A High-Precision Carbon-13 Shift Thermometer for the Temperature Range 100–300 K

✍ Scribed by Helmut Quast; Markus Heubes; Anita Dunger; Hans-Heinrich Limbach

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 134 KB
• Volume: 134
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.1998.1505

✦ Synopsis

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 ⌬␦ between averaged signals, e.g., d(⌬␦)/ dT ‫؍‬ 0.051 ppm K ؊1 at 300, 0.087 ppm K ؊1 at 200, and 0.175 ppm K ؊1 at 110 K for the quaternary carbon atoms C2 and C6. At 37 temperatures T, which were measured with calibrated platinum resistance thermometers, shift differences ⌬␦ 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 ⌬␦ vs T data to a polynomial equation of the fourth degree (Eq. [5], r 2 ‫؍‬ 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 ⌬␦ 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.