NMR diffusion measurements under chemical exchange between sites involving a large chemical shift difference

Abstract

This study concerns the thallium-205 cation in aqueous solution in the presence of a calixarene molecule. While the measurement of the self-diffusion coefficient of pure thallium (without calixarene in the aqueous solution) does not pose any particular problem, major difficulties are encountered with the standard method employing gradient strength increment as soon as thallium is partly complexed by calixarene. With static magnetic field gradients, the NMR signal is so weak that it prevents any reliable measurement whereas radio-frequency (rf) field gradients lead to an unrealistic value of the diffusion coefficient. This failure is explained by the fact that thallium is in fast exchange between two sites (complexed and free thallium) thus exhibiting a single NMR signal although, in the course of the experiment, two signals, with an important difference in resonance frequencies (due to the large thallium chemical shift range), are effectively involved. With the objective to understand these quite unexpected observations, the theory underlying NMR diffusion experiments is first reviewed and criteria of fast exchange are discussed for three parameters: chemical shifts, relaxation rates, diffusion coefficients. It turns out that off-resonance effects are responsible for unwanted defocusing due to rf pulses in the static magnetic field gradient method and for time-dependent gradients in the rf field gradient method. Concerning the latter, a remedy is proposed which consists in applying the stronger gradient and incrementing the gradient pulse durations. After correction for relaxation, the expected value of the diffusion coefficient is retrieved.2

Suppressing exchange effects in diffusion-ordered NMR spectroscopy

“…The primary reason is to minimise J-modulation: the STE stores spatially-encoded magnetization along the z-axis for most of the diffusion time, minimising the time for which J acts. Any exchange of magnetization during this storage period, whether by chemical exchange [18][19][20] or nuclear Overhauser effect (NOE) [21], will affect the attenuation as a function of gradient pulse area for the signals involved. This changes the apparent diffusion coefficients and complicates analysis.…”

Design of a three-loop asymmetric coil producing a homogeneous radiofrequency B1 field gradient along the axis of a vertical sample tube

“…This is not evidently the case for gradients whose rise and fall times are negligibly short. As a matter of fact, a failure of gradients may occur in particular situations while gradients still fit the bill [25]. The advantages of the gradient methodology have been well documented until now with however a gradient perpendicular to the sample tube.…”

Physical State of di-Nitrogen in the Presence of SBA and Various Zeolites Probed by 15N Spin–Lattice Relaxation of Pure 15N2 in the 30–300 K Temperature Range

“…These are very likely in fast exchange, as far as spin relaxation is concerned, since, in that case, fast exchange conditions imply that the residence time in each site is much smaller than any relaxation time. 28 These fast exchange conditions will be assumed for all samples studied here (as far as relaxation is concerned). The other spin-rotation contribution, which is present essentially at low temperature (actually below 77 K, the boiling point of pure dinitrogen), can be assigned to more pronounced (still relatively weak) interactions with the material.…”

“…In fact, the relaxation rates of gaseous dinitrogen measured at high temperature include very probably intercrystallite gaseous dinitrogen in addition to dinitrogen within the considered material. These are very likely in fast exchange, as far as spin relaxation is concerned, since, in that case, fast exchange conditions imply that the residence time in each site is much smaller than any relaxation time . These fast exchange conditions will be assumed for all samples studied here (as far as relaxation is concerned).…”