Toinvestigatethesefindings,wehaveperformedveryaccuratetheoreticalcalculationsanddevisedasimpleapproachtostudy theelectron displacement thataccompanies gas-phase binary intermolecular interactions inextreme detail.These calculations are based on a partial progressive integration of the electron density changes. The results unambiguously show that water's intermolecular interactions are not typical van der Waals complexes. Instead, these interactions possess a definite, strongly stereospecific CT component, even when very weak, where a water molecule may act as electron donor or acceptor depending on its orientation. CT is mediated by an asymmetric role played by the two hydrogen atoms, which causes strong orientation effects. The careful comparison of these calculations with the experimental results shows that the stabilization energy associated to CT is approximately 2� 3 eV per electron transferred and may make up for a large portion of the total interaction energy. A simple electron delocalization model helps to validate and explain these findings.

Revealing Charge-Transfer Effects in Gas-Phase Water Chemistry

Graphynes are novel two-dimensional carbon-based materials that have been proposed as molecular filters, especially for water purification technologies. We carry out first-principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne, and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. For graphdiyne, with a pore size almost matching that of water, a low barrier is found that in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is considered. Thus, in contrast with previous determinations, our results do not exclude graphdiyne as a promising membrane for water filtration. In fact, present calculations lead to water permeation probabilities that are 2 orders of magnitude larger than estimations based on common force fields. A new pair potential for the water-carbon noncovalent component of the interaction is proposed for molecular dynamics simulations involving graphdiyne and water.

Penetration Barrier of Water through Graphynes' Pores: First-Principles Predictions and Force Field Optimization.

Two-dimensional (2D) materials deriving from graphene, such as graphdiyne and 2D polyphenylene honeycomb (2DPPH), have been recently synthesized and exhibit uniformly distributed subnanometer pores, a feature that can be exploited for gas filtration applications. Accurate first-principles electronic structure calculations are reported showing that graphdiyne pores permit an almost unimpeded helium transport while it is much more difficult through the 2DPPH openings. Quantum dynamical simulations on reliable new force fields are performed in order to assess the graphdiyne capability for helium chemical and isotopic separation. Exceptionally high He/CH4 selectivities are found in a wide range of temperatures which largely exceed the performance of the best membranes used to date for helium extraction from natural gas. Moreover, due to slight differences in the tunneling probabilities of 3He and 4He, we also find promising results for the separation of the Fermionic isotope at low temperature.

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Graphdiyne Pores: “Ad Hoc” Openings for Helium Separation Applications

Energetic and structural properties of liquid water have been calculated using molecular dynamics simulations in order to investigate the effect of different formulations of the van der Waals (vdW) interaction on the behaviour of liquid water In particular, two model potentials, the SPC/E using a Lennard Jones (LJ) function and the AMPF using an Improved Lennard Jones function (ILJ) have been considered The flexibility of the ILJ function in the AMPF model has also been analysed, proving that its vdW component can match different parametrizations of the electrostatic component

On the suitability of the ILJ function to match different formulations of the electrostatic potential for water-water interactions

Graphynes are novel two-dimensional carbon-based materials that -due to their nanoweb-like structure- have been proposed as molecular filters, especially for water purification technologies. In this work we carry out first principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. Nevertheless, for graphdiyne, which presents a pore size almost matching that of water, a low barrier is found which in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is taken into account. These results support the possibility of using graphtriyne as an efficient membrane for water filtration but, in contrast with previous determinations, they do not exclude graphdiyne. In fact, the related first principles penetration barrier leads to water permeation probabilities which are at least two order of magnitude larger than those estimated by employing commonly used force fields. A new pair potential for the water--carbon non-covalent component of the interaction has been built and it is recommended for molecular dynamics simulation involving graphdiyne and water.

The penetration barrier of water through graphynes' pores: first-principles predictions and force field optimization

Scattering data, measured for rare gas-rare gas systems under high angular and energy resolution conditions, have been used to probe the reliability of a recently proposed interaction potential function, which involves only one additional parameter with respect to the venerable Lennard-Jones (LJ) model and is hence called Improved Lennard-Jones (ILJ). The ILJ potential eliminates most of the inadequacies at short- and long-range of the LJ model. Further reliability tests have been performed by comparing calculated vibrational spacings with experimental values and calculated interaction energies at short-range with those obtained from the inversion of gaseous transport properties. The analysis, extended also to systems involving ions, suggests that the ILJ potential model can be used to estimate the behavior of unknown systems and can help to assess the different role of the leading interaction components. Moreover, due to its simple formulation, the physically reliable ILJ model appears to be particularly useful for molecular dynamics simulations of both neutral and ionic systems.

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations.

Simona Brizi

Papers

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations.