Monitoring the Aging Dynamics of Glycidyl Azide Polyurethane by NMR Relaxation Times

Disorder and Oxide Ion Diffusion Mechanism in La1.54Sr0.46Ga3O7.27 Melilite from Nuclear Magnetic Resonance

Layered tetrahedral network melilite is a promising structural family of fast ion conductors that exhibits the flexibility required to accommodate interstitial oxide anions, leading to excellent ionic transport properties at moderate temperatures. Here, we present a combined experimental and computational magic angle spinning (MAS) nuclear magnetic resonance (NMR) approach which aims at elucidating the local configurational disorder and oxide ion diffusion mechanism in a key member of this structural family possessing the La1.54Sr0.46Ga3O7.27 composition. 17O and 71Ga MAS NMR spectra display complex spectral line shapes that could be accurately predicted using a computational ensemble-based approach to model site disorder across multiple cationic and anionic sites, thereby enabling the assignment of bridging/nonbridging oxygens and the identification of distinct gallium coordination environments. The 17O and 71Ga MAS NMR spectra of La1.54Sr0.46Ga3O7.27 display additional features not observed for the parent LaSrGa3O7 phase which are attributed to interstitial oxide ions incorporated upon cation doping and stabilized by the formation of five-coordinate Ga centers conferring framework flexibility. 17O high-temperature (HT) MAS NMR experiments capture exchange within the bridging oxygens at 130 °C and reveal coalescence of all oxygen signals in La1.54Sr0.46Ga3O7.27 at approximately 300 °C, indicative of the participation of both interstitial and framework oxide ions in the transport process. These results further supported by the coalescence of the 71Ga resonances in the 71Ga HT MAS NMR spectra of La1.54Sr0.46Ga3O7.27 unequivocally provide evidence of the conduction mechanism in this melilite phase and highlight the potential of MAS NMR spectroscopy to enhance the understanding of ionic motion in solid electrolytes.

Spin-spin interaction and relaxation in two trityl-nitroxide diradicals

Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear polarization. To optimize performance it is important to understand the impact of spin-spin interactions on relaxation times of the diradicals. CW spectra and electron spin relaxation were measured for two trityl-nitroxides that differ in the substituents on the amide linker and have different strengths of the exchange interaction J. Analysis of the EPR spectra in terms of overlapping AB spin-spin splitting patterns explains the impact of J on various regions of the spectra. Even modest values of J are large relative to the separation between trityl and nitroxide resonances for some nitrogen nuclear spin state. Two conformations for each diradical were observed in CW spectra in fluid solution at X-band and Q-band. For one diradical J = 15 G (83%) and 5 G (17%) at 293 K, and J = 27 G (67%) and 3 G (33%) with interspin distances of 16 Å and 12 Å, respectively, at 80 K. For the second diradical the exchange interaction is stronger: the two conformations in fluid solution at 293 K had J = 113 G (67%) and 59 G (33%) and at 80 K the value of J was 43 G and there were two conformations with interspin distances of 13 and 11.5 Å. The observation of two conformations for each diradical, with different values of J, demonstrates the dependence of their exchange interactions on through-bond orbital interactions. X-band values of spin relaxation rates 1/T₁ and 1/T_m at 80 to 120 K for the trityl-nitroxides are similar to values for nitroxide mono-radicals, and faster than for trityl radicals. These observations show that even for a relatively small value of J, the nitroxide is very effective in enhancing the relaxation of the more slowly relaxing trityl.

Electron Spin Relaxation of Tb3+ and Tm3+ Ions

Electron spin relaxation times T₁ and T_m of Tb³⁺ and Tm³⁺ in 1:1 water:ethanol and of Tb³⁺ doped (2%) in crystalline La₂(oxalate)₃ decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states. Echo-detected spectra are compared with CW spectra and with field-stepped direct-detected EPR spectra. Due to the strong temperature dependence of T₁, measurements were not made above 10 K. Between about 4.2 and 6 K T₁ is strongly concentration dependent between 1 and ~50 mM. T₁ values at 4.2 K are in the μs range which is orders of magnitude faster than for 3d transition metals. Phase memory times, T_m, are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. T_m is longer in the oxalate lattice which is attributed to the lower proton concentration in oxalate than in the organic solvent, which decreases nuclear spin diffusion. The rigidity of the crystalline lattice also may contribute to longer T_m.

Structural insights for vanadium catecholates and iron‑sulfur clusters obtained from multiple data analysis methods applied to electron spin relaxation data

Electron paramagnetic resonance (EPR) inversion recovery curves for vanadium catecholates and iron‑sulfur clusters were analyzed with three models: the sum of two exponentials, a stretched exponential, and a model-free distribution of exponentials (UPEN). For all data sets studied fits with a stretched exponential were statistically indistinguishable from the sum of two exponentials, and were significantly better than for single exponentials. UPEN provides insights into the structures of the distributions. For a vanadium(IV) tris catecholate the distribution of relaxation rates calculated with UPEN shows the contribution from spectral diffusion at low temperatures. The energy of the local mode for this complex, found from the temperature dependence of the spin lattice relaxation, is consistent with values expected for a metal-ligand vibration. For the [2Fe-2S]+ cluster in pyruvate formate lyase activating enzyme (PFL-AE) the small stretched exponential β values (0.3) at low temperature and the distributions calculated with UPEN reflect the contribution from a second rapidly relaxing species that could be difficult to detect by continuous wave EPR. The distributions in 1/T1 for the [4Fe-4S]+ clusters in Mycofactocin maturase were about a factor of four wider than for the three other systems studied. The very broad distribution of relaxation rates may be due to protein mobility and distributions in electronic energies and local environments for the clusters. UPEN provides insight into several situations that can result in low values of stretch parameter β including contributions from spectral diffusion, overlapping signals from distinguishable clusters, or very wide distributions.

The highly sensitive and facile colorimetric detection of the glycidyl azide polymer based on propargylamine functionalized gold nanoparticles using click chemistry

A highly sensitive and facile colorimetric sensor for the glycidyl azide polymer explosive was developed based on propargylamine functionalized gold nanoparticles using click chemistry.

19F fast magic-angle spinning NMR studies of perfluoroalkanoic acid self-assembled monolayers

The bonding and dynamic properties of perfluoroalkanoic acid self-assembled monolayers (SAMs) on zirconia and titania powders were characterized by Fourier transform infrared and solid-state 19F magic-angle spinning NMR spectroscopy. The perfluoro fatty acids investigated included C(n)F(2n+1)CO2H, n = 7, 13, 15 and 17. The acids bind to both metal oxides via ionic carboxylate bonds, but complete monolayers are only formed on the zirconia. The shift of the CF3 group from -83 ppm in the bulk state to -85 ppm for the adsorbed monolayers is assigned to CF3 groups located at the air/monolayer interface. With the exception of the perfluorooctanoic acid, 19F spin lattice relaxation measurements indicate that the fluorocarbon chains of the adsorbed acids, even in the case of densely packed monolayers, are significantly more mobile than those in the bulk state. The motions associated with the enhanced mobility of the adsorbed acids are proposed to involve reorientations along the long chain axes. No evidence for chain melting in the fluorocarbon SAMs is found for temperatures well above the melting temperature of the bulk acids.

Structural Analysis of a Melaminium Polyphosphate from X-ray Powder Diffraction and Solid-State NMR Data

The crystal structure of the environmentally friendly flame retardant melaminium polyphosphate (MPoly) (2,4,6-triamino-1,3,5-triazinium x PO(3))(n)was determined by a direct-space global optimization technique from X-ray powder diffraction data. Solid-state NMR was used to corroborate the proposed hydrogen-bonding model and to determine the average degree of polymerization (n > 100). An analysis of the crystal structure of MPoly reveals aspects of molecular geometry and packing that are characteristic for melamine-containing compounds and polyphosphate salts. A comparison of MPoly with the crystal structures of its precursors melaminium orthophosphate (MP) and melaminium dihydrogenpyrophosphate (MPy) provides insight in the mechanism of the endothermic dehydration processes that takes place in the reaction path MP --> MPy --> MPoly. Solid-state NMR characterization of various samples of the same batch showed inhomogeneities in the MPoly composition. Various quantities of orthophosphates were found, which cannot be assigned to be MP.