318 research outputs found
Classical stochastic diffusion theory for thermal desorption from solid surfaces
As a first step in the microscopic study of dynamic processes on surfaces and at interfaces, we have considered the thermal desorption of adsorbed species on solid surfaces. We review recent developments based on a classical stochastic diffusion formulation. Using this theory, we obtained a simple rate expression, R=(Omega0/2pi) f(T)exp(–De/kT), where Omega0 is the surface-adsorbate vibrational frequency and De the dissociation energy. For atoms f(T)=1, whereas for molecules f(T) depends on the parameters for the frustrated rotations at the surface. The effect of coverage on the rate of desorption and the process of desorption into a fluid are also examined. Finally, we discuss the relationship between our theory and the expressions obtained from activated complex (transition-state) theory
Density-Dependent Liquid Nitromethane Decomposition: Molecular Dynamics Simulations Based on ReaxFF
The decomposition mechanism of hot liquid nitromethane at various compressions was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two different initial thermal decomposition schemes is observed, depending on compression. At low densities, unimolecular C–N bond cleavage is the dominant route, producing CH_3 and NO_2 fragments. As density and pressure rise approaching the Chapman–Jouget detonation conditions (~30% compression, >2500 K) the dominant mechanism switches to the formation of the CH_(3)NO fragment via H-transfer and/or N–O bond rupture. The change in the decomposition mechanism of hot liquid NM leads to a different kinetic and energetic behavior, as well as products distribution. The calculated density dependence of the enthalpy change correlates with the change in initial decomposition reaction mechanism. It can be used as a convenient and useful global parameter for the detection of reaction dynamics. Atomic averaged local diffusion coefficients are shown to be sensitive to the reactions dynamics, and can be used to distinguish between time periods where chemical reactions occur and diffusion-dominated, nonreactive time periods
Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers
Decomposition of Condensed Phase Energetic Materials: Interplay between Uni- and Bimolecular Mechanisms
Activation energy for the decomposition of explosives is a crucial parameter of performance. The dramatic suppression of activation energy in condensed phase decomposition of nitroaromatic explosives has been an unresolved issue for over a decade. We rationalize the reduction in activation energy as a result of a mechanistic change from unimolecular decomposition in the gas phase to a
series of radical bimolecular reactions in the condensed phase. This is in contrast to other classes of explosives, such as nitramines and nitrate esters, whose decomposition proceeds via unimolecular reactions both in the gas and in the condensed phase. The thermal decomposition of a model
nitroaromatic explosive, 2,4,6-trinitrotoluene (TNT), is presented as a prime example. Electronic structure and reactive molecular dynamics (ReaxFF-lg) calculations enable to directly probe the condensed phase chemistry under extreme conditions of temperature and pressure, identifying the key bimolecular radical reactions responsible for the low activation route. This study elucidates the origin of the difference between the activation energies in the gas phase (∼62 kcal/mol) and the condensed phase (∼35 kcal/mol) of TNT and identifies the corresponding universal principle. On the basis of these findings, the different reactivities of nitro-based organic explosives are rationalized as an interplay between uni- and bimolecular processes
Molecular Dynamics Simulations of Weak Detonations
Detonation of a three-dimensional reactive non-isotropic molecular crystal is
modeled using molecular dynamics simulations. The detonation process is
initiated by an impulse, followed by the creation of a stable fast reactive
shock wave. The terminal shock velocity is independent of the initiation
conditions. Further analysis shows supersonic propagation decoupled from the
dynamics of the decomposed material left behind the shock front. The dependence
of the shock velocity on crystal nonlinear compressibility resembles solitary
behavior. These properties categorize the phenomena as a weak detonation. The
dependence of the detonation wave on microscopic potential parameters was
investigated. An increase in detonation velocity with the reaction
exothermicity reaching a saturation value is observed. In all other respects
the model crystal exhibits typical properties of a molecular crystal.Comment: 38 pages, 20 figures. Submitted to Physical Review
Direct MD Simulations of Terahertz Absorption and 2D Spectroscopy Applied to Explosive Crystals
A direct molecular dynamics simulation of the THz
spectrum of a molecular crystal is presented. A time-dependent
electric field is added to a molecular dynamics simulation of a crystal
slab. The absorption spectrum is composed from the energy dissipated
calculated from a series of applied pulses characterized by a carrier
frequency. The spectrum of crystalline cyclotrimethylenetrinitramine
(RDX) and triacetone triperoxide (TATP) were simulated with the
ReaxFF force field. The proposed direct method avoids the linear
response and harmonic approximations. A multidimensional extension
of the spectroscopy is suggested and simulated based on the nonlinear
response to a single polarized pulse of radiation in the perpendicular
polarization direction
Host-guest chemistry with water-soluble gold nanoparticle supraspheres
The uptake of molecular guests, a hallmark of the supramolecular chemistry of cages and containers, has yet to be documented for soluble assemblies of metal nanoparticles. Here we demonstrate that gold nanoparticle-based supraspheres serve as a host for the hydrophobic uptake, transport and subsequent release of over two million organic guests, exceeding by five orders of magnitude the capacities of individual supramolecular cages or containers and rivalling those of zeolites and metal-organic frameworks on a mass-per-volume basis. The supraspheres are prepared in water by adding hexanethiol to polyoxometalate-protected 4 nm gold nanoparticles. Each 200 nm assembly contains hydrophobic cavities between the estimated 27,400 gold building blocks that are connected to one another by nanometre-sized pores. This gives a percolated network that effectively absorbs large numbers of molecules from water, including 600,000, 2,100,000 and 2,600,000 molecules (35, 190 and 234 g l(-1)) of para-dichorobenzene, bisphenol A and trinitrotoluene, respectively
Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers
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