Complete quantum control of the population transfer branching ratio between two degenerate target states

A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.     

DSC characterisation of compression moulded PEEK-PTFE plaques

Thermal behaviour of substituted monoazo dyes I-XVI, have been investigated by means of differential thermal analysis (DTA) and thermogravimetry (TG). The thermal stabilities of these dyes have been determined using the DTA and TG curves and the influence of the substituents and intramolecular hydrogen bonds on the DTA and TG curves of the dyes together with the melting points were also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cycloadditions and annulations of transition metal carbene complexes

The synthetic aspects of several reactions from the multifaceted chemistry of Fischer carbene complexes are examined. Their benzannulation reactions with acetylenes are utilized in the synthesis of anthracyclinones via two approaches which differ by beginning at opposite ends of the molecule with either an aryl or an alkenyl substituted chromium carbene complex. The latter has been employed in a formal synthesis of daunomycinone. The Diels-Alder reactions of ,β-acetylenic chromium carbene complexes provide for a facile entry into substituted cyclohexenyl chromium carbene complexes that are subsequently employed in benzannulation reactions. These tandem cycloaddition/annulation reactions are incorporated into model studies for the synthesis of anthracyclinones and wentilactone A. Their potential is also demonstrated for coupling to yet a third reaction of organochromium compounds ; aromatic nucleophilic substitutions on arene chromium tricarbonyl complexes. The annulations of β,β-disubstituted alkenyl complexes provides for a regio- and stereoselective synthesis of 2,4-cyclohexadienones under neutral conditions at near ambient temperatures.     

An hermaphroditic [c2]daisy chain

A cyclic dimeric daisy chain compound, which has been assembled from a disfunctional [2]rotaxane in a sequence of noncovalent and covalent synthetic steps, the most important of which is a bis-Wittig reaction, has been characterised by X-ray crystallography.     

Rearrangements in the electron impact induced fragmentations of sulfonyl chlorides

The major fragmentation pattern obsrved in the mass spectra of simple alkane- and arylsulfonyl chlorides may be rationalized by loss of a chlorine atom from the molecular ion, followed by loss of SO₂ with concomitant carbocation formation. The mass spectra of α-mesyl sulfonyl chlorides and napthalenesulfonyl chlorides exhibit ions resulting from chlorine atom migration to the α-carbon atom with concomitant loss of SO₂. The mass spectra of α-mesyl sulfonyl chlorides also show ions which involve chlorine atom migration to the β-sulfonyl group.     

On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes

Mechanistic studies of a palladium-mediated decarboxylative olefination of arene carboxylic acids are presented, providing spectroscopic and, in two instances, crystallographic evidence for intermediates in a proposed stepwise process. Sequentially, the proposed pathway involves carboxyl exchange between palladium(II) bis(trifluoroacetate) and an arene carboxylic acid substrate, rate-determining decarboxylation to form an arylpalladium(II) trifluoroacetate intermediate (containing two trans-disposed S-bound dimethyl sulfoxide ligands in a crystallographically characterized form), then olefin insertion and beta-hydride elimination. Because of the unique mode of generation of the arylpalladium(II) trifluoroacetate intermediate, a species believed to be substantially electron-deficient relative to phosphine-containing arylpalladium(II) complexes previously studied, it has been possible to gain new insights into those steps that are common to the Heck reaction, namely, olefin insertion and beta-hydride elimination. The present results show that there are notable differences in reactivity between arylpalladium(II) intermediates generated by decarboxylative palladation and those produced in conventional Heck reactions. Specifically, we have found that more electron-rich alkenes react preferentially with an arylpalladium(II) trifluoroacetate intermediate formed by decarboxylative palladation, whereas an opposite trend is found in conventional Heck reactions. In addition, we have found that the aralkylpalladium(II) trifluoroacetate intermediates that are formed upon olefin insertion in the present study are stabilized with respect to beta-hydride elimination as compared to the corresponding phosphine-ligated aralkylpalladium(II) complexes. We have also crystallographically characterized an aralkylpalladium(II) trifluoroacetate intermediate derived from arylpalladium(II) insertion into norbornene, and this structure, too, contains an S-bound dimethyl sulfoxide ligand; the ipso-carbon of the transferred aryl group and trifluoroacetate function as the third and fourth ligands in the observed distorted square-planar palladium(II) complex.     

Structure, reactivity, and computational studies of a novel ruthenium hydrogen sulfide dihydride complex

Addition of 1 atm of H(2)S to [Ru(IMes)(2)(CO)(EtOH)H(2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) affords the air-stable hydrogen sulfide dihydride complex, [Ru(IMes)(2)(CO)(H(2)S)H(2)] 2. Treatment of 2 with excess H(2)S leads to formation of the 16-electron bis-hydrosulfido complex, [Ru(IMes)(2)(CO)(SH)(2)] 3. Preliminary studies show that 3 reacts with both H(2) and CO in solution as well as in the solid state. Both 2 and 3 have been structurally characterized     

Model discrimination for rotational energy transfer in the Ar-N2 system

The jackknife test of Rothstein et al. is applied to discrimination between several different models used to compute the rotationally inelastic cross sections for the Ar-N₂ system. The modified exponential models are the best models, except for the case where the energy gap is small, when power laws are best.     

Electrokinetic characterization of poly(acrylic acid) and poly(ethylene oxide) brushes in aqueous electrolyte solutions

Surfaces carrying hydrophilic polymer brushes were prepared from poly(styrene)-poly(acrylic acid) and poly(styrene)-poly(ethylene oxide) diblock copolymers, respectively, using a Langmuir-Blodgett technique and employing poly(styrene)-coated planar glass as substrates. The electrical properties of these surfaces in aqueous electrolyte were analyzed as a function of pH and KCl concentration using streaming potential/streaming current measurements. From these data, both the zeta potential and the surface conductivity could be obtained. The poly(acrylic acid) brushes are charged due to the dissociation of carboxylic acid groups and give theoretical surface potentials of -160 mV at full dissociation in 10(-)(3) M solutions. The surface conductivity of these brushes is enormous under these conditions, accounting for more than 93% of the total measured surface conductivity. However, the mobility of the ions within the brush was estimated from the density of the carboxylic acid groups and the surface conductivity data to be only about 14% of that of free ions. The poly(ethylene oxide) (PEO) brushes effectively screen the charge of the underlying substrate, giving a very low zeta potential except when the ionic strength is very low. From the data, a hydrodynamic layer thickness of the PEO brushes could be estimated which is in good agreement with independent experiments (neutron reflectivity) and theoretical estimates. The surface conductivity in this system was slightly lower than that of the polystyren substrate. This also indicates that no significant amount of preferentially, i.e., nonelectrostatically attracted, ions taken up in the brush.     

Preparation and X-ray crystal structure of hexakis(1-methylimidazoline-2(3H)-thione)tetrakis(nitrato)dilead(II)

The title complex, prepared by reacting lead(II) nitrate with a stoichiometric amount of 1-methylimidazoline-2(3H)-thione (mimt) in water, crystallizes in the triclinic space group P with a = 9.334(2), b = 11.340(4), c = 11.398(3) Å, = 108.24(2), β = 95.69(2), γ = 100.27(2)° and Z = 1. In the solid state, the molecules show an unusual dimeric structure, with the formula [(mimt)₂(NO₃)₂Pb(μ-mimt)₂Pb(NO₃)₂(mimt)₂].