Transmission of Helium Isotopes through Graphdiyne Pores: Tunneling versus Zero Point Energy Effects

7 pags.; 7 figs.; 1 tab.Recent progress in the production of new two-dimensional (2D) nanoporous materials is attracting considerable interest for applications to isotope separation in gases. In this paper we report a computational study of the transmission of 4 He and 3 He through the (subnanometer) pores of graphdiyne, a recently synthesized 2D carbon material. The He−graphdiyne interaction is represented by a force field parametrized upon ab initio calculations, and the 4 He/3 He selectivity is analyzed by tunneling-corrected transition state theory. We have found that both zero point energy (of the in-pore degrees of freedom) and tunneling effects play an extraordinary role at low temperatures (≈20−30 K). However, both quantum features work in opposite directions in such a way that the selectivity ratio does not reach an acceptable value. Nevertheless, the efficiency of zero point energy is in general larger, so that 4 He tends to diffuse faster than 3 He through the graphdiyne membrane, with a maximum performance at 23 K. Moreover, it is found that the transmission rates are too small in the studied temperature range, precluding practical applications. It is concluded that the role of the in-pore degrees of freedom should be included in computations of the transmission probabilities of molecules through nanoporous materials. © 2015 American Chemical SocietyThe work has been funded by Spanish MINECO grant FIS2013-48275-C2-1-P. Allocation of computing time by CESGA (Spain) and support by the COST-CMTS Action CM1405 “Molecules in Motion (MOLIM)” are also acknowledged.Peer reviewe

A novel nanoporous graphite based on graphynes: first-principles structure and carbon dioxide preferential physisorption

8 págs.; 4 figs.; 1 tab.Ubiquitous graphene is a stricly 2D material representing an ideal adsorbing platform due to its large specific surface area as well as its mechanical strength and resistance to both thermal and chemical stresses. However, graphene as a bulk material has the tendency to form irreversible agglomerates leading to 3D graphitic structures with a significant decrease of the area available for adsorption and no room for gas intercalation. In this paper, a novel nanoporous graphite formed by graphtriyne sheets is introduced; its 3D structure is theoretically assessed by means of electronic structure and molecular dynamics computations within the DFT level of theory. It is found that the novel layered carbon allotrope is almost as compact as pristine graphite but the inherent porosity of the 2D graphyne sheets and its relative stacking leads to nanochannels that cross the material and whose subnanometer size could allow the diffusion and storage of gas species. A molecular prototype of the nanochannel is used to accurately determine first-principles adsorption energies and enthalpies for CO2, N2, H2O, and H2 within the pores. The proposed porous graphite presents no significant barrier for gas diffusion and shows a high propensity for CO2 physisorption with respect to the other relevant components in both pre- and postcombustion gas streams.The work has been funded by the Spanish grant FIS2013- 48275-C2-1-P. Allocation of computing time by CESGA (Spain) is also acknowledged.Peer reviewe

Graphene Multi-Protonation: a Cooperative Mechanism for Proton Permeation

The interaction between protons and graphene is attracting a large interest due to recent experiments showing that these charged species permeate through the 2D material following a low barrier (~ 0.8 eV) activated process. A possible explanation involves the flipping of a chemisorbed proton (rotation of the C-H$^+$ bond from one to the other side of the carbon layer) and previous studies have found so far that the energy barriers (around 3.5 eV) are too high to explain the experimental findings. Contrarily to the previously adopted model assuming an isolated proton, in this work we consider protonated graphene at high local coverage and explore the role played by nearby chemisorbed protons in the permeation process. By means of density functional theory calculations exploiting large molecular prototypes for graphene it is found that, when various protons are adsorbed on the same carbon hexagonal ring, the permeation barrier can be reduced down to 1.0 eV. The related mechanism is described in detail and could shed a new light on the interpretation of the experimental observations for proton permeation through graphene.Comment: 16 pages, 5 figure

Low-energy structures of benzene clusters with a novel accurate potential surface

11 pags.; 9 figs.; 2 tabs.The benzene-benzene (Bz-Bz) interaction is present in severalchemical systems and it is known to be crucial in understand-ing the specificity of important biological phenomena. In thiswork, we propose a novel Bz-Bz analytical potential energysurface which is fine-tuned on accurate ab initio calculations inorder to improve its reliability. Once the Bz-Bz interaction ismodeled, an analytical function for the energy of the Bznclus-ters may be obtained by summing up over all pair potentials.We apply an evolutionary algorithm (EA) to discover thelowest-energy structures of Bznclusters (for n52225), and theresults are compared with previous global optimization studieswhere different potential functions were employed. Besidesthe global minimum, the EA also gives the structures of otherlow-lying isomers ranked by the corresponding energy. Addi-tional ab initio calculations are carried out for the low-lyingisomers of Bz3and Bz4clusters, and the global minimum isconfirmed as the most stable structure for both sizes. Finally, adetailed analysis of the low-energy isomers of the n 5 13 and19 magic-number clusters is performed. The two lowest-energy Bz13isomers show S6and C3symmetry, respectively,which is compatible with the experimental results available inthe literature. The Bz19structures reported here are all non-symmetric, showing two central Bz molecules surrounded by12 nearest-neighbor monomers in the case of the five lowest-energy structures.VC2015 Wiley Periodicals, Inc.Contract grant sponsor: Coimbra Chemistry Centre; Contract grant number: UID/QUI/00313/2013; Contract grant sponsor: Spanish“Ministerio de Ciencia e Innovacion”; Contract grant number: FIS2013-48275-C2-1-P; Contract grant sponsor: Italian Ministry of University and Research (MIUR) for PRIN 2010-2011; Contract grant number: 2010ERFKXL_002. The authors are grateful for the provision of computational time inthe supercomputer resources hosted at Laboratorio de Computación Avançada, Universidade de Coimbra. Allocation of computing timeby CESGA (Spain) is also acknowledged.Peer reviewe

Three-Dimensional Wave-Packet Calculations of the Transmission of He Isotopes through Graphynes Membranes

Mendoza, Argentina. 9th-13st of May 2016 ; http://photodynamics9.wixsite.com/phd9N

Graphdiyne based membranes: exceptional performances for helium separation applications

Graphdiyne is a novel two-dimensional material deriving from graphene that has been recently synthesized and featuring uniformly distributed sub-nanometer pores. We report accurate calculations showing that graphdiyne pores permit an almost unimpeded helium transport which can be used for its chemical and isotopic separation. Exceptionally high He/CH_4 selectivities are found which largely exceed the performance of the best membranes used to date for extraction from natural gas. Moreover, by exploiting 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

Helium Isotopes Quantum Sieving Through Graphtriyne Membranes

We report accurate quantum calculations of the sieving of Helium atoms by two-dimensional (2D) graphtriyne layers with a new interaction potential. Thermal rate constants and permeances in an ample temperature range are computed and compared for both Helium isotopes. With a pore larger than graphdiyne, the most common member of the gamma - graphyne family, it could be expected that the appearance of quantum effects were more limited. We find, however, a strong quantum behavior that can be attributed to the presence of selective adsorption resonances, with a pronounced effect in the low temperature regime. This effect leads to the appearance of some selectivity at very low temperatures and the possibility for the heavier isotope to cross the membrane more efficiently than the lighter, contrarily to what happened with graphdiyne membranes, where the sieving at low energy is predominantly ruled by quantum tunneling. The use of more approximate methods could be not advisable in these situations and prototypical transition state theory (TST) treatments might lead to large errors

Effect of the anisotropy on the glory structure of molecule-molecule scattering cross sections

Total (elastic + rotationally inelastic) integral cross sections are computed for O$_2(^3\Sigma_g^-)$-O$_2(^3\Sigma_g^-)$ using a recent ab initio potential energy surface. The sampled velocity range allows us a thorough comparison of the glory interference pattern observed in molecular beam experiments. The computed cross sections are about 10% smaller than the measured ones, however, a remarkable agreement in the velocity positions of the glory extrema is achieved. By comparing with models where the anisotropy of the interaction is reduced or removed, it is found that the glory pattern is very sensitive to the anisotropy, especially the positions of the glory extrema.Comment: 13 pages, 3 figure

Efficiency of Collisional O2 + N2 Vibrational Energy Exchange

10 pags.; 6 figs.; 5 tabs. In press.By following the scheme of the Grid Empowered Molecular Simulator (GEMS), a new O2 + N2 intermolecular potential, built on ab initio calculations and experimental (scattering and second virial coefficient) data, has been coupled with an appropriate intramolecular one. On the resulting potential energy surface detailed rate coefficients for collision induced vibrational energy exchanges have been computed using a semiclassical method. A cross comparison of the computed rate coefficients with the outcomes of previous semiclassical calculations and kinetic experiments has provided a foundation for characterizing the main features of the vibrational energy transfer processes of the title system as well as a critical reading of the trajectory outcomes and kinetic data. On the implemented procedures massive trajectory runs for the proper interval of initial conditions have singled out structures of the vibrational distributions useful to formulate scaling relationships for complex molecular simulations.The authors acknowledge financial support from the Phys4- entry FP7/2007-2013 project (Contract 242311), ARPA Umbria, INSTM, the EGI-Inspire project (Contract 261323), MIUR PRIN 2008 (2008KJX4SN 003) and 2010/2011 (2010ERFKXL_002), the ESA-ESTEC contract 21790/08/ NL/HE, and the Spanish CTQ2012-37404 and FIS2013- 48275-C2-1-P projects. Computations have been supported by the use of Grid resources and services provided by the European Grid Infrastructure (EGI) and the Italian Grid Infrastructure (IGI) through the COMPCHEM Virtual Organization. Thanks are also due to the COST CMST European Cooperative Project CHEMGRID (Action D37) EGI Inspire.Peer reviewe

Enhanced flexibility of the O2 + N2 interaction and Its effect on collisional vibrational energy exchange

12 págs.; 8 figs.; 1 app. This article is part of the Piergiorgio Casavecchia and Antonio Lagana Festschrift special issue.Prompted by a comparison of measured and computed rate coefficients of Vibration-to-Vibration and Vibration-to-Translation energy transfer in O2 + N2 nonreactive collisions, extended semiclassical calculations of the related cross sections were performed to rationalize the role played by attractive and repulsive components of the interaction on two different potential energy surfaces. By exploiting the distributed concurrent scheme of the Grid Empowered Molecular Simulator we extended the computational work to quasiclassical techniques, investigated in this way more in detail the underlying microscopic mechanisms, singled out the interaction components facilitating the energy transfer, improved the formulation of the potential, and performed additional calculations that confirmed the effectiveness of the improvement introduced.The authors acknowledge financial support from the Phys4entry FP7/2007-2013 project (Contract No. 242311), ARPA Umbria, INSTM, the EGI-Inspire project (Contract No. 261323), MIUR PRIN 2008 (2008KJX4SN 003) and 2010/2011 (2010ERFKXL_002), the ESA-ESTEC Contract No. 21790/ 08/NL/HE and the Spanish CTQ2012-37404 and FIS2013- 48275-C2-1-P projects. Computations have been supported by the use of Grid resources and services provided by the European Grid Infrastructure (EGI) and the Italian Grid Infrastructure (IGI) through the COMPCHEM Virtual Organization. Thanks are also due to the COST CMST European Cooperative Project CHEMGRID (Action D37) EGI Inspire.Peer reviewe