10 Search Results

Molybdenum is a body centered-cubic metal with a variety of high technology applications due to it’s thermal, mechanical, and chemical stability. In high pressure research, it is used as a calibration standard in diamond anvil cell (DAC) measurements. Many theoretical and experimental studies have been performed to study the Mo equation of state (EOS) to high pressure.

Here, we present a density functional theory study of cerium deuterides, varying deuterium content and temperature. Results are discussed in relation to ejecta experiments recently described in [J. Dynam. Mat. Behav. 1, 12 (2017)]. Using quantum molecular dynamics, we calculate diffusion coefficients, radial distribution functions, and heat capacities of these materials. Properties are calculated over temperatures ranging from 500-4000K at near ambient pressure and at 2.37 fold compression. We find that deuterium diffusion occurs rapidly accross this temperature and compression region and that heat capacity calculations are in agreement with approximations made in experiments. For cerium metal, a comparison ofmore » self-diffusion coefficient and viscosity with experimental data and analytical models shows excellent agreement. Lastly, we discuss the stucture of cerium deuterides with respect to radial distribution functions and provide fitted equations for diffusion coefficients over temperature.« less

Conjugated energetic molecules (CEMs) are a class of explosives with high nitrogen content that posses both enhanced safety and energetic performance properties and are ideal for direct optical initiation. As isolated molecules, they absorb within the range of conventional lasers. Crystalline CEMs are used in practice, however, and their properties can differ due to intermolecular interaction. Herein, time-dependent density functional theory was used to investigate one-photon absorption (OPA) and two-photon absorption (TPA) of monomers and dimers obtained from experimentally determined crystal structures of CEMs. OPA scales linearly with the number of chromophore units, while TPA scales nonlinearly, where a moremore » than 3-fold enhancement in peak intensity, per chromophore unit, is calculated. Cooperative enhancement depends on electronic delocalization spanning both chromophore units. An increase in sensitivity to nonlinear laser initiation makes these materials suitable for practical use. This is the first study predicting a cooperative enhancement of the nonlinear optical response in energetic materials composed of relatively small molecules. Finally, the proposed model quantum chemistry is validated by comparison to crystal structure geometries and the optical absorption of these materials dissolved in solution.« less

The effects of solvent on molecular processes such as excited state relaxation and photochemical reaction often occurs in a nonequilibrium regime. Dynamic processes such as these can be simulated using excited state molecular dynamics. In this paper, we describe methods of simulating nonequilibrium solvent effects in excited state molecular dynamics using linear-response time-dependent density functional theory and apparent surface charge methods. These developments include a propagation method for solvent degrees of freedom and analytical energy gradients for the calculation of forces. Finally, molecular dynamics of acetaldehyde in water or acetonitrile are demonstrated where the solute-solvent system is out of equilibriummore » due to photoexcitation and emission.« less

Time-dependent density functional theory was used to investigate optical absorption of novel Fe(II) coordination complexes with tetrazine ligands. These octahedral compounds absorb near-infrared (NIR) light and can be applied as secondary explosives with low laser-initiation thresholds compared to pentaerythritol tetranitrate. Here in this paper, numerous ligand architectures are studied to determine relationships between molecular structure and optical absorption in order to tune the low-energy charge transfer (CT) band. Geometrical structures and vertical excitation energies calculated with the TPSSh density functional and 6-311G basis set are in excellent agreement with experiment, with a maximum deviation from UV–vis spectra of 0.10 eV.more » By altering molecular substituents of the ligand scaffold, the CT band can be tuned between 500 and 1100 nm. Additional conjugation in the ligand scaffold pushes the CT band into the NIR region of the spectrum. Triazolo-tetrazine ligands shift the CT band by approximately 0.70 eV relative to that of Fe(II) coordinated with bipyridine ligands. Oxygenated analogues of several compounds are also studied in order to predict optical response, while improving explosive performance. A natural population analysis suggests that the high nitrogen content of the ligand scaffolds in these energetic compounds lessens their metal-to-ligand charge transfer character compared to that of Fe(II) coordinated with bipyridine ligands. The proposed model quantum chemistry is used to establish structure–property relationships for optical properties in this class of materials in order to make optical initiation with conventional lasers a more feasible approach.« less

This study describes variational energy expressions and analytical excited state energy gradients for time-dependent self-consistent field methods with polarizable solvent effects. Linear response, vertical excitation, and state-specific solventmodels are examined. Enforcing a variational ground stateenergy expression in the state-specific model is found to reduce it to the vertical excitation model. Variational excited state energy expressions are then provided for the linear response and vertical excitation models and analytical gradients are formulated. Using semiempiricalmodel chemistry, the variational expressions are verified by numerical and analytical differentiation with respect to a static external electric field. Lastly, analytical gradients are further tested by performingmore » microcanonical excited state molecular dynamics with p-nitroaniline.« less

Nitromethane (NM), a high explosive (HE) with low sensitivity, is known to undergo photolysis upon ultraviolet (UV) irradiation. The optical transparency, homogeneity, and extensive study of NM make it an ideal system for studying photodissociation mechanisms in conventional HE materials. The photochemical processes involved in the decomposition of NM could be applied to the future design of controllable photoactive HE materials. In this work, the photodecomposition of NM from the nπ* state excited at 266 nm is being investigated on the femtosecond time scale. UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) are combinedmore » with nonadiabatic excited state molecular dynamics (NA-ESMD) simulations to provide a unified picture of NM photodecomposition. The FSRS spectrum of the photoproduct exhibits peaks in the NO₂ region and slightly shifted C–N vibrational peaks pointing to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an nπ* state lifetime of ~20 fs were predicted from NA-ESMD simulations. Predicted time scales revealed that NO₂ dissociation occurs in 81 ± 4 fs and methyl nitrite formation is much slower having a time scale of 452 ± 9 fs corresponding to the excited state absorption feature with a decay of 480 ± 17 fs observed in the TA spectrum. Lastly, although simulations predict C–N bond cleavage as the primary photochemical process, the relative time scales are consistent with isomerization occurring via NO₂ dissociation and subsequent rebinding of the methyl radical and nitrogen dioxide.« less

In this study, an efficient method of treating solvent effects in excited state molecular dynamics (ESMD) is implemented and tested by exploring the solvatochromic effects in substituted p-phenylene vinylene oligomers. A continuum solvent model is used which has very little computational overhead. This allows simulations of ESMD with solvent effects on the scale of hundreds of picoseconds for systems of up to hundreds of atoms. At these time scales, solvatochromic shifts in fluoresence spectra can be described. Solvatochromic shifts in absorption and fluorescence spectra from ESMD are compared with time-dependent density functional theory calculations and experiments.