GEOMETRY AND TUNNELING DYNAMICS OF CHD2 GROUPS IN ASPIRIN - A SINGLE-CRYSTAL DEUTERON NUCLEAR-MAGNETIC-RESONANCE STUDY

CHD2 groups in aspirin are studied by single-crystal deuteron NMR at t emperatures between 7 and 290 K. They perform stochastic reorientation s which are governed by a rotational potential possessing three wells, two of which are almost equally deep whereas the third is significant ly higher. The three minima of the rotational potential are separated by angles substantially different from 120 degrees. Still, at least in the two lower wells, the geometry of the CHD2 groups is close to that of a symmetric rotor. By selective magnetization transfer experiments , the incoherent tunneling rate between the two lower potential wells is determined. At temperatures below 15 K, Raman processes are the dom inating rotor-phonon coupling mechanism, while at higher temperatures, thermally activated processes with activation energies close to the t otal potential height prevail. (C) 1998 American Institute of Physics. [S0021-9606(98)00340-7].