Molecular beam experiments are reported for collisions between oxygen molecules. Total integral cross sections have been measured as a function of the collision energy with the control of molecular alignment. The low collision energy (in the thermal and subthermal range) and the high angular resolution permit observation of the “glory” effect, manifestation of quantum-mechanical interference, which allows an accurate probe of intermolecular interactions. This first complete experimental characterization of the interaction yields a ground (singlet) state bond energy of 17.0 ± 0.8 meV for the most stable dimer geometry (the two oxygen molecules lying parallel at a distance of 3.56 ± 0.07 Å). Also the splittings among the singlet, the triplet, and the quintet surfaces are obtained, and a full representation of their angular dependence is reported via a novel harmonic expansion functional form for diatom−diatom interactions. These results indicate that most of the bonding in the dimer comes from van der Waals forces, but chemical (spin−spin) contributions in this open-shell/open-shell system are not negligible (∼15% of the van der Waals component of the interaction).

Two-dimensional (2D) materials deriving from graphene, such as graphdiyne and 2D polyphenylene honeycomb (2DPPH), have been recently synthesized and exhibit uniformly distributed sub-nanometer 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/CH 4 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 3 He and 4 He, we also find promising results for the separation of the fermionic isotope at low temperature.

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Molecular beam scattering measurements of total cross sections have been performed at a sufficiently low energy in the thermal range and at an angular resolution high enough to permit for the first time the observation of the "glory" interference effect in collisions of a benzene molecule with He, Ne, and Ar. Information on range, strength, and anisotropy of the interaction in the C 6 H 6rare gas dimers has been obtained from the analysis of the energy dependence of the total cross sections. In benzene-He, -Ne, and -Ar dimers bond energies are 0.98, 1.95, and 4.20 kJ/mol, respectively, for the most stable geometry, in all cases an out-ofplane configuration with the rare gas atom located on the 6-fold symmetry axis of benzene, at distances of 0.323, 0.331, and 0.359 nm, respectively. The results of the present investigation show that well depths for all three systems decrease by a factor 2 or 5 and corresponding distances increase by 40% or 70% for planar rare gas approaches respectively perpendicular to a C-C bond or collinear to a C-H bond (estimated uncertainties of 10% for bond energies and 3% for bond lengths). These experimental findings provide a crucial test of correlation formulas recently proposed (Chem. Phys. Lett. 2001, 350, 286-296) to estimate van der Waals minimum well depths and distances at selected approach geometries of rare gases on hydrocarbons.

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.