More than 100 solid-state compounds of the formula RTX2 (R = rare earth metal or Ca, Sr, Ba, T = Group 7-11 3d, 4d, and 5d transition metal, X = Si, Ge, Sn) are known. Most of them adopt the CeNiSi2 or the related LaMnSi2 structure. These two structural types are related, in containing square TX nets and polyacetylene-like X chains, but with a different disposition of the TX units in their nets. Using density functional computational tools, we have investigated the electronic origin of the site preference by the transition metal T and the main group atom X in these two crystal structures. Due to the larger band dispersion of the atomic orbitals at sites with closer neighbors and the stronger bonding feature in the lower half of the energy bands arising, and antibonding features of the upper half of the energy bands, the LaMnSi2 type is preferred for low d electron count, and the CeNiSi2 type for high d electron count. More generally, variation in dispersion (inherently greater in lattice sites with more nearby overlapping atoms) interacts with electron occupation, to explain structural preferences in these and other ternary compounds.

The following comments on the review process emerge out of many years of experience on both sides of the process, as reviewer and as author.They are idiosyncratic, indelibly colored by a world view that values expression and communication, and appreciates the penetration of the personal and emotional into science.

In 1965, Woodward and Hoffmann proposed a theory to predict the stereochemistry of electrocyclic reactions, which, after expansion and generalization, became known as the Woodward-Hoffmann Rules. Subsequently, Longuet-Higgins and Abrahamson used correlation diagrams to propose that the stereoselectivity of electrocyclizations could be explained by the correlation of reactant and product orbitals with the same symmetry. Immediately thereafter, Hoffmann and Woodward applied correlation diagrams to explain the mechanism of cycloadditions. We describe these discoveries and their evolution. We now report an investigation of various electrocyclic reactions using DFT and CASSCF. We track the frontier molecular orbitals along the intrinsic reaction coordinate and modeled trajectories and examine the correlation between HOMO and LUMO for thermally forbidden systems. We also investigate the electrocyclizations of several highly polarized systems for which the Houk group had predicted that donor-acceptor substitution can induce zwitterionic character, thereby providing low-energy pathways for formally forbidden reactions. We conclude with perspectives on the field of pericyclic reactions, including a refinement as the meaning of Woodward and Hoffmann's "Violations. There are none!" Lastly, we comment on the burgeoning influence of computations on all fields of chemistry.

We predict theoretically a carbon-based clathrate in the bipartite sodalite structure, SrB3C3, that is thermodynamically stable at high pressure. This clathrate is predicted to be a dynamically stable superconductor with an estimated Tc of 42 K at ambient pressure. Calculated stress-strain relations for SrB3C3 clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Boron substitution aids in the stabilization of SrB3C3 clathrate, and offers valuable insights into design guidelines for various carbon-based materials.

The Bergman cyclization of (Z)-hexa-3-ene-1,5-diyne to form the aromatic diradical p-benzyne has garnered attention as a potential antitumor agent due to its relatively low cyclization barrier and the stability of the resulting diradical. Here, we present a theoretical investigation of several ionic extensions of the fundamental Bergman cyclization: electrocyclizations of the penta-1,4-diyne anion, hepta-1,6-diyne cation, and octa-1,7-diyne dication, leveraging the spin-flip formulation of the equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD). Though the penta-1,4-diyne anion exhibits a large cyclization barrier of +66 kcal mol-1, cyclization of both the hepta-1,6-diyne cation and octa-1,7-diyne dication along a previously unreported triplet pathway requires relatively low energy. We also identified the presence of significant aromaticity in the triplet diradical products of these two cationic cyclizations.

W do we value as an academic and a scientific community? Do our core values include only the pursuit of facts and inventions, to the exclusion of other considerations? Or do we accept that scientists have a responsibility to serve society beyond simply expanding the knowledge base, and should therefore concern themselves (at least in part) with how their words and actions intersect and impact the human sphere? A scientist’s innovations might be profound, benefiting many, but if that person’s words or actions create an alienating or hostile workplace or learning environment, then how should the scientific community evaluate that person’s overall contribution to humanity? How should society view such a person? These questions lie at the heart of an emerging conversation regarding what equality means for the greater scientific enterprise as we pursue increased diversity and inclusion of underrepresented groups at our universities. The same questions are also central to recent debate regarding whether the scientific community should continue to retain “named” scientific phenomena in cases where the eponymous scientist has engaged in conduct that is inconsistent with contemporary values, even if that behavior is entirely separate from their scientific discoveries. Whether namesake buildings, lectures, and awards should be renamed is also under discussion, and similar questions arise regarding a controversial personal essay that was retracted in 2020 by the journal Angewandte Chemie. This conversation is interwoven with the emergence of historically marginalized voices within society, particularly on social media, along with the emergence and evolution of “cancel culture” as a new narrative. Efforts to call out inappropriate speech or behavior can lead to legal, professional, or social consequences for those accused; to some, this represents “cancel culture run amok”. To critics, social media call-outs inhibit open debate and thereby threaten traditional academic freedom to express unpopular views. In this Guest Commentary, we suggest that the aforementioned efforts by universities and scientific journals, which are aimed at promoting inclusivity, are nothing at all like the actions of a totalitarian government, as some have suggested. Diversity efforts, especially those targeting faculty hiring, have sometimes been mischaracterized as exercises in “critical race theory”, but this is equally hyperbolic in our view. The question that we address is whether inclusivity efforts generally constitute unreasonable censorship and political correctness, or whether they are instead manifestations of a long-overdue reckoning about values. Below, we elaborate using several case studies before turning to broader questions related to the pursuit of diversity, equity, and inclusion as part of a path toward excellence at our universities and within our scientific community.

Nanothreads are one-dimensional macromolecules formed by pressure-induced polymerization along stacks of multiply unsaturated (or highly strained) molecules such as benzene (or cubane). Borazine is isoelectronic to benzene yet with substantial bond polarity, thus motivating a theoretical examination of borazine-derived nanothreads with degrees of saturation of 2, 4, and 6 (defined as the number of four-coordinated boron and nitrogen atoms per borazine formula unit). The energy increases upon going from molecular borazine to degree-2 borazine-derived threads and then decreases for degree-4 and degree-6 nanothreads as more σ bonds are formed. With the constraint of no more than two borazine formula units within the repeat unit of the framework's bonding topology, there are only 13 fully saturated (i.e., degree-6) borazine-derived nanothreads that avoid energetically costly homopolar bonds (as compared to more than 50 such candidates for benzene-derived threads). Only two of these are more stable than borazine. Hypothetical pathways from molecular borazine to these two degree-6 borazine-derived nanothreads are discussed. This relative paucity of outcomes may assist in kinetic control of reaction products. Beyond the high mechanical strength also predicted for carbon-based threads, properties such as piezoelectricity and flexoelectricity may be accessible to the polar lattice of borazine-derived nanothreads, with intriguing prospects for expression in these extremely thin yet rigid objects.

A brief and personal history of the binding interactions between Hans-Beat Bürgi and Roald Hoffmann is given, and a potential rearrangement of their molecules is considered.

This theoretical study examines the formation, structure, and stability of two of the most ordered nanothreads produced yet, those derived from furan and thiophene. The energetic consequences and activation barriers of the first two steps of oligomerization via a Diels-Alder mechanism were examined. The ca. 20 GPa difference in the synthetic pressures (lower for furan) is explainable in terms of the greater loss of aromaticity by the thiophene. The effects of pressure on the reaction profiles, operating through a volume decrease along the reaction coordinate, are illustrated. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with opposing, cumulative dipole moments. The computed activation volumes are consistently more negative for furan, in accordance with the lower onset pressure of furan polymerization. The energetics of three ordered polymer structures were examined. The syn polymer, with all O/S atoms on the same side, if not allowed to distort, is at a high energy relative to the other two due to the O/S lone pair repulsion, understandably greater for S than for O at the 2.8/2.6 Å separation. Set free, the syn isomers curve or arch in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The syn polymer can also stabilize itself by twisting into zig-zag or helical energy minima. The release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of the crystalline order in the polymer as it is returned to ambient pressure.

Chemie Ingenieur TechnikVolume 94, Issue 3 p. 440-440 VorschauFree Access Vorschau: Chem. Ing. Tech. 4/2022 First published: 23 February 2022 https://doi.org/10.1002/cite.202270307AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume94, Issue3Special Issue: Herrn Professor Thomas Hirth zum 60. Geburtstag gewidmetMarch 2022Pages 440-440 RelatedInformation

Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. Over the past decade, computational predictions have revealed that compression to terapascal pressures may bring about counter-intuitive changes in the structure and bonding of solids as quantum mechanical forces grow in influence 1 – 6 . Although this behaviour has been observed at modest pressures in the highly compressible light alkali metals 7 , 8 , it has not been established whether it is commonplace among high-pressure solids more broadly. We used shaped laser pulses at the National Ignition Facility to compress elemental Mg up to 1.3 TPa, which is approximately four times the pressure at the Earth’s core. By directly probing the crystal structure using nanosecond-duration X-ray diffraction, we found that Mg changes its crystal structure several times with non-close-packed phases emerging at the highest pressures. Our results demonstrate that phase transformations of extremely condensed matter, previously only accessible through theoretical calculations, can now be experimentally explored.

We present a quantum mechanical model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment. Studies of 93 atoms predict drastic changes to ground-state electronic configurations and electronegativity in the pressure range of 0-300 GPa. This extension of atomic reference data assists in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts. For example, we can speculate on the existence of pressure-induced polarity (red-ox) inversions in various alloys. Our study confirms that the filling of energy levels in compressed atoms more closely follows the hydrogenic aufbau principle, where the ordering is determined by the principal quantum number. In contrast, the Madelung energy ordering rule is not predictive for atoms under compression. Magnetism may increase or decrease with pressure, depending on which atom is considered. However, Hund's rule is never violated for single atoms in the considered pressure range. Important (and understandable) electron shifts, s -> p, s -> d, and d -> f are essential chemical and physical consequences of compression. Among the specific intriguing changes predicted are an increase in the range between the most and least electronegative elements with compression; a rearrangement of electronegativities of the alkali metals with pressure, with Na becoming the most electropositive s(1) element (while Li becomes s -> p group element and K and heavier become transition metals); phase transitions in Ca, Sr, and Ba correlating well with transitions; spin-reduction in all d-block atoms for which the valence d-shell occupation is d(n) (4 <= n <= 8); d -> f transitions in Ce, Dy, and Cm causing Ce to become the most electropositive element of the f-block; f -> d transitions in Ho, Dy, and Tb and a s -> f transition in Pu. At high pressure Sc and Ti become the most electropositive elements, while Ne, He, and F remain the most electronegative ones.

Abstract Two rare earth oxysulfides Ln 5V3O7S6 (Ln = La, Ce) have been synthesized and their structures determined. The two isostructural compounds crystallize in the orthorhombic space group Pmmn (no. 59). The structure features one-dimensional edge-sharing VS4O2 octahedron chains parallel to the b axis. The bonding between V and S/O is covalent, and between Ln 3+ and the rest of the matrix ionic. Magnetic susceptibility measurement revealed that V is in a mixed valence state of V3+ and V4+. Its magnetic behavior follows the Curie-Weiss law.

In this theoretical study we examine several aspects of the formation, structure, and stability of the most ordered nanothreads yet made, those derived from furan and thiophene. First, we look at the enthalpic consequences and activation barriers of the first two steps of oligomerization by a Diels-Alder mechanism. The ca. 20 GPa difference in the synthetic pressures (furan lower) is explainable in terms of greater loss of aromaticity by the thiophene. Subsequent steps have understandably lower barriers. We show explicitly how pressure affects the reaction profiles, operating through the volume decrease in the transition state and onward to the product molecule. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with two opposing and cumulative dipole moments. The computed activation volumes are consistently more negative for likely initial furan (compared with thiophene) polymerization steps, in accord with the lower onset pressure of furan polymerization. In the second part of our study we examine the energetics of the likely polymers. Three ordered polymer structures compete in enthalpy -- a syn one, with all O/S on the same side, an anti one, S/O alternating, and a syn-anti isomer, with segments of four monomers repeating. The syn polymer, if not allowed to distort, is at high enthalpy relative to the other two. The origin of the destabilization is apparent, being S/O lone-pair repulsion, understandably greater for S than O at the 2.8/2.6Å separation. Set free, the syn isomers curve or arc, in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The syn polymer can also stabilize itself by the thread twisting into zig-zag or helical enthalpic minima. Release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of crystalline order in the polymer as it is returned to ambient pressure.

This article describes the memoirs and history of correspondence of two outstanding chemists, R. Hoffman and A.A.Pasynskii. A number of discussions related to isoloble principle and the theory of molecular orbitals were considered.

In the second part of this Essay, we leave philosophy, and begin by describing Roald's being trashed by simulation. This leads us to a general sketch of artificial intelligence (AI), Searle's Chinese room, and Strevens' account of what a go-playing program knows. Back to our terrain-we ask "Quantum Chemistry, † ca. 2020?" Then we move to examples of Big Data, machine learning and neural networks in action, first in chemistry and then affecting social matters, trivial to scary. We argue that moral decisions are hardly to be left to a computer. And that posited causes, even if recognized as provisional, represent a much deeper level of understanding than correlations. At this point, we try to pull the reader up, giving voice to the opposing view of an optimistic, limitless future. But we don't do justice to that view-how could we, older mammals on the way to extinction that we are? We try. But then we return to fuss, questioning the ascetic dimension of scientists, their romance with black boxes. And argue for a science of many tongues.

A quantum chemical model for the study of the electronic structure of compressed atoms lends itself to a perturbation-theoretic analysis. It is shown, both analytically and numerically, that the increase of the electronic energy with increasing compression depends on the electronic configuration, as a result of the variable spatial extent of the atomic orbitals involved. The different destabilization of the electronic states may lead to an isobaric change of the ground-state electronic configuration, and the same first-order model paves the way to a simple thermodynamical interpretation of this process.