79/81Br nuclear quadrupole resonance spectroscopic characterization of halogen bonds in supramolecular assemblies.

Despite the applicability of solid-state NMR to study the halogen bond, the direct NMR detection of 79/81Br covalently bonded to carbon remains impractical due to extremely large spectral widths, even at ultra-high magnetic fields. In contrast, nuclear quadrupole resonance (NQR) offers comparatively sharp resonances. Here, we demonstrate the abilities of 79/81Br NQR to characterize the electronic changes in the C-Br···N halogen bonding motifs found in supramolecular assemblies constructed from 1,4-dibromotetrafluorobenzene and nitrogen-containing heterocycles. An increase in the bromine quadrupolar coupling constant is observed, which correlates linearly with the halogen bond distance (dBr···N). Notably, 79/81Br NQR is able to distinguish between two symmetry-independent halogen bonds in the same crystal structure. This approach offers a rapid and reliable indication for the occurrence of a halogen bond, with experimental times limited only by the observation of 79/81Br NQR resonances.

Structure and solubility behaviour of zinc containing phosphate glasses.

The structure of phosphate glasses of general composition 10Na2O : (20 + x/2)ZnO : (20 + x/2)CaO : (50 -x)P2O5 (0 ≤x≤ 20) has been investigated using IR spectroscopy, 1D 31P and 43Ca MAS Bloch decay, 31P-31P double quantum MAS-NMR and 43Ca and 67Zn static NMR techniques, as well as neutron diffraction analysis. Zinc is shown to aid glass formation in this system. Glass transition temperature and density increase with increasing cation : phosphate ratio. However, free volume calculations show structures becoming significantly more compact from x = 5 to x = 10. The structural data confirm depolymerisation of the glasses with increasing cation : phosphate ratio. Zinc oxide is found to act in a network forming role in the system, with 67Zn NMR and neutron diffraction analysis confirming zinc exhibits predominantly four-coordinate geometry. Solubility in deionised water and tris/HCl buffer solution is seen to decrease significantly with increasing x-value. This is discussed in terms of water ingress and the degree of structural openness, associated with increased cross-linking and a decrease in concentration of P-O-P linkages. pH measurements confirm invert phosphate compositions maintain physiological pH levels on immersion in water and buffer solutions for up to four weeks.

Modeling 2hJiso(N, N) in nucleic acid base pairs: ab initio characterization of the 2hJ(N, N) tensor in the methyleneimine dimer as a function of hydrogen bond geometry.

The observation of 2hJiso(N, N) coupling has prompted considerable interest in this phenomenon from experimentalists and theoreticians due to the potential these couplings hold for the determination of secondary and tertiary structure in biologically important molecules. Here, we present an ab initio (MCSCF) study of the complete 2hJ(N, N) tensor for a model methyleneimine dimer system as a function of (i) the N-N separation, rNN, and (ii) the hydrogen bond angle, theta. This simple system models the 2hJ(N, N) tensor of nucleic acid base pairs. Results indicate that although the Fermi-contact mechanism dominates 2hJiso(N, N), the coupling tensor is anisotropic due to contributions from the Fermi-contact spin-dipolar cross term. The variation in 2hJiso(N, N) as a function of rNN is fit to an exponential decay. The influence of theta on the coupling constant is less pronounced but must be considered if experimental coupling constants are to be used for quantitative structure determination. Our results for this simple model system demonstrate that 2hJiso(N, N) is a valuable probe of hydrogen bonding in nucleic acid base pairs.