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Nanoscience and Nanotechnology (N&N), Madrid, mayo 17-20, 2022, Deuterium and tritium constitute relevant substances for many scientific and industrial applications. However, they cannot be easily separated from a mixture with H2, the most abundant hydrogen isotope, as standard methods are based on chemical properties, which are similar for isotopes. For this purpose, in this study, we have employed a quantum-mechanical method that takes into account mass-dependent effects. Graphdiyne, a recently synthesized porous derivative of graphene, offers a promising opportunity to enhance these effects, as its subnanometric pores enable a confinement of the hydrogen molecules when travelling through them. In our simulations the molecules have been treated as pseudoatoms and their interaction with the membrane is represented by an improved Lennard-Jones force field. By performing three-dimensional time-dependent wavepacket propagations, we have obtained the transmission probability of each isotope, which in turn has been used to compute the permeances at different temperatures. Finally, we have calculated the selectivity of deuterium and tritium over hydrogen as the quotient of their permeances: It is seen that the selectivities (favoring the heavier isotopes) increase as temperature decreases until reaching a maximum value. The maximum is a result of a compromise between zero point energy and tunnelling effects, which benefit the heavier and lighter species, respectively. These results are promising for efficient deuterium and tritium separation from hydrogen.

Nanoscience and Nanotechnology (N&N), Madrid, mayo 17-20, 2022, Recent discoveries that graphene permeates protons[1] and hydrogen atoms[2] have opened up prospects of new applications in hydrogen technologies, energy storage and conversion, isotope separation, etc. In addition, a great interest has risen for uncovering the microscopic mechanisms underlying such observations. In the presentation, we will address two processes involved in hydrogenated graphene, namely, the permeation or flipping of chemisorbed hydrogen atoms through a graphene layer and the recombination of these atoms (desorption and formation of hydrogen molecules, a reaction that can occur after permeation). With the aim to provide some insight into these processes, we have carried out density functional theory computations using large molecular prototypes of graphene. Firstly, we will discuss a new mechanism for the flipping of chemisorbed hydrogen atoms[3] or protons[4] (see Fig. 1), for which the estimated activation energies are of the order of the experimental findings[1,2]. Secondly, we will report reaction paths and rate coefficients for the recombination of hydrogen and deuterium and analyze the large isotopic substitution effects[5] observed in thermal desorption measurements[6,7]. Finally, some findings about the role of Stone-Wales defects on the studied processes will be outlined. References [1] S. Hu et al, ¿Proton transport through one-atom-thick crystals¿, Nature, 516 (2014) 227. [2] P. Z. Sun et al, ¿Limits on gas impermeability of graphene¿, Nature, 579 (2020) 229 [3] M. Bartolomei et al, ¿Permeation of chemisorbed hydrogen through graphene: a flipping mechanism elucidated¿, Carbon, 178 (2021) 718 [4] M. Bartolomei et al, ¿Graphene multi-protonation: A cooperative mechanism for proton permeation¿, Carbon, 144 (2019) 144, 724. [5] M. A. Valentín-Rodríguez et al, ¿Reaction paths and tunneling effects in the recombination of H2/D2 from graphene nanoflakes¿, in preparation (2022) [6] T. Zecho et al, ¿Adsorption of hydrogen and deuterium atoms on the (0001) graphite surface¿, J. Chem. Phys. 117, 8486 (2002) [7] L. Hornekaer et al, ¿Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface¿, Phys. Rev. Lett., 96 (2006) 156104

XIII Jornadas de jóvenes investigadores en física atómica y molecular (J²IFAM) , Santiago de Compostela, 16 de marzo 2022, Molecular deuterium is a relevant gas for many different purposes in industry and scientific research. Thus, it is crucial to separate it from other substances in a gas mixture, specially its isotopes. Standard methods have proved not to be very efficient for this separation as deuterium and hydrogen have similar chemical properties. In this work we have exploited a theoretical approach based on quantum sieving, which takes advantage of the enhancement of mass-dependent quantum effects in confined spaces. Subnanometric pores of graphdyine (GDY), a porous derivative of graphene, provide such a confined environment. In the simulations H2/D2 molecules have been treated as pseudoatoms and their interaction with the porous membrane has been modelled by a sum of atom-atom improved Lennard-Jones contributions, which have been obtained by exploiting the comparison with accurate ab initio estimations. With the latter, we have performed three-dimensional time-dependent wavepacket propagations, which yield the transmission probabilities of the gas molecules through GDY. These probabilities are used to compute the permeances (thermal flux divided by gas pressure) at different temperatures. The selectivity of deuterium over hydrogen is obtained as a quotient of their permeances. The selectivity increases as the temperature decreases reaching its highest value around 50 K. This is due to the quantization of the energy levels of the transition state at the centre of the pore. Preliminary results indicate that at lower temperatures the selectivity will eventually decrease because of the tunnelling effect favouring hydrogen over deuterium.

CECAM workshop “Modeling dilute matter in ultracold conditions: from quantum structure to dynamics and reactivity”, Grenoble, 17-19 May 2021 Online Meeting (France), This work has the support of the Spanish grant FIS2017-84391-C2-2-P. Allocation of computing time by CESGA is acknowledged.

cecam2021 : Modeling dilute matter under ultracold solvent: from quantum structure to dynamics and reactivity, 17-19 May 2021 Online Meeting (France)

CMD2020GEFES. 28th joint conference of Condensed Matter Division of the Spanish Royal Physics Society (RSEF-GEFES) and of the European Physical Society (EPS-C Universidad Autónoma de Madrid, Spain) Del 31 de Agosto al 04 de Septiembre de 2020. .- http://www.cmd2020gefes.eu, Graphene layers with carefully tailored nanopores could be useful tools as filters at the molecular level. Not only molecules of different size could be sieved but it might be also possible to use this 2D materials as appropriate filters for isotopic separation. We present first results on separation of 3He/4He on the nanoporous graphene (NPG) structure proposed in Ref

53 pags., 12 figs., 4 tabs. We review the solvation of atomic, molecular or cluster ions in HNDs. After briefly discussing the properties of snowballs in bulk helium we consider experimental conditions for the efficient synthesis of charged, doped HNDs. We show that the cluster ions observed in conventional mass spectrometers originate from fission of highly charged HNDs. The ionization threshold of HNDs doped with alkalis reveals the minimum cluster size required for full immersion. The abundance distributions of He (Formula presented.) X (Formula presented.) ions frequently reveal local anomalies or magic numbers. We demonstrate that the abundance is approximately proportional to the evaporation energy. Observed and calculated magic numbers will be compiled, including data for ions solvated in molecular hydrogen. Alternative methods to forming He (Formula presented.) X (Formula presented.) that do not employ HNDs will be summarized. Electronic excitation spectra of C (Formula presented.) and polycyclic aromatic hydrocarbon ions reveal the properties of the helium adsorption layer in quantitative detail. Next we discuss theoretical efforts to describe the interaction between ions and helium. We close with summarizing the size dependence of physical quantities computed for atomic alkali and alkaline earth cations in helium, such as binding energy, superfluid fraction, structural order, radial density profiles, and the existence of first and higher solvation shells. This work was supported by the Austrian Science Fund FWF (Project Nos. P31149 and P30355), the European Regional Development Fund (EFRE K-Regio FAENOMENAL, Project No. EFRE 2016- 4), the Swedish Research Council (Contract No. 2016-06625), and the MICINN with Grants No. FIS2017-83157-P and FIS2017-84391-C2-2-P

We report on a combined experimental and theoretical study of Li+ ions solvated by up to 50 He atoms., We report on a combined experimental and theoretical study of Li+ ions solvated by up to 50 He atoms. The experiments show clear enhanced abundances associated with HenLi+ clusters where n = 2, 6, 8, and 14. We find that classical methods, e.g. basin-hopping (BH), give results that qualitatively agree with quantum mechanical methods such as path integral Monte Carlo, diffusion Monte Carlo and quantum free energy, regarding both energies and the solvation structures that are formed. The theory identifies particularly stable structures for n = 4, 6 and 8 which line up with some of the most abundant features in the experiments.

Zaragoza, 15 -19 de julio de 2019, It has been recently found that pristine graphene and some other 2D materials can be permeated by protons and deuterium at room temperature [1,2], following a low barrier (~ 0.8 eV) activated process. Most of the theoretical attempts have found so far, that permeation of the H+ (D+) involves large energy barriers (around 3.5 eV) and therefore they are too high to explain experiments [3]. In previous models it was assumed an isolated proton permeating the 2D membrane. In this work however, we consider protonated graphene at high local coverage and explore the role played by nearby chemisorbed protons in the process. By using density functional theory calculations (DFT) applied to large molecular prototypes for graphene we have found [4] that when various protons are absorbed on carbons belonging to the same hexagonal ring, permeation barrier can be lowered down to 1.0 eV, thus making feasible the permeation of protons through pristine graphene. The proposed insertion mechanism necessarily need to count with the nearby protons and it could be of relevance not only to help in the understanding of the experimental results, but also in many other scenarios of basic and technological interest. References [1] S. Hu, M. Lozada-Hidalgo, F. C. Wang, A. Mishchenko, F. Schedin, R. R. Nair, E. W. Hill,D. W. Boukhvalov, M. I. Katsnelson, R. A. W. Dryfe, et al., Nature 516, 227 (2014). [2] M. Lozada-Hidalgo, S. Hu, O. Marshall, A. Mishchenko, A. N. Grigorenko, R. A. W. Dryfe,B. Radha, I. V. Grigorieva, and A. K. Geim, Science 351, 68 (2016). [3] Kroes et al., Phys. Chem. Chem. Phys. 19, 5813 (2017), Ekayanake et al. J. Phys.Chem C, 1231, 24335 (2017), Shi et al. J. Phys. Chem. Letts. 8, 4354 (2017), Feng et atl, J. Phys.Chem. Lett., 8 , 6009 (2017), Poltavsky et al. J. Chem. Phys., 148, 204707 (2018), Mazzuca et al, J. Chem. Phys., 148, 224301 (2018). [4] M. Bartolomei, M. I. Hernández, J. Campos-Martínez, R. Hernández-Lamoneda, Carbon, 144, 724-730 (2019).

Zaragoza, 15 -19 de julio de 2019, Interactions between molecular hydrogen and cations of metallic atoms are dominated by charge-quadrupole as well as induction forces, hence they are relatively strong despite being non-covalent. Due to these characteristics, one can ask whether hydrogen molecules would form dense, solid-like, solvation shells around the ion. In this work, the interactions between Cs+ and H2/D2 are investigated both experimentally and computationally. On the one hand, helium nanodroplets doped with cesium and hydrogen or deuterium are ionized by electron impact and the (H2/D2)nCs+ (up to n=30) clusters formed are identified by mass spectrometry. On the other hand, a new and accurate potential energy surface is reported and cluster energies and structures are computed by means of classical and quantum-mechanical Monte Carlo calculations. Dependence of the computed evaporation energies with the cluster size, n, is remarkably similar to the behavior of the measured ion abundances. Clusters (H2)12Cs+ and (D 2)12Cs+ stand out for their stability and quasi-rigid icosahedral structures. However, the first solvation shell involves thirteen or fourteen molecules for hydrogenated or deuterated clusters, respectively. It is found that these solvation layers exhibit the typical characteristics of the well-known Atkins snowballs. The role played by three-body induction interactions as well as the rotational degrees of freedom of the molecules is analyzed and it is found that, despite they are important at a quantitative level (10-15%), they become negligible once the first shell is completed.

IMAMPC2019, Madrid, 11 - 14 June, 2019

IMAMPC2019, Madrid, 11 - 14 June, 2019

EPoLM-4 Workshop, CSIC, Madrid, March 4-6 (2019), This work has been initially motivated by the experimental observation that proton permeation through graphene involves a low activation energy (~0.8 eV)[1], a discovery that contrasted with the common belief that this material is completely impermeable to all atoms under ambient conditions. A possible explanation involves the flipping of a chemisorbed proton from one to the other side of the graphene layer but the related energy barrier is too high (3.5 eV). In this work[2] we have found that this barrier can decrease to the 1.0-1.5 eV range if graphene is assumed to be initially protonated at high local coverage. For this purpose, DFT (PBE/cc-pVTZ) calculations were conducted (exploiting circumcoronene and circumcircumcoronene as graphene prototypes) for an increasing number of protons chemisorbed along a given carbon ring. As seen in Fig.1 (for a five-times protonated carbon ring), the proposed permeation mechanism involves the insertion of the flipping proton into the middle of an effectively broken C-C bond, a linkage that is restored in the products configuration, in analogy with processes originally reported for carbon nanotubes[3]. We believe that these findings can help to rationalize the proton permeation observations since, despite progress[4], theoretical insight is not complete or satisfactory yet. In addition, they can provide clues about properties of hydrogenated graphene as well as about astrochemical processes involving hydrogen and large PAHs[5]. To this end, preliminary results on related processes involving neutral atomic hydrogen will be also reported. [1] S. Hu, M. Lozada-Hidalgo, F. C. Wang, A. Mishchenko, F. Schedin, R. R. Nair et al, Nature 2014, 516, 227. [2] M. Bartolomei, M. I. Hernández, J. Campos-Martínez, R. Hernández-Lamoneda, Carbon 2019, 144, 724. [3] S. M. Lee, K. H. An, G. Seifert, Y. H. Lee, T. Frauenheim, J. Am. Chem. Soc. 2001, 123, 5059. [4] Y. Feng, J. Chen, W. Fang, E.-G. Wang, A Michaelides, X.-Z. Li, J. Phys. Chem. Lett. 2017, 8,6009. [5] P. Merino, M. Svec, J. Martínez, P. Jelinek, P. Lacovig, M. Dalmiglio, S. Lizzit, P. Soukiassian, J. Cernicharo, J. Martín-Gago, Nat. Commun. 2014, 5, 3054., This work has the support of the Spanish grant FIS2017-84391-C2-2-P. Allocation of computing time by CESGA is acknowledged