Neil Williams

Platinum catalysed regioselective ortho-silylation of benzylideneamines via intramolecular C–H activation

The Pt–P(OCH2)3CEt complex catalyses the ortho-silylation of benzylideneamines with disilanes via intramolecular C–H activation; both mono- and bis-silylated products are obtained.

Improving student achievement and satisfaction by adopting a blended learning approach to inorganic chemistry

A blended learning approach to the teaching of a level 2 inorganic chemistry module is presented. Lectures were replaced by study packs, which were supported by formative on-line assessment delivered via Blackboard and a programme of 20 workshops. Learning activities written using the Lockwood format were included in the study pack to facilitate active learning. The formative on-line assessments were designed to provide rapid and helpful feedback to the students in advance of the workshops. The tracking and gradebook facilities in Blackboard allowed staff to monitor student activity and progress. Attendance at the workshops was encouraged by including end-of workshop summative assessments. An analysis of module results revealed an improvement in performance compared to previous years and other core chemistry modules, after introducing blended learning to the level 2 inorganic chemistry module in 2004/5. The improvement was maintained in 2005/6. Module questionnaires revealed a significant improvement in student satisfaction with subject content, delivery and performance feedback on adopting a blended learning approach.

A comparative evaluation of fluoridated and non-fluoridated mouthrinses using a 5-day cycling enamel erosion model

OBJECTIVES To assess the relative protective effect of commercial mouthrinses containing 0-450 ppm fluoride on erosion progression in enamel using a simulated 5-day in vitro cycling model with concurrent monitoring of surface microhardness (SMH) and bulk tissue loss. METHODS Specimens were randomly assigned to six treatment groups (n=6). The model mimicked morning and evening use with rinse times (30 or 60 s) reflecting those prescribed on pack, interspersed with three cycles of demineralisation/remineralisation per simulated day. The latter comprised 1.0% citric acid monohydrate pH 3.2 for 300 s, with subsequent remineralisation in mucin-free artificial saliva pH7.0 for 120 min. SMH was determined by Vickers microindentation and bulk tissue loss using white-light interferometry. RESULTS From the end of day 1, fluoride-containing mouthrinses conferred statistically significant reductions in bulk tissue loss versus fluoride-free rinses (p<0.05), with lesion depth inversely proportional to fluoride concentration. From day 3, the mean lesion depth of specimens treated with the 450 ppm rinse were statistically significantly lower than all comparator treatment groups (p<0.05). Two distinct trends were apparent when comparing SMH changes in groups treated with fluoride-free versus fluoride-containing mouthrinses. In the latter, SMH levelled out over the final three simulated days in contrast to the former whose SMH continued to fall; differences were statistically significant at day 5 (p< 0.05). CONCLUSIONS Bulk tissue loss is inversely proportional to fluoride concentration in this cycling model. The plateau in SMH reflects stabilisation of mineral density as the study progresses and the number of fluoride binding sites and consequently uptake increases, in turn leading to enhanced lesion remineralisation.

Low temperature reaction of chlorine nitrate with water ice: Formation of molecular nitric acid

We have examined the reaction of ClONO2 with water ice at 140 K and found evidence for the formation of molecular nitric acid under conditions of reduced surface water. This is very different from the behaviour of ClONO2 on ice at 180 K, where substrate-induced pre-reaction ionisation of ClONO2 leads to a reaction mechanism involving the intermediacy of [H2OCl]+ and nitrate ions. These two results are not inconsistent with a single mechanism if a subtle change in the interplay between the availability of water molecules at the surface and the variation of the substrate/adsorbate interactions of ice/ClONO2 with temperature are taken into account. This implies that reaction mechanisms and product branching ratios in the atmosphere may vary widely over a range of temperatures and reactant partial pressures.

REFLECTION-ABSORPTION IR SPECTROSCOPIC INVESTIGATION OF THE PHOTOLYSIS OF THIN FILMS OF DICHLORINE MONOXIDE AND CHLORINE DIOXIDE

Reflection–absorption infrared (RAIR) spectroscopy and mass spectrometry have been employed in order to investigate the low-temperature photochemistry of thin films of chlorine dioxide, OClO, and dichlorine monoxide, Cl 2 O, grown on a gold foil in an ultra-high vacuum system. Photolysis of a neat film of OClO (λ>300 nm, 90–110 K) produces chloryl chloride, ClClO 2 . Irradiation of a co-deposited OClO/H 2 O film also produces chlorine superoxide, ClOO, which suggests that OClO isomerisation is the first step in the reaction producing ClClO 2 . Photolysis of Cl 2 O (300<λ<515 nm, 90–110 K) is shown to produce OClO, initially, which is subsequently converted to ClClO 2 . Analysis of the observed IR band intensities and consideration of the metal surface selection (MSS) rule indicates that the photochemically produced OClO intermediate aligns perpendicular to the gold substrate. Mechanistic details and the atmospheric implications of the chemistry are discussed.

The application of high resolution diffusion NMR for the characterisation and quantification of small molecules in saliva/dentifrice slurries

The application of DOSY (Diffusion Ordered Spectroscopy) NMR as a technique for the virtual separation of toothpaste adjuvants in model saliva is reported for the first time. In addition, the scope and limitations of DOSY NMR are considered using the DOSY Tool Box processing software, as is the quantification of the adjuvants and components of saliva by quantitative NMR (qNMR). These techniques represent a new and powerful tool for the evaluation of complex mixtures of natural products with a view to identifying biomarkers for disease within the oral cavity.

Synthesis of Silver (I) Complexes of Iminoalkyl Functionalised N-Heterocyclic Carbenes

A range of silver iminoalkyl imidazol-2-ylidene complexes have been isolated in good yield (50%–85%) and characterised by 1H and 13C NMR spectroscopy. A single crystal X-ray diffraction structure determination of 1-{2-(benzylhydrylidene-amino)-ethyl}-3-benzyl imidazol-2-ylidene silver bromide indicated monodentate coordination of the ligand.

Compounds containing a Spiro Phosphorus Atom

This chapter reviews compounds that are spiro fused with a phosphorus atom at the junction of the rings. The spirocyclic phosphorus atom may be four, five, or six coordinate. The range of compounds containing a spiro phosphorus includes spirophosphonium salts and ylides, four-coordinate spirophosphoranides, five-coordinate spirophosphoranes, and six-coordinate spiroperphosphoranides. Tricyclic six-coordinate compounds are also included. Further structural variations reviewed include tetracyclic compounds in which spiro rings are bridged. Phosphoranes, phosphines, and phosphine oxides which have donor functional groups that can undergo dative bonding with the phosphorus center to complete a spiro system are also considered. This chapter reviews the synthesis, reactivity, and structural investigation of such compounds. The major interest in these compounds is in mechanistic studies of phosphoryl transfer and hydrolysis of phosphates in biological systems, and as models for reactive intermediates in the Wittig reaction. In addition, chiral spiroperphosphoranides have a number of applications in chiral recognition and differentiation.

Palladium or platinum complex catalysed reactions of carbonyl and imine compounds with disilanes

The transition metal catalysed reactions of benzaldehydes and benzylideneamines with disilanes have been investigated. Palladium phosphine complexes catalyse the double silylation of the CO bond in benzaldehydes and the CN bond in benzylideneamines with 1,2-difluoro-1,1,2,2-tetramethyldisilane to yield α-(fluorodimethylsilyl)-α-(fluorodimethylsiloxy)toluene and N-methyl-N-(fluorodimethylsilyl)-α-(fluorodimethylsilyl)benzylamine respectively. When less activated disilanes such as 1,2-dichloro- and 1,2-dimethoxy-1,1,2,2-tetramethyldisilane were employed, the palladium phosphine complexes were less active and selective, resulting in extensive side reactions inclusive of 1,2-disiloxy-1,2-diphenylethane formation. The reaction of benzophenone with the difluorodisilane formed 2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane without affording the corresponding simple double silylation product. The formation of side products such as 1,2-disiloxy-1,2-diphenylethane in the reaction of benzaldehyde and 2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane in the reaction of benzophenone appears to suggest intermediacy of radical and silylene species. Tris(dibenzylideneacetone)diplatinum–etpo (etpo = 4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane catalyst system was more active for unactivated disilanes, catalysing double silylation of benzaldehydes with hexamethyldisilane. The same catalyst system was found to catalyse the ortho silylation of benzylideneamines with disilanes via intramolecular C–H activation; both mono- and bis-silylated products were obtained. Reaction rates and product distributions are rationalised in terms of the steric and electronic properties of the disilanes, substrates and the catalyst used.

An ab initio and experimental study of bromine on low-temperature water clusters and ice surfaces

A dual RAIR (reflection–absorption infrared) spectroscopy and ab initio study of the interaction between molecular bromine and water-ice has been performed. The spectra were measured in the temperature range 85 to 180 K and the results compared to theoretical models of Br–H2O clusters and hydrated H2OX+ and X2OH− ions. Particular theoretical attention was paid to the possibility that the interacting molecules modify their properties (geometry, partial atomic charge and vibrational frequencies) with the size of the water clusters i.e. Br2···(H2O)n. Additionally, ab initio methods were employed to investigate the actual mechanism of the bromine hydrolysis by evaluating the stability of possible reaction intermediates such as [H2OBr]+ in the gas-phase as well as upon solvation. The chemical structures and vibrational frequencies of the hydrated ion-pairs were also examined. The theoretical treatments employed show that the bromine and water molecules can form a weakly bonded binary complex with interaction between the Br- and O-atoms. Furthermore, the experimental evidence obtained for bromine hydrolysis occurring via an [H2OBr]+ ion on water-ice is investigated by the energy calculations, which show that the route is competitive with that occurring via an H3O+ ion. The vibrational frequencies calculated for a hydrated H2OBr+ Br− ion pair satisfactorily reproduce the set of RAIR spectroscopy experiments preformed at 180 K.