Referencing of the chemical shift scale in the NMR data

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Contents 1 Calibration of sample temperature 1.1 Technical notes 2 In the quest for the absolute chemical shift standard... 3 Bibliography

Calibration of sample temperature

Hydrogen - bonding protons have have especially pronounced dependence of resonance frequency on the sample temperature. Higher temperature results in weakening the hydrogen bonds and therefore lessening the electron withdrawing effect of the hydrogen bond acceptor on the proton. As a result the proton becomes more shielded and its chemical shift decreases (the resonance moves "upfield").

This dependence of chemical shift is used in the common "chemical shift" thermometers to calibrate the sample temperature, notably - methanol and ethylene glycol.

With such "thermometer" the temperature can be calculated with the formula given in the table below[1], where Δδ is a difference of chemical shifts of hydroxyl and methyl (or methylene) of the sensor molecule.

molecule	useful temperature range, K(oC)	forumula for T (K)
CH3OH (methanol)	250-320K (-23 ... +46 oC)	403.0 − 29.53Δδ − 23.87Δδ2
CH2OH-CH2OH (ethylene glycol)	300-370K (+27 ... +96 oC	466.0 − 101.6Δδ

Chemical shift of water proton also has a marked temperature dependence and it was proposed as an internal standard for the aqueous samples[2] using the equation:

δ(H2O) = 7.83 − T / 96.9, where temperature is measured in Kelvins.

This equation is valid at pH 5.5. Dependence of δ(H2O) on pH is about 0.02 ppm per pH unit.

Technical notes

On Varian NMR systems real temperature of methanol and ethylene glycol samples can be easily determined by recording a 1D spectrum, then placing two cursors on the two peaks and typing:

tempcal('m') for methanol and tempcal('e') - for ethylene glycol.

The system will report the true temperature. This way you can build a temperature calibration for your probe when you need it.

In the quest for the absolute chemical shift standard...

Chemical shift of commonly accepted standards does change with temperature, as can be seen from this section, however this change is small for many practical purposes.

Low density hydrocarbon, HBr, ¹²⁹Xe gases display very small variation of chemical shift with changes of temperature [3]. Using this property of low-density ethane gas, Cross and Schleich [3] measured chemical shift temperature coefficients for neat benzene, acetone, cyclohexane, 1,4-dioxane, dimethyl sulphoxide, chlorofom and acetonitrile. A sealed capillary containing ethane gas was placed into the NMR tube containing the solvent of interest. The temperature dependence observed was linear with the coefficient in the range of 0.001-0.0023 ppm/^oC in about 45-90^oC interval around room temperature (depending on the solvent). The same study included samples of 2,2-dimethyl-2-silapentane-5-sulfonate (DSS), 3-trimethylsilylpropionate-d₆ (TSP) and tetramethylammonium chloride (TMACl) dissolved in D₂O. DSS and TSP showed small (<0.002 ppm for DSS and < 0.005 ppm) change of proton chemical shift when changing temperature from 20 to 60^oC, while TMACl showed about 0.01 ppm change of chemical shift. For all three reference compounds dissolved in water chemical shift dependence was clearly non-linear.

Bibliography

1. Raiford, DS and Fisk, CL and Becker, ED. Calibration of methanol and ethylene glycol nuclear magnetic resonance thermometers. Analytical Chemistry 51(12):2050--2051, 1979. BibTeX [methanol]

2. Hartel, AJ and Lankhorst, PP and Altona, C. Thermodynamics of stacking and of self-association of the dinucleoside monophosphate m2 (6) AU from proton NMR chemical shifts: differential concentration temperature profile method.. European journal of biochemistry/FEBS 129(2):343, 1982. BibTeX [watershift]

3. Cross, BP, Schleich, T. Temperature dependence of the chemical shifts of commonly employed proton n.m.r. reference compounds. Organic Magnetic Resonance 10(1):82--85, 2005. BibTeX [cross05]

4. Hoffman, RE. Standardization of chemical shifts of TMS and solvent signals in NMR solvents. MAGNETIC RESONANCE IN CHEMISTRY 44(6):606, 2006. BibTeX [hoff06]

5. Wishart, DS and Bigam, CG and Yao, J and Abildgaard, F and Dyson, HJ and Oldfield, E and Markley, JL and Sykes, BD. 1 H, 13 C and 15 N chemical shift referencing in biomolecular NMR. Journal of Biomolecular NMR 6(2):135--140, 1995. BibTeX [wilshart95]

6. Van Geet, AL. Calibration of methanol nuclear magnetic resonance thermometer at low temperature. Analytical Chemistry 42(6):679--680, 1970. BibTeX [vangeet70]

7. Harris, Robin K and Becker, Edwin D and Cabral de Menezes, Sonia M and Granger, Pierre and Hoffman, Roy E and Zilm, Kurt W. Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008). Pure & Applied Chemistry 80(1):59--84, 2008. BibTeX [iupac08]