A high-precision carbon-13 shift thermometer for the temperature range 100-300 K

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 Deltadelta between averaged signals, e.g., d(Deltadelta)/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 Deltadelta 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 Deltadelta vs T data 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 Deltadelta 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. Copyright 1998 Academic Press.