17O NMR evidence for 180°-flips of (COOH)2units in dimethylmalonic acid

Looking into the dynamics of molecular crystals of ibuprofen and terephthalic acid using 17 O and 2 H nuclear magnetic resonance analyses

Oxygen-17 and deuterium are two quadrupolar nuclei that are of interest for studying the structure and dynamics of materials by solid-state nuclear magnetic resonance (NMR). Here, 17 O and 2 H NMR analyses of crystalline ibuprofen and terephthalic acid are reported. First, improved 17 O-labelling protocols of these molecules are described using mechanochemistry. Then, dynamics occurring around the carboxylic groups of ibuprofen are studied considering variable temperature 17 O and 2 H NMR data, as well as computational modelling (including molecular dynamics simulations). More specifically, motions related to the concerted double proton jump and the 180° flip of the H-bonded (-COOH)2 unit in the crystal structure were looked into, and it was found that the merging of the C=O and C-OH 17 O resonances at high temperatures cannot be explained by the sole presence of one of these motions. Lastly, preliminary experiments were performed with a 2 H-17 O diplexer connected to the probe. Such configurations can allow, among others, 2 H and 17 O NMR spectra to be recorded at different temperatures without needing to tune or to change probe configurations. Overall, this work offers a few leads which could be of use in future studies of other materials using 17 O and 2 H NMR.

Influence of N−H···O and C−H···O Hydrogen Bonds on the 17O NMR Tensors in Crystalline Uracil: Computational Study

We report a computational study for the 17O NMR tensors (electric field gradient and chemical shielding tensors) in crystalline uracil. We found that N-H...O and C-H...O hydrogen bonds around the uracil molecule in the crystal lattice have quite different influences on the 17O NMR tensors for the two C=O groups. The computed 17O NMR tensors on O4, which is involved in two strong N-H...O hydrogen bonds, show remarkable sensitivity toward the choice of cluster model, whereas the 17O NMR tensors on O2, which is involved in two weak C-H...O hydrogen bonds, show much smaller improvement when the cluster model includes the C-H...O hydrogen bonds. Our results demonstrate that it is important to have accurate hydrogen atom positions in the molecular models used for 17O NMR tensor calculations. In the absence of low-temperature neutron diffraction data, an effective way to generate reliable hydrogen atom positions in the molecular cluster model is to employ partial geometry optimization for hydrogen atom positions using a cluster model that includes all neighboring hydrogen-bonded molecules. Using an optimized seven-molecule model (a total of 84 atoms), we were able to reproduce the experimental 17O NMR tensors to a reasonably good degree of accuracy. However, we also found that the accuracy for the calculated 17O NMR tensors at O2 is not as good as that found for the corresponding tensors at O4. In particular, at the B3LYP/6-311++G(d,p) level of theory, the individual 17O chemical shielding tensor components differ by less than 10 and 30 ppm from the experimental values for O4 and O2, respectively. For the 17O quadrupole coupling constant, the calculated values differ by 0.30 and 0.87 MHz from the experimental values for O4 and O2, respectively.

A review of oxygen-17 solid-state NMR of organic materials--towards biological applications

17O solid state NMR of organic materials is developing rapidly. This article provides a snapshot of the current state of development of this field. The NMR techniques and enrichment protocols that are driving this progress are outlined. The (17)O parameters derived from solid-state NMR experiments are summarized and the structural sensitivity of the approach to effects such as hydrogen bonding highlighted. The prospects and challenges for (17)O solid-state NMR of biomolecules are discussed.

(1)H-(17)O nuclear-quadrupole double-resonance study of hydrogen disorder in 2-nitrobenzoic acid

Temperature dependence of (17)O nuclear quadrupole resonance frequencies was measured in solid 2-nitrobenzoic acid by a (1)H-(17)O nuclear quadrupole double resonance technique. The experimental results show the presence of a fast exchange of hydrogen atoms between two nonequivalent positions within the O-H ellipsis O hydrogen bonds. The hydrogen disorder is ascribed to concerted jumps of two hydrogen atoms within the hydrogen bonds connecting two molecules in a dimer. The energy difference DeltaE of the two hydrogen configurations is equal to DeltaE = 60 meV = 5.8 kJ/mol. The dipole structure of the (17)O NQR lines from the C-O-H oxygen positions was also measured at -100 degrees C and at room temperature. The orientation of the principal axes of the electric field gradient tensor with respect to the O-H bond and the sign of the quadrupole coupling constant were determined. The oxygen-hydrogen distance R(O-H), as determined from the dipole structure of the (17)O NQR lines is at -100 degrees C equal to 0.099 nm. At room temperature we observe a longer distance, R(O-H) = 0.101 nm, in agreement with the hydrogen intrabond exchange.