Insight into Hydrazinium Nitrates, Azides, Dicyanamide, and 5-Azidotetrazolate Ionic Materials from Simulations and Experiments

A transferrable, polarizable, quantum chemistry (QC) based force field has been developed for hydrazinium (N2H5+), monomethylhydrazinium ((CH3)N2H4+), and dimethylhydrazinium ((CH3)2N2H3+) cations in combination with the nitrate (NO3–), azide (N3–), dicyanamide (N(CN)2–), and 5-azidotetrazolate (CN7–) anions. Inclusion of the off-atom charge center to represent a lone pair on the hydrazinium-based cations significantly improved the electrostatic potential description around cations and led to overall a more accurate prediction of ionic crystal cell parameters in molecular dynamics (MD) simulations. Seven different ionic systems have been investigated: [N2H5][NO3], [(CH3)N2H4][NO3], [(CH3)2N2H3][NO3], [N2H5][CN7], [(CH3)N2H4][N3], [(CH3)2N2H3][N3], [N2H5][N(CN)2]. For all but [(CH3)2N2H3][NO3] and [N2H5][N(CN)2], QC calculations of a single, gas-phase ion pair predicts spontaneous deprotonation of the cation. Crystal lattice parameters obtained from MD simulations for these seven ionic crystals were compared with the previously published experimental data as well as the crystal structure of [N2H5][N(CN)2] determined in this work from X-ray data. In general, MD simulations predicted crystal lattice vectors/angles (volumes) within a 5% (3%) absolute margin of error from experiments, with outlying volume deviations of 5–6.6% for three crystals [(CH3)N2H4][N3], [N2H5][NO3], and [(CH3)N2H4][NO3] with combinations of particularly small anions and/or cations. Structural comparisons between ionic materials in the liquid and crystalline states are made, including the observation of two crystalline systems where the crystalline state induces conformational changes in the methylated hydrazinium cations between the gas-phase and liquid states. Elastic constants and estimated shear and bulk moduli were extracted from MD simulations for all seven ionic crystals and correlated with the structural motifs of ion interactions in the crystals

Influence of electrostatic interactions on the properties of cyanobiphenyl liquid crystals predicted from atomistic molecular dynamics simulations

<p>The influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4ʹ-pentylbiphenyl (5CB) systems have been investigated in the 292–368 K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (<i>T</i><i>_NI</i>) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the <i>T</i><i>_NI</i> by about 30 K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of <i>T</i><i>_NI</i> as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.</p

Role of Plasticity in Mechanical Failure of Solid Electrolyte Interphases on Nanostructured Silicon Electrode: Insight from Continuum Level Modeling

Understanding the failure mechanisms of solid electrolyte interphases (SEI) is important for silicon electrodes because their volume expands substantially during lithiation. This work discusses material point method simulations of SEI failure during lithiation of silicon nanopillars. We demonstrate that considering SEI films as brittle, elastic materials does not allow fracture that would be consistent with experimental observations. However, constitutive models that include plastic deformation and result in ductile fracture are in very good agreement with trends observed in experiments. The insight gained from these results allows suggestion of possible strategies for design of SEI with improved failure resistance under lithiation-induced electrode expansion, where modification of the SEI leading to increased Young’s modulus and/or strain hardening without compromising the underlying ductility of the material presents a desirable outcome for chemical and/or processing modifications designed to modify SEI response

Photoinduced and Thermal Relaxation in Surface-Grafted Azobenzene-Based Monolayers: A Molecular Dynamics Simulation Study

Extensive atomistic molecular dynamics simulations have been employed to study the structure and molecular orientational relaxation of azobenzene-based monolayers grafted to a solid substrate. Systems with surface coverage of 0.6 nm²/molecule were investigated over a wide temperature range ranging from 298 K, where the mesogens show local ordering and the monolayer dynamics was found to be glassy, up to 700 K, where the azobenzene groups have a nearly isotropic orientational distribution, with a subnanosecond characteristic orientational relaxation time scale. Biased simulations that model single-molecule thermal excitation and conformational isomerization have been conducted to obtain insight into the mechanisms for photoinduced athermal fluidization and monolayer reorganization observed experimentally in this system. Our simulations clearly indicate that <i>trans</i>–<i>cis</i> conformational isomerization transitions of azobenzene units can lead to reorientation of mesogens and to the formation of a monolayer with strong macroscopic in-plane nematic order. While local heating created by excitation process can facilitate this process, thermal excitation alone is not sufficient to induce ordering in the monolayer. Instead, the work done by a molecule undergoing <i>cis–trans</i> isomerization on the cage of neighboring molecules is the key mechanism for photofluidization and orientational ordering in dMR monolayers exposed to linearly polarized light leading to relaxation dynamics that can be described in terms of higher effective temperature. The obtained simulation results are discussed in light of recent experimental data reported for these systems

Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds

<div><p>Recently, new thermotropic ionic liquid crystals (LCs) with a hexyl-linked tris(imidazolium bromide) core and two terminal alkyl chains were synthesised and characterised. To explore the effect of different counter-ions on the LC behaviour of this system, derivatives with BF₄⁻ and Tf₂N⁻ counter-ions were prepared and analysed. Five of the BF₄⁻ derivatives were found to exhibit thermotropic LC behaviour. The 12-, 14- and 16-carbon tail BF₄⁻ compounds form SmA phases. The 18- and 20-carbon tail homologues form what appears to be a smectic phase but are weakly mesogenic and harder to characterise. Only two of the Tf₂N⁻ derivatives exhibited mesogenic behaviour. The 18-carbon tail Tf₂N⁻ compound forms an as-yet unidentified, highly periodic smectic phase with positional order while the 20-carbon tail homologue forms a periodic SmA phase. The Tf₂N⁻ mesogens have much lower clearing points even though their LC phases have more order than the Br⁻ and BF₄⁻ mesogens. X-ray diffraction showed that these mesogens have different amounts of tail interdigitation between the smectic layers depending on the counter-ion present. Atomistic molecular dynamics simulations indicated that counter-ion size plays an important role in defining the density of the ionic region, which in turn affects the amount of interdigitation in the smectic phases.</p></div