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 m ay lead to a variety of anomalous phenomena in prolonged multiple-puls e NMR experiments. They are investigated by applying inversion-recover y pulse sequences to Xe-129 of xenon gas dissolved in deuterated cyclo hexane and acetonitrile, and to F-19 in xenon difluoride (XeF2) dissol ved in deuterated acetonitrile. If convection is present, the recovery of the magnetization after the pi pulse may be very different from th e recovery due to the spin-lattice relaxation alone. It may be much fa ster, very sensitive to temperature, and nonexponential, exhibiting ev en oscillatory behavior. In addition, the shape of the spectral lines may be seriously distorted. The results show that convection and the r esulting anomalies can be effectively eliminated by rotating the sampl e tube at a spinning speed on the order of 10 Hz. These phenomena may provide novel methods for investigating thermal convection. (C) 1996 A cademic Press, Inc.