Solid-state N14 nuclear magnetic resonance techniques for studying slow molecular motions

Abstract

A variety of transition-selective solid-state N14 (I=1)NMR techniques are demonstrated for the first time to be useful for quantitatively describing slow molecular motions in the solid state. These techniques are validated by quantitative measurements of molecular reorientation by tetrahedral jumps in hexamethylenetetramine (HMT). A new four-site magnetization-exchange model, capable of being generalized to n-sites, which includes the effects of spin-lattice relaxation is developed. This model provides the limiting conditions under which the orientation dependence of spin-lattice relaxation values T1 can be safely neglected. The model is used to analyze results from a frequency-selective DANTE train used to burn a hole in the spectrum, that provide a direct indication of the existence of 4-site exchange. The measured correlation time for the motion in HMT of 103±6 ms at room temperature agrees well with previous studies by other techniques. In a novel application to molecular dynamics, the repeated hole-burning pulse trains of the SINK experiment are used to measure a magnetization recovery time constant due to N14 spin-lattice relaxation in HMT of 0.99±0.12 s. Both experiments employ frequency-selective as well as transition-selective radio frequency pulses on a relatively small frequency region (<100 kHz) of the entire quadrupolar powder pattern of HMT (NQCC=4.414 MHz, η=0). The Hahn spin-echo used for detection can be understood in terms of the fictitious spin-1/2 formalism. Quantitative dynamical information is obtained from measurements at only one frequency position of a wide inhomogeneously broadened powder pattern. Because we are operating in this unusual regime, the sensitivity can be significantly improved by replacing the DANTE hole-burning train with a series of π/2 pulses that saturate all observable magnetization. Results from such an experiment compare well with those obtained using DANTE trains.