Apparatus for low-temperature magic-angle spinning NMR

Abstract

Low-temperature magic-angle spinning (MAS) NMR experiments are of considerable interest since they permit the study of a variety of phenomena such as chemical exchange, phase transitions, and Knight shifts. In addition, the recent development of techniques for determining distances and distinguishing among molecular conformations in solids provides opportunities to examine the structure of reactions in intermediates (i.e., photolysis products and enzyme/inhibitor or substrate complexes) trapped at low temperatures. In this Note we describe a system that we have developed and successfully employed for performing MAS experiments for extended periods of continuous operation in the temperature range of 150-300 K. The desirable features of a low-temperature MAS system are severalfold. First, it should provide setable and stable spinning speeds for extended periods. For low temperatures, this dictates that at least the drive-bearing gas must be at room temperature and supplied to a spinner-speed controller before cooling in order to maintain constant speeds. Second, the spinning speed must be maintained when the temperature is changed. Third, facilities for replenishing the coolants and the drive and bearing gases must be provided to accommodate operation for extended periods. The first requirement above was initially addressed simply by circulating the drive and bearing gases through copper coils immersed in liquid nitrogen, the temperature being determined by adjusting the size of the coils and the flow rate. This is the method used at present in most laboratories and in most commercial equipment. However, with this arrangement the temperature and spinning speed cannot be adjusted independently. In addition, at the high pressures which are necessary to achieve high spinning speeds, condensation of the nitrogen occurs, resulting in pulsations of the spinning. Kendrick et al. (2) demonstrated that this problem could be circumvented by pressurizing the liquid nitrogen dewar above the pressure of the gas flow. However, this procedure has the disadvantages that it involves a large pressurized vessel (3) subject to strict safety rules in most operating environments, and that it is difficult to replenish the coolant. It is therefore unsatisfactory for samples requiring extended signal averaging periods.