. TMS is the reference compound recommended by IUPAC and was used to reference the spectra. To determine the temperature, a sample of neat methanol-d4 and the equation reported by Findeisen et al were used. The sample was allowed to equilibrate for five minutes at each temperature before recording the spectra. The graph below plots the chloroform 1H chemical shift against temperature.
At 298oK the chemical shift is very close to 7.260 ppm. The dependence of chemical shift on temperature is obvious, though the scale of the effect is fairly small. The slope of the correlation here is -0.43 ppb/K, close to the -0.5 ppb/K reported by Harris et al. This data shows that at 298oK setting the residual chloroform peak to 7.260 ppm should reference spectra correctly.
The spectrometer software references spectra to residual solvent peaks using a predefined table of values. It does not account for temperature. In most cases this works fairly well, but if precise chemical shifts are important for your project an internal reference is usually the best way to go. Many people are unwilling to add another compound to a sample they have worked hard to purify but, with a boiling point of 28oC, TMS is more volatile than chloroform and can easily be removed.
References
NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities
Hugo E. Gottlieb, Vadim Kotlyar, and Abraham Nudelman
J Org Chem. 1997 62(21) 7512-7515
NMR Chemical Shifts of Trace Impurities: Industrially Preferred Solvents Used in Process and Green Chemistry
Nicholas R. Babij, Elizabeth O. McCusker,
Gregory T. Whiteker, Belgin Canturk, Nakyen Choy, Lawrence C. Creemer, Carl V. De Amicis, Nicole M. Hewlett, Peter L. Johnson, James A. Knobelsdorf, Fangzheng Li, Beth A. Lorsbach, Benjamin M. Nugent, Sarah J. Ryan, Michelle R. Smith, and Qiang Yang
Org Process Res Dev. 2016 20(3) 661-667
Further conventions for NMR shielding and chemical shifts
R.K. Harris, E.D. Becker, S.M. Cabral de Menezes, P. Granger, R.E Hoffman, K.W. Zilm
Pure Appl Chem. 2008 80 59-84
A 1H‐NMR thermometer suitable for cryoprobes.
M. Findeisen, T. Brand, and S. Berger
Magn Reson Chem. 2007 45 175-178
Temperature dependence of the 1H chemical shift of tetramethylsilane in chloroform, methanol, and dimethylsulfoxide.
Roy E. Hoffman and Edwin D. Becker
J Magn Reson. 2005 176(1) 87-98
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