The Offset-Saturation NMR Experiment for Measuring Spin Relaxation Times and Application to Chemical Exchange

Abstract

<p>NMR spin relaxation times T$\sb1$ and T$\sb2$ provide useful information regarding molecular motions and dynamics. The spin-spin relaxation time T$\sb2$ in particular is important for measuring rates of chemical exchange, but has proven difficult to measure. A facile offset-saturation experiment for the simultaneous measurement of T$\sb1$ and T$\sb2$ NMR spin relaxation times is presented in this thesis. Errors determined as the 95% confidence limits for the relaxation times measured by offset-saturation experiments are on the order of 5%, provided that the saturating field B$\sb2$ is well calibrated, and the sampling points are judiciously chosen. The relaxation times for acidified distilled water determined by a non-linear least-squares fit to offset-saturation data are in good agreement with the results obtained by inversion-recovery and CPMG techniques. A qualitative guideline for the optimal choice of sampling points for offset-saturation experiments is illustrated.</p> <p>The offset-saturation experiment is shown to be a viable method for the study of fast chemical exchange processes. Applications to the chair-chair interconversion of cyclohexane, and the hindered C-N bond rotation in N-acetylpyrrole, show that the offset-saturation method can be used to obtain rate constants on the order of 10$\sp5$ s$\sp{-1}.$ Exchange rates can be determined over several orders of magnitude when the offset-saturation method is used in combination with other dynamic NMR methods for the fast and slow exchange regimes.</p> <p>The single-spin double resonance experiment is shown to be a simple and precise method for the calibration of the saturating field B$\sb2$ used for offset-saturation experiments. The use of pulsed homodecoupling radiofrequency sources to provide the saturating rf field has entailed a theoretical study regarding the equivalence of pulsed and continuous-wave radiofrequency radiation. An analytical expression for the z magnetization as a function of irradiation resonance offset for a series of repeated soft pulses is obtained, which in the limit of small duty cycles and pulse flip angles is equivalent to the Bloch equations.</p>