Quasilinear Molecule par Excellence, SrCl2: Structure from High-Temperature Gas-Phase Electron Diffraction and Quantum-Chemical Calculations—Computed Structures of SrCl2⋅Argon Complexes

From Molecules to Solids: A vdW-DF-C09 Case Study of the Mercury Dihalides

The mercury dihalides show a remarkable diversity in the structural preferences in their minimum energy structure types, spanning molecular to strongly bound ionic solids. A challenge in the development of density functional methods for extended systems is to arrive at strategies that serve equally well such a broad range of bonding modes or structural preferences. The chemical bonding and the stabilities of mercury dihalides and the general utility and reliability of the van der Waals density functional with C09 exchange (vdW-DF-C09) in predicting or describing the energetics and structural preferences in these metal dihalides is examined. We show that, in contrast with the uncorrected generalized gradient approximation of the Perdew-Burke-Erzenhoff (PBE) exchange-correlation functional, qualitative and quantitative patterns in the bonding of the mercury dihalide solids are well reproduced with vdW-DF-C09 for the full series of HgX₂ systems for X = F, Cl, Br, and I. The possible existence of a low-temperature cotunnite polymorph for HgF₂ and PbF₂ is posited.

Structural effects in molecular metal halides

Metal halides are a relatively large class of inorganic compounds that participate in many industrial processes, from halogen metallurgy to the production of semiconductors. Because most metal halides are ionic crystals at ambient conditions, the term "molecular metal halides" usually refers to vapor-phase species. These gas-phase molecules have a special place in basic research because they exhibit the widest range of chemical bonding from the purely ionic to mostly covalent bonding through to weakly interacting systems. Although our focus is basic research, knowledge of the structural and thermodynamic properties of gas-phase metal halides is also important in industrial processes. In this Account, we review our most recent work on metal halide molecular structures. Our studies are based on electron diffraction and vibrational spectroscopy, and increasingly, we have augmented our experimental work with quantum chemical computations. Using both experimental and computational techniques has enabled us to determine intriguing structural effects with better accuracy than using either technique alone. We loosely group our discussion based on structural effects including "floppiness", relativistic effects, vibronic interactions, and finally, undiscovered molecules with computational thermodynamic stability. Floppiness, or serious "nonrigidity", is a typical characteristic of metal halides and makes their study challenging for both experimentalists and theoreticians. Relativistic effects are mostly responsible for the unique structure of gold and mercury halides. These molecules have shorter-than-expected bonds and often have unusual geometrical configurations. The gold monohalide and mercury dihalide dimers and the molecular-type crystal structure of HgCl(2) are examples. We also examined spin-orbit coupling and the possible effect of the 4f electrons on the structure of lanthanide trihalides. Unexpectedly, we found that the geometry of their dimers depends on the f electron configuration. Metal halides are unique in exhibiting strong vibronic interactions such as the Jahn-Teller effect and the related Renner-Teller effect. Some metal trihalide molecules have an almost T-shape due to static Jahn-Teller distortions. The nonlinear structure with a 150 degree bond angle of the chromium dichloride molecule demonstrates the Renner-Teller effect. Finally, we present a few examples of unknown structures that appear to be thermodynamically stable, including gold and silver triiodides and all silver subhalides. The combination of experimental and computational techniques has brought new insights to the structural chemistry of metal halides. We expect that the continuing progress in computational chemistry will shed further light on the intricate details of these and other molecular structures.

Why Are Some ML2 Molecules (M = Ca, Sr, Ba; L = H, F, Cl, Br) Bent while Others are Linear? Implications of the Pseudo Jahn−Teller Effect

The unexpected bent geometries of some alkaline earth dihalides and dihydrides, ML(2) (M = Ca, Sr, Ba; L = H, F, Cl, Br) have been explained in the literature using various models that attribute the effect to different phenomena like covalency, metal core polarization, sd-hybridization, and electron pair repulsion. We employ (based on first principles) the pseudo Jahn-Teller effect, as the only source of instability of high-symmetry configurations in nondegenerate states, to analyze the origin of the geometry of these systems and show that this approach explains all of their main structural features, including the topology of the Laplacian of the electron density and the vibrational frequencies. The main contribution to the distortion of the linear configuration is due to the pseudo Jahn-Teller mixing by bending of the sigma(u) HOMO formed by the ligand orbitals with the unoccupied pig orbitals of the metal (with main d(xz) and d(yz) character), resulting in new covalency which stabilizes the bent configuration. We show that the model approaches to the problem, mentioned above, are either restricted particular cases of the pseudo Jahn-Teller interaction, or they yield very small contributions to the instability that do not explain the origin of the bending. All of our conclusions are supported by high-quality ab initio calculations.

Molecular Constants of Aluminum Monohalides: Caveats for Computations of Simple Inorganic Molecules

Quantum chemical calculations have become an everyday tool in chemistry. There are commercial program packages and downloadable basis sets for most needs. However, many chemists rarely go beyond the routine use of these programs, rarely, if ever, checking the original references for basis sets. In this letter, we point out some of the pitfalls of such an approach. Structural parameters of the aluminum-monohalides, AlF, AlCl, AlBr, and AlI, have been calculated using the Gaussian 03 program package and different basis set combinations.