Remotely detected MRI velocimetry in microporous bead packs

Many NMR and MRI methods probe fluid dynamics within macro- and mesoporous materials, but with few exceptions, they report on its macroscopically averaged properties. MRI methods are generally unable to localize microscopic features of flow within macroscopic samples because the fraction of the enclosing detector volume occupied by these features is so small. We have recently overcome this problem using remotely detected MRI velocimetry, a technique in which spatial, chemical, and velocity information about elements of the flow is encoded with a conventional NMR coil and detected sensitively at the sample outflow by a volume-matched microdetector. Here, we apply this method to microporous model systems, recording MRI images that correlate local velocity, spin relaxation, and time-of-flight in microscopic resolution and three spatial dimensions. Our results illustrate that remotely detected MRI is an effective approach to elucidate flow dynamics in porous materials including bead pack microreactors and chromatography columns.     

Electronic excited states of tetracyanonickelate(II)

We revisit the assignment of the absorption spectrum of tetracyanonickelate(II) by calculating energies of excitations with time-dependent density functional theory. Our results give strong evidence that the original assignment of the spectrum is only partially correct. We thus propose an alternative assignment consistent with our theoretical calculations and all available experimental evidence. In particular, we reassign the bands at 22 400 and 32 300 cm(-1) to the (1)A(1g) --> (3)A(2g) (b(2g) --> b(1g)) and (1)A(1g) --> (1)A(2g) (b(2g) --> b(1g)) excitations.     

Probing amide bond nitrogens in solids using 14N NMR spectroscopy

A novel two-dimensional nuclear magnetic resonance (NMR) experiment is proposed for indirect observation of 14N nuclei in various types of nitrogen-containing solids. In a method somewhat similar to the heteronuclear single-quantum correlation (HSQC) experiment widely used for protein structure determination in solutions, this technique correlates spin S=1/2 nuclei, e.g., 1H, 13C, with the 14N spin I=1 nucleus in solids. The present experiment, however, transfers coherence from neighboring 1H or 13C nuclei to 14N via a combination of J-couplings and residual dipolar splittings (RDS). Projections of the two-dimensional NMR spectra onto the 14N dimension yield powder patterns that reflect the 14N quadrupolar interaction, which can be used to study molecular structure and dynamics. Indirect detection of amide nitrogen-14 via 1H and 13C is shown experimentally on a model compound of N-acetyl-glycine.