Higher hydrogen fractions in dielectric polymers boost self-healing in electrical capacitors.

Electrical capacitors are omnipresent in modern electronic devices, in which they swiftly release large portions of energy on demand. The capacitors may suffer from arc discharges due to local structural heterogeneities in their components and inappropriate exploitation practices. High energies of the arc discharge are transferred as phonons to the electrode and dielectric film, which burn out locally. The dielectric breakdown takes place. The complete burnout leads to the isolation of the failed region and the capacitor's self-healing. The emerging soot can form a semiconducting channel and damage the capacitor. The efficiency of self-healing depends on the dielectric properties of the soot and its amount. We employ reactive molecular dynamics simulations to reveal the regularities of the high-temperature polymer destruction and record by-products emerging during this process. We found the formation of multiple volatile low-molecular compounds and contaminated quantum carbon dots (CQD) designated as soot. The percentage of carbon in soot is higher compared to the polymer. Furthermore, the CQD contains numerous unsaturated C-C bonds and aromatic C6-rings suggesting an enhanced electrical conductivity. The size of the CQD depends on the available volume, i.e., on the spatial scale of the dielectric breakdown. The elemental composition of the soot is unique for each polymer. Polypropylene undergoes the most efficient self-healing thanks to containing a large molar fraction of hydrogen atoms. The results are addressed to the experts in electrical engineering and polymer fine-tuning.

Chemical similarity of dialkyl carbonates and carbon dioxide opens an avenue for novel greenhouse gas scavengers: cheap recycling and low volatility via experiments and simulations.

Global warming linked to the industrial emissions of greenhouse gases may be the end of mankind unless it is adequately and timely handled. To prevent irreversible changes to the climate of the Earth, numerous research groups are striving to develop robust CO2 sorbents. Dialkyl carbonates (DACs) and CO2 exhibit obvious chemical similarities in their structure and properties. The degrees of oxidation of all atoms composing DACs and CO2 are identical resulting in very similar nucleophilicities and electrophilicities of all interaction centers. While both compounds possess relatively high partial atomic charges on their polar moieties, the molecular geometries prevent tight binding of the head groups. The computed DAC-DAC binding energies are ∼40 kJ mol-1, whereas the effect of the alkyl chain length is marginal. The phase transition points and shear viscosities of DACs are very low. We herein hypothesize and numerically rationalize that DACs represent noteworthy physical sorbents for CO2 thanks to the similar sorbent-CO2 and sorbent-sorbent interaction energies. By reporting in silico-derived sorption thermodynamics at various conditions, spectral and structural properties, and experimentally derived CO2 capacities and recyclabilities, we highlight the mutual affinity of DACs and CO2. Indeed, the experimentally determined CO2 sorption capacity of 0.88 mol% (diethyl carbonate) at 278.15 K and 30 bar is competitive. The unprecedentedly low DAC-CO2 binding energies, ∼14 kJ mol-1, suggest a low-cost desorption process and outstanding recyclability of the sorbent. We also note that DACs possessing long alkyl chains (butyl, hexyl, octyl) exhibit negligible volatilities, while preserving the liquid aggregate state over a practically important temperature range. The reported results may foster the development of a new class of CO2 scavengers with possibly quite peculiar characteristics.

Extensively amino-functionalized graphene captures carbon dioxide.

The development of robust carbon dioxide (CO2) scavengers is a challenging but paramount problem of modern humanity. In the present work, we report a prospective CO2 sorbent based on amino-functionalized graphene (FG). Amino-FG retains the favorable physicochemical properties of graphene and acquires the capability of chemically fixing CO2via the carbamic acid formation mechanism. In the present work, we comprehensively investigate CO2 capturing prospects by extensively amino-FG using hybrid density functional theory. We show that up to six amino groups can be grafted, remain stable, and subsequently chemisorb CO2 per benzene ring. Two functional groups above the benzene ring and four such groups below the benzene ring represent a thermodynamically stable molecular configuration in which the number of carbon atoms is equal to the number of functional groups. The thermochemistry of chemisorption is, in general, negatively impacted by the increase in the density of functional groups. However, a less favorable Gibbs free energy is compensated by a several fold higher number of prospective reaction sites. The thermochemistry results are rationalized by considering steric hindrances on the surface of graphene in the context of the states of hybridization and genuine geometries of the amino- and carboxamido functional groups. The functionalization and chemisorption decrease the hydrophobicity of graphene derivatives and, therefore, foster the development of novel and more robust chemical engineering setups.

Carbon Dioxide Chemisorption by Ammonium and Phosphonium Ionic Liquids: Quantum Chemistry Calculations.

Carbon capture and storage is an important technological endeavor aiming to improve the ecology by combating global warming. The present work investigates reaction paths that are responsible for CO2 chemisorption by the ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic anion 2-cyanopyrrolide. We exemplify that 2 mol of CO2 per 1 mol of the gas scavenger can be theoretically fixed by such ionic liquids. Both the cation and anion participate in the chemisorption. The corresponding standard enthalpies and potential energies are moderately negative. The chemisorption reaction, as revealed by the simulations of competing pathways, is started by the donation of the proton from the cation to the anion. The double covalent bond in the cation's structure emerges. The barriers to all reactions involving the phosphonium-based cation are relatively small and favor practical applications of the considered sorbents. The performance of the ammonium-based cation is less favorable due to the inherent instability of the tetraalkylammonium ylide. The role of phosphonium ylide in the mechanism of the reaction is carefully characterized. The performance of the aprotic anion as a CO2 scavenger is unaffected by the chemical identity of the counterion. The essential heights of the identified steric barriers underline the necessity to simulate the entire structures of the reacting species to obtain a reliable description of chemisorption. The reported results foster a fundamental understanding of the outstanding CO2 sorption performance of the quaternary ammonium- and phosphonium-based 2-cyanopyrrolides.

Triethylsulfonium-based ionic liquids enforce lithium salt electrolytes.

The demand for cheap production of energy and its efficient storage is huge nowadays. Sulfonium-based ionic liquids have exhibited a useful set of physical-chemical and electrochemical properties, which make them good prospective electrolytes for electrochemical double-layer capacitors and rechargeable lithium batteries. The ability of the researchers to correctly describe local ionic structural patterns in the electrochemical systems is a cornerstone of achieving sustainable progress in this field. Herein, we report an in silico investigation of a few lithium-triethylsulfonium electrolytes and correlate our results with the recently published electrochemical study. All chosen organic and inorganic anions have been recently used in the supercapacitor and lithium-battery electrolyte systems: bis(trifluoromethylsulfonyl)imide, perchlorate, hexafluorophosphate, and trifluoromethanesulfonate. Analyzing potential energy surfaces, ion-ionic coordination, electron density distributions, and structure properties, we identified that the best-performing electrolyte system is lithium bis(trifluoromethylsulfonyl)imide dissolved in triethylsulfonium bis(trifluoromethylsulfonyl)imide. In the mentioned system, we found the weakest cation-anion binding that resulted in the fastest ionic transport. The lithium-ion plays a paramount role in the coordination of all investigated anions, whereas the impact of the triethylsulfonium cation is relatively insignificant. The lithium-induced structural changes in the local order of the electrolyte are reflected by computed vibrational spectra. In the lithium-free systems, the anions strongly bind the triethylsulfonium cation via its electron-deficient α-methyl groups. Some of these electrostatically driven interactions may be classified as medium-strength hydrogen bonds. The computed cohesion energies explain the conductivity and viscosity trends obtained for similar electrolyte compositions in the recent experiments. The reported results will be interesting for researchers who develop Li-based energy storage devices that use room-temperature ionic liquids as non-volatile and electrochemically stable media.

Ammonium-, phosphonium- and sulfonium-based 2-cyanopyrrolidine ionic liquids for carbon dioxide fixation.

The development of carbon dioxide (CO2) scavengers is an acute problem nowadays because of the global warming problem. Many groups around the globe intensively develop new greenhouse gas scavengers. Room-temperature ionic liquids (RTILs) are seen as a proper starting point to synthesize more environmentally friendly and high-performance sorbents. Aprotic heterocyclic anions (AHA) represent excellent agents for carbon capture and storage technologies. In the present work, we investigate RTILs in which both the weakly coordinating cation and AHA bind CO2. The ammonium-, phosphonium-, and sulfonium-based 2-cyanopyrrolidines were investigated using the state-of-the-art method to describe the thermochemistry of the CO2 fixation reactions. The infrared spectra and electronic and structural properties were simulated at the hybrid density functional level of theory to characterize the reactants and products of the chemisorption reactions. We conclude that the proposed CO2 capturing mechanism is thermodynamically allowed and discuss the difference between different families of RTILs. Quite unusually, the intramolecular electrostatic attraction plays an essential role in stabilizing the zwitterionic products of the CO2 chemisorption. The difference in chemisorption performance between the families of RTILs is linked to sterical hindrances and nucleophilicities of the α- and ß-carbon atoms of the aprotic cations. Our results rationalize previous experimental CO2 sorption measurements (Brennecke et al., 2021).

Binary mixtures of novel sulfoxides and water: intermolecular structure, dynamic properties, thermodynamics, and cluster analysis.

Higher molecular weight dialkyl sulfoxides attract interest in the context of biomedical sciences due to their ability to penetrate phospholipid bilayers, dissolve drugs, and serve as cryoprotectants. Intermolecular interactions with water, a paramount component of the living cell, determine the performance of the sulfoxide-based artificial systems in their prospective applications. Herein, we simulated a wide composition range of sulfoxide/water mixtures, up to 85 w/w% sulfoxide, using classical molecular dynamics to determine structure, dynamics, and thermodynamics as a function of the mixture composition. As found, both diethyl sulfoxide (DESO) and ethyl methyl sulfoxide (EMSO) are strongly miscible with water. DESO- and EMSO-based aqueous mixtures exhibit similar structure and thermodynamic properties, however, quite different dynamic properties over the entire range of compositions. Strong deviations from an ideal mixture of between 30-50 mol% (based on molar volume) of sulfoxide content lead to relatively high dynamic viscosities of the mixtures. The free energy of mixing with water is only slightly more favorable for EMSO than for DESO. The results, for the first time, quantify high miscibilities of both sulfoxides with water and motivate comprehensive in vivo investigation of the proposed mixtures.

Hydration peculiarities of graphene oxides with multiple oxidation degrees.

Hydration properties of graphene oxide (GO) are essential for most of its potential applications. In this work, we employ atomistic molecular dynamics simulations to investigate seven GO compositions with different levels of oxygenation. Two atomic charge models for GO are compared: (1, a simplified model) sp2 carbons are purely Lennard-Jones sites; (2, a CHELPG model) sp2 carbon charges are consistent with the CHELPG scheme. Structural properties were found to depend insignificantly on the charge model, whereas thermodynamics appeared very sensitive. In particular, the simplified model provides systematically stronger GO/water coupling, as compared to the more accurate model. For all GO compositions, hydration free energies are in the range of -5 to -45 kJ mol-1 indicating that hydration is thermodynamically favourable even for modest oxidation degrees, thus differing drastically from the cases of pristine graphene and graphite. In general, it has been observed that as R increases the high oxidation degree obstructs the formation of new hydrogen bonds, which considerably affects their hydration properties. Although both the used charge models are qualitatively equivalent, the energy and number of hydrogen bonds have been shown to be sensitive to the charge set employed. In particular, the comparison shows that the simplified model tends to overestimate the GO/water interaction energy. The results and discussion presented herein provide a physical background for modern applications of GO, e.g. in electrodes of supercapacitors and inhibitors in processes involving biological molecules.

Imidazolium Ionic Liquid Mediates Black Phosphorus Exfoliation while Preventing Phosphorene Decomposition.

Forthcoming applications in electronics and optoelectronics make phosphorene a subject of vigorous research efforts. Solvent-assisted exfoliation of phosphorene promises affordable delivery in industrial quantities for future applications. We demonstrate, using equilibrium, steered and umbrella sampling molecular dynamics, that the 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] ionic liquid is an excellent solvent for phosphorene exfoliation. The presence of both hydrophobic and hydrophilic moieties, as well as substantial shear viscosity, allows [EMIM][BF4] simultaneously to facilitate separation of phosphorene sheets and to protect them from getting in direct contact with moisture and oxygen. The exfoliation thermodynamics is moderately unfavorable, which indicates that an external stimulus is necessary. Unexpectedly, [EMIM][BF4] does not coordinates phosphorene by π-electron stacking with the imidazole ring. Instead, the solvation proceeds via hydrophobic side chains, while polar imidazole rings form an electrostatically stabilized protective layer. The simulations suggest that further efforts in solvent engineering for phosphorene exfoliation should concentrate on use of weakly coordinating ions and grafting groups that promote stronger dispersion interactions and on elongation of nonpolar chains.

Microwave reduction of graphene oxide rationalized by reactive molecular dynamics.

Obtaining graphene (GRA) in industrial quantities is among the most urgent goals in today's nanotechnology. Elegant methods involve the oxidation of graphite with its subsequent solvent-assisted exfoliation. The reduction of graphene oxide (GO) is challenging leading to a highly-disordered oxygen-rich material. A particularly successful microwave-induced reduction of GO was reported recently (Science, 2016, 353, 1413-1416). We mimic the experiment by reactive molecular dynamics and establish the molecular mechanisms of reduction and their time scales as functions of temperature. We show that the rapid removal of oxygen groups achieved by microwave heating leaves GRA sheets intact. The epoxy groups are most stable within GO. They can rearrange into the carbonyl groups upon quick heating. It is important to avoid creating holes upon graphite oxidation. They cannot be healed easily and undermine GRA thermal stability and electronic properties. The edge oxygen groups cannot be removed by irradiation, but their effect is marginal on the properties of µm GRA sheets. We demonstrate that different oxygen groups are removed from GO at drastically different temperatures. Therefore, it is possible to obtain separate fractions, e.g. carbonyl-, hydroxyl- and carboxyl-free partially reduced GO. Our results guide the improvement of the GO reduction methods and can be tested directly by experiment.

Understanding weakly coordinating anions: tetrakis(pentafluorophenyl)borate paired with inorganic and organic cations.

Efficient design of ionic compounds requires a systematic understanding of cation-anion interactions. Weakening of electrostatic attraction is essential to increase the liquid range of the ionic compound and decrease its melting point. Here, we report simulations of the closest-approach cation-anion distances in a variety of ion pairs containing the tetrakis(pentafluorophenyl)borate (TFPB-) anion. Small alkali cations (Li+, Na+) penetrate the TFPB- core, whereas K+ and larger organic cations do not. In the latter case, the shortest possible distance from the cations to the boron atom of TFPB- ranges from 0.50 nm to 0.63 nm. TFPB- was shown to be substantially rigid, providing a steric hindrance to thermodynamically efficient cation-anion coordination. Our results prove that TFPB- is more efficient for electrostatic charge confinement than the tetraoctylammonium cation, whereas the perfluorophenyl group is more efficient than linear alkyl chains. These simulations will motivate development of TFPB--based ionic liquids with low phase transition points. Graphical Abstract Ionic configuration of the equilibrated "TFPB + K"system.

Solubility origin at the nanoscale: enthalpic and entropic contributions in polar and nonpolar environments.

Nanostructures are known to be poorly soluble, irrespective of their elemental composition, shape, electronic structure, dipole moment, hydrophobicity/hydrophilicity and the employed solvent. The methods of colloid chemistry allow for preparing suspensions - metastable systems, the stabilities of which differ greatly from one another - but not real solutions. A systematic investigation of the solubility origin at the nanoscale is hereby reported in terms of its fundamental constituents: enthalpy and entropy. Slightly different one-dimensional solutes - narrow carbon nanotubes (CNTs) of different lengths - were considered in hydrophilic (water) and hydrophobic (benzene) environments. We decompose the process of solvation into the solid → gas transition (sublimation) and the gas → liquid transition (condensation). Sublimation is a thermodynamically unfavorable process under room conditions, while the condensation transition depends on the solvent-solute interactions (enthalpic contribution). Unlike solvation of small molecules, solvation of the nanostructures results in a significant alteration of entropy. This alteration is proportional to the linear dimensions of the nanostructure. If the solvent exhibits peculiar solvent-solvent interactions (such as hydrogen bonding in water), solvation is entropically forbidden, irrespective of the solute nature and its nanoscale dimensions. In the case of the hydrophobic solvent (benzene), the condensation transition can be both enthalpically and entropically favorable. The free energy of solvation is in direct proportion to the CNT length. While highlighting principal difficulties in solvating nanostructures, this paper discusses an optimal choice of solvents for solutes exhibiting hydrophobic and hydrophilic interactions with their environments. Our results allow us to predict the solvation of an arbitrary nanostructure using its small, about 2 nm, atomistic model.

Laser-Induced Explosion of Nitrated Carbon Nanotubes: Nonadiabatic and Reactive Molecular Dynamics Simulations.

Laser-initiated decomposition of carbon nanotubes (CNTs) can lead to medical, military, and other applications. In medicine, CNTs give rise to efficient remedies against diseases and malignant cells, since they encapsulate drug molecules, can be delivered inside living organisms, and absorb light that penetrates through biological tissues. As explosives, pyrotechnics, and propellants, CNTs can be activated remotely by a visible or infrared laser, avoiding the need for a detonating cord. The reported non-equilibrium investigation demonstrates the possibility of photoinduced polynitro-CNT explosion and provides a detailed chemical mechanism of the decomposition process, explicitly in the time domain. Nonadiabatic molecular dynamics (MD) performed with real-time time-dependent tight-binding density functional theory demonstrates that the photogenerated exciton deposits its energy into a broad range of phonon modes within less than a picosecond, resulting in a rapid polynitro-CNT heating. Following the heating, reactive MD demonstrates an explosion, during which the local temperature of polynitro-CNTs and its fragments rises as high as 4000 K. Photoexcitation of nitro groups by a high-energy laser is not required; the energy can be delivered to polynitro-CNTs using near-infrared light within the biological window. Furthermore, the explosion is possible both with and without an external oxygen source. Anaerobic explosion could be particularly beneficial in confined biological and nanoscale environments. The products of the polynitro-CNT decomposition are nontoxic: carbon dioxide and molecular nitrogen. The in silico demonstration of the laser-induced polynitro-CNT explosion, its chemical mechanism, and the time scales of physical and chemical transformations can be tested experimentally using time-resolved laser techniques.

Protein remains stable at unusually high temperatures when solvated in aqueous mixtures of amino acid based ionic liquids.

Using molecular dynamics simulations, we investigated the thermal stability and real-time denaturation of a model mini-protein in four solvents: (1) water, (2) 1-ethyl-3-methylimidazolium alaninate [EMIM][ALA] (5 mol% in water), (3) methioninate [EMIM][MET] (5 mol% in water), and (4) tryptophanate [EMIM][TRP] (5 mol% in water). Upon analyzing the radius of gyration, the solvent-accessible surface area, root-mean-squared deviations, and inter- and intramolecular hydrogen bonds, we found that the mini-protein remains stable at 30-40 K higher temperatures in aqueous amino acid based ionic liquids (AAILs) than in water. This thermal stability was correlated with the thermodynamics and shear viscosity of the AAIL-containing mixtures. These results suggest that AAILs are generally favorable for protein conservation. Graphical Abstract Conformation of the [TRP]-cage mini-protein in an aqueous amino acid based ionic liquid (AAIL).

Atomically precise understanding of nanofluids: nanodiamonds and carbon nanotubes in ionic liquids.

A nanofluid (NF) is composed of a base liquid and suspended nanoparticles (NPs). High-performance NFs exhibit significantly better heat conductivities, as compared to their base liquids. In the present work, we applied all-atom molecular dynamics (MD) simulations to characterize diffusive and ballistic energy transfer mechanisms within nanodiamonds (NDs), carbon nanotubes (CNTs), and N-butylpyridinium tetrafluoroborate ionic liquid (IL). We showed that heat transfer within both NDs and CNTs is orders of magnitude faster than that in the surrounding IL, whereas diffusion of all particles in the considered NF is similar. Intramolecular heat transfer in NPs is a key factor determining the difference of NFs from base liquids. Solvation free energy of NDs and CNTs in ILs was estimated from MD simulations. The geometric dimensions of NPs were shown to be a major source of entropic penalty. Temperature adjusts the entropic factor substantially by modifying a genuine local structure of the bulk base liquid. Our work contributes to engineering more stable and productive suspensions of NPs in ILs, which are necessary for essential progress in the field of NFs.

Boron doping of graphene-pushing the limit.

Boron-doped derivatives of graphene have been intensely investigated because of their electronic and catalytic properties. The maximum experimentally observed concentration of boron atoms in graphite was 2.35% at 2350 K. By employing quantum chemistry coupled with molecular dynamics, we identified the theoretical doping limit for single-layer graphene at different temperatures, demonstrating that it is possible to achieve much higher boron doping concentrations. According to the calculations, 33.3 mol% of boron does not significantly undermine thermal stability, whereas 50 mol% of boron results in critical backbone deformations, which occur when three or more boron atoms enter the same six-member ring. Even though boron is less electro-negative than carbon, it tends to act as an electron acceptor in the vicinity of C-B bonds. The dipole moment of B-doped graphene depends strongly on the distribution of dopant atoms within the sheet. Compared with N-doped graphene, the dopant-dopant bonds are less destructive in the present system. The reported results motivate efforts to synthesize highly B-doped graphene for semiconductor and catalytic applications. The theoretical predictions can be validated through direct chemical synthesis.

Peculiar Aqueous Solubility Trend in Cucurbiturils Unraveled by Atomistic Simulations.

Cucurbiturils (CBs) compose a family of macrocycles whose elementary unit is glycouril (GLYC). CBs are of high interest in chemistry and biology due to their versatile applications, ranging from sensors to advanced drug-delivery systems. Here, we report a systematic hydration study of all currently known CBs by classical molecular dynamics simulations to understand their different aqueous solubilities, as revealed in the experiments. Water readily penetrates CBs, including the smallest CB, that is, CB[5]. The number of CB[n]-water hydrogen bonds can be assessed as 2 × n. The hydration enthalpies of the CBs were found to be significantly favorable, due to a number of strong hydrogen bonds with water. However, these enthalpy gains are not enough to compensate for an even larger entropic penalty due to modifying a genuine bulk arrangement of water molecules. We found that the free energy of hydration moderately but uniformly increases with the number of GLYCs. Therefore, the better solubility of odd-numbered CBs is independent of the CB-water interactions, either an enthalpic or entropic contribution. The higher solubilities of CB[n]s with n = 5, 7, or 9 occur exclusively because of their amorphous solid states. Our results allow the prognosis of the aqueous solubilities of not-yet-synthesized CBs.

Free energy of solvation of carbon nanotubes in pyridinium-based ionic liquids.

Numerous prospective applications require the availability of individual carbon nanotubes (CNTs). Pristine CNTs, strongly hydrophobic in nature, are known to be either totally insoluble or poorly dispersible. While it is unlikely to be possible to prepare a real solution of CNTs in any solvent, the ability of certain solvents to maintain dispersions of CNTs for macroscopic times constitutes great research interest. In the present work, we characterize two pyridinium-based ionic liquids (ILs), N-butylpyridinium chloride [BPY][Cl] and N-butylpyridinium bis(trifluoromethanesulfonyl)imide [BPY][TFSI], with respect to their potential to solvate CNTs of different diameters, from the CNT (10,10) to the CNT (25,25). Using a universal methodology, we found that both ILs exhibit essentially the same solvation performance. Solvation of CNTs is strongly prohibited entropically, whereas the energy penalty increases monotonically with the CNT diameter. Weak van der Waals interactions, which guide enthalpy alteration upon the CNT solvation, are unable to compensate for the large entropic penalty from the destruction of the IL-IL electrostatic interactions. The structure of ILs inside and outside CNTs is also discussed. The reported results are necessary for gaining a fundamental understanding of the CNT solvation problems, thereby inspiring the search for more suitable solvents.

Sodium-ion electrolytes based on ionic liquids: a role of cation-anion hydrogen bonding.

Recent success of the sodium-ion batteries fosters an academic interest for their investigation. Room-temperature ionic liquids (RTILs) constitute universal solvents providing non-volatility and non-flammability to electrolytes. In the present work, we consider four families of RTILs as prospective solvents for NaBF4 and NaNO3 with an inorganic salt concentration of 25 and 50 mol%. We propose a methodology to rate RTILs according to their solvation capability using parameters of the computed radial distribution functions. Hydrogen bonds between the cations and the anions of RTILs were found to indirectly favor sodium solvation, irrespective of the particular RTIL and its concentration. The best performance was recorded in the case of cholinium nitrate. The reported observations and correlations of ionic structures and properties offer important assistance to an emerging field of sodium-ion batteries. Graphical Abstract Sodium-ion electrolytes.

Ab Initio Molecular Dynamics of Dimerization and Clustering in Alkali Metal Vapors.

Alkali metals are known to form dimers, trimers, and tetramers in their vapors. The mechanism and regularities of this phenomenon characterize the chemical behavior of the first group elements. We report ab initio molecular dynamics (AIMD) simulations of the alkali metal vapors and characterize their structural properties, including radial distribution functions and atomic cluster size distributions. AIMD confirms formation of Men, where n ranges from 2 to 4. High pressure sharply favors larger structures, whereas high temperature decreases their fraction. Heavier alkali metals maintain somewhat larger fractions of Me2, Me3, and Me4, relative to isolated atoms. A single atom is the most frequently observed structure in vapors, irrespective of the element and temperature. Due to technical difficulties of working with high temperatures and pressures in experiments, AIMD is the most affordable method of research. It provides valuable understanding of the chemical behavior of Li, Na, K, Rb, and Cs, which can lead to development of new chemical reactions involving these metals.