Temperature Dependent Properties of the Aqueous Electron

The temperature‐dependent properties of the aqueous electron have been extensively studied using mixed quantum‐classical simulations in a wide range of thermodynamic conditions based on one‐electron pseudopotentials. While the cavity model appears to explain most of the physical properties of the aqueous electron, only a non‐cavity model has so far been successful in accounting for the temperature dependence of the absorption spectrum. Here, we present an accurate and efficient description of the aqueous electron under various thermodynamic conditions by combining hybrid functional‐based molecular dynamics, machine learning techniques, and multiple time‐step methods. Our advanced simulations accurately describe the temperature dependence of the absorption maximum in the presence of cavity formation. Specifically, our work reveals that the red shift of the absorption maximum results from an increasing gyration radius with temperature, rather than from global density variations as previously suggested.

Shallow and deep trap states of solvated electrons in methanol and their formation, electronic excitation, and relaxation dynamics

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (e_met⁻) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of e_met⁻ are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (e_aq⁻). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Condensed-phase first-principles molecular dynamics simulations elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons.

Surface tension of liquids and binary mixtures from molecular dynamics simulations

In this work we assess and extend strategies for calculating surface tension of complex liquids from molecular dynamics simulations: the mechanical route and the instantaneous liquid interface (ILI) approach. The former employs the connection between stress tensor and surface tension, whereas the latter involves computation of instantaneous density field. Whereas the mechanical route is general, the ILI method involves system-dependent parameters restricting its original application to liquid water only. Here we generalize the approach to complex molecular liquids using atomic van der Waals radii. The performance of the approaches is evaluated on two liquid systems: acetonitrile and water-methanol mixture. In addition, we compare the effect of the computational models for interaction potentials based on semi-empirical electronic structure theory and classical force fields on the estimate of the surface tension within both stress tensor and ILI approaches.

Subsystem Density Functional Theory Augmented by a Delta Learning Approach to Achieve Kohn-Sham Accuracy

Simulations based on electronic structure theory naturally include polarization and have no transferability problems. In particular, Kohn-Sham density functional theory (KS-DFT) has become the method of reference for ab initio molecular dynamics simulations of condensed matter systems. However, the high computational cost often poses strict limits on the affordable system size as well as on the extension of sampling (number of configurations). In this work, we propose an improvement to the subsystem density functional theory approach, known as the Kim-Gordon (KG) scheme, thus enabling the sampling of configurations for condensed molecular systems keeping the KS-DFT level accuracy at a fraction of computer time. Our scheme compensates the known KG shortcomings of the electronic kinetic energy term by adding a simple correction and can match KS-DFT accuracy in energies and forces. The computationally cheap correction is determined by means of a machine learning procedure. The proposed KG scheme is applied within a linear scaling self-consistent field formalism and is assessed by a series of molecular dynamics simulations of liquid water under different conditions. Although system-dependent, the correction is transferable between system sizes and temperatures.

Formulation and Implementation of Density Functional Embedding Theory Using Products of Basis Functions

The representation of embedding potential using products of atomic orbital basis functions has been developed in the context of density functional embedding theory. The formalism allows to treat pseudopotential and all-electron calculations on the same footing and enables simple transfer of the embedding potential in a compact matrix form. In addition, a cost-reduction procedure for the basis set and potential reduction based on population analysis has been proposed. Implemented for the condensed-phase and molecular systems within Gaussian and plane-waves and Gaussian and augmented plane-waves formalisms, the scheme has been tested for proton-transfer reactions in the cluster and the condensed phase and projected density of states of carbon monoxide adsorbed on platinum surface. With the computational scaling of the embedding potential optimization similar to that of hybrid density functional theory with a significantly reduced prefactor, the method allows for large-scale applications to extended systems.

Decomposition of сarbon tetrachloride under the action of a dielectric barrier discharge of atmospheric pressure in an oxygen atmosphere

In this study, the process of decomposition of carbon tetrachloride (CCl₄) vapor in oxygen DBD at atmospheric pressure and its kinetic regularities have been studied. In the course of the experiments, it was shown that the efficiency of the decomposition of carbon tetrachloride in DBD can reach 100%. Depending on the conditions of the experiments, the effective rate constants were equal to (0.16-0.59) s^-1, and the decomposition energy yields were (0.001-0.012) molecules per 100 eV of the inputed energy. The main decomposition products were CO₂ and Cl₂ molecules. The formation of a solid on the internal electrode of the reactor was also found. The substance contains atoms of carbon, oxygen, chlorine (C:O:Cl) = 1:0.38:0.01, as well as hydrogen atoms. The substance also contains functional groups -CH, -CH₂, -OH and dimers of carboxylic (chlorocarboxylic) acids. Based on the solution of the Boltzmann equation for electrons, it is shown that for the compositions of a gas containing O₂ molecules, ССl₄, and decay products, the kinetic and transport characteristics of electrons are the same as in a pure oxygen discharge. Using the kinetic characteristics of electrons and the reaction rate constants the mechanisms of reactions leading to the found reaction products are proposed. It was shown that the primary reaction of destruction is the reaction of dissociation of CCl₄ by electron impact, leading to the formation of CCl₃^• and Cl and the reaction with the O (¹D) atom, as a result of which CCl₃^• and ClO^• are formed.

Simulating the ghost: quantum dynamics of the solvated electron

The nature of the bulk hydrated electron has been a challenge for both experiment and theory due to its short lifetime and high reactivity, and the need for a high-level of electronic structure theory to achieve predictive accuracy. The lack of a classical atomistic structural formula makes it exceedingly difficult to model the solvated electron using conventional empirical force fields, which describe the system in terms of interactions between point particles associated with atomic nuclei. Here we overcome this problem using a machine-learning model, that is sufficiently flexible to describe the effect of the excess electron on the structure of the surrounding water, without including the electron in the model explicitly. The resulting potential is not only able to reproduce the stable cavity structure but also recovers the correct localization dynamics that follow the injection of an electron in neat water. The machine learning model achieves the accuracy of the state-of-the-art correlated wave function method it is trained on. It is sufficiently inexpensive to afford a full quantum statistical and dynamical description and allows us to achieve accurate determination of the structure, diffusion mechanisms, and vibrational spectroscopy of the solvated electron.

Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron

Aqueous solvated electron (e_aq^-), a key species in radiation and plasma chemistry, can efficiently reduce CO₂ in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase molecular dynamics based on the correlated wave function and an accurate density functional theory (DFT) approximation. Here, we design and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion. The carbon dioxide reduction proceeds via a cavity intermediate, which is separated from the product (CO₂^-) by an energy barrier due to the bending of CO₂ and the corresponding solvent reorganization energy. The formation of the intermediate is caused by solvated electron's diffusion, whereas the intermediate transformation to CO₂^- is triggered by hydrogen bond breaking in the second solvation shell of the solvated electron. This picture of an activation-controlled e_aq^- reaction is very different from both rapid barrierless electron transfer and proton-coupled electron transfer, where key transformations are caused by proton migration.

Double-Hybrid DFT Functionals for the Condensed Phase: Gaussian and Plane Waves Implementation and Evaluation

Intermolecular interactions play an important role for the understanding of catalysis, biochemistry and pharmacy. Double-hybrid density functionals (DHDFs) combine the proper treatment of short-range interactions of common density functionals with the correct description of long-range interactions of wave-function correlation methods. Up to now, there are only a few benchmark studies available examining the performance of DHDFs in condensed phase. We studied the performance of a small but diverse selection of DHDFs implemented within Gaussian and plane waves formalism on cohesive energies of four representative dispersion interaction dominated crystal structures. We found that the PWRB95 and ωB97X-2 functionals provide an excellent description of long-ranged interactions in solids. In addition, we identified numerical issues due to the extreme grid dependence of the underlying density functional for PWRB95. The basis set superposition error (BSSE) and convergence with respect to the super cell size are discussed for two different large basis sets.

TREATMENT OF WASTEWATER CONTAINING 2,4-DICHLOROPHENOL IN DIELECTRIC BARRIER DISCHARGE PLASMA

In the work, the processes of destruction of aqueous solutions of 2,4-dichlorophenol in a dielectric barrier discharge of atmospheric pressure in oxygen were studied. It has been experimentally shown that 2,4-dichlorophenol is destroyed in plasma quite efficiently (the degree of destruction reaches 80 %), which confirms the earlier studies on the decomposition of various organic pollutants in a dielectric barrier discharge plasma. The kinetic parameters were estimated and the main intermediate and final products of the decomposition of 2,4-dichlorophenol under the action of active plasma particles were determined. The destruction of the starting compound is described by a first order kinetic equation. The effective rate constant depends weakly on the experimental conditions and it equals to 0.56 s–1. The composition of the degradation products was studied by gas chromatography, as well as by fluorescence, spectrophotometric and potentiometric methods. Cl- in the liquid phase, as well as СО and СО2 in the gas phase, were identified as the final degradation products. And carboxylic acids and aldehydes were intermediate degradation products. But their concentrations are not high relative to СО and СО2. No molecular chlorine was detected in the gas phase. It was found that ozone does not make a significant contribution to the oxidative destruction of 2,4-dichlorophenol. The hydroxyl radicals and atomic oxygen are main active particles involved in oxidative processes.  An increase in the frequency of the discharge current from 50 to 800 Hz, as well as the absence of a hydrophobic coating of the internal electrode, leads to a decrease in the decomposition rate by a factor of 1.7 (from 227 to 135 μmol/(l∙s)).

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post-Hartree-Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

Ionization of Water as an Effect of Quantum Delocalization at Aqueous Electrode Interfaces

The enhanced probability of water dissociation at the aqueous electrode interfaces is predicted by path-integral ab initio molecular dynamics. The ionization process is observed at the aqueous platinum interface when nuclear quantum effects are introduced in the statistical sampling, while minor effects have been observed at the gold interface. We characterize the dissociation mechanism of the formed water ions. In spite of the fact that the concentration and lifetime of the ions might be challenging to experimentally detect, they may serve as a guide to future experiments. Our observation might have a significant impact on the understanding of electrochemical processes occurring at the metal electrode surface.

Distribution of Policyclic aromatic hydrocarbons in a snow cover in the territory of Ivanovo city, Russia

The paper presents the results of a study of the content of 12 polyaromatic hydrocarbons (PAHs) in the snow cover of the city of Ivanovo (Russian Federation). It is shown that their average content exceeds the background level by 6.6 times, which made it possible to identify for which compounds the admission channels are associated with transboundary transport (naphthalene, pyrene, benz [b]fluorantin, benzo [a]pyrene and dibenz [a,h]anthracene), and for which with local emission sources (anthracene, phenanthrene, fluoranthene, chrysene, benz [k]fluorantin, and benzo [g,h,i]perylene). According to the known indicator ratios of the concentrations of PAHs, the main sources of release (pyrogenic and mixed) PAHs into the environment were estimated. The combination of experimental data in combination with factor analysis allowed identifying priority PAHs (naphthalene, fluoren, fluoranthene, benzo [a]pyrene and benzo [g,h,i]perylene), which should be included in the environmental monitoring programs of the region. Environmental risk assessments are given, which showed that the level of pollutant does not always adequately reflect the environmental impact for the territories. Thus, the contribution to the total PAH concentration of benz [b]fluorantin is only 9%, and to the amount of environmental risk - 51%. This must be taken into account in order to prioritize the control of individual components of PAHs in environmental objects.

COMPARATIVE STUDY OF ELECTRICAL AND PHYSICAL PARAMETERS OF GLOW DISCHARGE UNDER WATER SOLUTIONS OF ANIONIC AND CATIONIC SURFACTANTS

The results of experimental study of the electrical and physical parameter of the gas discharge in air under the solutions of the two types of surfactants are given. The discharge was ignited between metal needle anode and liquid cathode. As the cathode the solutions of anionic surfactant C12H25SO4Na (SLS) and cationic surfactant C12H46ClN, (AOTC) were used. In the concentration range of 5·10-3-10 g/L and discharge current range of 20-100 mA the phenomenology of discharge was studied, the current densities and inputted power, cathode voltage drops, vibrational temperatures of N2(C3Πu), gas temperatures were obtained and reduced electric field strength was calculated. It was show that increase of the SLS concentration leads to the change in the discharge color due to transfer of Na atoms from liquid to the gas phase. In the same time there are no any new emission lines or band for the AOTC solutions were obtained. It was established that for cationic surfactants the cathode voltage drop is lower than for anionic surfactant. The gas temperatures and vibrational temperatures don’t depend on solution type. Reduced electric field is in the range of 10-15 Td and the increase of the concentration leads to decrease of the E/N. The increase of the discharge current results in the growth of reduced electric field.

Dynamics of the Bulk Hydrated Electron from Many-Body Wave-Function Theory

The structure of the hydrated electron is a matter of debate as it evades direct experimental observation owing to the short life time and low concentrations of the species. Herein, the first molecular dynamics simulation of the bulk hydrated electron based on correlated wave‐function theory provides conclusive evidence in favor of a persistent tetrahedral cavity made up by four water molecules, and against the existence of stable non‐cavity structures. Such a cavity is formed within less than a picosecond after the addition of an excess electron to neat liquid water, with less regular cavities appearing as intermediates. The cavities are bound together by weak H−H bonds, the number of which correlates well with the number of coordinated water molecules, each type of cavity leaving a distinct spectroscopic signature. Simulations predict regions of negative spin density and a gyration radius that are both in agreement with experimental data.

OXIDATIVE-REDUCING PROCESSES WITH PARTICIPATION OF MANGANESE IONS INITIATED BY AN ELECTRIC DISCHARGE IN AQUEOUS SOLUTION

The results of experimental studies of the kinetics of oxidation-reduction of Mn7+ ions (MnO4-) in aqueous solutions initiated by the action of a discharge of a direct current of atmospheric pressure in air are analyzed in the article. A solution of potassium permanganate served as a discharge cathode. The range of initial solution concentrations for Mn7+ ions was (0.44-2.5) mmol/l, and discharge currents (20-60) mA. It was found that the discharge action leads to the reduction of Mn7+ ions and discoloration of the solution. At the same time, dark solid particles with a size of 0.1 μm to 20 μm are formed. X-ray diffraction analysis showed that the particles are amorphous, and energy dispersive X-ray analysis showed that the powder is manganese oxide (IV). The kinetics of reduction-oxidation of Mn7+ ions is measured. It is shown that the obtained data on the kinetics of the reduction of Mn7+ ions in the best way (the determination coefficient R2≈0.99) can be described by the scheme X↔Y↔Z, where X is the starting material, and Y and Z are the reaction products. The processing of kinetic curves on the basis of this scheme found the effective rate constants of the corresponding stages. It was found that the effective rate constants depend on the initial concentration of the solution. At a discharge current of 20 mA, an increase in the concentration from 0.44 to 2.5 mol/l led to a decrease in the rate constant for the reduction of Mn7+ ions from (2.48 ± 0.5) ·10-2 to (7.2 ± 1.5) ·10-3 s-1, respectively. Possible mechanisms of processes are discussed. It is assumed that the main particles involved in the oxidation reactions of the reduction of manganese ions are H2O2, HO2, OH and solvated electrons that are formed in the solution under the action of a discharge. For citation: Shutov D.A., Sungurova A.V., Smirnova K.V., Manukyan A.S., Rybkin V.V. Oxidative-reducing processes with participation of manganese ions initiated by electric discharge in aqueous solution. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 9-10. P. 23-29

Application of classical simulations for the computation of vibrational properties of free molecules

In this study, we investigate the ability of classical molecular dynamics (MD) and Monte-Carlo (MC) simulations for modeling the intramolecular vibrational motion. These simulations were used to compute thermally-averaged geometrical structures and infrared vibrational intensities for a benchmark set previously studied by gas electron diffraction (GED): CS₂, benzene, chloromethylthiocyanate, pyrazinamide and 9,12-I₂-1,2-closo-C₂B₁₀H₁₀. The MD sampling of NVT ensembles was performed using chains of Nose-Hoover thermostats (NH) as well as the generalized Langevin equation thermostat (GLE). The performance of the theoretical models based on the classical MD and MC simulations was compared with the experimental data and also with the alternative computational techniques: a conventional approach based on the Taylor expansion of potential energy surface, path-integral MD and MD with quantum-thermal bath (QTB) based on the generalized Langevin equation (GLE). A straightforward application of the classical simulations resulted, as expected, in poor accuracy of the calculated observables due to the complete neglect of quantum effects. However, the introduction of a posteriori quantum corrections significantly improved the situation. The application of these corrections for MD simulations of the systems with large-amplitude motions was demonstrated for chloromethylthiocyanate. The comparison of the theoretical vibrational spectra has revealed that the GLE thermostat used in this work is not applicable for this purpose. On the other hand, the NH chains yielded reasonably good results.

DESTRUCTION OF OIL HYDROCARBONS IN WATER SOLUTIONS WITH OXYGEN DIELECTRIC BARRIER DISCHARGE OF ATMOSPHERIC PRESSURE

This paper reports the experimental study of destruction kinetics of oil hydrocarbons (engine oil of the M8 trademark) dissolved in water by dielectric barrier discharge in oxygen at the atmospheric pressure. The range of initial concentrations of oil in the aqueous solution was (12- 91) mg/l. All experiments were carried out under the applied voltage of 7.9 kV and discharge current of 0.45 mA (rms values) with a frequency of 50 Hz. The inputted power was 0.35 W/cm3.  The gas flow rate was 1 l/min. It was shown that the kinetics of oil decomposition can be formally described by the pseudo-first order law on oil concentration. The effective rate constant for decomposition of petroleum products was 0.0016 s-1. The rate of destruction in the investigated concentration range of M8 was varied in the range (3.8 - 28.9) · 10-6 mol / (l·min). The oil decomposition degree achieved 80 %, and the decomposition energetic yield was 0.16 molecules per 100 eV of inputted energy. The kinetics of the formation of ozone and carbon dioxide was examined as well. The obtained experimental data allowed to determine the degree of completeness of oil oxidation, which at the maximum was 54% (for carbon dioxide), this results indicates the formation of other carbon-containing compounds. The dynamics of the change in the pH of the treated solution is revealed. Under the experimental conditions, the pH was reduced from 6 to 4. It was found that the concentration of ozone formed in the reactor was deficiency for oil oxidative degradation. This results confirms that in the system are formed other active particles involved in the destruction of petroleum products, such as the OH* radicals and atomic oxygen. The possible mechanism of degradation processes is discussed.Forcitation:Grinevich V.I., Rybkin V.V., Lyubimov V.A., Gushchin A.A. Destruction of oil hydrocarbons in water solutions with oxygen dielectric barrier discharge of atmospheric pressure. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 8. P. 20-27.

Franck-Condon Theory of Quantum Mechanochemistry

The spectroscopic Franck-Condon (FC) principle is extended to mechanochemistry. If the external force is applied rapidly (the sudden-force regime), then the transition between the potential energy surface and the force-modified potential energy surface is analogous to the optical electronic transition. Such a transition produces a nonequilibrium ensemble of vibrationally excited molecules. This excess of vibrational energy is another activation source in addition to the well-known reaction barrier modulation by the external force. In the same time, the nonequilibrium vibrational distribution implies nonstatistical kinetics of a mechanochemical transformation. Mechanochemical FC principle thus provides a conceptual picture for the sudden-force mechanochemistry and opens possibilities for quantitative calculations of the mechanochemical rates and mechanisms. Here we use it to compute the dissociation rates of a model diatomic molecule and to explain the selectivity in mechanochemical bond breaking in n-butane. The approach is predicted to be relevant for large-magnitude external forces, applied instantaneously. Otherwise, the excess vibrational energy will dissipate due to intramolecular vibrational redistribution and interaction with environment.

Gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene: cog-wheel-type vs. independent internal rotation and influence of dispersion interactions

The gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene (1,8-BTMSA) was determined by a combined gas electron diffraction (GED)/mass spectrometry (MS) experiment as well as by quantum-chemical calculations (QC). DFT and dispersion corrected DFT calculations (DFT-D3) predicted two slightly different structures for 1,8-BTMSA concerning the mutual orientation of the two -C-C[triple bond, length as m-dash]C-SiMe₃ units: away from one another or both bent to the same side. An attempt was made to distinguish these structures by GED structural analysis. To probe the structural rigidity, a set of Born-Oppenheimer molecular dynamics (BOMD) calculations has been performed at the DFT-D level. Vibrational corrections Δr = r_a - r_e were calculated by two BOMD approaches: a microcanonically (NVE) sampled ensemble of 20 trajectories (BOMD(NVE)) and a canonical (NVT) trajectory thermostated by the Noose-Hoover algorithm (BOMD(NVT)). In addition, the conventional approach with both, rectilinear and curvilinear approximations (SHRINK program), was also applied. Radial distribution curves obtained with models using both MD approaches provide a better description of the experimental data than those obtained using the rectilinear (SHRINK) approximation, while the curvilinear approach turned out to lead to physically inacceptable results. The electronic structure of 1,8-BTMSA was investigated in terms of an NBO analysis and was compared with that of the earlier studied 1,8-bis(phenylethynyl)anthracene. Theoretical and experimental results lead to the conclusion that the (trimethylsilyl)ethynyl (TMSE) groups in 1,8-BTMSA are neither restricted in rotation nor in bending at the temperature of the GED experiment.

Nuclear Quantum Effects on Aqueous Electron Attachment and Redox Properties

Nuclear quantum effects (NQEs) on the reduction and oxidation properties of small aqueous species (CO₂, HO₂, and O₂) are quantified and rationalized by first-principles molecular dynamics and thermodynamic integration. Vertical electron attachment, or electron affinity, and detachment energies (VEA and VDE) are strongly affected by NQEs, decreasing in absolute value by 0.3 eV going from a classical to a quantum description of the nuclei. The effect is attributed to NQEs that lessen the solvent response upon oxidation/reduction. The reduction of solvent reorganization energy is expected to be general for small solutes in water. In the thermodynamic integral that yields the free energy of oxidation/reduction, these large changes enter with opposite sign, and only a small net effect (0.1 eV) remains. This is not obvious for CO₂, where the integrand is strongly influenced by NQEs due to the onset of interaction of the reduced orbital with the conduction band of the liquid during thermodynamic integration. We conclude that NQEs might not have to be included in the computation of redox potentials, unless high accuracy is needed, but are important for VEA and VDE calculations.

The effect of molecular dynamics sampling on the calculated observable gas-phase structures

In this study, we compare the performance of various ab initio molecular dynamics (MD) sampling methods for the calculation of the observable vibrationally-averaged gas-phase structures of benzene, naphthalene and anthracene molecules. Nose-Hoover (NH), canonical and quantum generalized-Langevin-equation (GLE) thermostats as well as the a posteriori quantum correction to the classical trajectories have been tested and compared to the accurate path-integral molecular dynamics (PIMD), static anharmonic vibrational calculations as well as to the experimental gas electron diffraction data. Classical sampling methods neglecting quantum effects (NH and canonical GLE thermostats) dramatically underestimate vibrational amplitudes for the bonded atom pairs, both C-H and C-C, the resulting radial distribution functions exhibit nonphysically narrow peaks. This deficiency is almost completely removed by taking the quantum effects on the nuclei into account. The quantum GLE thermostat and a posteriori correction to the canonical GLE and NH thermostatted trajectories capture most vibrational quantum effects and closely reproduce computationally expensive PIMD and experimental radial distribution functions. These methods are both computationally feasible and accurate and are therefore recommended for calculations of the observable gas-phase structures. A good performance of the quantum GLE thermostat for the gas-phase calculations is encouraging since its parameters have been originally fitted for the condensed-phase calculations. Very accurate molecular structures can be predicted by combining the equilibrium geometry obtained at a high level of electronic structure theory with vibrational amplitudes and corrections calculated using MD driven by a lower level of electronic structure theory.

Mechanochemistry: the effect of dynamics

Dynamical effects on the mechanochemistry of linear alkane chains, mimicking polyethylene, are studied by means of molecular dynamics simulations. Butane and octane are studied using density-functional theory (DFT), whereas higher homologues are studied using a simple one-dimensional model in which the molecules are represented by a linear chain of Morse potentials (LCM). The application of a fixed external force to a thermodynamically pre-equilibrated molecule leads to a preference for cleavage of the terminal C-C bonds, whereas a sudden application of the force favors bond breaking in the central part of the chain. In all cases, transition-state theory predicts higher bond-breaking rates than found from the more realistic molecular dynamics simulations. The event of bond dissociation is related to dynamic states involving symmetric vibrational modes. Such modes do in general have lower frequencies of vibration than antisymmetric modes, which explains the deviation between the statistical theory and the dynamics simulations. The good qualitative agreement between the DFT and LCM models makes the latter a useful tool to investigate the mechanochemistry of long polymer chains.

The Dalton quantum chemistry program system

Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, Møller-Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

Insights into the dynamics of evaporation and proton migration in protonated water clusters from large-scale Born-Oppenheimer direct dynamics

Large-scale on-the-fly Born-Oppenheimer molecular dynamics simulations using recent advances in linear scaling electronic structure theory and trajectory integration techniques have been performed for protonated water clusters around the magic number (H(2)O)(n)H(+) , for n = 20 and 21. Besides demonstrating the feasibility and efficiency of the computational approach, the calculations reveal interesting dynamical details. Elimination of water molecules is found to be fast for both cluster sizes but rather insensitive to the initial geometry. The water molecules released acquire velocities compatible with thermal energies. The proton solvation shell changes between the well-known Eigen and Zundel motifs and is characterized by specific low-frequency vibrational modes, which have been quantified. The proton transfer mechanism largely resembles that of bulk water but one interesting variation was observed.

Surface oxidation of polyethylene using an atmospheric pressure glow discharge with liquid electrolyte cathode

This study investigated the action of an atmospheric pressure air glow discharge (APGD) with aqueous electrolyte cathode onto the surface of polyethylene (PE) films. Distilled water and aqueous solutions of KCl and HCl were utilized as a cathode. The surface properties of PE were characterized by contact angle measurement followed by surface free energy calculation, Fourier transform infrared by attenuated total reflectance (FTIR-ATR), and XPS. After treating the PE surface, we observed OH groups, CO groups in ester, ketone, and carboxyl groups, and CO groups in unsaturated ketones and aldehydes. For a treatment time of 20 min and a discharge current of 40 mA, atomic concentrations of O and N were 12% and 2%, respectively, under distilled water application. Modification processes were able to improve the surface free energy of PE.