(741g) Species-Specific Measurement of Molecular Transport Using Heteronuclear Magnetic Resonance Methods

Pulsed field gradient (PFG) NMR methods allow for the non-invasive measurement of translational molecular motion. Discrimination between individual chemical species is typically achieved on the basis of chemical shift (i.e., resonant frequency). Frequently, PFG experiments involve the observation of ¹H nuclei. However, in multiphase systems, the often-favored ¹H NMR measurement may fail to yield sufficient frequency resolution. Magnetic susceptibility gradients arising at phase interfaces broaden spectral lines such that resonances of individual chemical species cannot be unambiguously identified. Superior chemical shift resolution can be obtained by adopting a multinuclear approach. By observing a nucleus exhibiting a larger chemical shift range (¹³C, for example), signal from multiple species can be resolved. Additionally, when observing ¹³C, signal from aqueous solvents are naturally and efficiently suppressed. Observation of ¹⁵N, ³¹P and other system-appropriate nuclei provide further chemical resolution of the PFG measurement via isotopic selectivity. Beyond aiding in chemical resolution, observation of a non-¹H isotope with favorable NMR properties provides the possibility of probing translational motion over increased timescales. In this work, we demonstrate the implementation and application of heteronuclear PFG methods employing (i) solution-state polarization transfer for signal enhancement, (ii) multinuclear detection for chemical selectivity, and (iii) rapid acquisition schemes for substantial reductions in experiment duration. Variants of these heteronuclear approaches include cyclic cross polarization for chemical shift selective, ¹H-detected ¹³C PFG measurements, as well as signal-enhanced direct detection of rare isotopes at natural abundance. Phase encoded imaging can be integrated into the set of multinuclear methods such that the measurements of molecular transport may be spatially resolved. The heteronuclear methods have been validated against standard ¹H PFG-NMR measurements and have been shown to yield excellent agreement with this more traditional approach, as well as yielding quantitative results at timescales exceeding those accessible via ¹H NMR.