Theoretical Insights into the ²⁷Al NMR Parameters of Binary Aluminosilicate Glass and Their Relationship to the Atomic and Electronic Structure

Al-rich 60Al2O3–40SiO2 glass is a candidate for technological applications in electronic and optical devices. Though the amorphous structure of the glass has been studied using solid-state NMR and simulation approaches, the atomic and electronic structure have not been fully revealed. Solid-state 27Al NMR spectra reflect the 27Al environment, though a comprehensive understanding of the spectra and local structure is challenging when interpreting the broadened peak shapes of the amorphous state. Here, an accurate atomic structure of 60Al2O3–40SiO2 glass was modeled using ab initio molecular dynamics (AIMD) simulations containing 418 atoms and employing the melt-quenching route with 15 K/ps. This simulation approach reproduced X-ray diffraction data better than classical molecular dynamics (CMD) simulations. The structure of the polyhedra formed by O bonded to Al was quantitatively analyzed by evaluating bond-angle distributions and the degree of symmetry using spherical harmonic functions. The relationship between chemical shifts and charge-balancing mechanisms was explored through the analysis of electronic structures obtained from AIMD-derived structures. Interestingly, the Al partial charge and the spatial electron distribution of Al–O bonds were independent of the Al coordination number, implying that valence electrons are not localized to specific atoms but are rather distributed throughout the glass network. The theoretical distribution of 27Al NMR parameters was obtained through statistical analysis of theoretically calculated NMR parameters for 100 AIMD-derived structures. By comparing the experimental 27Al NMR data with the theoretical distribution, the previously unclear relationship between 27Al NMR parameters and local structure was elucidated

Theoretical Insights into the ²⁷Al NMR Parameters of Binary Aluminosilicate Glass and Their Relationship to the Atomic and Electronic Structure

Al-rich 60Al2O3–40SiO2 glass is a candidate for technological applications in electronic and optical devices. Though the amorphous structure of the glass has been studied using solid-state NMR and simulation approaches, the atomic and electronic structure have not been fully revealed. Solid-state 27Al NMR spectra reflect the 27Al environment, though a comprehensive understanding of the spectra and local structure is challenging when interpreting the broadened peak shapes of the amorphous state. Here, an accurate atomic structure of 60Al2O3–40SiO2 glass was modeled using ab initio molecular dynamics (AIMD) simulations containing 418 atoms and employing the melt-quenching route with 15 K/ps. This simulation approach reproduced X-ray diffraction data better than classical molecular dynamics (CMD) simulations. The structure of the polyhedra formed by O bonded to Al was quantitatively analyzed by evaluating bond-angle distributions and the degree of symmetry using spherical harmonic functions. The relationship between chemical shifts and charge-balancing mechanisms was explored through the analysis of electronic structures obtained from AIMD-derived structures. Interestingly, the Al partial charge and the spatial electron distribution of Al–O bonds were independent of the Al coordination number, implying that valence electrons are not localized to specific atoms but are rather distributed throughout the glass network. The theoretical distribution of 27Al NMR parameters was obtained through statistical analysis of theoretically calculated NMR parameters for 100 AIMD-derived structures. By comparing the experimental 27Al NMR data with the theoretical distribution, the previously unclear relationship between 27Al NMR parameters and local structure was elucidated

<i>Ab Initio</i> Molecular Dynamics Simulations and GIPAW NMR Calculations of a Lithium Borate Glass Melt

The atomic structure of a molten 0.3Li₂O–0.7B₂O₃ glass at 1250 K was investigated using <i>ab initio</i> molecular dynamics (AIMD) simulations. The gauge including projector augmented wave (GIPAW) method was then employed for computing the chemical shift and quadrupolar coupling constant of ¹¹B, ¹⁷O, and ⁷Li from 764 AIMD derived structures. The chemical shift and quadrupolar coupling constant distributions were directly estimated from the dynamical structure of the molten glass. ¹¹B NMR parameters of well-known structural units such as the three-coordinated ring, nonring, and four-coordinated tetrahedron were found to be in good agreement with the experimental results. In this study, more detailed classification of B units was presented based on the number of O species bonded to the B atoms. This highlights the limitations of ¹¹B NMR sensitivity for resolving ¹¹B local environment using the experimentally obtained spectra only. The ¹⁷O NMR parameter distributions can theoretically resolve the bridging and nonbridging O atoms with different structural units such as nonring, single boroxol ring, and double boroxol ring. Slight but clear differences in the number of bridging O atoms surrounding Li that have not been reported experimentally were observed in the theoretically obtained ⁷Li NMR parameters

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion–water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H⁺(H₂O)_<i>n</i>), nonprotonated hydrogen (H)-bonded water (H₂O···H₂O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O–H bending band, it was found that the volume fraction <i>f</i>_{<i>i</i> (<i>i</i>=each deconvoluted band)} of H-bonded water for elevated temperatures (>80–100 °C) increased remarkably higher than for 60 °C

Theoretical Studies of Pendant Effects on the Properties of Sulfonated Hydrocarbon Polymer Electrolyte Membranes

Six model compounds of hydrocarbon polymer electrolyte membrane (PEM) with different neighboring pendants have been investigated using density functional theory (DFT). The effect of the neighboring pendant on the proton dissociation properties of the PEMs and on the chemical stability of the key adjacent bond containing a sulfonic group was evaluated. Results of the proton dissociation properties of the six model compounds indicate that the introduction of a strong electron-withdrawing group, such as CF, CF₂, or CN, on the neighboring pendant of the acid group can improve the proton dissociation properties of PEMs. The calculated p<i>K</i>_a values confirm the relative acid strength of the six model compounds, whose properties are, to some extent, related to the proton conductivity. Our results demonstrate that a model compound containing a strong electron-withdrawing group in the neighboring pendant has stronger acid strength. DFT calculations on the C–S bond degradation reactions caused by OH or H radicals show that a −CF₂CF₂– group in the neighboring pendant improves the stability of the C–S bond against attack from a radical, while introduction of a −CH₂CH₂CH₂– or a CN group has little influence on the stability of the C–S bond

First-Principles Molecular Dynamics Study of a Hydrocarbon Copolymer for Use in Polymer Electrolyte Membrane Fuel Cells

The structural and dynamic properties of a brush-type hydrocarbon copolymer are investigated using first-principles molecular dynamics simulations. Two model compounds, one with mainly hydrophilic domains and one with mainly hydrophobic domains, were selected and used in the simulations. A series of radial distribution functions of different groups, such as water–water, sulfonic group–hydrogen, and ether–hydrogen, is obtained to investigate the structure of the whole systems. The radial distribution functions of sulfonic groups, <i>g</i>_S–S(<i>r</i>), and the structure of water clusters indicate the formation of a well-developed water channel in the studied copolymer. Analysis of proton dissociation reveals that the protons in both systems are not completely dissociated when the number of water molecules per sulfonic group is equal to 4. The low dissociation nature of this copolymer compared with that of Nafion is explained by its intrinsic acid strength and the presence of ineffective hydrogen bonds in the system, where ineffective hydrogen bonds indicate hydrogen bonds that do not contribute strongly to proton transport. The proton conductivity of this copolymer is comparable to that of Nafion, which is ascribed to the formation of good water channels. In addition, the calculated electrical conductivity of the two model compounds shows good agreement with the measured proton conductivity of this copolymer