Radiofrequency-induced temperature increase as a function of cross polarization contact time in 8CB

In this work we analyse the non-negligible heating effect that can be produced by the radiofrequency during the contact time in Hartmann-Hahn cross polarization experiments in liquid crystals. We perform the analysis in the nematic phase of 4-n-octyl-4'-cyanobiphenyl (8CB). The heating effect is quantified through the chemical shifts of the carbon resonances, which move towards their respective positions in the isotropic phase when increasing the contact time. The calibration procedure involves an independent measurement of chemical shift dependence on externally controlled bath temperature. This variation was performed in the range between 300 and 313 K covering the smectic A and nematic mesophases. An overall heating rate of 0.16 K ms(-1) is observed during the contact time in a cross polarization experiment performed at omega(1)/2pi similar or equal to 80 kHz. This rate goes to approximately one-half for omega(1)/2pi similar or equal to 60 kHz and becomes unobservable for omega(1)/2pi < 50 kHz under typical experimental conditions. Copyright (C) 2002 John Wiley Sons, Ltd.