Nuclear Quadrupole Splittings in Rotational Spectra for the Possible Detection of Fine Structure Constant Variations: The Diatomic Gold Halides and Gold Hydride as Case Studies

The dependence of nuclear quadrupole coupling constants CNQC(α) on the fine-structure constant α for various diatomic gold molecules AuX (X = H, F, Cl, Br, and I) at the density functional level of theory is investigated. While the electric field gradient at gold is very sensitive to the density functional applied, the derivative with respect to α is less sensitive. From this, we can estimate the upper limit for the α variation in time, ∂CNQC/∂t, for the 197Au nuclear quadrupole coupling constant, which is on the order of 10–9 Hz/year. This is currently beyond the limit of high-precision spectroscopy. I demonstrate that α∂CNQC∂α can be estimated from relativistic effects in CNQC, which will be useful in further investigations

Xenon Suboxides Stable under Pressure

We present results from first-principles calculations on solid xenon–oxygen compounds under pressure. We find that the xenon suboxide Xe₃O₂ is the first compound to become more stable than the elements, at around <i>P</i> = 75 GPa. Other, even more xenon-rich compounds follow at higher pressures, while no region of enthalpic stability is found for the monoxide XeO. We establish the spectroscopic fingerprints of a variety of structural candidates for a recently synthesized xenon–oxygen compound at atmospheric pressure and, on the basis of the proposed stoichiometry XeO₂, suggest an orthorhombic structure that comprises extended sheets of square-planar-coordinated xenon atoms connected through bent Xe–O–Xe linkages

100 Years of the Lennard-Jones Potential

It is now 100 years since Lennard-Jones published his first paper introducing the now famous potential that bears his name. It is therefore timely to reflect on the many achievements, as well as the limitations, of this potential in the theory of atomic and molecular interactions, where applications range from descriptions of intermolecular forces to molecules, clusters, and condensed matter

100 Years of the Lennard-Jones Potential

It is now 100 years since Lennard-Jones published his first paper introducing the now famous potential that bears his name. It is therefore timely to reflect on the many achievements, as well as the limitations, of this potential in the theory of atomic and molecular interactions, where applications range from descriptions of intermolecular forces to molecules, clusters, and condensed matter

Trends in Inversion Barriers of Group 15 Compounds. 3. Are Fluorinated Pyridone Derivatives Planar or Nonplanar?

Fluorinated compounds of 4-pyridone are studied using the semiempirical PM3 method, and the ab initio HF and MP2 methods. The perfluorinated derivative of 4-pyridone is predicted to have a nonplanar ring structure with the fluorine ligand at the nitrogen atom lying above the pyridine ring. The inversion barrier for the pentafluoro-4-pyridone is predicted to be 26 kJ/mol similar to that found for NH3. This distortion corresponds to a static second-order Jahn−Teller effect and is expected to be experimentally detectable at low temperatures. N-Fluoro-4-pyridone is predicted to be nonplanar and has a small inversion barrier of 0.2 kJ/mol at the MP2 level. However, the maximum point of this barrier lies below the lowest zero-point out-of-plane inversion vibrational mode (1/2 84 cm-1 ≡ 0.5 kJ/mol). This corresponds to a dynamic Jahn−Teller effect and thus is experimentally difficult to verify. The MP2 calculations indicate that at least one fluorine atom is required at the ring nitrogen in order to achieve nonplanarity. Schleyer's negative-independent chemical shift method (NICS) is applied, and the results are used to discuss aromaticity in fluorinated pyridones. The NICS values show that succesive fluorination increases aromaticity. The vibrational spectra of all fluorinated pyridone derivatives are predicted. The vibrational spectrum of 4-pyridone is discussed in detail using a normal-mode analysis defined within a set of nonredundant internal coordinates

F₂Al(μ-η²:η²-O₂)AlF₂: An Unusual, Stable Aluminum Peroxo Compound

The oxidation processes in the industrial production of aluminum from cryolite melts are not fully understood. Oxidation of AlOF2- leads initially to AlOF2 radicals. The structure of the AlOF2 dimer and several oxidized and reduced forms of this compound are investigated by theoretical methods and compared to analogous boron and gallium compounds. The thermodynamic stability of these compounds is investigated. It is shown that the dimeric compound of AlOF2 contains a symmetric peroxo bridge and is unexpectedly stable toward decomposition

Helium Tunneling through Nitrogen-Functionalized Graphene Pores: Pressure- and Temperature-Driven Approaches to Isotope Separation

Recently, we showed that nitrogen-functionalized nanopores obtained by removing two rings from a perfect graphene sheet provide suitable barriers for a separation of fermionic helium-3 from its bosonic counterpart helium-4 [<i>J. Phys. Chem. Lett.</i> <b>2012</b>, <i>3</i>, 209–213]. In this follow-up Article, we provide potential curves for helium passing through several different types of pores, discuss the relation of the barrier height to the effective pore size, give estimations for bound states of helium attached to the pores, and analyze the effects of isomeric and stoichiometric variations of the pore-rim nitrogen-passivation on the gas separation performance. Slight deviations in the tunneling probability for the two helium isotopes can lead to a high selectivity at an industrially acceptable gas flux if the gas temperature is kept sufficiently low. We also recently showed that the mass-dependence of quantum tunneling and zero-point energy differences at the top of the potential energy barrier allow for a classically prohibited steady-state thermally driven isotope separation [<i>Chem. Phys. Lett.</i> <b>2012</b>, <i>521</i>, 118–124]. The nitrogen-passivated nanopores studied here give rise to larger steady-state isotopic enrichment than that in previous work and are dominated by zero-point energy differences at both high and low temperatures

Nucleation of Antiferromagnetically Coupled Chromium Dihalides: from Small Clusters to the Solid State

The nucleation of chromium dihalide clusters is investigated by studying clusters of the form CrnX2n (n ≤ 4, X = F, Cl, Br, and I) for different spin states and the corresponding low temperature solid-state modifications using density functional theory. Using both wave function based (coupled cluster) and density functional theory, we predict that in all cases the ground state of the CrX2 monomer is a bent 5B2 state arising from a weakly Renner−Teller distorted 5Πg state of the linear CrX2 unit. These quintet units can form antiferromagnetically coupled, two-dimensional chains with chromium being bridged by two halides and a nucleation growth pattern that resembles the structural motif found for the solid state. Deviations from this two-dimensional chain growth are only found for the trimers and tetramers of CrBr2 and CrI2, where a “triangular” three-dimensional geometry takes slight precedence over the planar ribbon motif. We find that each single CrX2 unit adds an almost constant amount of energy between 45 and 50 kcal/mol to the cluster growth. This is in accordance with the calculated sublimation energies for the solid state which gave 58 kcal/mol for CrF2, and between 41 and 46 kcal/mol for CrCl2, CrBr2, and CrI2. The large deviation of the calculated from the experimental sublimation energy for CrF2 is due to the high electronegativity of fluorine ligand, which substantially increases the ionic interactions, resulting in a much more tightly packed solid-state structure, which is not so well described by spin-broken density functional theory. In accordance with this, CrF2 shows an unusually large bulk modulus (395 kbar) compared to the heavier halides CrCl2 (82 kbar), CrBr2 (40 kbar), and CrI2 (18 kbar)

It’s Complicated: On Relativistic Effects and Periodic Trends in the Melting and Boiling Points of the Group 11 Coinage Metals

While the color of metallic gold is a prominent and well-investigated example for the impact of relativistic effects, much less is known regarding the influence on its melting and boiling point (MP/BP). To remedy this situation, this work takes on the challenging task of exploring the phase transitions of the Group 11 coinage metals Cu, Ag, and Au through nonrelativistic (NR) and scalar/spin–orbit relativistic (SR/SOR) Gibbs energy calculations with λ-scaled density-functional theory (λDFT). At the SOR level, the calculations provide BPs in excellent agreement with experimental values (1%), while MPs exhibit more significant deviations (2–10%). Comparing SOR calculations to those conducted in the NR limit reveals some remarkably large and, at the same time, some surprisingly small relativistic shifts. Most notably, the BP of Au increases by about 800 K due to relativity, which is in line with the strong relativistic increase of the cohesive energy, whereas the MP of Au is very similar at the SOR and NR levels, defying the typically robust correlation between MP and cohesive energy. Eventually, an inspection of thermodynamic quantities traces the trend-breaking behavior of Au back to phase-specific effects in liquid Au, which render NR Au more similar to SOR Ag, in line with a half-a-century-old hypothesis of Pyykkö

Molecular Geometry of Vanadium Dichloride and Vanadium Trichloride: A Gas-Phase Electron Diffraction and Computational Study

The molecular geometries of VCl2 and VCl3 have been determined by computations and gas-phase electron diffraction (ED). The ED study is a reinvestigation of the previously published analysis for VCl2. The structure of the vanadium dichloride dimer has also been calculated. According to our joint ED and computational study, the evaporation of a solid sample of VCl2 resulted in about 66% vanadium trichloride and 34% vanadium dichloride in the vapor. Vanadium dichloride is unambiguously linear in its 4Σg+ ground electronic state. For VCl3, all computations yielded a Jahn−Teller-distorted ground-state structure of C2v symmetry. However, it lies merely less than 3 kJ/mol lower than the 3E′′ state (D3h symmetry). Due to the dynamic nature of the Jahn−Teller effect in this case, rigorous distinction cannot be made between the planar models of either D3h symmetry or C2v symmetry for the equilibrium structure of VCl3. Furthermore, the presence of several low-lying excited electronic states of VCl3 is expected in the high-temperature vapor. To our knowledge, this is the first experimental and computational study of the VCl3 molecule