393 research outputs found
Exploding Nitromethane in silico, in real time
Nitromethane (NM) is widely applied in chemical technology as a solvent for
extraction, cleaning and chemical synthesis. NM was considered safe for a long
time, until a railroad tanker car exploded in 1958. We investigate detonation
kinetics and reaction mechanisms in a variety of systems consisting of NM,
molecular oxygen and water vapor. State-of-the-art reactive molecular dynamics
allows us to simulate reactions in time-domain, as they occur in real life.
High polarity of the NM molecule is shown to play an important role, driving
the first exothermic step of the reaction. Presence of oxygen is important for
faster oxidation, whereas its optimal concentration is in agreement with the
proposed reaction mechanism. Addition of water (50 mol%) inhibits detonation;
however, water does not prevent detonation entirely. The reported results
provide important insights for improving applications of NM and preserving
safety of industrial processes.Comment: arXiv admin note: text overlap with arXiv:1408.372
A New Model of Chemical Bonding in Ionic Melts
We developed a new physical model to predict macroscopic properties of
inorganic molten systems using a realistic description of inter-atomic
interactions. Unlike the conventional approach, which tends to overestimate
viscosity by several times, our systems consist of a set of ions with an
admixture of neutral atoms. The neutral atom subsystem is a consequence of the
covalent/ionic state reduction, occurring in the liquid phase. Comparison of
the calculated macroscopic properties (shear viscosity and self-diffusion
constants) with the experiment demonstrates good performance of our model. The
presented approach is inspired by a significant degree of covalent interaction
between the alkali and chlorine atoms, predicted by the coupled cluster theory
Electron-nuclear correlations for photo-induced dynamics in molecular dimers
Ultrafast photoinduced dynamics of electronic excitation in molecular dimers
is drastically affected by the dynamic reorganization of inter- and intra-
molecular nuclear configuration modeled by a quantized nuclear degree of
freedom [Cina et. al, J. Chem Phys. {118}, 46 (2003)]. The dynamics of the
electronic population and nuclear coherence is analyzed by solving the chain of
coupled differential equations for %mean coordinate, population inversion,
electron-vibrational correlation, etc. [Prezhdo, Pereverzev, J. Chem. Phys.
{113} 6557 (2000)]. Intriguing results are obtained in the approximation of a
small change of the nuclear equilibrium upon photoexcitation. In the limiting
case of resonance between the electronic energy gap and the frequency of the
nuclear mode these results are justified by comparison to the exactly solvable
Jaynes-Cummings model. It is found that the photoinduced processes in the model
dimer are arranged according to their time scales: (i) fast scale of nuclear
motion, (ii) intermediate scale of dynamical redistribution of electronic
population between excited states as well as growth and dynamics of
electron-nuclear correlation, (iii) slow scale of electronic population
approach to the quasi-equilibrium distribution, decay of electron-nuclear
correlation, and decrease of the amplitude of mean coordinate oscillation. The
latter processes are accompanied by a noticeable growth of the nuclear
coordinate dispersion associated with the overall nuclear wavepacket width. The
demonstrated quantum relaxation features of the photoinduced vibronic dynamics
in molecular dimers are obtained by a simple method, applicable to systems with
many degrees of freedom
Polarization versus Temperature in Pyridinium Ionic Liquids
Electronic polarization and charge transfer effects play a crucial role in
thermodynamic, structural and transport properties of room-temperature ionic
liquids (RTILs). These non-additive interactions constitute a useful tool for
tuning physical chemical behavior of RTILs. Polarization and charge transfer
generally decay as temperature increases, although their presence should be
expected over an entire condensed state temperature range. For the first time,
we use three popular pyridinium-based RTILs to investigate temperature
dependence of electronic polarization in RTILs. Atom-centered density matrix
propagation molecular dynamics, supplemented by a weak coupling to an external
bath, is used to simulate the temperature impact on system properties. We show
that, quite surprisingly, non-additivity in the cation-anion interactions
changes negligibly between 300 and 900 K, while the average dipole moment
increases due to thermal fluctuations of geometries. Our results contribute to
the fundamental understanding of electronic effects in the condensed phase of
ionic systems and foster progress in physical chemistry and engineering
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