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In this Letter, we report the experimental detection of likely the largest ordered structure of helium atoms surrounding a monatomic impurity observed to date using a recently developed technique. The mass spectrometry investigation of HeNCa2+ clusters, formed in multiply charged helium nanodroplets, reveals magic numbers at N = 12, 32, 44, and 74. Classical optimization and path integral Monte Carlo calculations suggest the existence of up to four shells surrounding the calcium dication which are closed with well-ordered Mozartkugellike structures: He12Ca2+ with an icosahedron, the second at He32Ca2+ with a dodecahedron, the third at He44Ca2+ with a larger icosahedron, and finally for He74Ca2+, we find that the outermost He atoms form an icosidodecahedron which contains the other inner shells. We analyze the reasons for the formation of such ordered shells in order to guide the selection of possible candidates to exhibit a similar behavior., The Journal of Physical Chemistry Letters, 14 (13), ISSN:1948-7185

9 pags., 10 figs. -- This article is part of the themed collection: Stability and properties of new-generation metal and metal-oxide clusters down to subnanometer scale, We report a novel method to reversibly attach and detach hydrogen molecules to positively charged sodium clusters formed inside a helium nanodroplet host matrix. It is based on the controlled production of multiply charged helium droplets which, after picking up sodium atoms and exposure to H2 vapor, lead to the formation of Nam+(H2)n clusters, whose population was accurately measured using a time-of-flight mass spectrometer. The mass spectra reveal particularly favorable Na+(H2)n and Na2+(H2)n clusters for specific “magic” numbers of attached hydrogen molecules. The energies and structures of these clusters have been investigated by means of quantum-mechanical calculations employing analytical interaction potentials based on ab initio electronic structure calculations. A good agreement is found between the experimental and the theoretical magic numbers., This work was supported by the Austrian Science Fund, FWF (project number P31149) and the Spanish MICINN with Grants FIS2016-79596-P and PID2019-105225GB-I00 (JB, JHR); PID2020- 114654GB-I00/AEI/10.13039/501100011033, 2021-2024 (TGL,MB) and PID2020-114957GB-I00/AEI/10.13039/501100011033 (JCM, MIH). Collaboration has also been supported by the CSIC under i-Link+ program LINKB20041. Allocation of computing time by CESGA (Spain) is also acknowledged.

4 figs., 4 tabs., The impermeability of defect-free graphene to all gases has been recently con tested since experimental evidence (see Nature 579, 229-232 (2020)) of hydrogen transmission through a two-dimensional carbon layer has been obtained. By means of density functional theory computations here we elucidate a flipping mechanism which involves the insertion of a chemisorbed hydrogen atom in the middle of a C-C bond via a transition state that is relatively stable due to a sp2 rehybridization of the implicated carbon atoms. Present results suggest that transmission for hydro genated graphene at low local coverage is highly unlikely since other outcomes such as hydrogen diffusion and desorption exhibit quite lower activation enthalpies. How ever, at high local coverage, with a given graphenic ring tending to saturation, the proposed flipping mechanism becomes competitive leading to a significantly exother mic process. Moreover, for a specific arrangement of four neighboring chemisorbed hydrogen atoms the flipping of one of them becomes the most likely outcome with a low activation enthalpy (about 0.8 eV), which is in the range of the experimental estimation. The effect of charge doping is investigated and it is found that electrondoping can help to reduce the related activation enthalpy and to slightly enhance its exothermicity. Finally, an analysis of corresponding results for deuterium substitution is also presented., The work has been funded by the Spanish grant FIS2017-84391-C2-2-P. Allocation ofcomputing time by CESGA (Spain) is also acknowledged by MB. GG acknowledges PRACE for awarding the access to the Marconi system based in Italy at CINECA and the Italian ISCRA program.

12 pags., 8 figs., 2 tabs.-- This article is part of the themed collection: New Trends and Challenges Low Dimensional Systems and Surface Phenomena, Graphdiyne (GDY) has emerged as a very promising two-dimensional (2D) membrane for gas separation technologies. One of the most challenging goals is the separation of deuterium (D2) and tritium (T2) from a mixture with the most abundant hydrogen isotope, H2, an achievement that would be of great value for a number of industrial and scientific applications. In this work we study the separation of hydrogen isotopes in their transport through a GDY membrane due to mass-dependent quantum effects that are enhanced by the confinement provided by its intrin sic sub-nanometric pores. A reliable improved Lennard-Jones force field, optimized on accurate ab initio calculations, has been built to describe the molecule-membrane interaction, where the molecule is treated as a pseudoatom. The quantum dynamics of the molecules impacting on the membrane along a complete set of incidence directions have been rigorously addressed by means of wave packet calculations in the 3D space, which have allowed us to obtain transmis sion probabilities and, in turn, permeances, as the thermal average of the molecular flux per unit pressure. The effect of the different incidence directions on the probabilities is analyzed in detail and it is concluded that restricting the simulations to a perpendicular incidence leads to reason able results. Moreover, it is found that a simple 1D model -using a zero-point energy-corrected interaction potential- provides an excellent agreement with the 3D probabilities for perpendicular incidence conditions. Finally, D2/H2 and T2/H2 selectivities are found to reach maximum values of about 6 and 21 at ≈ 50 and 45K, respectively, a feature due to a balance between zero-point energy and tunneling effects in the transport dynamics. Permeances at these temperatures are below recommended values for practical applications, however, at slightly higher temperatures (77 K) they become acceptable while the selectivities preserve promising values, particularly for the separation of tritium, The work has been funded by Spanish Agencia Estatal de Investigación PID2020-114957GB-I00/AEI/10.13039/501100011033 and PID2020-114654GB-I00/AEI/10.13039/501100011033. E.G-A thanks financial support from Comunidad de Madrid (Spain), under the “Garantía Juvenil” Program (PEJ-2019- AI/IND-14267). Allocation of computing time by CESGA (Spain) is also acknowledged

13 pags., 6 figs. -- This article belongs to the Special Issue Intermolecular Forces: From Atoms and Molecules to Nanostructures, We present a combined experimental and theoretical investigation on Ca+ ions in helium droplets, HeNCa+. The clusters have been formed in the laboratory by means of electron-impact ionization of Ca-doped helium nanodroplets. Energies and structures of such complexes have been computed using various approaches such as path integral Monte Carlo, diffusion Monte Carlo and basin-hopping methods. The potential energy functions employed in these calculations consist of analytical expressions following an improved Lennard-Jones formula whose parameters are finetuned by exploiting ab initio estimations. Ion yields of HeNCa+ -obtained via high-resolution mass spectrometry- generally decrease with N with a more pronounced drop between N = 17 and N = 25, the computed quantum HeNCa+ evaporation energies resembling this behavior. The analysis of the energies and structures reveals that covering Ca+ with 17 He atoms leads to a cluster with one of the smallest energies per atom. As new atoms are added, they continue to fill the first shell at the expense of reducing its stability, until N = 25, which corresponds to the maximum number of atoms in that shell. Behavior of the evaporation energies and radial densities suggests liquid-like cluster structures, This research was funded by the Austrian Science Fund FWF project number P31149 (P.M., P.S.) and the Spanish MICINN with Grants FIS2016-79596-P and PID2019-105225GB-I00 (J.B., J.H.-R.), FIS2017-83157-P (T.G.-L.) and FIS2017-84391-C2-2-P (M.B., J.C.-M., M.I.H.).

9 pags., 9 figs., We study the growth of rare gases (Rg) Ne, Ar and Kr on coronene (Cor) molecules, Cor-(Rg). The study is taken up to sizes N=60, and we show the global energy minima for those clusters. Improved Lennard-Jones and atom-bond potentials are used to represent the Rg-Rg and Rg-Cor interactions, respectively. The Basin-Hopping (BH) global optimization technique is employed to locate the putative global energy minimum structures. Results are presented for Cor-(Ne), Cor-(Ar) and Cor-(Kr) systems. Both classical Cor-(Ar) and Cor-(Kr) clusters present the same “magic numbers” for N=3, 6, 10, 14, 19 and 38. This last number marks the first layer of solvation. For Cor-(Ne) clusters, we consider quantum effects adding the zero-point energy (ZPE) into the classical potential energy minima (BH-ZPE). This semiclassical approximation is compared with quantum diffusion Monte Carlo (DMC) calculations up to N=20, obtaining a good agreement and showing that this approximation works reasonably well. In this BH-ZPE approach, high stability configurations for Cor-(Ne) clusters are obtained at N=6, 14, 42, 51 and 57. However, we have not found any evidence of a first solvation layer using either the semiclassical BH-ZPE or the classical BH one., This work has been supported by the Spanish “Ministerio de Ciencia e Innovación” with Grants PID2019- 105225GB-100 (J.B, J.H-R.), FIS2017-83157-P (T.G-L.), FIS2017- 84391-C2-2-P (J.C.-M, MI.H. E.G.-A). E.G-A thanks fnancial support from Comunidad de Madrid (Spain), under the “Garantía Juvenil” Program (PEJ-2019-AI/IND-14267). Computation time by “Centro de Supercomputación de Galicia” (CESGA) and cluster TRUENO (CSIC) is also acknowledge.

Oxygen in its elemental form shows a variety of magnetic properties in its condensed phases; in particular, the epsilon solid phase loses its magnetism. These phenomena reflect the nature of the intermolecular forces present in the solid and the changes that arise with variations in pressure and temperature. In this study, we use intermolecular potentials obtained with unrestricted ab initio methods to model the singlet state of the oxygen tetramer [(O

A new spin-averaged potential energy surface (PES) for non-reactive O

Aims. In this work, we present a pruned set of state-to-state rate coefficients (STS rates) for inelastic H2:H2 collisions in the thermal range from 10 to 1000 K. The set includes all relevant rates needed for diagnostics based on the simulation of quadrupole infrared spectra of H2. Methods. The reported set was obtained from a quantum scattering close-coupling calculation employing a recent version of the MOLSCAT code, a high-level potential energy surface, and rotational energies of the H2 molecules with spectroscopic accuracy. These improvements have led to a significant increase in the accuracy with respect to previous computational results. The accuracy of the present STS rates is tested against recently reported experimental rates. Most dominant rates agree with the experiment within a 2 uncertainty (2 to 6%). Results. In addition to the tables given in the main text, three machine-readable tables are available at the CDS. These tables include all the relevant numerical results of the paper, namely, the excitation and de-excitation STS rates for H2:H2 inelastic collisions at selected temperatures between 10 and 1000 K, and their functional description for interpolation at any intermediate temperature.

11 pags., 7 figs., Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes, and compare for the first time with rigorous quantum calculations. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of 4He and 3He. In particular, a remarkable tunneling effect is reported on the Gr2 membrane at 10 K, leading to a much larger permeation of the lighter species as compared to the heavier isotope. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture., Y. V. S. and S. B. acknowledge the support of the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Projects: INFRASTRUCTURE/ 1216/0070 and Cy-Tera NEA YPODOMH/STPATH/0308/31). Y. V. S. was also supported by the Russian Foundation for Basic Research grant number 20-03-00833. J. C.-M. and M. I. H. were supported by Spanish MICINN with Grant FIS2017-84391-C2-2-P and PID2020- 114957GB-I00. We are also indebted to ‘‘Centro de Supercomputacio´n de Galicia’’ (CESGA) and specially to the people in the help desk.

13 pags., 11 figs., 5 tabs., Oxygen in its elemental form shows a variety of magnetic properties in its condensed phases; in particular, the epsilon solid phase loses its magnetism. These phenomena reflect the nature of the intermolecular forces present in the solid and the changes that arise with variations in pressure and temperature. In this study, we use intermolecular potentials obtained with unrestricted ab initio methods to model the singlet state of the oxygen tetramer [(O2)4], which is the unit cell, consistent with the non-magnetic character of this phase. To do this, we perform an analysis of the coupled–uncoupled representations of the spin operator together with a pairwise approximation and the Heisenberg Hamiltonian. We start from unrestricted potentials for the dimer calculated at a high level as well as different density functional theory (DFT) functionals and then apply a finite model to predict the properties of the epsilon phase. The results obtained in this way reproduce well the experimental data in the entire pressure range below 60 GPa. Additionally, we show the importance of calculating the singlet state of the tetramer as opposed to previous DFT periodic calculations, where the unrestricted description leads to a mixture of spin states and a poor comparison with the experiment. This point is crucial in the recent discussion about the coexistence of two epsilon phases: one where the identity of each O2 with spin S = 1 is retained within the tetramer unit vs another at higher pressures where the tetramer behaves as a single unit with a closed-shell character., M.A.V.-R. would like to thank CONACYT for a Ph.D. scholarship with No. 307123 and funding through the SNI program of research associates. We also acknowledge the Spanish Grant No. FIS2017-84391-C2-2-P for financial support.

The properties of molecular oxygen including its condensed phases continue to be of great relevance for the scientific community. The richness and complexity of its associated properties stem from the fact that it is a very stable diradical. Its open-shell nature leads to low-lying multiplets with total electronic spin S = 0, 1, 2 in the case of the dimer, (O

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 γ-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 treatments might lead to large errors., This work was supported by the Spanish MICINN with Grant FIS2017-84391-C2-2-P.

9 pags., 7 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 3.0. -- This article is part of the themed collection: 2018 PCCP HOT Articles, 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., This work has been funded by the MINECO with Grants FIS2014-51993-P, FIS2016-79596-P, FIS2017-84391-C2-2-P and FIS2017-83157-P, the Austrian Science Fund FWF (projects P26635 and W1259), and the Swedish Research Council (Contract No. 2016-06625). JOZ would like to thank Programa Operativo de Empleo Juvenil 2014–2020 of Fondo Social Europeo.

The noncovalent intermolecular interaction of Helium, Neon and Argon with naphthalene is investigated for selected dimer configurations by means of electronic structure calculations at the MP2C level of theory and exploiting complete basis set extrapolation techniques. An analytical formulation of the global potentials, based on simple and efficient atom–atom expressions, is also provided by optimizing few and physically meaningful parameters on the calculated benchmark interaction energies. The proper representation of the interaction so obtained has been exploited to predict and analyze energies and structures of the low-lying minima in naphthalene-RgN (N = 1–8) clusters by using Basin-Hopping and Diffusion Monte Carlo approaches.

10 pags., 8 figs., 2 tabs. -- Published as part of The Journal of Physical Chemistry virtual special issue “F. Javier Aoiz Festschrift”, Rotationally inelastic collisions of O2 with He in the 10−34 K thermal range are investigated by means of an experimental procedure based on supersonic gas jets probed by Raman spectroscopy. The procedure employs a kinetic master equation (MEQ) that describes the time evolution of the rotational populations of O2 along supersonic jets of O2 and He mixtures. The MEQ is expressed in terms of experimental quantities (number density and rotational populations) measured here, and state-to-state rate coefficients for the O2:He inelastic collisions calculated here, plus those for O2:O2 collisions from the literature. An agreement with the experiments is accomplished for temperatures between 10 and 34 K. Within this thermal range, the role of the fine structure due to electron spin in the collision dynamics of O2 is discussed., This work has been supported by the Spanish Ministerio de Economıá y Competitividad (MINECO), grants FIS2017- 84391-C2 and CONSOLIDER-ASTROMOL CSD2009-0038. F.G. thanks the Spanish Regional Government of Andalucıá for a postdoctoral grant (project P07-FQM-2600).

9 pags., 4 figs., 3 tabs., Complexes between metal cations and molecular hydrogen are systems quite amenable for precise spectroscopic and theoretical studies, and at the same time, they are relevant for applications in hydrogen storage and astrochemistry. In this work, we report new intermolecular potential energy surfaces and rovibrational states calculations for complexes involving molecular hydrogen and alkaline metal cations, M+−H2 (M+ = Na+, K+, Rb+, Cs+). The intermolecular potentials, formulated in an internally consistent way to emphasize differences in the properties of the systems, are represented by simple analytical expressions whose parameters have been optimized from comparison with accurate ab initio calculations. Properties of the low-lying bound statesbinding energies, frequencies, and rotational constantsare compared with previous measurements or computations and an overall good agreement is achieved, supporting the reliability of the present formulation. Variations of these properties as a function of the cation size and isotopic substitution, with a proper sequence of ortho and para rotational levels, are also discussed., The work has been funded by Spanish MINECO Grants FIS2017-84391-C2-2-P and FIS2017-83157-P. F.P. thanks the MIUR and the Universita degli Studi di Perugia for financial support to the project AMIS, through the program Dipartimenti di Eccellenza 2018−2022. Allocation of computing time by CESGA (Spain) is also acknowledged

Interactions of atomic cations with molecular hydrogen are of interest for a wide range of applications in hydrogen technologies. These interactions are fairly strong despite being non-covalent, hence one can ask whether hydrogen molecules would form dense, solid-like, solvation shells around the ion (snowballs) or rather a more weakly bound compound. In this work, the interactions between Cs+ and H2 are studied both experimentally and computationally. Isotopic substitution of H2 by D2 is also investigated. 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 via mass spectrometry. On the other hand, a new analytical potential energy surface, based on ab initio calculations, is developed and used to study cluster energies and structures by means of classical and quantum-mechanical Monte Carlo methods. The most salient features of the measured ion abundances are remarkably mimicked by the computed evaporation energies, particularly for the clusters composed of deuterium. This result supports the reliability of the present potential energy surface and allows us to recommend its use in related systems. Clusters with either twelve H2 or D2 molecules 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. This shell retains its internal structure when extra molecules are added to the second shell and is nearly solid-like, especially for the deuterated clusters. The role played by three-body induction interactions as well as the rotational degrees of freedom is analyzed and they are found to be significant (up to 15% and 18%, respectively) for the molecules belonging to the first solvation shell.

Solvation of Cs

11 pags., 13 figs., Solvation of Cs+ ions inside helium droplets has been investigated both experimentally and theoretically. On the one hand, mass spectra of doped helium clusters ionized with a crossed electron beam, HeNCs+, have been recorded for sizes up to N = 60. The analysis of the ratio between the observed peaks for each size N reveals evidences of the closure of the first solvation shell when 17 He atoms surround the alkali ion. On the other hand, we have obtained energies and geometrical structures of the title clusters by means of basin-hopping, diffusion Monte Carlo (DMC), and path integral Monte Carlo (PIMC) methods. The analytical He–Cs+ interaction potential employed in our calculations is represented by the improved Lennard-Jones expression optimized on high level ab initio energies. The weakness of the existing interaction between helium and Cs+ in comparison with some other alkali ions such as Li+ is found to play a crucial role. Our theoretical findings confirm that the first solvation layer is completed at N = 17 and both evaporation and second difference energies obtained with the PIMC calculation seem to reproduce a feature observed at N = 12 for the experimental ion abundance. The analysis of the DMC probability distributions reveals the important contribution from the icosahedral structure to the overall configuration for He12Cs+., This work was supported by the Austrian Science Fund, FWF (Project Nos. P23657 and P31149); the European Commission (ELEvaTE H2020 Twinning Project, Project No. 692335); and the MINECO with Grant Nos. FIS2014-51993-P, FIS2016-79596-P, FIS2017-84391-C2-2-P, and FIS2017-83157-P.

7 pags., 5 figs. -- Open Access funded by Creative Commons Atribution Licence 3.0, Interactions of atomic cations with molecular hydrogen are of interest for a wide range of applications in hydrogen technologies. These interactions are fairly strong despite being non-covalent, hence one can ask whether hydrogen molecules would form dense, solid-like, solvation shells around the ion (snowballs) or rather a more weakly bound compound. In this work, the interactions between Cs+ and H2 are studied both experimentally and computationally. Isotopic substitution of H2 by D2 is also investigated. 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 via mass spectrometry. On the other hand, a new analytical potential energy surface, based on ab initio calculations, is developed and used to study cluster energies and structures by means of classical and quantum-mechanical Monte Carlo methods. The most salient features of the measured ion abundances are remarkably mimicked by the computed evaporation energies, particularly for the clusters composed of deuterium. This result supports the reliability of the present potential energy surface and allows us to recommend its use in related systems. Clusters with either twelve H2 or D2 molecules 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. This shell retains its internal structure when extra molecules are added to the second shell and is nearly solid-like, especially for the deuterated clusters. The role played by three-body induction interactions as well as the rotational degrees of freedom is analyzed and they are found to be significant (up to 15% and 18%, respectively) for the molecules belonging to the first solvation shell., work has been funded by Spanish MINECO grants (FIS2014-51993-P, FIS2016-79596-P, FIS2017-84391-C2-2-P and FIS2017-83157-P) and by the Austrian Science Fund, Wien (FWF Projects P26635, P31149, and W1259). Allocation of computing time by the CESGA (Spain) is also acknowledged. JOZ would like to acknowledge Programa Operativo de Empleo Juvenil 2014-2020 from the European Social Fund., We acknowledge support of the publication fee by the CSIC Open Access Support Initiative through its Unit of Information Resources for Research (URICI)

16 pags., 11 figs., 3 tabs. -- This paper is part of the JCP Special Collection in Honor of Women in Chemical Physics and Physical Chemistry, A new spin-averaged potential energy surface (PES) for non-reactive O2(ςg-3) + O2(ςg-3) collisions is presented. The potential is formulated analytically according to the nature of the principal interaction components, with the main van der Waals contribution described through the improved Lennard-Jones model. All the parameters involved in the formulation, having a physical meaning, have been modulated in restricted variation ranges, exploiting a combined analysis of experimental and ab initio reference data. The new PES is shown to be able to reproduce a wealth of different physical properties, ranging from the second virial coefficients to transport properties (shear viscosity and thermal conductivity) and rate coefficients for inelastic scattering collisions. Rate coefficients for the vibrational inelastic processes of O2, including both vibration-to-vibration (V-V) and vibration-to-translation/rotation (V-T/R) energy exchanges, were then calculated on this PES using a mixed quantum-classical method. The effective formulation of the potential and its combination with an efficient, yet accurate, nuclear dynamics treatment allowed for the determination of a large database of V-V and V-T/R energy transfer rate coefficients in a wide temperature range., This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA17030100) and the National Natural Science Foundation of China through Grant Nos. 11372325 and 91116013. M.B. acknowledges the FIS2017-84391-C2-2-P Spanish grant for fundings.

7 pags., 5 figs., 1 tab., 1 app., 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 CH+ 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., The work has been funded by the Spanish grant FIS2017-84391-C2-2-P. Allocation of computing time by CESGA (Spain) is also acknowledged. RHL would like to thank CONACYT for a sabbatical scholarship

11 pags., 5 fgs., 4 tabs., Benchmark interaction energies between coronene, CH, and molecular hydrogen, H, have been computed by means of high level electronic structure calculations. Binding energies, equilibrium distances and strengths of the long range attraction, evaluated for the basic configurations of the H-CH complex, indicate that the system is not too affected by the relative orientations of the diatom, suggesting that its behavior can be approximated to that of a pseudoatom. The obtained energy profiles have confirmed the noncovalent nature of the bonding and serve to tune-up the parameters of a new force field based on the atom-bond approach which correctly describes the main features of the H-coronene interaction. The structure and binding energies of (para-H)-coronene clusters have been investigated with an additive model for the above mentioned interactions and exploiting basin-hopping and path integral Monte Carlo calculations for N = 1-16 at T = 2 K. Differences with respect to the prototypical (rare gas)-coronene aggregates have been discussed., The work has been funded by the Spanish grants FIS2013- 48275-C2-1-P, FIS2014-51993-P, FIS2013-41532-P and FIS2016- 79596-P. Allocation of computing time by CESGA (Spain) is also acknowledged. F. P. acknowledges Dipartimento di Chimica, Biologia e Biotecnogie di Perugia for financial support.

7 pags., 5 figs., Quantum phenomena are relevant to the transport of light atoms and molecules through nanoporous membranes. Indeed, confinement provided by (sub)nanometer pores enhances quantum effects such as tunneling and zero point energy (ZPE), even leading to quantum sieving of different isotopes of a given element. In this work, we apply rigorous wave packet techniques to the simulation in the three-dimensional space of the passage of atoms through periodic rigid one-atom-thick membranes. We aim to assess more approximate quantum models such as one-dimensional tunneling calculations or transition state theory (TST). Calculations are reported for the transmission of 3He and 4He through graphdiyne as well as through a holey graphene model. For He–graphdiyne, conclusions from tunneling-corrected TST are confirmed: both tunneling and ZPE effects are significant, but competition between each other leads to a moderately small 4He/3He selectivity, in stark contrast with predictions from one-dimensional tunneling calculations. For the transport of He isotopes through leaky graphene, the computed transmission probabilities are highly structured suggesting widespread selective adsorption resonances and, this time, rate coefficients and selectivity ratios do not agree well with estimations from TST. This study intends to go one step further toward a better understanding of the quantum-mechanical processes in the transport of gases through nanoporous membranes., The 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.

10 pags., 6 figs., 4 tabs., Clusters formed by the combination of rare gas (RG) atoms of He, Ne, Ar, and Kr on coronene have been investigated by means of a basin-hopping algorithm and path integral Monte Carlo calculations at T = 2 K. Energies and geometries have been obtained and the role played by the specific RG-RG and RG-coronene interactions on the final results is analysed in detail. Signatures of diffuse behavior of the He atoms on the surface of the coronene are in contrast with the localization of the heavier species, Ar and Kr. The observed coexistence of various geometries for Ne suggests the motion of the RG atoms on the multi-well potential energy surface landscape offered by the coronene. Therefore, the investigation of different clusters enables a comparative analysis of localized versus non-localized features. Mixed Ar-He-coronene clusters have also been considered and the competition of the RG atoms to occupy the docking sites on the molecule is discussed. All the obtained information is crucial to assess the behavior of coronene, a prototypical polycyclic aromatic hydrocarbon clustering with RG atoms at a temperature close to that of interstellar medium, which arises from the critical balance of the interactions involved. Published by AIP Publishing., This work has been supported by MICINN Grant No. FIS2011-29596-C02-01, FIS2013-48275-C2-1-P and FIS2014-51993-P. RR-C acknowledges funding from the Grant No. JAE-Pre-2010-01277. J. H.-R. and J. B. acknowledge the financial support from Ministerio de Economía y Competitividad under Grants No. FIS2013-41532-P. FP acknowledges Dipartimento di Chimica, Biologia e Biotecnogie di Perugia for financial support.

A detailed characterization of the interaction between the most abundant molecules in air is important for the understanding of a variety of phenomena in atmospherical science. A completely ab initio global potential energy surface (PES) for the O2((3)Σg(–)) + N2((1)Σg(+)) interaction is reported for the first time. It has been obtained with the symmetry-adapted perturbation theory utilizing a density functional description of monomers [SAPT(DFT)] extended to treat the interaction involving high-spin open-shell complexes. The computed interaction energies of the complex are in a good agreement with those obtained by using the spin-restricted coupled cluster methodology with singles, doubles, and noniterative triple excitations [RCCSD(T)]. A spherical harmonics expansion of the interaction potential containing a large number of terms due to the anisotropy of the interaction has been built from the ab initio data. The expansion coefficients, which are functions of the intermolecular distance, are matched in the long-range with the analytical functions based on the recent ab initio calculations of the electric properties of the monomers [M. Bartolomei et al. J. Comput. Chem. 2011, 32, 279]. The PES is tested against the second virial coefficient B(T) data and the integral cross sections measured with rotationally hot effusive beams, leading in both cases to a very good agreement. The lowest lying states of the complex have been computed and relevant spectroscopic features of the interacting complex are reported. A comparison with a previous experimentally derived PES is also provided.

5 pags.; 4 figs., We present density functional theory (DFT) calculations of the (O 2)4 cluster to shed light into the applicability of DFT methods to studies of the ε phase of solid oxygen. For the lower pressure range, loose clusters, DFT calculations cannot reproduce the nature of the interaction, producing artificial overbinding. For more compact geometries, single configuration character appears and some of the functionals studied agree well with benchmark calculations. In this situation -corresponding to higher pressures - a simple structural model of the solid leads to quite good agreement of the DFT predictions with measurements of the pressure dependence of the intra- and inter-clusters distances. © 2013 Elsevier B.V. All rights reserved., This Letter has been supported by Conacyt Grant 167921 and by Spanish Grant FIS2010-22064-C02-02.

Global potentials for the physisorption of rare-gas atoms on graphene and graphite, amenable for a variety of dynamics simulations, are reported. An atom-bond pairwise additive form of the potential is used, where the interaction pairs, represented by proper analytical functions, are constituted by the Rg atom (Rg = He, Ne, Ar, Kr) and the C-C bonds of the graphene sheet(s). The parameters of the atom-bond pair potential, derived from the polarizability of the interacting partners, are fine-tuned, exploiting calculations of the prototypical Rg-coronene system using high-level electronic structure methods and large basis sets. The atom-graphene/graphite potential is further expanded in a Fourier series, and it is found that for an accurate representation of the interaction only a small number of corrugation terms need to be added to the laterally averaged potential. Furthermore, this corrugation part of the potential is both identical for Rg-graphene and Rg-graphite; in other words, inner layers of graphite only play a role in the laterally averaged Rg-graphite potential. For all systems, the hollow at the center of the carbon ring is the preferred adsorption site, although diffusion barriers are low. The present results compare well with previous data regarding well depths and equilibrium distances at different adsorption sites and, for graphite, the long-range dispersion coefficient C3. In addition, binding energies (eigenvalues of the laterally averaged potentials) are in a fairly good agreement with experimental determinations, providing further support for the reliability of the potentials. © 2013 American Chemical Society., The work has been funded by Spanish grants FIS2010-22064- C02-02 and CSD2009-00038. Allocation of computing time by CESGA (Spain) and the COST-CMTS Action CM1002 “Convergent Distributed Environment for Computational Spectroscopy (CODECS)” are also acknowledged. F.P. acknowledges financial support from the Italian Ministry of University and Research (MIUR) for PRIN contracts.

10 págs.; 7 figs.; 3 tabs., Feynman−Hibbs (FH) effective potentials constitute an appealing approach for investigations of many-body systems at thermal equilibrium since they allow us to easily include quantum corrections within standard classical simulations. In this work we apply the FH formulation to the study of NeN-coronene clusters (N = 1−4, 14) in the 2−14 K temperature range. Quadratic (FH2) and quartic (FH4) contributions to the effective potentials are built upon Ne−Ne and Ne-coronene analytical potentials. In particular, a new corrected expression for the FH4 effective potential is reported. FH2 and FH4 cluster energies and structuresobtained from energy optimization through a basin-hopping algorithm as well as classical Monte Carlo simulationsare reported and compared with reference path integral Monte Carlo calculations. For temperatures T > 4 K, both FH2 and FH4 potentials are able to correct the purely classical calculations in a consistent way. However, the FH approach fails at lower temperatures, especially the quartic correction. It is thus crucial to assess the range of applicability of this formulation and, in particular, to apply the FH4 potentials with great caution. A simple model of N isotropic harmonic oscillators allows us to propose a means of estimating the cutoff temperature for the validity of the method, which is found to increase with the number of atoms adsorbed on the coronene molecule. © XXXX American Chemical Society, The work has been funded by Spanish MINECO Grant Nos. FIS2013-48275-C2-1-P, FIS2014-51993-P, and FIS2013- 41532-P. R.R.-C. is thankful for support from FIS2013- 48275-C2-1-P. Allocation of computing time by CESGA (Spain) and support by the COST-CMTS Action CM1405 “Molecules in Motion” are also acknowledged.

Under the terms of the Creative Commons Attribution (CC BY) license to their work., Coronene-doped helium clusters have been studied by means of classical and quantum mechanical (QM) methods using a recently developed He–C24H12 global potential based on the use of optimized atom-bond improved Lennard-Jones functions. Equilibrium energies and geometries at global and local minima for systems with up to 69 He atoms were calculated by means of an evolutive algorithm and a basin-hopping approach and compared with results from path integral Monte Carlo (PIMC) calculations at 2 K. A detailed analysis performed for the smallest sizes shows that the precise localization of the He atoms forming the first solvation layer over the molecular substrate is affected by differences between relative potential minima. The comparison of the PIMC results with the predictions from the classical approaches and with diffusion Monte Carlo results allows to examine the importance of both the QM and thermal effects., This work has been supported by MICINN Grant Nos. FIS2011-29596-C02-01, FIS2013-48275-C2-1-P, and FIS2014-51993-P. R.R.-C. acknowledges funding from the Grant No. JAE-Pre-2010-01277. J.H.-R. and J.B. acknowledge the financial support from Ministerio de Economía y Competitividad under Grant No. FIS2013-41532-P.

An optimal procedure for obtaining spherically averaged potentials for the interaction between two linear centrosymmetric molecules (Koide and Kihara, Chem Phys 1974, 5, 34) is extended here for the calculation of the leading anisotropy term of the interaction. It is found that by computing the potential just at nine specific relative orientations of the monomers, the main anisotropy term of the spherical harmonic expansion can be calculated exactly provided that the expansion does not involve more than the first 14 terms. The approach also serves to investigate, in an affordable manner, the dependence of the interaction potential on the intramolecular degrees of freedom, and, in this way, to go beyond the commonly used rigid rotor approximation. Application has been performed for the [O2(3Σ)]2 dimer by means of high-level ab initio calculations (RCCSD(T) method), and results are compared with a previous accurate rigid rotor potential energy surface. It is found that both the present isotropic and main anisotropy terms are very reliable and that the effect of the corrected vibrationally averaged potential is of about 1%. In addition, the dependence of the intermolecular potentials with the intramolecular vibrational modes is analyzed for the different geometries studied. The treatment can be quite useful in a realistic description of the main features of intermolecular interactions in more complex systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

The quantum mechanical theory for the scattering of two identical rigid rotors is reviewed and applied to the collision of O2(3Sigma(g)-) molecules using a new accurate ab initio potential energy surface (PES) for the quintet state of the composite system. The PES is based on calculations using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] [Bartolomei; et al. J. Chem. Phys. 2008, 128, 214304.]. This PES is extended here for large intermolecular distances using the ab initio long-range coefficients of Hettema et al. [J. Chem. Phys. 1994, 100, 1297.]. Elastic and rotationally inelastic integral cross sections have been obtained by means of close coupling calculations in the subthermal energy range (center-of-mass velocities below 500 m/s). Results are compared with those obtained using a PES derived from molecular beam experiments [Aquilanti; et al. J. Am. Chem. Soc. 1999, 121, 10794.]. General agreement is found between both PESs, although the experimentally derived PES appears as somewhat more anisotropic at least for the studied energy range. There is, however, a significant difference in the absolute value of the elastic cross sections that is due to differences in the long-range dispersion interaction. The performance of the ab initio PES for higher velocities (relevant to experiments) is also explored by retaining just the isotropic component of the interaction. A satisfactory agreement is found for the shape of the glory pattern but shifted toward lower absolute values of the cross sections.

The quenching of CO2(010) by thermal collisions with ground-state atomic oxygen is a crucial process in determining the cooling rates of the thermospheres of Earth, Venus and Mars and then to predict changes due to the increase of this green-house gas. One of the questions raised by the experiments and aeronomical observations is with regard to the physical origin of the non-Landau–Teller and negligible temperature dependence of the vibrational relaxation rate at the temperatures existing in the thermospheres (150K≤T≥550 K). In this context, we extend our previous work on quantum scattering calculations performed on high-quality ab initio potential energy surfaces (PESs) [M. P. de Lara-Castells, Marta I. Hernandez, G. Delgado-Barrio, P. Villarreal, and M. Lopez-Puertas, J. Chem. Phys. 124, 164302 (2006)]. and report translational-energy averaged quenching probabilities for the previously reported C2 v PES as well as for additional orientational approaches of the oxygen atom. Our results reveal the spin–or...

7 págs.; 2 figs.; 2 tabs., We report pressure broadening (PB) coefficients for the 21 electric–dipole transitions between the eight lowest rotational levels of ortho-H2O and para-H2O molecules by collisions with He at temperatures from 20 to 120 K. These coefficients are derived from recently published experimental state-to-state rate coefficients for H2O:He inelastic collisions, plus an elastic contribution from close coupling calculations. The resulting coefficients are compared to the available experimental data. Mostly due to the elastic contribution, the PB-coefficients differ much from line to line, and increase markedly at low temperature. The present results are meant as a guide for future experiments and astrophysical observations., This work has been supported by the Spanish Ministerio de Economía y Competitividad (MINECO), grants FIS2013- 48275-C2 and CONSOLIDER-ASTROMOL CSD2009-0038., CONSOLIDER-ASTROMOL CSD2009-0038

8 pags. ; 7 figs. ; 4 tabls. ; Supporting material: machine-readable tables, State-to-state rate coefficients for ortho-H2O:He and para-H2O:He inelastic collisions in the 20-120 K thermal range are investigated by means of an improved experimental procedure. This procedure is based on the use of a kinetic master equation (MEQ) which describes the evolution of populations of H2O rotational levels along a supersonic jet of H2O highly diluted in helium. The MEQ is expressed in terms of experimental observables and rate coefficients for H2O:He inelastic collisions. The primary experimental observables are the local number density and the populations of the rotational energy levels of H2O, quantities which are determined along the jet with unprecedented accuracy by means of Raman spectroscopy with high space resolution. Sets of rate coefficients from the literature and from present close-coupling calculations using two different potential energy surfaces (PESs) have been tested against the experiment. The Green et al. rate coefficients are up to 50% too low compared to the experiment, while most rates calculated here from the Hodges et al. PES and the Patkowski et al. PES are much closer to the experimental values. Experimental rates with an estimated accuracy on the order of 10% have been obtained for ortho-H2O:He and para-H2O:He inelastic collisions between 20 and 120 K by scaling and averaging the theoretical rates to the experiment. 2015. C The American Astronomical Society, This work has been supported by the Spanish Ministerios de Innovación (MICINN) and Economía y Competitividad (MINECO) through the research projects FIS2010-22064-C01, FIS2010-22064-C02, FIS2013-48275-C2-1-P, and FIS2013-48275-C2-2-P, and CONSOLIDER-ASTROMOL CSD2009-0038.

Accurate intermolecular potentials for the lowest three multiplet states of O2-O2 dimer have been produced on the basis of ab initio calculations. The quintet potential was taken from previous highly correlated CCSD(T) calculations. In this work, we perform MRCI calculations, with large basis sets including bond functions, of the singlet and triplet states, which are of multireference character. As expected the size inconsistency and lack of higher order excitations limit the accuracy of the MRCI potentials specifically in describing the long range interactions. We show that the Heisenberg Hamiltonian provides an accurate representation of the exchange interactions in this system and this enables us to combine the accurate CCSD(T) potentials with the MRCI spin-exchange parameter to obtain accurate singlet and triplet potentials. The reliability of these potentials is tested by computing integral cross sections and comparing them with the detailed experimental study of the Perugia group, with excellent results. More interestingly, comparison with the experimentally derived potential shows important discrepancies for some angular orientations including that corresponding with the global minima, indicating the need for further work, both theoretical and experimental, to clarify their origin.

The intermolecular potential of the quintet state of molecular oxygen dimer has been calculated using highly accurate ab initio calculations. Comparison with the best experimental intermolecular potential shows an excellent agreement for the equilibrium distances and binding energies of most geometrical configurations studied. Significant discrepancies occur for some configurations, most striking is the equilibrium distance for the global minimum and the binding energy of the linear configuration. Further analysis is realized by calculating the isotropic and anisotropic terms of the theoretical potential and comparing it with the experiment.

Isotopic substitution effects for H2(v 1 = high)+H2(v 2 = low) collisions are studied by means of time-dependent wave packet calculations. A three degree-of-freedom reduced dimensionality model was employed, which can account for collision-induced dissociation as well as for four centre exchange processes, for reactants in selected vibrational states. Mass effects are studied in two ways: by substituting the collider H2(v 2=low) by D2 in the same vibrational state, or by substituting the target H2(v 1 = high) by D2 in a state with a similar vibrational energy. For the collider in its ground vibrational state, we have found that four centre probability profiles are quite similar but, conversely, thresholds for dissociation probabilities are quite different for the different mass combinations. The H2(v 1 = high)+D2(v 2 = low) combination was found to be the most effective one for the dissociation process. In addition, the process of dissociation of the vibrationally cold diatom was found to be quite importa...

The van der Waals intermolecular potential for the dimer O2–O2 is determined as a function of both intermolecular and bond coordinates from ab initio calculations for two selected orientations of the diatomic molecules. A recent empirical method based on correlation formulae between the polarizability of the diatoms and some potential features is also employed and extended to determine a model potential which incorporates the dependence on the bond coordinate. The main features of the ab initio and empirical potential energy surfaces are compared and discussed. Quantum scattering calculations of the vibrational relaxation of O2(v) are performed by using both types of potentials for each selected geometry, providing evidence for the key role played by the representation of the vibrational dependence of the potential.

5 pags.; 3 figs., 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. © 2014 American Chemical Society., The work has been funded by Spanish Grant FIS2010-22064- C02-02. Allocation of computing time by CESGA (Spain) and the COST-CMTS Action CM1002 “Convergent Distributed Enviroment for Computational Spectroscopy (CODECS)” are also acknowledged.

7 pags.; 4 figs. ;*S Supporting Information Periodic DFT calculations as well as intermolecular potentials related to different configurations used for graphdiyne pore−methane computations. This material is available free of charge via the Internet at http://pubs.acs.org., 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. © 2014 American Chemical Society, The work has been funded by Spanish Grants FIS2010-22064-C02-02 and FIS2013-48275-C2-1-P. Allocation of computing time by CESGA (Spain) is also acknowledged. F.P. acknowledges financial support from the Italian Ministry of University and Research (MIUR) for PRIN 2010-2011, Grant 2010 ERFKXL_002.

Collision induced dissociation and four center exchange reaction in H2(v1=high)+H2(v2=low) are studied by means of time-dependent wave packet calculations and within a three-degree-of-freedom reduced dimensionality model. The role of both—vibrationally excited and vibrationally cold—collision partners is examined varying v1 between 10 and 14 and v2 between 0 and 2, respectively. From the analysis of the results, a clear picture of the main mechanisms of dissociation and reaction has been obtained, and the regions of the potential energy surface most sensitive to the dynamics have been identified. In this way, reaction bottlenecks are found to significantly depend on the initial v1 state, owing to the anharmonicity of these v1 states near dissociation and the different regions of the potential explored by the associated wave packets. The topography of such bottlenecks provide a basis for the existence of tunneling in (v1=10,12, v2=0–2) reactions. Regarding the dissociation process, we find that there are t...

Rate constants have been calculated using quasi-classical trajectories (QCT) for the vibrational relaxation of highly excited O2:O2(X3Σḡ, vi) + O2(X3Σḡ,0) → O2(X3Σḡ, vf) + O2(X3Σḡ) with 22 ≤vi≤28. The present full-dimensional QCT results agree very well with recent quantum reduced dimensionality calculations, giving further support to the hypothesis that the observed experimental jump in depletion rates would be due partially to enhanced vibrational relaxation. © 2000 Taylor & Francis Group, LLC. Fil: Campos-Martínez, José. Instituto de Matemática y Física Fundamental; España Fil: Carmona-Novillo, Estela. Instituto de Matemática y Física Fundamental; España Fil: Echave, Julián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Centro de Estudios e Investigación; Argentina Fil: Hernández, Marta I.. Instituto de Matemática y Física Fundamental; España Fil: Palma, Juliana Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Centro de Estudios e Investigación; Argentina

In this work we explore the application of a time-dependent Hartree (TDH) scheme to study the vibrational predissociation of Ne2I2 van der Waals clusters. The present approach is based on equations of motion extracted from the usual variational principle where the Hamiltonian has been previously represented in a set of diatomic vibrational states. The procedure leads to a set of coupled equations for the different modes on each diabatic state with, however, explicit separation between those modes. The application on a problem that inherently requires long-time propagation is shown to be successful. Calculated lifetimes compare well with previous calculations as well as with available experimental data. A more detailed mechanism, as the breath of the angular mode on the different vibrational channels, is better described.

16 p., The rovibrational structure of the He2Cl2 van der Waals cluster in the X and B electronic states is studied by means of full imensional quantum-mechanical calculations. He2Cl2 is the smallest cluster containing helium for which rotationally state-resolved data are available and for which the effects of Bose statistics are important. The He2Cl2 wave functions exhibit quite large amplitude motions, particularly for the He–He bending mode @associated with the angle formed between the two He–~center of mass of Cl2) bonds#. The preferred geometry of the ground van der Waals state is planar, with the He–He axis perpendicular to the Cl2 axis. It is shown that a reduced dimension model for the He–He bending vibration together with a rigid structural model reproduces well the low-lying energy levels of the complex and allows us to assign proper statistical weights to the asymmetric top transitions of the B←X spectra. In particular, the symmetry under He exchange of the rigid rotor levels is shown to depend on the He–He bending evel. The observed excitation spectra are successfully simulated using the proposed model. The effective rigid structure that best reproduces the rotationally excited levels with a rigid rotor analysis is a distorted tetrahedron where the He–He angle is approximately 130°. This difference from the 180° most probable configuration is due to the complex dependence of the moments of inertia on the internal degrees of freedom for such a floppy molecule. Therefore, structural information obtained from observed spectra of this or similar clusters should be carefully analyzed to avoid reaching misleading conclusions. Fragmentation rates for He2Cl2(B,v510– 13) are, for the first time, extracted from experimental data, confirming that the dissociation process is mainly sequential. © 2000 American Institute of Physics., This work has been supported by the D.G.I.C.Y.T. Grant No. PB95-0071 (Spain), CNRS (France), NSF ~USA!, and the Spanish–French cooperation program No. HF1999-0132.

A first-order diatomics-in-molecule potential energy surface (PES) for the excited B state of the HeBr2 Van der Waals complex is implemented for the calculation of its B←X vibronic spectra at high vibrational excitations v of the Br2(B) fragment. The parameters of the PES, which should describe true He–Br interactions, are determined empirically. It is found that three-body interactions included in the model markedly change the topological properties of the PES with respect to a simple pairwise one. In particular, they are responsible for the decrease of the frequency shift at v>33 observed experimentally. Better agreement with the experimental spectrum is also attained at very high vibrational excitations where the effects of intramolecular vibrational relaxation are essential.