Unknowns in the Chemistry of boron

Structural links between benzenoid aromatics and graphite as well as saturated hydrocarbons and diamond are seen in high school text books. A similar understanding is only beginning to emerge in the chemistry of boron. Unlike benzenoid aromatics where the condensation is usually by edge-sharing, there are several ways of condensing polyhedral boranes. These include edge-sharing, triangular face sharing, four atom sharing and a single atom sharing. An electron counting mno rule similar to the Huckel 4n + 2 π electron counting rule will be presented for mono and condensed polyhedral boranes. Application of this rule shows that the structural details of β-rhombohedral boron, such as the vacancies and extra occupancies in the unit cell are a consequence of the electronic requirements rather than defects in the structure. While this is the beginning of a general understanding, there is a long way to go before the quantitative details emerge in this area, along with the synthesis and chemistry of a variety of condensed boranes. These ideas also provide a relationship between boron and fullerenes. Further, designing stable boron-rich candidates for physiological applications and materials for use in extreme conditions are areas that are waiting to be explored

Exocyclobutadieneite

This communication describes a novel structural-type that could be the basis for a potentially new allotrope of C. The novel structural-type is called “exocyclobutadieneite”, and it is thus named for the 1,3-dimethylenecyclobutane generating fragment that the lattice is based upon. It is a 3-,4-connected network consisting of slightly distorted tetrahedral vertices, and slightly distorted pairs of trigonal planar vertices. The lattice can be derived from a known mineral structure called Cooperite (PtS or PdO) by a topologically isomorphic substitution of trigonal planar atom pairs, for square planar vertexes, in the parent Cooperite unit cell. As such, the new pattern bears a distinct counterpoint relationship with its sibling structural-type called the glitter lattice, which has already been described by the authors in several other papers. And whereas glitter is generated by a topologically isomorphic substitution of trigonal planar atom pairs for the square planar vertices in the Cooperite unit cell, in a fashion that extends the unit cell vertically along the crystallographic c-axis, the exocyclobutadieneite structure is generated, in counterpoint, by such an isomorphic substitution that extends the unit cell horizontally along the a- and b-axes. Both the resulting glitter and exocyclobutadieneite structural-types possess AB2 stoichiometry (where A is a tetrahedral vertex and B is a trigonal planar vertex) and both occur in the tetragonal symmetry space group P42/mmc (#131). The networks differ, however, in the special positions adopted by the vertices in the resultant unit cells, and in their respective topology, as is evidenced by consideration of the Wells point symbols and Schläfli symbols for the 2 tetragonal networks. These differences are illustrated further in the course of the discussion to follow.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasConsejo Nacional de Investigaciones Científicas y Técnica

Structural phase transitions in perovskite BaCeO3 with data mining and first-principles theoretical calculations

Several experiments conducted over decades have revealed that the perovskite-structured BaCeO3 goes through a series of temperature-induced structural phase transitions. However, it has been frequently observed that the number of phases and the sequence in which they appear as a function of temperature differ between experiments. Insofar as neutron diffraction and Raman spectroscopy experiments are concern, four structures are well characterized with three transitions: Pnma to Imma [563 K] to R-3c [673 K] to Pm-3m [1173 K]. In contrast, thermoanalytical methods showed multiple singularities corresponding to at-least three more structural transitions at around 830 K, 900 K, and 1030 K. In account of these conflicting experimental findings, we computed free energy phase diagram for BaCeO3 employing crystal structure data mining in conjunction with first principles electronic structure and phonon lattice dynamics. A total of 34 polymorphs have been predicted, the most stable of which follows the Glazer classification of the perovskite tilt system. It has been predicted that the Cmcm and P4/mbm phases surpass Pnma at 666 K and 1210 K, respectively. At any temperature, two alternate tetragonal phases (P42/nmc and I4/mcm) are also found to be 20 to 30 meV less favored than the Pnma. While the calculated stability order of the predicted polymorphs is in acceptable agreement with the results of neutron diffraction, the transitions observed in thermoanalytical studies could be ascribed to the development of four novel phases (Cmcm, P4/mbm, P42/nmc, and I4/mcm) at intermediate temperatures. However, we analyze that the R-3c phase predominantly stabilized over a broad temperature field, masking all subsequent phases up until the cubic Pm-3m. Consequently, the novel phases predicted to occur in thermoanalytical studies are only fleetingly metastable.Comment: 20 pages, 5 figures, 1 tabl

Energy Landscapes 2019

This article summarizes the presentations delivered at the Energy Landscapes Conference held in Belgrade, Serbia, from 26 to 30 August 2019. The focus of the conference was on the present state of the art in theoretical energy landscape approaches, and their applications in the fields of chemistry, physics, biology, and materials science in general. The presentations were organized around some of the hot topics, such as applications from spectroscopy to the solid-state, folding and misfolding of proteins, DNA and RNA, multiscale modeling, materials under extreme pressure/temperature conditions, designing landscapes for self-assembly and multifunctional systems, landscapes for machine learning, atomic, molecular, colloidal and nanoalloy clusters

Pooled analysis of WHO Surgical Safety Checklist use and mortality after emergency laparotomy

Background The World Health Organization (WHO) Surgical Safety Checklist has fostered safe practice for 10 years, yet its place in emergency surgery has not been assessed on a global scale. The aim of this study was to evaluate reported checklist use in emergency settings and examine the relationship with perioperative mortality in patients who had emergency laparotomy. Methods In two multinational cohort studies, adults undergoing emergency laparotomy were compared with those having elective gastrointestinal surgery. Relationships between reported checklist use and mortality were determined using multivariable logistic regression and bootstrapped simulation. Results Of 12 296 patients included from 76 countries, 4843 underwent emergency laparotomy. After adjusting for patient and disease factors, checklist use before emergency laparotomy was more common in countries with a high Human Development Index (HDI) (2455 of 2741, 89.6 per cent) compared with that in countries with a middle (753 of 1242, 60.6 per cent; odds ratio (OR) 0.17, 95 per cent c.i. 0.14 to 0.21, P <0001) or low (363 of 860, 422 per cent; OR 008, 007 to 010, P <0.001) HDI. Checklist use was less common in elective surgery than for emergency laparotomy in high-HDI countries (risk difference -94 (95 per cent c.i. -11.9 to -6.9) per cent; P <0001), but the relationship was reversed in low-HDI countries (+121 (+7.0 to +173) per cent; P <0001). In multivariable models, checklist use was associated with a lower 30-day perioperative mortality (OR 0.60, 0.50 to 073; P <0.001). The greatest absolute benefit was seen for emergency surgery in low- and middle-HDI countries. Conclusion Checklist use in emergency laparotomy was associated with a significantly lower perioperative mortality rate. Checklist use in low-HDI countries was half that in high-HDI countries.Peer reviewe

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

Stuffed fullerenelike boron carbide nanoclusters

Viable stuffed fullerenelike boron carbide nanoclusters, C50B34, C48B36-2, and their isomers based on an icosahedral B84 fragment of elemental &#946;-rhombohedral boron have been investigated using density functional theory calculations. The structure and the stability of these clusters are rationalized using the polyhedral skeletal electron counting and ring-cap orbital overlap compatibility rules. The curvature of the fullerene was found to play a vital role in achieving the most stable isomer C50B34(3B). The large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, three dimensional aromaticity, and electron detachment energies support their high stability. Further, the IR and Raman active modes were recognized

Stuffing improves the stability of fullerenelike boron clusters

First-principles electronic structure calculations show that boron clusters B98, B99, B100, B101, and B102 based on icosahedral-B12 stuffed fullerenes are more stable than the fullerenelike boron clusters. These more stable structures are envisaged as an icosahedral B12 each vertex of which is connected to the apex of a pentagonal pyramid (B6) via radial 2c-2e &#963; bonds. The resulting B84 (B12@B12@B60) retains the same symmetry as C60, and the Bn (n=84-116) clusters are generated around it. We further project the possibility of producing such B12 based giant clusters

Boron and MgB<SUB>2</SUB> analogs of fullerenes and carbon nanotubes: a density functional theory study

The structural analogy between fullerene and fragments of elemental boron allotropes extends to the structure of MgB2, the simplest bulk high Tc (39 K) superconductor, and graphite. Electronic structure studies on equivalent structures in the boron regime relate the chemistry of carbon and boron. The fullerene and nanotube analogs, B84H50, Mg36B60, Mg30B60, Mg32B60 and (6,6) boron and MgB2 nanotubes were studied using first principles DFT calculations. Structures with all Mg atoms outside the B60 cage are more stable than the structures with Mg atoms inside. The energy difference between MgB2 sheet and nanotubes is close to that between graphite and carbon nanotubes. The &#945;-boron allotrope is calculated to be about 2.4 eV lower in energy than the boron nanotube. Analysis of band structures indicates that boron and MgB2 nanotubes are metallic