High-resolution magnetic relaxation dispersion measurements of solute spinprobes using a dual-magnet system

The magnetic field dependence of the nuclear spin-lattice relaxation rate p rovides a detailed report of the spectral density functions that characteri ze the intra- and intermolecular fluctuations that drive magnetic relaxatio n. Rie have addressed the difficult sensitivity and resolution problems ass ociated with low magnetic field strengths by using two magnets in close pro ximity and shielded from each other. The sample is stored in the high magne tic field, pneumatically driven to the variable satellite field, then retur ned to the high field for detection at high resolution. A magnetic shield e ffectively decouples the two magnets so that varying the satellite field st rength has minimal effect on the field strength and shim of the high field magnet. The disadvantage of the sample-shuttle magnet-pair system is the re striction imposed on the relaxation times by the finite shuttle times. Expe riments not described here have shown this rate maximum to be about 20 s(-1 ) for most practical solutions. However, we demonstrate here that the sensi tivity gains over switched-current magnet systems permit characterization o f solute inter- and intramolecular dynamics over the time scale range from tens of microseconds to less than a picosecond. This range permits investig ation of a number of crucial chemical dynamics questions, while high sensit ivity permits examination of a variety of solute spins. Representative data are presented for H-1, Cd-111, and Li-7. (C) 1999 Academic Press.