A HIGH-PRECISION C-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 semibullv alene valence tautomers. The temperature dependence of the equilibrium constant is reflected in strongly temperature-dependent shift differe nces Delta delta between averaged signals, e.g., d(Delta delta)/ 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, whic h were measured with calibrated platinum resistance thermometers, shif t differences Delta delta were taken from nondecoupled carbon-13 spect ra recorded from solutions of 1 in mixtures of chlorodifluoromethane a nd deuterated dimethyl ether without spinning. The least-squares fit o f these Delta delta vs T data to a polynomial equation of the fourth d egree (Eq. [5], (r)2 = 0.9999) allows the calculation of temperatures from measured shift differences with a standard deviation of 0.46 K an d an estimated error of about 1 K. The heating effects of WALTZ-16 dec oupling and the influence of solvents on Delta delta are investigated. A comparison with existing NMR thermometers demonstrates the superior performance of the new carbon-13 shift thermometer with respect to pr ecision and the accessible temperature range. (C) 1998 Academic Press.