G. Scoles

Intermolecular forces in simple systems

Abstract A new, semi-empirical, method for the prediction of intermolecular forces introduced recently [Chem. Phys. Letters 36 (1975) 451] is assessed in the light of recently published dispersion forces coefficients and high precision SCF HF calculations of the repulsive interaction, performed by us, on closed shell spherical systems. The method is found to give very accurate predictions for systems with a small number of electrons and leads to the conclusion that those systems behave as the internal atomic correlation energy was almost independent from the interatomic distance. The extension of the method to heavier systems is discussed. Its application to the prediction of the potential in a mixture when the pure gas interactions are known is presented and the results shown to be satisfactory. The isotropic part of the interaction between two hydrogen molecules can also be described quite well using the same procedure.

Towards extending the applicability of density functional theory to weakly bound systems

While the attempts currently in progress in several groups for the rigorous inclusion of dispersion interactions in density functional theory (DFT) calculations mature and evolve into practical methodology, we contribute to the debate on the applicability of current functionals to the calculation of weak interaction with a systematic investigation of a few, typical, weakly bound systems. We have used both pure DFT and a hybrid approach in which the total interaction energy is partitioned into two parts: (a) the dispersion energy which, in a first approximation is the contribution due to intermonomer correlations and (b) all other interactions. The first component is accurately obtained at all distances of interest by means of a well-known damped multipolar expansion of the dispersion energy while for the second component different approximations will be evaluated. The need to avoid double counting a fraction of the correlation energy when using the hybrid approach and the choice of the appropriate functio...

Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces

Water is the renewable, bulk chemical that nature uses to enable carbohydrate production from carbon dioxide. The dream goal of energy research is to transpose this incredibly efficient process and make an artificial device whereby the catalytic splitting of water is finalized to give a continuous production of oxygen and hydrogen. Success in this task would guarantee the generation of hydrogen as a carbon-free fuel to satisfy our energy demands at no environmental cost. Here we show that very efficient and stable nanostructured, oxygen-evolving anodes are obtained by the assembly of an oxygen-evolving polyoxometalate cluster (a totally inorganic ruthenium catalyst) with a conducting bed of multiwalled carbon nanotubes. Our bioinspired electrode addresses the one major challenge of artificial photosynthesis, namely efficient water oxidation, which brings us closer to being able to power the planet with carbon-free fuels.

Intermolecular forces via hybrid Hartree–Fock–SCF plus damped dispersion (HFD) energy calculations. An improved spherical model

Molecular interactions are partitioned in SCF and correlation energy parts. It is shown that for atomic systems, one can join high quality ’’a priori’’ SCF calculations with a semiempirical estimate of the correlation energy, made using the standard long range multipolar expansion and corrected assuming the 3Σ+u state of H2 as a model (scaling the ’’size’’ of the atomic charge distribution as the ionization potential to the negative 2/3 power) to obtain very good agreement with the available experimental information.

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

In this article, recent developments in helium nanodroplet isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its ani...

X-ray Diffraction and Computation Yield the Structure of Alkanethiols on Gold(111)

The structure of self-assembled monolayers (SAMs) of long-chain alkyl sulfides on gold(111) has been resolved by density functional theory–based molecular dynamics simulations and grazing incidence x-ray diffraction for hexanethiol and methylthiol. The analysis of molecular dynamics trajectories and the relative energies of possible SAM structures suggest a competition between SAM ordering, driven by the lateral van der Waals interaction between alkyl chains, and disordering of interfacial Au atoms, driven by the sulfur-gold interaction. We found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first gold surface layer contains gold atom vacancies (which are partially redistributed over different sites) as well as gold adatoms that are laterally bound to two sulfur atoms.

Molecular order at the surface of an organic monolayer studied by low energy helium diffraction

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface‐sensitive and nondestructive technique shows that the CH3‐terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 A), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Superlattice structure at the surface of a monolayer of octadecanethiol self‐assembled on Au(111)

We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self‐assembled monolayer of long‐chain n‐alkanethiols. Our helium diffraction measurements for a monolayer of n‐octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 A containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [(√3×√3)R30°] array of lattice parameter 5.01 A previously observed for monolayers of other n‐alkanethiols on gold. No temperature‐dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n‐alkanes.

Infrared laser spectroscopy of molecular beams

A new technique suitable for infrared laser spectroscopy of molecular beams is presented and tested. The metod is based on the detection of the power absorbed, as a modulated increase of power carried by the molecules to a microcalorimeter (a cryogenic bolometer), after the molecular beam has been crossed with the modulated output of a semiconductor diode laser. Sensitivity, resolution, and possible applications are briefly discussed.

A simple but reliable method for the prediction of intermolecular potentials

Abstract A simple but reliable method for the prediction of intermolecular potentials is presented and tested for the case of the lighter noble gases and some of their mixtures.