A High-Precision Carbon-13 Shift Thermometer for the Temperature Range 100–300 K |
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Authors: | Helmut Quast Markus Heubes Anita Dunger Hans-Heinrich Limbach |
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Institution: | aInstitut für Organische Chemie der Universität Würzburg, Am Hubland, D-97074, Würzburg, Germany;bInstitut für Organische Chemie, Freie Universität Berlin, Takusstraße 3, D-14195, Berlin, Germany |
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Abstract: | The first carbon-13 shift thermometer for the temperature range of 100–300 K is based on the very rapid equilibration of a pair of semibullvalene valence tautomers. The temperature dependence of the equilibrium constant is reflected in strongly temperature-dependent shift differences Δδ between averaged signals, e.g.,d(Δδ)/dT= 0.051 ppm K−1at 300, 0.087 ppm K−1at 200, and 0.175 ppm K−1at 110 K for the quaternary carbon atoms C2 and C6. At 37 temperaturesT, which were measured with calibrated platinum resistance thermometers, shift differences Δδ were taken from nondecoupled carbon-13 spectra recorded from solutions of 1 in mixtures of chlorodifluoromethane and deuterated dimethyl ether without spinning. The least-squares fit of these Δδ vsTdata to a polynomial equation of the fourth degree (Eq. 5],r2= 0.9999) allows the calculation of temperatures from measured shift differences with a standard deviation of 0.46 K and an estimated error of about 1 K. The heating effects of WALTZ-16 decoupling and the influence of solvents on Δδ are investigated. A comparison with existing NMR thermometers demonstrates the superior performance of the new carbon-13 shift thermometer with respect to precision and the accessible temperature range. |
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Keywords: | NMR thermometer low-temperature carbon-13 spectroscopy WALTZ-16 decoupling semibullvalenes Cope rearrangement |
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