TEMPORAL CHARACTERISTICS OF NMR SIGNALS FROM SPIN-3 2 NUCLEI OF INCOMPLETELY DISORDERED-SYSTEMS/

Anisotropic nuclear quadrupole interactions can produce residual quadr upole splitting in the NMR spectra of rapidly moving quadrupolar nucle i in incompletely disordered aqueous heterogeneous systems. Such syste ms may include hydrated sodium nuclei in biological tissue and biopoly mer gels. To describe the NMR signals from such samples, we use a doma in model in which each domain is characterized by a quadrupole frequen cy and a residence time of the nucleus. We show that the signals from each domain after one pulse, the quadrupole echo sequence, and the var ious multiple quantum filters (MQFs) can be expressed as a linear comb ination of five different phase coherences. To simulate the effect of various distributions (Pake powder pattern, Gaussian, etc.) of quadrup ole frequencies for different domains on the NMR signal, we have writt en the computer program CORVUS. CORVUS also includes the effects of ex change between different domains using diffusion and random jump model s. The results of computer simulations show that the Gaussian and Fake powder pattern quadrupole frequency distributions produce very differ ent phase coherences and observable NMR signals when the exchange rate (1/tau(e)) between different domains is slow. When 1/tau(e) is simila r to the root mean square quadrupole frequency (a), the signals from t he two distributions are similar. When 1/tau(e) is an order of magnitu de greater than a, there is no apparent evidence of quadrupole splitti ng in the shape of the signal following one pulse, but the residual ef fects of the quadrupole splitting make a significant contribution to t he fast transverse relaxation rate. Therefore, in this case, it is ina ppropriate to use the observed biexponential relaxation rates to obtai n a single correlation time. The quadrupole echo and the various MQF s ignals contain an echo from the satellite transitions in the presence of quadrupole splitting. The peak of this echo is very sensitive to 1/ tau(e). The time domain analysis of these signals is more direct and l ess ambiguous than the frequency domain analysis because the echo does not occur at the beginning of data acquisition. The quadrupole echo p ulse sequence is the most sensitive detector of residual quadrupole sp litting and exchange of sodium ions between different domains. However , if the sample is compartmentalized so that only a fraction of the nu clei have quadrupole splitting, the double quantum magic angle filter (DQ-MA) is more suitable. This is because the DQ-MA signal contains on ly the contributions from satellite transitions. Use of simulations to analyze signals from various one-pulse, quadrupole echo, and multiple quantum filter pulse sequences can yield information on substrate ord er and aid in quantitation of multiple quantum filter signals. (C) 199 8 Academic Press.