Natural abundance solid-state 67Zn NMR characterization of microporous zinc phosphites and zinc phosphates at ultrahigh magnetic field

Zinc-phosphite and -phosphate based microporous materials are crystalline open framework materials with potential industrial applications. Although (31)P MAS NMR has been used for characterization of these materials, the local environments around zinc centres have never been directly probed by solid-state NMR due to the many unfavourable NMR characteristics of (67)Zn. In this work, we have characterized the local structure around the Zn centres in several representative microporous zinc phosphites and zinc phosphates by acquiring natural abundance (67)Zn solid-state NMR spectra at ultrahigh magnetic field of 21.1 T. The observed line-shapes are mainly determined by the second order quadrupolar interaction. The NMR tensor parameters were extracted from the spectra and are related to the local geometry around the Zn centre. Computational study of the electric field gradient (EFG) tensor at Zn was performed using hybrid density functional theory (DFT) calculations at B3LYP level of theory on model clusters. The calculations using Projector Augmented-Wave (PAW) method were also carried out with the CASTEP code wherever it was possible. The work has shown that it is possible to study Zn environments in porous materials which often have very low Zn concentration by natural abundance (67)Zn SSNMR at very high magnetic fields.     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.     

Solid‐State 73Ge NMR Spectroscopy of Simple Organogermanes

Germanium‐73 is an extremely challenging nucleus to examine by NMR spectroscopy due to its unfavorable NMR properties. Through the use of an ultrahigh (21.1 T) magnetic field, a systematic study of a series of simple organogermanes was carried out. In those cases for which X‐ray structural data were available, correlations were drawn between the NMR parameters and structural metrics. These data were combined with DFT calculations to obtain insight into the structures of several compounds with unknown crystal structures.     

Synthesis of a Four-Strand N2O2-Donor Ligand and its Transition Metal Complexation Probed by Electrospray Ionisation Mass Spectrometry

The four-strand and potentially N₂O₂-donor ligand 2,3-endo,endo-bis(aminomethyl)-5,6-endo,endo-bis(hydroxymethyl)bicyclo[2.2.1]heptane (L1), a close analogue of the known tetraalcohol 2,3,5,6-endo,endo,endo,endo-tetrakis(hydroxymethyl)bicyclo[2.2.1]heptane (L2), has been prepared via a multi-step synthesis and isolated as the copper(II) complex [Cu(L1)₂](ClO₄)₂. An ESI-MS study of the complex and metal ion exchange with other transition metal ions (Fe²⁺, Co²⁺, Ni²⁺, Mn²⁺ or Zn²⁺) indicates that 1:1 complexes form readily. Apparently special stability for the Ni²⁺ species observed in the ESI-MS study suggests strong encapsulation of this ion.     

Synthesis and characterization of cationic selenium-nitrogen heterocycles from tert-butyl-DAB (DAB = 1,4-di-tert-butyl-1,3-diazabutadiene) and SeX4 via the reductive elimination of X2 (X = Cl, Br): a distinct contrast with tellurium

The synthesis and comprehensive characterisation of a series of 1,2,5-selenadiazolium salts, which were generated from the direct reaction between the neutral bidentate ligand tert-butyl-DAB and a variety of heavy chalcogen halides, are reported. The formation of the cationic heterocycle from the reaction of SeX4 (X = Cl, Br) and the ligand results in a two electron redox process where the chalcogen undergoes a two electron reduction concomitant with the elimination of X2, the oxidation by-product. A reaction pathway for this chemistry has been proposed necessitating several key intermediates. These species have been synthesized and used in a stepwise series of transformations that strongly supports this very unusual reactivity for the chalcogens. In contrast, the reaction between tert-butyl DAB and TeX4 (X = Cl, Br, I), does not result in redox, rather an octahedral Te(iv) x DAB complex is formed or no reaction was observed.     

Glass transition broadening via nanofiller-contiguous polymer network in aromatic thermosetting copolyester nanocomposites

The glass transition is a genuine imprint of temperature-dependent structural relaxation dynamics of backbone chains in amorphous polymers, which can also reflect features of chemical transformations induced in macromolecular architectures. Optimization of thermophysical properties of polymer nanocomposites beyond the state of the art is contingent on strong interfacial bonding between nanofiller particles and host polymer matrix chains that accordingly modifies glass transition characteristics. Contemporary polymer nanocomposite configurations have demonstrated only marginal glass transition temperature shifts utilizing conventional polymer matrix and functionalized nanofiller combinations. We present nanofiller-contiguous polymer network with aromatic thermosetting copolyester nanocomposites in which carbon nanofillers covalently conjugate with cure advancing crosslinked backbone chains through functional end-groups of constituent precursor oligomers upon an in situ polymerization reaction. Via thoroughly transformed backbone chain configuration, the polymer nanocomposites demonstrate unprecedented glass transition peak broadening by about 100 °C along with significant temperature upshift of around 80 °C. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1595–1603     

Determination of Acid Dissociation Constants of Neamine by Potentiometric and Electrospray Mass Spectral Techniques

The acid dissociation constants of neamine have been measured using potentiometric titrations. The pK_a1, pK_a2, and pK_a3 values of neamine are 6.35 ± 0.2, 7.73 ± 0.15, and 8.62 ± 0.08, respectively. Neamine is readily characterized using positive-ion electrospray ionization–mass spectrometry (ESI–MS). Various protonated species and their solvated ions are mainly observed. Nevertheless, the abundances of the observed speciation over the pH range 5.0–9.8 do not reflect the variation in the bulk solution. Hence, reaction quotients determined from the ESI–MS investigations vary significantly from the pK_a's determined from potentiometric titrations, thereby illustrating that care must be taken in determining thermodynamic properties using the former technique.