Convection in liquid-state NMR: expect the unexpected

Abstract

Temperature gradients in liquid-state NMR samples are unavoidable, but undesirable: they lead to sample convection, and consequently to signal attenuation in experiments that use field gradients. This paper illustrates how widely the dependence of sample convection velocity on the temperature at which the sample is maintained can differ between different probes and different spectrometers, including the first such results for cryoprobe systems, and highlights the importance of understanding this dependence if the effects of sample convection are to be kept to an acceptable minimum. It is sometimes thought that efficient sample temperature control should suffice to avoid convection: alas, this is not true, and rapid sample convection can occur even with the best hardware. Previous experiments have shown that the effects of convection can sometimes be avoided by setting the sample temperature regulation to one particular temperature; here it is shown that no such temperature exists in some probes. The issue of convection is all too often swept under the carpet; these results confirm that it is a more general problem than is commonly realized.

Excitation of singlet–triplet coherences in pairs of nearly-equivalent spins

“…38 enough for the molecules to diffuse across the whole sample assuming typical convection rates of ca. 0.01 mm/s in NMR measurements 39 . For this reason, it is important to restrict the sample size within the active volume of the NMR coil.…”

1H and 19F NMR in drug stress testing: the case of voriconazole

“…The diffusion experiment was performed using a sample in a 3 mm NMR tube, 29,30 to minimize convection, with a net diffusion-encoding gradient pulse width (d) of 2.4 ms, a diffusion delay (D) of 60 ms, and sixteen nominal gradient amplitudes ranging from 4.8 to 38.5 G cm À1 . The delay for gradient recovery (d16) and duration of the gradient purge pulse (p19) were xed at 0.2 and 0.6 ms, respectively.…”

Practical aspects of real‐time pure shift HSQC experiments

“…It should be noted that convection is present and can lead to signal attenuation, in almost all NMR experiments. Its effects can be reduced by minimising sample heating, by using constricted NMR sample tubes (e.g., 3‐mm tubes, heavy‐walled tubes, or 5‐mm tubes fitted with a 2‐mm capillary), and/or by selecting solvents with high viscosity and/or low thermal expansion coefficient . For example, DMSO‐ d 6 is generally preferable to CDCl 3 as a solvent for real‐time pure shift NMR experiments.…”

“…Its effects can be reduced by minimising sample heating, by using constricted NMR sample tubes (e.g., 3-mm tubes, heavy-walled tubes, or 5-mm tubes fitted with a 2-mm capillary), and/or by selecting solvents with high viscosity and/or low thermal expansion coefficient. [49,50] For example, DMSO-d 6 is generally preferable to CDCl 3 as a solvent for real-time pure shift NMR experiments. A simpler, and very effective, solution is to apply the field gradient pulses perpendicular to the commonly used vertical axis, [51] but this requires special hardware (triaxial field gradients).…”