Temperature-Dependent Hyperfine Coupling Constant of the Dianion Radical of Fremy's Salt, a Convenient Internal Thermometer for EPR Spectroscopy

Abstract

The hyperfine coupling constant,A0, of peroxylamine disulfonate in aqueous solutions depends upon the temperature and the concentration of added K2CO3buffer. Near room temperature,A0varies from 13.036 to 13.201 G for K2CO3concentrations varying from zero to near saturation. For samples prepared in 50.0 mMK2CO3, the results are independent of Fremy's salt concentration in the range 0.005–2.8 mMas follows:A0(T) = 12.978 + 0.00311 T, where the hyperfine coupling constant at temperatureT, A0(T), is given in gauss when the temperature is given in °C. This temperature dependence is an order of magnitude larger and of opposite sign than that found for doxylstearic acid esters and is proposed as the basis of an internal thermometer for EPR spectroscopy. The variation ofA0with solvent polarity is found to be a factor of about 27–30 less than for the neutral radicals di-tert-butyl nitroxide and doxylstearic acid esters. It is shown that microwave heating of aqueous samples at high microwave powers can be monitored by measuringA0while conventional thermometry can lead to significant errors even for a thermocouple immersed within the sample just above the microwave cavity. The value ofA0(25°C) = 13.056 ± 0.002 G is significantly different than previously used standards; therefore, some previous data may require recalibration. Correction procedures for nonlinearities in the field sweep are presented.