Marcelo A. Carignano

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosols.

Structure and dynamics of [PF6][P1,2,2,4] from molecular dynamics simulations

Diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate, [PF6][P(1,2,2,4)], is an organic ionic plastic crystal with potential uses as a solid electrolyte in storage and light harvesting devices. In this work, we present a molecular dynamics simulation study for this material covering an extended temperature range, from 175 to 500 K. The simulations predict a transition from the crystalline to a semi plastic phase at 197 K, the onset of cation jump-like rotations at 280 K, a third transition at 340 K to a full plastic phase, and melting to 450 K. Overall, the simulations show a good agreement with the experimental findings, providing a wealth of detail in the structural and dynamic properties of the system.

Atomic partial charges on CH3NH3PbI3 from first-principles electronic structure calculations

We calculated the partial charges in methylammonium (MA) lead-iodide perovskite CH3NH3PbI3 in its different crystalline phases using different first-principles electronic charge partitioning approaches, including the Bader, ChelpG, and density-derived electrostatic and chemical (DDEC) schemes. Among the three charge partitioning methods, the DDEC approach provides chemically intuitive and reliable atomic charges for this material, which consists of a mixture of transition metals, halide ions, and organic molecules. The DDEC charges are also found to be robust against the use of hybrid functionals and/or upon inclusion of spin–orbit coupling or dispersive interactions. We calculated explicitly the atomic charges with a special focus on the dipole moment of the MA molecules within the perovskite structure. The value of the dipole moment of the MA is reduced with respect to the isolated molecule due to charge redistribution involving the inorganic cage. DDEC charges and dipole moment of the organic part rema...

Molecular kinetics of solid and liquid CHCl3

Abstract We present a detailed analysis of the molecular kinetics of CHCl 3 in a range of temperatures covering the solid and liquid phases. Using nuclear quadrupolar resonance we determine the relaxation times for the molecular rotations in solid at pre-melting conditions. Molecular dynamics simulations are used to characterize the rotational dynamics in the solid and liquid phases and to study the local structure of the liquid in terms of the molecular relative orientations. We find that in the pre-melting regime the molecules rotate about the C–H bond, but the rotations are isotropic in the liquid, even at supercooled conditions.

A surface-stabilized ozonide triggers bromide oxidation at the aqueous solution-vapour interface

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO−] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

Understanding lithium solvation and transport in ionic liquids is important due to their possible application in electrochemical devices. Using first-principles simulations aided by a metadynamics approach we study the free-energy landscape for lithium ions at infinite dilution in ethylammonium nitrate, a protic ionic liquid. We analyze the local structure of the liquid around the lithium cation and obtain a quantitative picture in agreement with experimental findings. Our simulations show that the lowest two free energy minima correspond to conformations with the lithium ion being solvated either by three or four nitrate ions with a transition barrier between them of 0.2u2005eV. Other less probable conformations having different solvation pattern are also investigated.

Tuning the Stereoselectivity and Solvation Selectivity at Interfacial and Bulk Environments by Changing Solvent Polarity: Isomerization of Glyoxal in Different Solvent Environments

Conformational isomerism plays a central role in organic synthesis and biological processes; however, effective control of isomerization processes still remains challenging and elusive. Here, we propose a novel paradigm for conformational control of isomerization in the condensed phase, in which the polarity of the solvent determines the relative concentration of conformers at the interfacial and bulk regions. By the use of state-of-the-art molecular dynamics simulations of glyoxal in different solvents, we demonstrate that the isomerization process is dipole driven: the solvent favors conformational changes toward conformers having molecular dipoles that better match its polar character. Thus, the solvent polarity modulates the conformational change, stabilizing and selectively segregating in the bulk vs the interface one conformer with respect to the others. The findings in this paper have broader implications affecting systems involving compounds with conformers of different polarity. This work suggests novel mechanisms for tuning the catalytic activity of surfaces in conformationally controlled (photo)chemical reactions and for designing a new class of molecular switches that are active in different solvent environments.

Theoretical insights into hybrid perovskites for photovoltaic applications

In this paper, we examine recent theoretical investigations on 3D hybrid perovskites (HOP) that combine concepts developed for classical bulk solid-state physics and empirical simulations of their optoelectronic properties. In fact, the complexity of HOP calls for a coherent global view that combines usually disconnected concepts. For the pseudocubic high temperature reference perovskite structure that plays a central role for 3D HOP, we introduce a new tight-binding Hamiltonian, which specifically includes spin-orbit coupling. The resultant electronic band structure is compared to that obtained using state of the art density functional theory (DFT). Next, recent experimental investigations of excitonic properties in HOP will be revisited within the scope of theoretical concepts already well implemented in the field of conventional semiconductors. Last, possible plastic crystal and orientational glass behaviors of HOP will be discussed, building on Car-Parrinello molecular dynamics simulations.

Dynamic Heterogeneity in the Monoclinic Phase of CCl4

Carbon tetrachloride (CCl4) is one of the simplest compounds having a translationally stable monoclinic phase while exhibiting a rich rotational dynamics below 226 K. Recent nuclear quadrupolar resonance experiments revealed that the dynamics of CCl4 is similar to that of the other members of the isostructural series CBrnCl4-n, suggesting that the universal relaxation features of canonical glasses such as α and β relaxation are also present in nonglass formers. Using molecular dynamics simulations we studied the rotational dynamics in the monoclinic phase of CCl4. The molecules undergo C3-type jump-like rotations around each one of the four C-Cl bonds. The rotational dynamics is very well described with a master equation using as the only input the rotational rates measured from the simulated trajectories. It is found that the heterogeneous dynamics emerges from faster and slower modes associated with different rotational axes, which have fixed orientations relative to the crystal and are distributed among the four nonequivalent molecules of the unit cell.

Starch-Lipid and Starch-Protein Complexes and Their Application

Amylose is a linear polysaccharide derived from D-glucopyranose units from α-1,4-glycosidic linkages, which can form complexes with some inorganic or organic groups to form helical introns. In this chapter, starch copolymers and the creation, properties, and applications of starch-protein and starch-lipid complexes were introduced. Because of the low toxicity, excellent biocompatibility, and solubility, amylose/α-linoleic acid/β-lactoglobulin triplex has attracted significant interest in food nutraceuticals or functional compounds in food delivery systems. Cornstarch was washed with n-butanol and isoamyl alcohol to remove impurities, followed by vacuum drying to obtain highly pure amylose. The amylose was dissolved by heating and stirring. α-Linoleic acid dissolved in absolute ethanol was added first and stirred prior to the addition of β-lactoglobulin aqueous solution. After stirring, the mixture was cooled to room temperature and filtered and vacuum-dried to prepare the triplex complex. The resulting nanoparticles had exhibited very good stability. Then, in order to understand the mechanism of the self-assembling actions of such a ternary system (interaction among amylose, β-lactoglobulin, and α-linoleic acid) deeply, all-atom molecular dynamics simulations were performed to analyze the self-assembling of the three components by the Gromacs software. Molecular dynamics simulations confirmed the importance of dynamic structural changes during stable complex formation of ternary nanoparticles made from maize amylose/β-lactoglobulin/α-linoleic acid. These results provided valuable insight into the formation of amphipathic ternary nanoparticle structure at the atomic level. Amylose in complex with lipids and proteins is of great importance to the protection of bioactive or aromatic compounds by its ability to increase solubility and bioavailability. These findings indicate that starch-lipid and starch-protein complexes have broad applications in the food, cosmetics, and pharmaceutical industries.