Does He2 Exist?

For the electronic ground state X ¹Σ⁺_g, the potential-energy function of He₂ reported by Aziz et al. has been transformed into the form V(z), containing only eight parameters, which is more suitable for the investigation of the existence of states of discrete energy. We found no evidence that a bound vibration-rotational state of the stable diatomic molecule ³He₂ or ⁴He₂, even if rotating, can exist in the electronic ground state.     

CO vibration as a probe of ligand dissociation and transfer in myoglobin

We report femtosecond visible pump, midinfrared probe, spectrally integrated experiments resolving the dynamics of CO in myoglobin upon photodissociation. Our results show a progressive change in absorption strength of the CO vibrational transition during its transfer from the heme to the docking site, whereas the vibrational frequency change is faster than our time resolution. A phenomenological model gives good qualitative agreement with our data for a time constant of 400 fs for the change in oscillator strength. Density-functional calculations demonstrate that indeed vibrational frequency and absorption strength are not linearly coupled and that the absorption strength varies in a slower manner due to charge transfer from the heme iron to CO.     

Ribonucleotide analogues having novel internucleotide linkages

The conversion of nucleoside phosphites into a number of novel nucleotide analogues is described.     

Preparation of ribonucleoside 3′-O-phosphoramidites and their application to the automated solid phase synthesis of oligonucleotides

Oligoribonucleotides, six to fifteen units long, were synthesized on a controlled pore glass support using ribonucleoside 3′-O-phosphoramidite reagents. Average coupling yields of up to 98% were obtained with diisopropylaminophosphoramidite derivatives. fa]Some preliminary results of this work have been communicated by N. Usman & K.K. Ogilvie at the CIC-AIC Conference June 1984, Montreal, Canada.     

Goldstone modes on Wilson's surface

In three-dimensional compact QED in its continuum formulation, Goldstone modes associated with surface fluctuations cause Wilson's surface to wander infinitely far from the minimal surface. On the lattice, this is only true for small coupling, due to the roughening transition.     

Phosphoramidate analogues of diribonucleoside monophosphates

The synthesis of thio, seleno, and imino analogues of diribonucleoside phosphoramidates is described.     

Directed ortho metalation-boronation and Suzuki-Miyaura cross coupling of pyridine derivatives: a one-pot protocol to substituted azabiaryls

A general method for the synthesis of azabiaryls 19a-t by a one-pot procedure involving a Directed ortho metalation (DoM)-boronation-Suzuki-Miyaura cross coupling sequence is described. Aside from the three isomeric pyridyl carboxamides 15a-c, chloro-, fluoro-, and O-carbamoyl pyridines are adapted to this method providing a range of azabiaryls (Table 2). The method has an advantage in that it avoids the recognized difficult isolation of pyridyl boronic acids and their instability toward deboronation. The efficient synthesis of hydroxypicolinamides 12-14 (Scheme 3) by a one-pot metalation-boronation-oxidation sequence with the LDA-B(OiPr)3 in situ procedure that avoids self-condensation of incipient ortho-metalated species (Scheme 2) is delineated. The conversion of azabiaryls 19b,e,h,l into azafluorenones 20b,e,h,l by a directed remote metalation protocol is demonstrated (Table 3). A comprehensive survey of pyridyl boronates, of considerable interest in contemporary heterocyclic synthetic chemistry, is given (Figure 1).     

Chemically resistant microfluidic valves from Viton® membranes bonded to COC and PMMA

We present a reliable technique for irreversibly bonding chemically inert Viton? membranes to PMMA and COC substrates to produce microfluidic devices with integrated elastomeric structures. Viton? is widely used in commercially available valves and has several advantages when compared to other elastomeric membranes currently utilised in microfluidic valves (e.g. PDMS), such as high solvent resistance, low porosity and high temperature tolerance. The bond strength was sufficient to withstand a fluid pressure of 400 kPa (PMMA/Viton?) and 310 kPa (COC/Viton?) before leakage or burst failure, which is sufficient for most microfluidic applications. We demonstrate and characterise on-chip pneumatic Viton? microvalves on PMMA and COC substrates. We also provide a detailed method for bonding fluorinated Viton? elastomer, a highly chemically compatible material, to PMMA and COC polymers. This allows the production of microfluidic devices able to handle a wide range of chemically harsh fluids and broadens the scope of the microfluidic platform concept.     

Nanomolar detection with high sensitivity microfluidic absorption cells manufactured in tinted PMMA for chemical analysis

We describe a novel, cost effective and simple technique for the manufacture of high sensitivity absorption cells for microfluidic analytical systems. The cells are made from tinted polymethyl methacrylate (PMMA) in which microfluidic channels are fabricated. Two windows (typically 250 μm thick, resulting in little optical power loss) are formed at either end of the channel through which light is coupled. Unwanted stray light from the emitter passes through a greater thickness of the tinted substrate (typically the length of the cell) and is preferentially absorbed. In effect, this creates a pin-hole configuration over the length of the absorption cell, providing improved performances (sensitivity, S/N ratios, baseline noise and limit of detection) when used as an absorption cell compared to clear substrates. The method is used to achieve a LOD of 20 nM with a colourimetric iron assay and a LOD of 0.22 milli-absorption units with a pH assay.     

Single-isomer trisubstituted olefins from a novel reaction of (E)-beta-chloro-alpha-iodo-alpha,beta-unsaturated esters and amides

(E)-beta-chloro-alpha-iodo-alpha,beta-unsaturated esters are converted to single isomer trisubstituted olefins bearing three different carbon substituents by cross-coupling under reflux. Mechanistic investigations suggest that this process transfers a hydrogen from the boronic acid to the alpha-position of the substrate and then introduces an aryl group to the beta-position of the intermediate template while replacing chloride. The reaction is highly stereoselective, showing preference for the E-isomer. The process proceeds through (E)-beta-chloro-alpha-aryl-alpha,beta-unsaturated esters that are transformed efficiently into the corresponding E-products through stereoselective Suzuki-type reactions giving single isomers. The observed stereochemistry is apparently enabled by the intermediacy of a palladium allenoate. The reaction involves a catalytic cycle in which Pd(II) is reduced to Pd(0) through the formation of biaryl-coupled products.