Has AuF7 Been Made?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.     

Has monopotassium phthalocyanine, KPc, been synthesized?

A recent publication described the synthesis and characterization of monopotassium phthalocyanine, KPc. A reevaluation of the original EPR, magnetic susceptibility, and X-ray diffraction data, along with a comparison to the powder diffraction pattern of an authentic sample of Na2Pc, indicates that the material was actually Na2Pc.     

Gaseous carbide theory. Has it been buried prematurely?

The status of the gaseous carbide theory of spike formation is discussed with due account of the recent critical publications. An analysis of the results of quadrupole mass spectrometric studies permits the conclusion that the principal assumption of the theory, namely, that gaseous carbides are present in over-equilibrium amounts, remains valid. A critical consideration of the alternative model of spike formation which is based on autocatalytic reduction of Al₂O₃ by aluminium vapor shows it to be inconsistent with thermodynamic and kinetics restrictions.     

Has the trimethylene radical cation been observed?: An Ab initio molecular orbital study

Ab initio calculations have been performed on the cyclopropane radical cation and the trimethylene radical cation. The former radical cation has been claimed to undergo irreversible ring opening to the latter in CFCl₂CF₂Cl matrices at 80 K. However, the computational results reported here show that the energy of the cyclopropane radical cation is much lower than that of the (0,0) trimethylene radical cation, thus casting doubt on the possibility of irreversible ring opening of the former to the latter. It is suggested that the ring-opened species that is observed in the matrix by EPR has a nucleophile strongly coordinated to the carbocationic center.     

Are metals made from molecules?

In the traditional view, covalently bound materials differ in a fundamental way from metallic substances. Though both are built from more basic units that are, in turn, constructed from a small number of atoms, for these two materials classes the nature of these units is thought to be quite different. For covalent solids and liquids, these units are considered to be molecular, meaning that they possess properties and bonding that are retained in the condensed phase and thus they continue to be identifiable within the larger system. For metallic materials, these basic units are considered to be mere constructs that are not observable against the delocalized bonding of metals or alloys. The perceived dissimilarity of metallic and covalently bound materials has fostered distinctly different approaches to their design and improvement. Here, the delocalized view of metallic bonding is examined. This examination suggests that much of the rationale used in the design of molecular materials my be applied to metals and alloys as well.     

Can AuF be synthesized? A theoretical study using relativistic configuration interaction and plasma modeling techniques

Theoretical studies using spin—orbit averaged relativistic configuration interaction and plasma modeling techniques indicate AuF to be a stable gas-phase compound and the dominant gas-phase species in the reaction of gold with O₂ and CF₄ between 1000 and 2700 K. Accurate spectroscopic properties are predicted and a synthesis for AuF is proposed.     

What is the smallest saturated acyclic alkane that cannot be made?

Saturated acyclic alkanes may show a high degree of strain if they have many branches close to each other. We report calculations which indicate how strained a molecule may become before it falls apart rapidly at room temperature and so allow us to identify the simplest alkane which cannot be made.     

Matrix Infrared Spectroscopy and Quantum‐Chemical Calculations for the Coinage‐Metal Fluorides: Comparisons of Ar?AuF,Ne?AuF,and Molecules MF2 and MF3

The reactions of laser‐ablated Au, Ag, and Cu atoms with F₂ in excess argon and neon gave new absorptions in the M? F stretching region of their IR spectra, which were assigned to metal‐fluoride species. For gold, a Ng? AuF bond was identified in mixed neon/argon samples. However, this bonding was much weaker with AgF and CuF. Molecules MF₂ and MF₃ (M=Au, Ag, Cu) were identified from the isotopic distribution of the Cu and Ag atoms, comparison of the frequencies for three metal fluorides, and theoretical frequency calculations. The AuF₅ molecule was characterized by its strongest stretching mode and theoretical frequency calculations. Additional evidence was observed for the formation of the Au₂F₆ molecule.     

Arsenic calamity in the Indian subcontinent What lessons have been learned?

Groundwater arsenic (As) contamination in West Bengal (WB, India) was first reported in December 1983, when 63 people from three villages of two districts were identified by health officials as suffering from As toxicity. As of October 2001, the authors from the School of Environmental Studies (SOES) have analyzed >105 000 water samples, >25 000 urine/hair/nail/skin-scale samples, screened approximately 86 000 people in WB. The results show that more than 6 million people in 2700 villages from nine affected districts (total population approximately 42 million) of 18 total districts are drinking water containing >/=50 mug l(-1) As and >300 000 people may have visible arsenical skin lesions. The As content of the physiological samples indicates that many more may be sub-clinically affected. Children in As-affected villages may be in special danger. In 1995, we had found three villages in two districts of Bangladesh where groundwater contained >/=50 mug l(-1) As. The present situation is that in 2000 villages in 50 out of total 64 districts of Bangladesh, groundwater contains As above 50 mug l(-1) and more than 25 million people are drinking water above >/=50 mug l(-1) As. After years of research in WB and Bangladesh, additional affected villages are being identified on virtually every new survey. The present research may still reflect only the tip of iceberg in identifying the extent of As contamination. Although the WB As problem became public almost 20 years ago, there are still few concrete plans, much less achievements, to solve the problem. Villagers are probably in worse condition than 20 years ago. Even now, many who are drinking As-contaminated water are not even aware of that fact and its consequences. 20 years ago when the WB government was first informed, it was a casual matter, without the realization of the magnitude this problem was to assume. At least up to 1994, one committee after another was formed but no solution was forthcoming. None of the expert reports has suggested solutions that involve awareness campaigns, education of the villagers and participation of the people. Initially, international aid agencies working in the subcontinent simply did not consider that As could be present in groundwater. Even now, while As in drinking water is being highlighted, there have been almost no studies on how additional As is introduced through the food chain, as large amounts of As are present in the agricultural irrigation water. Past mistakes, notably the ceaseless exploitation of groundwater for irrigation, continue unabated today; at this time, more groundwater is being withdrawn than ever before. No efforts have been made to adopt effective watershed management to harness the extensive surface water and rainwater resources of this region. Proper watershed management and participation by villagers are needed for the proper utilization of water resources and to combat the As calamity. As in groundwater may just be nature's initial warning about more dangerous toxins yet to come. What lessons have we really learned?     

Crystal Growth and Crystal Structures of Gold(V) Compounds: Cu(AuF6)2, Ag(AuF6)2, and O2(CuF)3(AuF6)4 ⋅ HF

Crystal growth from anhydrous hydrogen fluoride solutions of M²⁺ (M=Cu, Ag) and [AuF₆]⁻ gave M(AuF₆)₂ salts (M=Cu, Ag). Similar attempts to prepare single crystals of the corresponding nickel, zinc and magnesium salts failed. The crystal structure of Cu(AuF₆)₂ consists of layers of Cu²⁺ cations connected by [AuF₆]⁻ anions, thus forming slabs. Only van der Waals interactions exist between adjacent slabs. The crystal structure of Ag(AuF₆)₂ consists of a three-dimensional framework in which Ag⁺ cations are linked by [AuF₆]⁻ anions. Both structures are members of the M^II(X^VF₆)₂ family, in which seven different structure types have been observed to date. In the crystal structure of O₂(CuF)₃(AuF₆)₄ ⋅ HF, the bridging AuF₆ units connect [−Cu−F−Cu−F−]_∞ chains to form stacks between which O₂⁺ cations and HF molecules are located.     

Polymer modeling: where has it been and where is it going?

The development of the area of polymer modeling often referred to as molecular modeling has been reviewed from its early beginnings to the present day. Key forces influencing the development include computational power, algorithmic advances and access to computational resources. The desire to apply modeling techniques to predict the properties of increasingly complex polymer-containing systems, taken in conjunction with a number of current limitations discussed in this brief review, is expected to define in part some essential future developments.     

Regiochemistry in the Pauson-Khand reaction: has a trans effect been overlooked?

Alkyne-dicobalt hexacarbonyl complexes have been studied by DFT to examine whether electronic differences in the acetylenic substituents could play a role in determining the regiochemical outcome in the Pauson-Khand reaction. It appears that in some instances the regiochemistry is, in fact, governed exclusively by the electronic nature of the acetylenic substituents through the discriminant loss of a carbon monoxide ligand.     

How Has the Bridge Fragment Chosen to Design Charge Transfer Molecular Device?

Applying the time-dependent density functional theory (TD-DFT) method the design of molecular logical functions from the dithieno[3,2-b:2,3-d]thiophene (thiophene) and azo-dye Disperse Orange 3 (DO3) molecules joined with various electron-insulator bridges is performed. Based on the results of the investigations, the main design rules for the selection of the bridge fragment between a photo-acceptor and photo-donor of a charge transfer molecular device are proposed.     

Insertion of noble-gas atom (Kr and Xe) into noble-metal molecules (AuF and AuOH): are they stable?

The structure and the stability of a new class of insertion compounds of noble-gas atoms of the type AuNgX (Ng=Kr, Xe and X=F, OH) have been investigated theoretically through ab initio molecular-orbital calculations. All the species are found to have a linear structure with a noble-gas-noble-metal bond, the distance of which is comparable to covalent bond length except the AuKrOH system, for which it lies in between the covalent and van der Waals limits. The dissociation energies corresponding to the lowest-energy fragmentation products, AuX+Ng have been computed to be -166.2, -276.0, -194.4, and -257.6 kJ/mol for AuXeF, AuKrF, AuXeOH, and AuKrOH, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (Au-Ng-X bending mode) have been calculated to be 119.1, 74.9, 160.7, and 141.6 kJ/mol. However, three of these species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the Au+Ng+X fragments have been calculated to be 112.9, 3.0, and 18.7 kJ/mol for AuXeF, AuKrF, and AuXeOH, respectively, at the same level of theory. An analysis of the nature of interactions involved in the Au-Ng-X systems has been performed using Bader's topological theory of atoms-in-molecules (AIM). Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of Au-Ng bonds in these systems. This work might have important implications in the preparation of a new class of insertion compounds of noble-gas atoms containing noble-gas-noble-metal bond.     

On the accuracy of explicitly correlated coupled-cluster interaction energies--have orbital results been beaten yet?

The basis set convergence of weak interaction energies for dimers of noble gases helium through krypton is studied for six variants of the explicitly correlated, frozen geminal coupled-cluster singles, doubles, and noniterative triples [CCSD(T)-F12] approach: the CCSD(T)-F12a, CCSD(T)-F12b, and CCSD(T)(F12*) methods with scaled and unscaled triples. These dimers were chosen because CCSD(T) complete-basis-set (CBS) limit benchmarks are available for them to a particularly high precision. The dependence of interaction energies on the auxiliary basis sets has been investigated and it was found that the default resolution-of-identity sets cc-pVXZ/JKFIT are far from adequate in this case. Overall, employing the explicitly correlated approach clearly speeds up the basis set convergence of CCSD(T) interaction energies, however, quite surprisingly, the improvement is not as large as the one achieved by a simple addition of bond functions to the orbital basis set. Bond functions substantially improve the CCSD(T)-F12 interaction energies as well. For small and moderate bases with bond functions, the accuracy delivered by the CCSD(T)-F12 approach cannot be matched by conventional CCSD(T). However, the latter method in the largest available bases still delivers the CBS limit to a better precision than CCSD(T)-F12 in the largest bases available for that approach. Our calculations suggest that the primary reason for the limited accuracy of the large-basis CCSD(T)-F12 treatment are the approximations made at the CCSD-F12 level and the non-explicitly correlated treatment of triples. In contrast, the explicitly correlated second-order Mo?ller-Plesset perturbation theory (MP2-F12) approach is able to pinpoint the complete-basis-set limit MP2 interaction energies of rare gas dimers to a better precision than conventional MP2. Finally, we report and analyze an unexpected failure of the CCSD(T)-F12 method to deliver the core-core and core-valence correlation corrections to interaction energies consistently and accurately.     

Why have serine/threonine/tyrosine kinases been evolutionarily selected in eukaryotic signaling cascades?

The signal transduction systems of eukaryotes are different from those of prokaryotes with respect to their structures and mechanisms. The main signal transduction system of prokaryotes called the two-component system (TCS) is a one-step phosphorelay system composed of a histidine kinase (HK) while the central signal transduction system of eukaryotes called the mitogen-activated protein kinase (MAPK) cascade system (MCS) is a multi-step phosphorelay system composed of serine/threonine/tyrosine kinases (STYKs). The two signal transduction systems are also different in their transphosphorylation mechanisms. HK in the TCS transfers its own phosphate group to the response regulator protein while STYKs in the MCS phosphorylate other proteins using ATP. We were intrigued by the different dynamics resulting from such differences and wondered why STYKs instead of HKs have been evolutionarily selected in eukaryotic signaling cascades. In this paper, we compared the dynamical characteristics of two mathematical models which reflect such differences between the TCS and the MCS, and found that STYKs are more appropriate for cascade structures in eukaryotic signal transduction than HK with respect to the duration and settling time of response signals.