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.

In complex mixtures, proton nuclear magnetic resonance (NMR) spectra are often very crowded, making spectral analysis complicated or even impossible, particularly when detailed structural information about the mixture components is needed. A new 1D NMR method (fluorine-edited selective TOCSY acquisition, FESTA) is introduced that facilitates the structural analysis of mixtures of species that contain fluorine. It allows simplified H spectra to be obtained that show only those protons that are in a spin system coupled to fluorine of interest. The new method is illustrated by factorizing a complexH spectrum into subspectra for individual spin systems involving different F sites.

In this work, a new approach to studying coupling pathways for the Fermi contact term of NMR spin-spin coupling constants (SSCCs) is presented. It is based on the known form of propagating the Fermi hole through a canonical molecular orbital (CMO). It requires having an adequate spatial description of the relevant canonical molecular orbitals, which are obtained by expanding CMOs in terms of natural bond orbitals (NBOs). For detecting the relevant contributions of CMOs to a given Fermi contact (FC) pathway, the description of the FC in terms of the triplet polarization propagator (PP) is used. To appreciate the potential of this approach, dubbed FCCP-CMO (Fermi contact coupling pathways-CMO), it is applied to analyze the through-space transmission of the FC term of J(PP) SSCCs by overlap of the P lone pairs. This method can be applied using well-known quantum chemistry software without any further modification, which makes it appealing for use as a complement to SSCC measurements by NMR spectroscopy.