Computational Study of the Properties of Acetonitrile/Water-in-Salt Hybrid Electrolytes as Electrolytes for Supercapacitors.

Normal and water-in-salt Li-bis(trifluoromethane) sulfonimide anion-based electrolytes were modeled using atomistic molecular dynamics simulations. Their acetonitrile (ACN) mixtures, in various concentrations, were also studied to evaluate the impact of a cosolvent on the structural, dynamical, and electrical properties of the electrolytes using liquid electrolyte and supercapacitor models. Our simulations for pure and ACN-based electrolytes revealed a drastic difference that exists between normal electrolytes and water-in-salt electrolytes and a systematic reduction of the diffusion of species by approximately a factor of 2 because of the ACN impact. Electrolytic cells for each electrolyte were built with graphene as the electrode. Our results for capacitance reveal an asymmetry between the electrode capacitances, with negative electrode capacitance systematically higher than those of the positive electrode. The total capacitance of the electrode exhibited negligible variations regardless of the concentration and composition of the electrolyte.

Storing Energy in Biodegradable Electrochemical Supercapacitors.

The development of green and biodegradable electrical components is one of the main fronts of research to overcome the growing ecological problem related to the issue of electronic waste. At the same time, such devices are highly desirable in biomedical applications such as integrated bioelectronics, for which biocompatibility is also required. Supercapacitors for storage of electrochemical energy, designed only with biodegradable organic matter would contemplate both aspects, that is, they would be ecologically harmless after their service lifetime and would be an important component for applications in biomedical engineering. By means of atomistic simulations of molecular dynamics, we propose a supercapacitor whose electrodes are formed exclusively by self-organizing peptides and whose electrolyte is a green amino acid ionic liquid. Our results indicate that this supercapacitor has a high potential for energy storage with superior performance than conventional supercapacitors. In particular its capacity to store energy was estimated to be almost 20 times greater than an analogue one of planar metallic electrodes.

GIAO-DFT-NMR characterization of fullerene-cucurbituril complex: the effects of the C60@CB[9] host-guest mutual interactions.

Magnetic shielding constants for an isolated fullerene C60, cucurbituril CB[9], and the host-guest complex C60@CB[9] were calculated as a function of separation of the monomers. Our results in the gas phase and water indicate a significant variation of the magnetic properties for all atoms of the monomers in the complex and after liberation of fullerene C60 from the interior of the CB[9] cavity. The interaction between the two monomers results in a charge transfer that collaborates with a redistribution of electron density to deshield the monomers. Graphical Abstract NMR spectroscopy alteration on C60@CB[9] host-guest mutual interactionsᅟ.

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.

Predicting the properties of a new class of host-guest complexes: C60 fullerene and CB[9] cucurbituril.

DFT, semi-empirical and classical molecular dynamics methods were used to describe the structure and stability of the inclusion complex formed by the fullerene C60 and the cucurbituril CB[9]. Our results indicate a high structural compatibility between the two monomers, which is evident from the potential energy curve for the inclusion process of the C60 into the CB[9] cavity. The interaction between the two monomers is mainly of the van der Waals type and leads to a highly stable complex. Thermal contributions and environmental interaction are taken into account by the free energy of binding of -224 kJ mol(-1), indicating that even in aqueous medium the complex remains very stable.

Assessing the hydration free energy of a homologous series of polyols with classical and quantum mechanical solvation models.

Molecular dynamics (MD) simulations associated with the thermodynamic integration (TI) scheme and the polarizable continuum model (PCM) in combination with the SMD solvation model were used to study the hydration free energy of the homologous series of polyols, C(n)H(n+2)(OH)n (1 ≤ n ≤ 7). Both solvation models predict a nonlinear behavior for the hydration free energy with the increase of the number of hydroxyl groups. This study also indicates that there is a sizable solute polarization in aqueous solution and that the inclusion of the polarization effect is important for a reliable description of the free energy differences considered here.

Ab initio study of weakly bound halogen complexes: RX⋯PH3.

Ab initio calculations were employed to study the role of ipso carbon hybridization in halogenated compounds RX (R=methyl, phenyl, acetyl, H and X=F, Cl, Br and I) and its interaction with a phosphorus atom, as occurs in the halogen bonded complex type RX⋯PH3. The analysis was performed using ab initio MP2, MP4 and CCSD(T) methods. Systematic energy analysis found that the interaction energies are in the range -4.14 to -11.92 kJ mol(-1) (at MP2 level without ZPE correction). Effects of electronic correlation levels were evaluated at MP4 and CCSD(T) levels and a reduction of up to 27% in interaction energy obtained in MP2 was observed. Analysis of the electrostatic maps confirms that the PhCl⋯PH3 and all MeX⋯PH3 complexes are unstable. NBO analysis suggested that the charge transfer between the moieties is bigger when using iodine than bromine and chlorine. The electrical properties of these complexes (dipole and polarizability) were determined and the most important observed aspect was the systematic increase at the dipole polarizability, given by the interaction polarizability. This increase is in the range of 0.7-6.7 u.a. (about 3-7%).

Prediction of the hydration properties of diamondoids from free energy and potential of mean force calculations.

Molecular dynamics simulations were used to predict the thermodynamical properties of the hydration process of the adamantane, diamantane, and trimantane, the first three members of the series of diamondoids. Free-energy results suggest that the water solubility of these molecules is low. The hydration free energy increases with size of the diamondoid. As for the alkane hydrocarbons, hydration free energy correlates linearly with the surface accessible solvent area; however, here it has been shown that small diamondoids present hydration free energy significantly lower than the n-alkanes of similar molecular weights. The decomposition of the hydration free energy in enthalpic and entropic terms revealed that the hydration process of the small diamondoids is entropic driven. The potential of mean-force calculations indicates that the aggregation of these species in the aqueous medium should occur spontaneously and that the contribution of the solvent is greater the larger the diamondoid.

Molecular dynamics simulation of liquid trimethylphosphine.

Structural and dynamical properties of liquid trimethylphosphine (TMP), (CH(3))(3)P, as a function of temperature is investigated by molecular dynamics (MD) simulations. The force field used in the MD simulations, which has been proposed from molecular mechanics and quantum chemistry calculations, is able to reproduce the experimental density of liquid TMP at room temperature. Equilibrium structure is investigated by the usual radial distribution function, g(r), and also in the reciprocal space by the static structure factor, S(k). On the basis of center of mass distances, liquid TMP behaves like a simple liquid of almost spherical particles, but orientational correlation due to dipole-dipole interactions is revealed at short-range distances. Single particle and collective dynamics are investigated by several time correlation functions. At high temperatures, diffusion and reorientation occur at the same time range as relaxation of the liquid structure. Decoupling of these dynamic properties starts below ca. 220 K, when rattling dynamics of a given TMP molecules due to the cage effect of neighbouring molecules becomes important.

The gas-phase alcoholysis of protonated homoleptic alkoxysilanes.

Tetra-alkoxysilanes are common and useful reagents in sol-gel processes and understanding their reactivity is important in the design of new materials. The mechanism of gas-phase reactions that mimic alcoholyis of Si(OMe)(4) (usually known as TMOS) under acidic conditions have been studied by Fourier transform ion cyclotron resonance techniques and density functional calculations at the B3LYP/6-311+G(d,p) level. The proton affinity of TMOS has been estimated at 836.4 kJ mol(-1) and protonation of TMOS gives rise to an ionic species that is best represented as trimethoxysilyl cations associated with a methanol molecule. Protonated TMOS undergoes rapid and sequential substitution of the methoxy groups in the gas-phase upon reaction with alcohols. The calculated energy profile of the reaction indicates that the substitution reaction through an S(N)2 type mechanism may be more favorable than frontal attack at silicon. Furthermore, the sequential substitution reactions are promoted by a mechanism that involves proton shuttle from the most favorable protonation site to the oxygen of the departing group mediated by the neutral reagent molecule.

Effects of hydroxyl group distribution on the reactivity, stability and optical properties of fullerenols.

We investigate the impact of hydroxyl groups on the properties of C(60)(OH)(n) systems, with n = 1, 2, 3, 4, 8, 10, 16, 18, 24, 32 and 36 by means of first-principles density functional theory calculations. A detailed analysis from the local density of states has shown that adsorbed OH groups can induce dangling bonds in specific carbon atoms around the adsorption site. This increases the tendency to form polyhydroxylated fullerenes (fullerenols). The structural stability is analyzed in terms of the calculated formation enthalpy of each species. Also, a careful examination of the electron density of states for different fullerenols shows the possibility of synthesizing single molecules with tunable optical properties.

Structure and UV-vis spectrum of C(60) fullerene in ethanol: a sequential molecular dynamics/quantum mechanics study.

A molecular dynamics simulation combined with semiempirical quantum mechanics calculations has been performed to investigate the structure, dynamical, and electronic properties of pure C60 in liquid ethanol. The behavior of the fullerene alcoholic solution was obtained by using the NPT ensemble under ambient conditions, including one C60 fullerene immersed in 1000 ethanol molecules. Our analyzed center-of-mass pairwise radial distribution function indicated that, on average, there are 32, 72, 132, and 187 ethanol molecules around, respectively, the first, second, third, and fourth solvation shells of the C60 molecule. To investigate the UV-vis transition energies of C60 in the presence of ethanol, we have considered constituents of the time uncorrelated supramolecular structures of the first solvation shell, i.e., clusters of C60@{EtOH}32 types. The semiempirical calculations were performed at the intermediate neglect of differential overlap level with configuration interaction singles (INDO/CIS). Our results have pointed out that the characteristic C60 UV-vis absorbance peaks are slightly shifted to longer wavelengths, as compared to the isolated molecule. These findings are in connection with the weak donor-acceptor character of the interactions involving electron lone pairs of oxygen atoms on the solvent and the fullerene surface.

Electronic changes due to thermal disorder of hydrogen bonds in liquids: pyridine in an aqueous environment.

Combined Metropolis Monte Carlo computer simulation and first-principles quantum mechanical calculations of pyridine in water are performed to analyze the role of thermal disorder in the electronic properties of hydrogen bonds in an aqueous environment. The simulation uses the NVT ensemble and includes one pyridine and 400 water molecules. Using a very efficient geometric-energetic criterion, the hydrogen bonds between pyridine and water C5H5N---H2O are identified and separated for subsequent quantum mechanical calculations of the electronic and spectroscopic properties. Statistically uncorrelated configurations composed of one pyridine and one water molecule are used to represent the configuration space of the hydrogen bonds in the liquid. The quantum mechanical calculations on these structures are performed at the correlated second-order perturbation theory level and all results are corrected for basis-set superposition error. The results are compared with the equivalent electronic properties of the hydrogen bond in the minimum-energy configuration. Charge transfer, dipole moment, and dipole polarizabilities are calculated for the thermally disordered and minimum-energy structures. In addition, using the mean and anisotropic polarizabilities, the Rayleigh depolarizations are obtained. All properties obtained for the thermally disordered structures are represented by a statistical distribution and a convergence of the average values is obtained. The results indicate that the charge transfer, dipole moment, and average depolarization ratios are systematically decreased in the liquid compared to the optimized cluster. This study quantifies, using ab initio quantum mechanics and statistical analysis, the important aspect of the thermal disorder of the hydrogen bond in a liquid system.