A theoretical study of C4B isomers. The interconversion of CCBCC and CCCCB via cyclic C4B

Theory suggests that CCBCC (1) will rearrange to planar cyclo-C4B (19) if the excess energy of 1 is greater than or equal to16.1 kcal mol(-1) [calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-31G(d) level of theory]. Cyclo-C4B lies only 1.1 kcal mol(-1) above CCBCC. The planar nature of symmetrical cyclo-C4B is attributed to multicentered bonding involving boron. If cyclo-C4B (19) has an excess energy of greater than or equal to24.4 kcal mol-1, it may ring open to form CCCCB (3).

Reactions of the hydroperoxide anion with dimethyl methylphosphonate in an ion trap mass spectrometer : evidence for a gas phase alpha-effect

The gas phase degradation reactions of the chemical warfare agent (CWA) simulant, dimethyl methylphosphonate (DMMP), with the hydroperoxide anion (HOO(-)) were investigated using a modified quadrupole ion trap mass spectrometer. The HOO(-) anion reacts readily with neutral DMMP forming two significant product ions at m/z 109 and m/z 123. The major reaction pathways correspond to (i) the nucleophilic substitution at carbon to form \[CH(3)P(O)(OCH(3))O](-) (m/z 109) in a highly exothermic process and (ii) exothermic proton transfer. The branching ratios of the two reaction pathways, 89% and 11% respectively, indicate that the former reaction is significantly faster than the latter. This is in contrast to the trend for the methoxide anion with DMMP, where proton transfer dominates. The difference in the observed reactivities of the HOO(-) and CH(3)O(-) anions can be considered as evidence for an a-effect in the gas phase and is supported by electronic structure calculations at the B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d) level of theory that indicate the S(N)2(carbon) process has an activation energy 7.8 kJ mol(-1) lower for HOO(-) as compared to CH(3)O(-). A similar alpha-effect was calculated for nucleophilic addition-elimination at phosphorus, but this process an important step in the perhydrolysis degradation of CWAs in solution - was not observed to occur with DMMP in the gas phase. A theoretical investigation revealed that all processes are energetically accessible with negative activation energies. However, comparison of the relative Arrhenius pre-exponential factors indicate that substitution at phosphorus is not kinetically competitive with respect to the S(N)2(carbon) and deprotonation processes.

Comparison of mechanical and ultrasound elastic modulus of ovine tibial cortical bone

Finite element models of bones can be created by deriving geometry from anx-ray CT scan. Material properties such as the elastic modulus can then be applied using either a single or set of homogeneous values, or individual elements can have local values mapped onto them. Values for the elastic modulus can be derived from the CT density values using an elasticityversus density relationship. Many elasticity–density relationships have been reported in the literature for human bone. However, while ovine in vivo models are common in orthopaedic research, no work has been done to date on creating FE models of ovine bones. To create these models and apply relevant material properties, an ovine elasticity-density relationship needs to be determined. Using fresh frozen ovine tibias the apparent density of regions of interest was determined from a clinical CT scan. The bones were the sectioned into cuboid samples of cortical bone from the regions of interest. Ultrasound was used to determine the elastic modulus in each of three directions – longitudinally, radially and tangentially. Samples then underwent traditional compression testing in each direction. The relationships between apparent density and both ultrasound, and compression modulus in each directionwere determined. Ultrasound testing was found to be a highly repeatable non-destructive method of calculating the elastic modulus, particularly suited to samples of this size. The elasticity-density relationships determined in the longitudinal direction were very similar between the compression and ultrasound data over the density range examined.A clear difference was seen in the elastic modulus between the longitudinal and transverse directions of the bone samples, and a transverse elasticity-density relationship is also reported.

Nb2O5 Schottky based ethanol vapour sensors : effect of metallic catalysts

The aim of this paper is to compare the performances of the highly porous Nb2O5 Schottky based sensors formed using different catalytic metals for ethanol vapour sensing. The fabricated sensors consist of a fairly ordered nano-vein like porous Nb2O5 prepared via an elevated temperature anodization method. Subsequently, Pt, Pd and Au were sputtered as both Schottky contacts and catalysts for the comparative studies. These metals are chosen as they have large work functions in comparison to the electron affinity of the anodized Nb2O5. It is demonstrated that the device based on Pd/Nb2O5 Schottky contact has the highest sensitivity amongst the developed sensors. The sensing behaviors were studied in terms of the Schottky barrier height variations and properties of the metal catalysts.

The loss of carbon dioxide from activated perbenzoate anions in the gas phase : Unimolecular rearrangement via epoxidation of the benzene ring

The unimolecular reactivities of a range of perbenzoate anions (X-C6H5CO3-), including the perbenzoate anion itself (X=H), nitroperbenzoates (X=para-, meta-, ortho-NO2), and methoxyperbenzoates (X=para-, meta-OCH3) were investigated in the gas phase by electrospray ionization tandem mass spectrometry. The collision-induced dissociation mass spectra of these compounds reveal product ions consistent with a major loss of carbon dioxide requiring unimolecular rearrangement of the perbenzoate anion prior to fragmentation. Isotopic labeling of the perbenzoate anion supports rearrangement via an initial nucleophilic aromatic substitution at the ortho carbon of the benzene ring, while data from substituted perbenzoates indicate that nucleophilic attack at the ipso carbon can be induced in the presence of electron-withdrawing moieties at the ortho and para positions. Electronic structure calculations carried out at the B3LYP/6311++G(d,p) level of theory reveal two competing reaction pathways for decarboxylation of perbenzoate anions via initial nucleophilic substitution at the ortho and ipso positions, respectively. Somewhat surprisingly, however, the computational data indicate that the reaction proceeds in both instances via epoxidation of the benzene ring with decarboxylation resulting-at least initially-in the formation of oxepin or benzene oxide anions rather than the energetically favored phenoxide anion. As such, this novel rearrangement of perbenzoate anions provides an intriguing new pathway for epoxidation of the usually inert benzene ring.

A mass spectrometry study of XCO+, X = Si, Ge : Is SiCO+ a main group carbonyl? Comments on the bonding in ground state SiCO and the Si,C,O (+) potential energy surface

The cation\[Si,C,O](+) has been generated by 1) the electron ionisation (EI) of tetramethoxysilane and 2) chemical ionisation (CI) of a mixture of silane and carbon monoxide. Collisional activation (CA) experiments performed for mass-selected \[Si,C,O](+), generated by using both methods, indicate that the structure is not inserted OSiC+; however, a definitive structural assignment as Si+-CO, Si+-OC or some cyclic variant is impossible based on these results alone. Neutralisation-reionisation (+NR+) experiments for EI-generated \[Si,C,O](+) reveal a small peak corresponding to SiC+, but no detectable SiO+ signal, and thus establishes the existence of the Si+-CO isomer. CCSD(T)//B3LYP calculations employing a triple-zeta basis set have been used to explore the doublet and quartet potential-energy surfaces of the cation, as well as some important neutral states The results suggest that both Si+-CO and Si+ - OC isomers are feasible; however, the global minimum is (2)Pi SiCO+. Isomeric (2)Pi SiOC+ is 12.1 kcal mol(-1) less stable than (2)Pi SiCO+, and all quartet isomers are much higher in energy. The corresponding neutrals Si-CO and Si-OC are also feasible, but the lowest energy Si - OC isomer ((3)A") is bound by only 1.5 kcal mol(-1). We attribute most, if nor all, of the recovery signal in the +NR' experiment to SiCO+ survivor ions. The nature of the bonding in the lowest energy isomers of Si+ -(CO,OC) is interpreted with the aid of natural bond order analyses, and the ground stale bonding of SiCO+ is discussed in relation to classical analogues such as metal carbonyls and ketenes.

Ion-molecule reactions of O,S-Dimethyl methylphosphonothioate : Evidence for intramolecular sulfur oxidation during VX perhydrolysis

The alkaline perhydrolysis of the nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) was investigated by studying the ion-molecule reactions of HOO(-) with O,S-dimethyl methylphosphonothioate in a modified linear ion-trap mass spectrometer. In addition to simple proton transfer, two other abundant product ions are observed at m/z 125 and 109 corresponding to the S-methyl methylphosphonothioate and methyl methylphosphonate anions, respectively. The structure of these product ions is demonstrated by a combination of collision-induced dissociation and isotope-labeling experiments that also provide evidence for their formation by nucleophilic reaction pathways, namely, (i) S(N)2 at carbon to yield the S-methyl methylphosphonothioate anion and (ii) nucleophilic addition at phosphorus affording a reactive pentavalent intermediate that readily undergoes internal sulfur oxidation and concomitant elimination of CH(3)SOH to yield the methyl methylphosphonate anion. Consistent with previous Solution phase observations of VX perhydrolysis, the toxic P-O cleavage product is not observed in this VX model system and theoretical calculations identify P-O cleavage to be energetically uncompetitive. Conversely, intramolecular sulfur oxidation is calculated to be extremely exothermic and kinetically accessible explaining its competitiveness with the facile gas phase proton transfer process. Elimination of a sulfur moiety deactivates the nerve agent VX and thus the intramolecular sulfur oxidation process reported here is also able to explain the selective perhydrolysis of the nerve agent to relatively nontoxic products.

Differences in membrane acyl phospholipid composition between an endothermic mammal and an ectothermic reptile are not limited to any phospholipid class

This study examined questions concerning differences in the acyl composition of membrane phospholipids that have been linked to the faster rates of metabolic processes in endotherms versus ectotherms. In liver, kidney, heart and brain of the ectothermic reptile, Trachydosaurus rugosus, and the endothermic mammal, Rattus norvegicus, previous findings of fewer unsaturates but a greater unsaturation index (UI) in membranes of the mammal versus those of the reptile were confirmed. Moreover, the study showed that the distribution of phospholipid head-group classes was similar in the same tissues of the reptile and mammal and that the differences in acyl composition were present in all phospholipid classes analysed, suggesting a role for the physical over the chemical properties of membranes in determining the faster rates of metabolic processes in endotherms. The most common phosphatidylcholine (PC) molecules present in all tissues (except brain) of the reptile were 16:0/18:1, 16:0/18:2, 18:0/18:2, 18:1/18:1 and 18:1/18:2, whereas arachidonic acid (20:4), containing PCs 16:0/ 20: 4, 18: 0/ 20: 4, were the common molecules in the mammal. The most abundant phosphatidylethanolamines ( PE) used in the tissue of the reptile were 18:0/18:2, 18:0/20:4, 18:1/18:1, 18:1/18:2 and 18:1/20:4, compared to 16: 0/ 18: 2, 16: 0/ 20: 4, 16: 0/ 22: 6, 18: 0/ 20: 4, 18: 0/ 22: 6 and 18:1/20: 4 in the mammal. UI differences were primarily due to arachidonic acid found in both PC and PEs, whereas docosahexaenoic acid (22:6) was a lesser contributor mainly within PEs and essentially absent in the kidney. The phospholipid composition of brain was more similar in the reptile and mammal compared to those of other tissues.

New approaches to the synthesis of strapped porphyrin containing bipyridinium [2]rotaxanes

Inspired by the interesting photo- and electrochemical properties observed in bipyridinium and porphyrin containing interlocked catenanes, herein we describe new approaches towards the synthesis of related rotaxanes. Previous efforts in this domain had been hampered by the limited range of chemical reactions that are compatible with these motifs, however the use of a “click” methodology, together with a better understanding of the size of these strapped porphyrin macrocycles, resulted in the synthesis of a bipyridinium porphyrin [2]rotaxane in modest yields. X-ray crystallography of the zinc metalloporphyrin macrocycle used in this study revealed that in the solid state, these strapped porphyrins adopt a 1-dimensional coordination polymer, in which an oxygen atom in the strap of one macrocycle is coordinated to the zinc metal center in an adjacent porphyrin ring

Phosphorus-based functional groups as hydrogen bonding templates for rotaxane formation

We report on the use of the hydrogen bond acceptor properties of some phosphorus-containing functional groups for the assembly of a series of [2]rotaxanes. Phosphinamides, and the homologous thio– and selenophosphinamides, act as hydrogen bond acceptors that, in conjunction with an appropriately positioned amide group on the thread, direct the assembly of amide-based macrocycles around the axle to form rotaxanes in up to 60% yields. Employing solely phosphorus-based functional groups as the hydrogen bond accepting groups on the thread, a bis(phosphinamide) template and a phosphine oxide-phosphinamide template afforded the corresponding rotaxanes in 18 and 15 % yields, respectively. X-Ray crystallography of the rotaxanes shows the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and various hydrogen bond accepting groups on the thread, including rare examples of amide-to-phosphonamide, -thiophosphinamide and -selenophosphinamide groups. With a phosphine oxide-phosphinamide thread, the solid state structure of the rotaxane is remarkable, featuring no direct intercomponent hydrogen bonds but rather a hydrogen bond network involving water molecules that bridge the H-bonding groups of the macrocycle and thread through bifurcated hydrogen bonds. The incorporation of phosphorus-based functional groups into rotaxanes may prove useful for the development of molecular shuttles in which the macrocycle can be used to hinder or expose binding ligating sites for metal-based catalysts.

Sulfur-containing amide-based [2]rotaxanes and molecular shuttles

We report the synthesis, structure and properties of [2]rotaxanes prepared by the assembly of benzylic amide macrocycles around a series of amide and sulfide-/sulfoxide-/sulfone-containing threads. The efficacy of rotaxane formation is related to the hydrogen bond accepting properties of the various sulfur-containing functional groups in the thread, with the highest yields (up to 63% with a rigid vinyl spacer in the template site) obtained for sulfoxide rotaxanes. X-Ray crystallography of a sulfoxide rotaxane, 5, shows that the macrocycle adopts a highly symmetrical chair-like conformation in the solid state, with short hydrogen bonds between the macrocycle isophthalamide NH-protons and the amide carbonyl and sulfoxide S-O of the thread. In contrast, in the X-ray crystal structures of the analogous sulfide (4) and sulfone (6) rotaxanes the macrocycle adopts boat-like conformations with long intercomponent NH…O=SO and NH…S hydrogen bonds (in addition to several intercomponent amide-amide hydrogen bonds). Taking advantage of the different hydrogen bonding modes of the sulfur-based functional groups, a switchable molecular shuttle was prepared in which the oxidation level of sulfur determines the position of the macrocycle on the thread.

Minimization of bauxite residue neutralization products using nanofiltered seawater

Currently, open circuit Bayer refineries pump seawater directly into their operations to neutralize the caustic fraction of the Bayer residue. The resulting supernatant has a reduced pH and is pumped back to the marine environment. This investigation has assessed modified seawater sources generated from nanofiltration processes to compare their relative capacities to neutralize bauxite residues. An assessment of the chemical stability of the neutralization products, neutralization efficiency, discharge water quality, bauxite residue composition, and associated economic benefits have been considered to determine the most preferable seawater filtration process based on implementation costs, savings to operations and environmental benefits. The mechanism of neutralization for each technology was determined to be predominately due to the formation of Bayer hydrotalcite and calcium carbonate, however variations in neutralization capacity and efficiencies have been observed. The neutralization efficiency of each feed source has been found to be dependent on the concentration of magnesium, aluminium, calcium and carbonate. Nanofiltered seawater with approximately double the amount of magnesium and calcium required half the volume of seawater to achieve the same degree of neutralization. These studies have revealed that multiple neutralization steps occur throughout the process using characterization techniques such as X-ray diffraction (XRD), infrared (IR) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Improvements to bauxite residue neutralisation efficiency through the use of alternative feed sources

Currently, open circuit Bayer refineries pump seawater directly into their operations to neutralise the caustic fraction of the Bayer residue. The resulting supernatant has a reduced pH and is pumped back to the marine environment. This investigation has assessed modified seawater sources generated from different ion filtration processes to compare their relative capacities to neutralise bauxite residues. An assessment of the chemical stability of the neutralisation products, neutralisation efficiency, discharge water quality, bauxite residue composition, and associated economic benefits have been considered to determine the most preferable seawater filtration process based on implementation costs, savings to operations and environmental benefits. The mechanism of neutralisation for each technology was determined to be predominately due to the formation of Bayer hydrotalcite and calcium carbonate, however variations in neutralisation capacity and efficiencies have been observed. The neutralisation efficiency of each feed source has been found to be dependent on the concentration of magnesium, aluminium, calcium and carbonate. These studies have revealed that multiple neutralisation steps occur throughout the process. Environmental, economic and social advantages and disadvantages of the different filtration technologies have been explored to determine the most sustainable method for the neutralisation of bauxite residues. The relative degree of “green” associated with nanofiltered seawater and reverse osmosis filtered seawater are discussed.

Chemo-enzymatic synthesis of unnatural amino acids

A general chemo-enzymatic process has been developed to prepare enantiomerically pure L- and D-amino acids in high yield by deracemisation of racemic starting materials. The method has been developed from initial academic studies to be a robust, scalable industrial process. Unnatural amino acids, in high optical purity, are a rapidly growing class of intermediates required for pharmaceuticals, agrochemicals and other fine chemical applications. However, no single method has proven sufficiently adaptable to prepare these compounds generally at large scale. Our approach uses an enantioselective oxidase biocatalyst and a non-selective chemical reducing agent to effect the stereoinversion of one enantiomer and can result in an enantiomeric excess of > 99 % from a starting racemate, and product yields over 90 %. The current approach compares very favourably to resolution methods which have a maximum single pass yield of 50 %. Efficient methods have been developed to adapt the biocatalyst used in this process towards new target compounds and to optimise key factors which improve the process efficiency and offer competitive economics at scale.

Hyperbranched polymer-gold nanoparticle assemblies : role of polymer architecture in hybrid assembly formation and SERS activity

Plasmonic gold nano-assemblies that self-assemble with the aid of linking molecules or polymers have the potential to yield controlled hierarchies of morphologies and consequently result in materials with tailored optical (e.g. localized surface plasmon resonances (LSPR)) and spectroscopic properties (e.g. surface enhanced Raman scattering (SERS)). Molecular linkers that are structurally well-defined are promising for forming hybrid nano-assemblies which are stable in aqueous solution and are increasingly finding application in nanomedicine. Despite much ongoing research in this field, the precise role of molecular linkers in governing the morphology and properties of the hybrid nano-assemblies remains unclear. Previously we have demonstrated that branched linkers, such as hyperbranched polymers, with specific anchoring end groups can be successfully employed to form assemblies of gold NPs demonstrating near-infrared SPRs and intense SERS scattering. We herein introduce a tailored polymer as a versatile molecular linker, capable of manipulating nano-assembly morphologies and hot-spot density. In addition, this report explores the role of the polymeric linker architecture, specifically the degree of branching of the tailored polymer in determining the formation, morphology and properties of the hybrid nano-assemblies. The degree of branching of the linker polymer, in addition to the concentration and number of anchoring groups, is observed to strongly influence the self-assembly process. The assembly morphology shifts primarily from 1D-like chains to 2D plates and finally to 3D-like globular structures, with increase in degree of branching. Insights have been gained into how the morphology influences the SERS performance of these nano-assemblies with respect to hot-spot density. These findings supplement the understanding of the morphology determining nano-assembly formation and pave the way for the possible application of these nano-assemblies as SERS bio-sensors for medical diagnostics.