Effects of thermal convection on NMR and their elimination by sample rotation

It is shown that the presence of thermal convection in a sample tube may lead to a variety of anomalous phenomena in prolonged multiple-pulse NMR experiments. They are investigated by applying inversion-recovery pulse sequences to Xe-129 of xenon gas dissolved in deuterated cyclohexane and acetonitrile, and to F-19 in xenon difluoride (XeF2) dissolved in deuterated acetonitrile. If convection is present, the recovery of the magnetization after the pi pulse may be very different from the recovery due to the spin-lattice relaxation alone. It may be much faster, very sensitive to temperature, and nonexponential, exhibiting even oscillatory behavior. In addition, the shape of the spectral lines may be seriously distorted. The results show that convection and the resulting anomalies can be effectively eliminated by rotating the sample tube at a spinning speed on the order of 10 Hz. These phenomena may provide novel methods for investigating thermal convection. (C) 1996 Academic Press, Inc.