Reshaping two-dimensional MoS2 for superior magnesium-ion battery anodes.

Few-atom-thick two-dimensional (2D) molybdenum disulfide (MoS2) monolayers possess numerous crucial applications in energy storage. Usually, the strategy of activating interfacial electron transfer was employed to promote their performance. Herein, we reshape the structure of materials to excite their subinterfacial and interfacial electron transfer for superior metal-ion batteries. As an example, we rationally design and reconfigure the structure of 2D MoS2 and propose a new stable structure, B-MoS2, which has an S-Mo-S sandwich structure with a buckled square lattice. The B-MoS2 monolayer is a promising anode material for magnesium-ion batteries (MgIBs) with a high capacity (921.3 mA h g-1) and a low averaged open circuit voltage (0.154 V). Multiscale underlying mechanisms for the storage of Mg and Li ions in MoS2 are provided. Based on the electronic level, the high capacity is ascribed to the occurrence of interfacial and subinterfacial electron transfer between metal ions and B-MoS2. Based on the atomic level, the insertion-adsorption mechanism or adsorption-insertion mechanism is determined for different ion storage at B-MoS2. The intrinsic metallic property of B-MoS2 and the enhanced electronic conductivity of Mg/B-MoS2 systems as well as low migration barriers (∼0.604 eV) of Mg ions at MoS2 suggest that the B-MoS2 anode has fast charge/discharge rates. This work offers novel concepts (i.e. subinterfacial electron transfer and its activation) for superior energy storage materials, and proposes new multiscale underlying mechanisms for ion storage in the MoS2 family.

Effect of chemical aging of aqueous organic aerosols on the rate of their steady-state nucleation.

We present the steady-state solution of the kinetic equation for the size and composition distribution of an ensemble of aqueous organic droplets, evolving via nucleation and concomitant chemical aging. The partial differential equation of second order for the temporal evolution of this distribution can be reduced to the canonical form of the multidimensional Fokker-Planck equation, which can be solved analytically by using the method of complete separation of variables. Its solution for the steady-state process provides the stationary distribution of droplets in the vicinity of the saddle point of the free-energy surface as well as the stationary nucleation rate in the form of the product "kinetic (Zeldovich) factor × normalization factor × exp(-free energy of nucleus formation)". Our numerical evaluations for the formation of aqueous organic aerosols in the air containing the vapors of water, 2-methylglyceric acid, and 3-methyl-4 -hydroxy-benzoic acid, as well as typical atmospheric gaseous species, indicate that the steady-state nucleation rate of such aerosols can be significantly enhanced by their concomitant chemical aging. Thus, one can expect that the application of our approach to the formation and evolution of atmospheric aqueous organic aerosols (via concurrent nucleation and chemical aging) will make aerosol models more adequate and may, once implemented in climate models, improve their forecasting accuracy.

Kinetic equation of concurrent nucleation and chemical aging of an ensemble of aqueous organic aerosols.

Using the formalism of classical nucleation theory, we derive a kinetic equation for the size and composition distribution of an ensemble of aqueous organic droplets evolving via nucleation and concomitant chemical aging. This distribution can be drastically affected by the enthalpy of heterogeneous chemical reactions and the depletion of organic trace gases absorbed by aerosols. A partial differential equation of second order for the temporal evolution of this distribution is obtained from the discrete equation of balance via Taylor series expansions. Once reduced to the canonical form of the multidimensional Fokker-Planck equation, this kinetic equation can be solved via the method of complete separation of variables. This kinetic equation opens a new direction in the development of the kinetic theory of first-order phase transitions, while its applications to the formation and evolution atmospheric organic aerosols via concurrent nucleation and chemical aging may drastically improve the accuracy of global climate models.

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH3 Radicals.

Lignin is the most complex component of biomass, and development of a detailed chemical kinetic model for biomass pyrolysis mainly relies on the understanding of the lignin decomposition kinetics. para-Coumaryl alcohol (p-CMA, HOPh-CH═CH-CH2OH), the focus of our analysis, is the simplest of the lignin monomers (monolignols) containing a typical side-chain double bond and both alkyl- and phenolic-type OH-groups. In parts I and II of our work (Asatryan, R. J. Phys. Chem. A 2019, 123, 2570-2585; Hudzik, J. M. J. Phys. Chem. A 2020, current issue), we created a detailed potential energy surface (PES) and performed a kinetic analysis of chemically activated, unimolecular, and bimolecular reactions pathways for p-CMA + OH. Reaction pathways analyzed include dissociation, intramolecular abstraction, group transfer, and elimination processes. The α- and ß-carbon addition reactions generate 1,3- (RA1) and 1,2-diol (RB1) adduct radicals, respectively. Well depths are approximately 29 and 41 kcal/mol below the p-CMA + OH entrance level. Kinetic analysis aides in determining the major pathways for our conventional and fractional pyrolysis experiments. The current paper focuses on the H-abstraction reactions via H, OH, and CH3 light ("pool") radicals from p-CMA. The thermochemical properties of all stable, radical, and transition-state species were determined using the ωB97XD density functional theory (DFT) and higher-level CBS-QB3 composite methods. Barrier heights from the prereaction complexes, for OH-radical abstractions, to the transition states for the propanoid side chain are compared to the model H-abstraction reactions of allyl alcohol (AA) with OH and p-CMA with H and CH3 radicals. The lowest-energy, most stable, p-CMA radical formed is at the C9 allylic position (p-CMA-C9j) with exothermicity of 26.63, 41.32, and 27.34 kcal/mol for H, OH, and CH3, respectively. For OH-radical abstraction at this position, our findings are consistent with corresponding data on AA + OH at 37.44 kcal/mol and similar to that of RB1. A similar stable radical with an exothermicity of 34.95 kcal/mol occurs for the phenol hydroxyl group, generating the p-CMA-O4j radical. H-abstraction pathways are considered in relation to other major pathways previously considered for p-CMA + OH reactions including H-atom shifts, dehydration, and ß-scission reactions. Derived rate coefficients for substituted phenols can be utilized in detailed kinetic models for lignin/biomass pyrolysis.

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis.

Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semidetailed kinetic models, however, are lacking the comprehensive decomposition kinetics of these key intermediates in order to advance toward the fundamentally based detailed chemical-kinetic models of biomass pyrolysis. para-Coumaryl alcohol (HOPh-CH═CH-CH2OH, p-CMA) is the simplest of the three basic monolignols containing a typical side-chain double bond and both alkyl and phenolic type OH groups. The two other monomers additionally contain one and two methoxy groups, respectively, attached to the benzene ring. Previously, we developed a detailed fundamentally based mechanism for unimolecular decomposition of p-CMA (as well as its truncated allyl and cinnamyl alcohol models) and explored its reactivity toward H radicals generated during pyrolysis. The reactions of p-CMA with pyrolytic OH radicals is another set of key reactions particularly important for understanding the formation mechanisms of a wide variety of oxygenates in oxygen-deficit (anaerobic) conditions and the role of the lignin side groups in pyrolysis pathways. In Part I of the current study (J. Phys. Chem. A, 2019, 123, 2570-2585), we reported a detailed potential energy (enthalpy) surface analysis of the reaction OH + p-CMA with suggestions for a variety of chemically activated, unimolecular, and bimolecular reaction pathways. In Part II of our work, we provide a detailed kinetic analysis of the major reaction channels to evaluate their significance and possible impacts on product distributions. Temperature- and pressure-dependent rate constants are calculated using the quantum Rice-Ramsperger-Kassel method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using density functional theory and higher-level composite methods for stable molecules, radicals, and transition-state species. A significant difference between well depths for the chemically activated adduct radicals, [p-CMA-OH]*, is found for the α- and ß-carbon addition reactions to generate the 1,3- and 1,2-diol radicals, respectively. This is due to the synergistic effect from conjugation of the proximal radical center with the aromatic ring and the strong H-bonding interaction between vicinal OH groups in the ß-adduct (1,2-diol radical). Both adducts undergo isomerization and low-energy transformations, however, with different kinetic efficiencies because of the difference in stabilization energies. Reaction pathways include dissociation, intramolecular abstraction, atom and group transfers, and elimination. Of particular interest is a roaming-like low-energy dehydration reaction to form O-centered intermediate radicals. The kinetic analysis demonstrated the feasible formation of various products detected in pyrolysis experiments, suggesting that the gas-phase reactions of OH radicals can be a key process to form major products and complex oxygenates during lignin pyrolysis. Our preliminary experiments involving pyrolysis of the vaporized monomers support this basic statement. A novel mechanism for the formation of benzofuran, identified in experimentation, is also provided based on the potential conversions of hydroxyphenylacetaldehyde and corresponding isomers, which are kinetically favored products.

Bond Number Revisited: Axisymmetric Macroscopic Pendant Drop.

The numbers Rbθ and Rbr introduced recently (Berim, G.O.; Ruckenstein, E. J. Phys. Chem. 2019, 123, 10294-10300) to characterize the cylindrical pendant drops hanging from the solid surface are modified for characterization of axisymmetric pendant drops which are more common in experiment. Similar to the case of cylindrical drops, Rbθ and Rbr are defined on the basis of rigorous solution of Young-Laplace equation for the drop profile. As a consequence, they contain only the input parameters of the drop-solid system such as the volume of the drop, gravitational acceleration, fluid densities, surface tension, and contact angle. This constitutes the main difference between Rb numbers and traditional Bond and Bond-like numbers which involve the dimensions of the drop unknown prior to the experiment (e.g., height of the drop or radius of curvature at the drop apex). It is shown, that the new numbers can be used, for example, for predicting the breakup conditions of the drop, determining the surface tension, and estimating the deviation of drop shape from the circular one. A good agreement between theoretical predictions and experimental results known from the literature was demonstrated.

Comment on "Dry reforming of methane by stable Ni-Mo nanocatalysts on single-crystalline MgO".

Song et al (Reports, 14 February 2020, p. 777) ignore the reported efficient Ni/MgO solid-solution catalysts and overstate the novelty and importance of the Mo-doped Ni/MgO catalysts for the dry reforming of methane. We show that the Ni/MgO solid-solution catalyst that we reported in 1995, which is efficient and stable for the dry reforming, is superior to the Mo-doped Ni/MgO catalyst.

Bond Number Revisited: Two-Dimensional Macroscopic Pendant Drop.

The definition of the Bond number, which is widely used for the qualitative description of the shape and stability of a macroscopic pendant drop, is revisited. Two new numbers, named Rbθ and Rbλ, are introduced for two kinds of two-dimensional pendant drops, which have a constant contact angle and a constant width of drop-solid contact, respectively. These numbers contain only input parameters such as the volume of the drop, gravitational acceleration, fluid densities, surface tensions, and contact angles. This distinguishes them from the traditional Bond number, which involves, in addition to the input parameters, prior unknown experimentally derived characteristics of the drop, e.g., height of the drop or radius of the curvature at the drop apex. It is shown that, when properly defined, the new numbers can be used, in particular, for predicting the breakup conditions of the drop and estimating their shape.

Mechanical deformation: A feasible route for reconfiguration of inner interfaces to modulate the high performance of three-dimensional porous carbon material anodes in stretchable lithium-Ion batteries.

The rapid development of stretchable electronics, which have wide applications from clinical applications to stretchable smart phones, requires numerous advanced stretchable energy technologies, such as stretchable batteries. However, maintaining performance in such batteries during deformation and developing stretchable batteries with suitable mechanical robustness for industrial applications remain challenges. In this work, by using first-principles calculations, the performance of three-dimensional (3D) topological semimetal porous carbon material bct-C40 anodes in stretchable lithium-ion batteries (LIBs) is investigated. We find that the mechanical deformation is a feasible route for reconfiguration of inner surfaces of porous carbon material anodes to modulate their high performance in stretchable LIBs. The bct-C40 anode delivers a high theoretical capacity of 893 mA h/g, which is approximately 2.4 times larger than that of the commercial graphite anode (372 mA h/g). Adsorption-activation-adsorption mechanism and (de)activation-adsorption mechanism are proposed for the capacities of the anode under strain-free and strained states, respectively. Under the strain-free state, the adsorption of Li atoms changes the size of porous of bct-C40 at the atomic scale and readjusts the electron distribution on bct-C40 at the electronic scale, activating more adsorption sites. Large tensile strains expand its inner space and inner surface area, forming new adsorption sites and boosting its high capacities. Large compressive strains undermine its inner surface and deactivate some adsorption sites, reducing its capacities. Small compressive and tensile strains play a little role in the inner surface and do not affect adsorption sites, retaining its high capacities. More excitingly, diffusion barriers under strain-free and strained states, which are sensitive to the inner surface, are (ultra)low, demonstrating that the anode has (ultra)fast charge/discharge rates. This work provides new insights for the modulatable performance of 3D porous carbon material anodes, and offers an approach to innovate high performance stretchable metal-ion battery anodes with suitable mechanical robustness.

An analog to Bond number for pendant nanodrops.

It is argued that the Bond number, which is widely used for the qualitative description of the shape and stability of a pendant macroscopic drop, is not appropriate to characterize a nanodrop. The new number, called Rb, is introduced in a heuristic manner using the results obtained for nanodrops on the basis of density functional theory. Unlike the Bond number which involves surface tension of the liquid, Rb depends on microscopic parameters of the liquid-liquid and liquid-solid interactions. It also involves gravitational acceleration G, number of molecules in the system, as well as roughness of the solid surface. Some possible applications of Rb are discussed. In particular, it is shown that Rb can be used for estimation of the range of the nanodrop stability with respect to the magnitude of G.

Mechanical deformation induced charge redistribution to promote the high performance of stretchable magnesium-ion batteries based on two-dimensional C2N anodes.

Stretchable batteries play a central role in stretchable electronics such as healthcare devices and sensors. However, challenges in stretchable batteries, such as unstable performance during the deformation process and their mechanical resistances, slow down their applications. In this paper, by adopting state-of-the-art first-principles calculations, the high performance of two-dimensional (2D) nitrogenated holey graphene C2N based stretchable magnesium-ion batteries (MgIBs) and its origin are determined. Their maximum capacity and average open circuit voltage are 1175 mA h g-1 and 0.447 eV, respectively, in the strain-free state, which are much more superior to most of the previously reported 2D anode materials. Mechanical activation is a facile and effective approach to induce charge redistribution to promote their high performance, i.e. their capacities are promoted under large compressive and tensile strains, whereas their high capacities are maintained under small compressive and tensile strains. The origin of the high performance of stretchable MgIBs is also proposed based on the evidence of electron localization function and charge density difference. Compressive and tensile strains remodulate the structures of electrode materials at the atomic scale and redistribute the electrons at the electronic scale, resulting in the reactivation of adsorption sites in electrode materials and maintaining the high performance of stretchable MgIBs. Additionally, large compressive strains enhance the stability of the C2N/Mg system, further increasing its capacities. The stretchable battery shows a two-stage diffusion mechanism in strain-free and strained states, i.e. in the first stage, the out-of-plane Mg ions diffuse quickly, and in the second stage, the in-plane Mg ions migrate moderately. Compared with the stretchable battery in the strain-free state, its barrier energies are lowered in the strained states. This study provides new insights and microscale mechanisms for non-stretchable and stretchable batteries with high performance and facilitates the innovation of low cost non-stretchable and stretchable batteries with 2D material anodes.

Depletion of atmospheric organic trace gases due to their uptake by an ensemble of aqueous aerosols evolving via concurrent condensation and chemical aging.

In the framework of classical nucleation theory (CNT), we demonstrate that an ensemble of aqueous hydrophilic-hydrophobic organic droplets, containing both soluble and insoluble surfactants and evolving via concurrent condensation and chemical aging, may deplete the surrounding air of low-volatility organic trace gases and thus noticeably decrease their saturation ratios. At a given liquid water content in the air, this depletion becomes stronger with increasing dispersion of the liquid phase, i.e., with increasing collective surface-to-volume ratio of droplets; this dependence becomes particularly sharp for droplets of submicron to micron radii R⪅ 1 µm. One can thus suggest that the adsorption of organic molecules on the surface of droplets may play an important (if not crucial) role in this phenomenon. As a result of such depletion, the height of the nucleation barrier and the size of a critical droplet (nucleus) will sharply increase; with a decrease of ∼0.01% of a saturation ratio, the barrier height and number of molecules in the nucleus both increase by a factor of ∼102. This may trigger the redistribution of condensable matter in the system. Some smaller supercritical, previously growing droplets will become subcritical, evaporating ones, with the newly available condensable matter enhancing the growth of larger droplets. Thus, the uptake of organic trace gases by an ensemble of aqueous organic aerosols may drastically affect their distribution with respect to size and chemical composition. Therefore, a CNT-based theoretical model, taking this effect into account and implemented in atmospheric aerosol models, would allow one to improve the forecasting accuracy of climate models.

A heuristic approach for nanodrops on a smooth solid surface.

A heuristic approach is developed to obtain a simple equation for the contact angle of a nanodrop on a smooth planar solid surface. First, nanodrops of various fluids in contact with various solid surfaces are considered on the basis of nonlocal density functional theory (DFT). Along with the traditional (apparent) contact angle, θa, which the drop profile makes with the solid surface, another one, θd, formed by the smooth part of the drop profile and the horizontal plane separating that part from the oscillatory part of the profile was examined. For each of the contact angles, a separate simple equation resembling the Young equation for the macroscopic drops but containing, instead of surface tensions, the microscopic parameters of intermolecular interactions, temperature, and average density of the fluid was hypothesized and the parameters of this equation were determined using the results of DFT calculations. It was shown that predictions of these equations coincide with the results provided by DFT.

Two-Dimensional Carbon-Based Auxetic Materials for Broad-Spectrum Metal-Ion Battery Anodes.

Auxetic materials possess special applications due to their unique negative Poisson's ratios (NPRs). As a classic 2D carbon material, the NPR of graphene is still deliberated. Introducing the NPR in graphene would increase its extraordinary properties, and the NPR together with other properties would bring more significant applications for graphene. In this Letter, on the basis of first-principles calculations, we reconfigure the structure of graphene, and, as an example, we propose a new 2D planar carbon allotrope, xgraphene, which is constructed by 5-6-7 carbon rings. Our theoretical calculations indicate that xgraphene has an NPR and constitutes a broad spectrum of metal ion battery anodes with high performance. Its maximum storage capacities are 930/1302/744/1488 mAh/g for Li/Na/K/Ca-ion batteries. It has low metal-ion diffusion energy barriers (≤0.49 eV) and low average open-circuit voltages (≤0.53 V). Our density functional theory results also showed that it is intrinsically metallic and possesses dynamic, thermal, and mechanical stabilities. Its intrinsic NPR, which stems from the weakness of coupling of carbon-carbon bonds, is found upon loading the uniaxial strain along the armchair direction. This work not only opens up a new direction for the design of the next-generation broad-spectrum energy-storage materials with low cost and high performance but also offers a class application for auxetic materials.

OH-Initiated Reactions of p-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part I. Potential Energy Surface Analysis ⊥.

Cinnamyl alcohols such as p-coumaryl alcohol ( p-CMA) are lignin models and precursors (monolignols) and the most important primary products of lignin pyrolysis. However, the detection of monomers is not straightforward since they either undergo secondary transformations or repolymerize to contribute to the char formation. Both concerted-molecular and free-radical pathways are involved in these processes. Our recent fundamentally based theoretical and low-temperature matrix-isolation-EPR studies of cinnamyl alcohols highlighted the role of side-chain reactivity in diversity of pyrolysis products and provided a network of the chemically activated H + p-CMA reactions ( Asatryan J. Phys. Chem. A, 2017 , 121 , 3352 - 3371 ). The readily available hydroxyl radicals also can trigger a cascade of free-radical processes. Here, we present a comprehensive potential energy surface (PES) analysis of the OH + p-CMA reaction using various DFT and ab initio protocols. Since the p-CMA involves both an alkyl OH-group and a side-chain double bond, the title reaction can also serve as a relevant model for reactions of unsaturated alcohols with hydroxyl radicals to form various oxygenates including polyhydric alcohols which are abundant in nature. The newly identified pathways suggest certain alternatives to the known radical reactions. Of particular interest are the roaming-like low-energy dehydration reactions to generate a variety of O- and C-centered intermediate radicals, which are primarily transformed into the phenolic compounds observed in pyrolysis experiments. Several concerted unimolecular decomposition pathways for p-CMA are also revealed, not considered previously, such as the migration of terminal OH-group, and/or its splitting over the ipso-C and ortho-C atoms of the benzene ring to form bicyclic oxispiro- and chromene compounds represented in natural lignin.

Formation and evolution of aqueous organic aerosols via concurrent condensation and chemical aging.

We review recent results on the formation and evolution of aqueous organic aerosols via concurrent nucleation/condensation and chemical aging processes obtained mostly using the formalism of classical nucleation theory In this framework, an aqueous organic aerosol was modeled as a spherical particle of liquid solution of water and hydrophilic and hydrophobic condensable organic compounds; besides these compounds, the surrounding air contained some chemically reactive, non-condensable species. Hydrophobic organic molecules on the aerosol surface can be processed by chemical reactions with some atmospheric species; this affects the hygroscopicity of the aerosol and hence its ability to become a cloud droplet. Such processing is most probably triggered by atmospheric hydroxyl radicals that abstract hydrogen atoms from surfactant molecules located on the aerosol surface (first step), resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). These two reactions play a crucial role in the enhancement of the Köhler activation of the aerosol. Taking them and a third reaction (next in the multistep chain of relevant heterogeneous reactions) into account, one can derive an explicit expression for the free energy of formation of a four-component aqueous droplet on a ternary aqueous organic aerosol as a function of four independent variables of state of a droplet. This approach was also applied to study a large subset of primary marine aerosols which can be initially treated using an "inverted micelle" model whereof the core consists of aqueous "salt" solution. Numerical evaluations suggest that the formation of cloud droplets on such (both aqueous hydrophilic/hydrophobic organic and marine) aerosols is most likely to occur via Köhler activation rather than via nucleation. The models allow one to determine the threshold parameters necessary for the Köhler activation of such aerosols. Furthermore, heterogeneous chemical reactions involved in the chemical aging of aerosols are most likely exothermic. Due to the release of the enthalpy of these reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. The analysis of the characteristic timescales of four most important processes involved suggests that this effect may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and, hence, must be taken into account in atmospheric aerosol and global climate models.

Functionalization: An Effective Approach to Open and Close Channels for Electron Transfer in Nitrogenated Holey Graphene C2N Anodes in Sodium-Ion Batteries.

Electron transfer plays a crucial role in energy storage materials, such as metal-ion batteries (MIBs). Numerous approaches have been developed to facilitate the electron transfer for MIBs, and most of them extend the special surface areas, which promotes the contact between metal ions and the electrodes. Herein, we report the formation of intramolecular channels for electron transfer to open and close intermolecular channels in sodium-ion batteries (SIBs) through functionalization, modulating the cell performance of two-dimensional (2D) nitrogenated holey graphene C2N anodes. This also activates the inactive metal ions in the anodes, reducing their production costs. Upon increasing the concentration of hydrogen atoms, the intramolecular electron transfer increases, lowering the intermolecular electron transfer between C2N and metal ions and thus weakening their interaction and reducing the capacities of the anodes. A high concentration of hydrogen atoms introduced in C2N would further promote the intramolecular electron transfer and block the channels for intermolecular electron transfer. For functionalized C2N monolayers that can be used as anodes in SIBs, they possess high specific capacities, high conductivities, and low open-circuit voltages. This work proposes the fabrication of 2D energy storage materials with tunable macroscale behaviors (e.g., performance) through the relationships between the macroscale and microscale levels and between intramolecular and intermolecular electron transfer.

Roaming-like Mechanism for Dehydration of Diol Radicals.

Diol radicals (DRs) are important intermediates in biocatalysis, atmospheric chemistry, and biomass combustion. They are particularly generated from photolysis of halogenated diols and addition of hydroxyl radical to a double bond of unsaturated alcohols, such as lignols. The energized DRs further isomerize/decompose to form products, including water. Aqueous-phase dehydration in radiolytic and biomimetic systems typically occurs at low temperatures, with or without catalysis, whereas the gas-phase dehydration is usually considered energetically unfavorable. In the present study, we propose a new low-energy, roaming-like mechanism based on a detailed dispersion-corrected DFT and ab initio level analysis of the gas-phase dehydration of DRs obtained from the combination of OH radicals with allyl alcohol (AA, CH2═CHCH2OH)-the simplest relevant model of the unsaturated alcohols. The roaming pathways involve a nearly dissociated OH-group, which subsequently abstracts an H atom of the remaining fragment to form water and [C3H5O] radical via a transition state (TS) with energy close to the C-O bond fission asymptote. Two types of roaming-like first-order saddle points (SP) are identified for unimolecular dehydration of 1,2- and 1,3-DR radical adducts involving either both hydroxyl groups of diol radicals to generate an oxygen-centered radical, or ß-OH group and a skeletal α-hydrogen atom of the 1,2-DR to form a resonantly stabilized hydroxyallyl radical. Two higher energy conventional (tight) transition states, along with the pathways to 1,2-OH-migration, as well as direct H-abstraction, are also identified and analyzed. Most of the traditional density functional theory methods that have been successfully employed in the literature to locate so-far-known roaming SPs were also able to identify the new mechanism, in accord with dispersion-corrected double hybrid B2PLYP-D3(BJ) and mPW2PLYPD methods involving MP2-correlation corrections. However, the MP2 method itself failed to locate any of them, which seems to be typical for MP2 method for loose TS structures, confirmed here for a flat region of PES connecting direct and roaming saddle points. However, MP2 method correctly locates an identical roaming SP for a larger p-coumaryl alcohol model involving hydroxyphenyl substituent at Cγ atom of AA. Two types of interfragmental interactions are identified that stabilize the roaming SPs: (a) H-bonding of the leaving OH radical either with the H atom of the remaining OH group, or with π-cloud of the double bond; (b) direct interaction of π-electrons with the lone-pair electrons of the heteroatom in the leaving OH group through the TS-ring. The alternative TSs are qualitatively characterized by "collinearity" angle of the OH radical attack on the O-H/C-H bonds of the substrate in abstraction-like O-H-O geometry, attributed to the improved orbital overlaps. The proposed mechanism presents broader implications to signify, particularly, a larger role in atmospheric and combustion processes, especially biomass pyrolysis.

Does the Enthalpy of Heterogeneous Chemical Reactions Affect the Formation of Aqueous Secondary Organic Aerosols?

The chemical aging of liquid organic aerosols most likely occurs via exothermic heterogeneous chemical reactions on the aerosol's surface. Because of the enthalpy of reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. We attempt to shed light on this aspect of the formation of secondary organic aerosols considering their nucleation and chemical aging as concomitant processes. Using the model of aqueous hydrophilic-hydrophobic organic aerosols in the framework of classical nucleation theory, we evaluate characteristic time scales of the four most important processes involved in this complex phenomenon. Their analysis suggests that the release of the enthalpy of heterogeneous chemical reactions during the chemical aging of organic aerosols may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and hence must be taken into account in atmospheric aerosol and global climate models.

Shape and Stability of a Pendant Nanodrop.

The shape and stability of a pendant nanodrop attached to a smooth or rough solid surface are considered on the basis of the microscopic density functional theory in the presence of a strong external force normal to the solid surface. The drop profile, width, and height of the drop and the contact angle that a nanodrop makes with the solid surface are calculated and some interesting features of the drop shape and stability are identified. In particular, a linear relationship between the height of the drop and the contact angle, as well as one between the critical external force at which the drop loses its stability and the strength of the fluid-solid interaction, was found. In some cases, a fixed point on the drop profile was observed whose spacial location does not depend on the value of external force. It is also argued that the Bond number that is a characteristic of the pendant macroscopic drop is not applicable to nanodrops.