Theoretical results for the excited states of ozone

On the basis of ab initio calculations (configuration interaction and generalized valence bond), we find eight excited states of ozone with vertical excitation energies less than 7 eV. Using these results the various experimental transitions are assigned. One state has an equilateral triangle as the minimum energy geometry (1.5 eV above the ground state).     

Determination of methadone and its metabolites by high performance liquid chromatography following solid-phase extraction in rat plasma.

This paper describes a rapid, quantitative liquid chromatographic analysis and extraction of methadone and its two major metabolites from rat plasma, using difenoxin as the internal standard. Using a C18 column, resolution of all sample components and the internal standard is achieved with a mobile phase of 25:75 acetonitrile-0.08% diethylamine in 1000 mL water, pH 2.3, at a flow rate of 1.5 mL/min. The injection volume is 100 microL. Standards are linear over the range 25-100 ng, with a lower limit of detection for methadone of 0.25 ng. Within- and between-run coefficients of variation (CV) are 1.24% and 2.94%, respectively. Extraction of methadone and its metabolites from rat plasma uses a solid-phase extraction technique that is highly efficient. Extraction efficiencies of 90.3%, 99.6%, 85.9% and 93.8% were achieved for methadone, its primary and secondary metabolites, and difenoxin, respectively.     

Synthesis and polymerisation of lipophilic peptide nucleic acids derived from stearic acid and pentacosa-10,12-diynoic acid

The adenine, cytosine and thymine peptide nucleic acid (PNA) monomers and PNA T₁₀ oligomers bearing either a diacetylenic or stearoyl moiety at the N- or C-terminus have been successfully prepared. The resulting thymine monomeric and T₁₀-mer derivatives have been subsequently incorporated into polydiacetylene-containing liposomes.     

Silanization and non-aqueous electrochemistry of two oxide states on platinum electrodes

When the surface of a Pt electrode is oxidized in aqueous 1 M H₂SO₄ at +0.8 to +2.0 V vs. SSCE for a few seconds to 10 min, disconnected, washed and dried, and placed in CH₃CN solvent, a negative potential scan shows that the surface oxide is reduced in two waves at potentials between?1.2 and?1.6 V vs. SSCE. The combined charge of the two waves amounts to 0.3–2.5 layers of oxide, depending on anodization potential and time. The more easily reduced oxide becomes non-reducible after reaction of the electrode with methyltrichlorosilane. The two oxide waves are interpreted as surface and subsurface oxide layers.     

Near-IR luminescence of monolayer-protected metal clusters

Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.     

Chrysomelidial by chromyl chloride oxidation: A revised structure for gastrolactone

Non-conjugated $\text{gem}$-dialkylated alkenes were oxidized to aldehydes in the presence of α,β-unsaturated carbonyl functional groups, providing a new synthesis of $\text{1}$ and a synthesis of $\text{2}$ that led to a revised structure for gastrolactone.     

Entropy-driven hydrogen bonding: stereodynamics of a protonated, N,N-chiral "proton sponge"

The C2-symmetric ("[DL]") and achiral ("[meso]") diastereoisomers of the hydrogen iodide salt of 1,8-bis-(N-benzyl-N-methylamino)naphthalene ([2H]-[I] ) interconvert in solution. Direct interconversion of the diastereoisomers of [2H]+ must involve hydrogen bond fission (to give "[nonHB-2H+]") and rotation-inversion of the non-protonated nitrogen centre. The global activation parameters (deltaH++ and deltaS++) for diastereoisomer interconversion in [D7]DMF have been determined from rate data obtained by temperature-drop and magnetisation-transfer 13C NMR spectroscopy over a temperature range of 170 degrees C. The process is found to have a high entropy of activation in both directions (deltaS++=163(+/-4) and 169(+/-4) JK(-1)mol(-1)) and this is suggested to arise through hydrogen bonding of the ammonium centre in [nonHB-2H+] with the solvent ([D7]DMF). Comparison of the enthalpy of activation (deltaH++) with that earlier found for diastereoisomer interconversion of the free-base form 2 suggests that the intramolecular hydrogen bond in [2H]+ is roughly equal in enthalpic strength (deltaH) with that made with the solvent ([D7]DMF) in the non-hydrogen-bonded intermediate [nonHB-2H+]. As such, the hydrogen bonding in [2H]+ may be considered as predominantly an entropically driven process, without any unusual enthalpic strength.     

Electronic structure of axially ligated rhodium carboxylates: π back-bonding revisited

Fenske-Hall molecular orbital calculations have been applied to a new, structurally isolable analog of the intermediate involved in catalytic rhodium carboxylate carbenoid transformations. Results from the structural characterization of axially ligated rhodium acetate phenylisonitrile complexes have been utilized in approximate molecular orbital calculations. The results from the calculations suggest that a significant degree of π back-bonding exists between the metal and isonitrile fragments which, by analogy, implies that π back-bonding should also exist in the rhodium carbenoid intermediate. Sensitivity of the Fenske-Hall method to the magnitude of π back-donation in these complexes was gauged through calculations involving different phenylisonitrile groups in which the π back-bonding ability was modulated through derivativization. The reliability of the Fenske-Hall method was evaluated through a comparison to a high-level calculation. Received: 13 December 1996 / Accepted: 18 March 1997     

The acid catalysed rearrangement of a diterpenoid epoxide

A study has been made of the principal rearrangement products resulting from formic acid treatment of 19-hydroxy-ent-beyerene epoxide (3). In concentrated solutions 3 has been found to undergo a deep seated rearrangement to the allylic alcohol (14). A mechanism for the formation of 14 is proposed involving a novel 1,4-hydride shift in the bicyclo[3:2:1]octane C/D ring system following cleavage of the C₁₅O bond. Supporting evidence has been obtained from a study of the specifically labelled epoxide (4), the deuterium in 14 appearing exclusively at C₁₂. Four products (7 and 11–13) emanating from the known beyerane → kaurane interconversion have been identified.