The site of ionization in the acetylation of aliphatic esters by [(CH3CO)3]+

A series of esters RCOOR′ (where R, R′ = CH₃, CH₃CH₂, (CH₃)₂CH, (CH₃)₃C) were reacted with the [(CH₃CO)₃]⁺ ion from biacetyl in an ion cyclotron resonance spectrometer. A steric effect influences the rate of formation of stable products [RCOOR′·CH₃CO]⁺ and is used to determine that either oxygen of the ester may be initially acylated by [(CH₃CO)₃]⁺.     

X=y-zh systems as potential 1,3-dipoles—5 : Intramolecular cycloadditions of imines of α-amino acid esters

Azomethine ylides are readily generated from imines of α-amino acid esters by a formal 1,2-H shift. A suitably positioned unactivated double or triple bond in either of the two precursors of the imines (aldehyde or α-amino ester) leads to an intramolecular cycloaddition generating fused ring systems in good yield. Cis stereochemistry is assigned to the newly created ring junction of the cycloadducts based on NOE difference spectroscopy and, in the case of 8a, by a single crystal X-ray structure. Equilibration of the kinetically formed dipole leads to mixtures of epimeric cycloadducts for imines of phenylglycine methyl ester but equilibration is not observed for other imines. Reasons for this are discussed. The intramolecular cycloaddition is sensitive to ring size with 6/5 and 5/5 cis-fused systems being most easily formed depending in which moiety (aldehyde or amino acid) the dipolarophile is located. Intramolecular trapping of the azomethine ylide by an alkyne is accompanied by variable amounts of aromatized pyrrolic products.     

Spin-independent three-body effective valence-shell operators: Application to molecular oxygen

A mathematical construction is presented that uniquely defines a set of spin-independent effective valence-shell Hamiltonian (H^v) three-body matrix elements. These spin-independent H^v matrix elements separate direct and exchange portions of the three-body H^v matrix elements and therefore provide the most natural form for comparisons with parameterization schemes of semiempirical electronic structure methods in which the three-body matrix elements are incorporated into semiempirical one- and two-body Hamiltonian matrix elements in an averaged manner. Ab initio H^v three-body matrix elements of O₂ are computed through third order of quasidegenerate perturbation theory and are analyzed as a function of internuclear distance and atomic orbital overlap to aid in understanding how these three-body matrix elements may be averaged into semiempirical one- and two-body matrix elements. © 1992 John Wiley & Sons, Inc.     

Horeau's Coupling of Enantiomers Revisited. The Reversible Coupling of Enantiomers to Form Diastereoisomers in Kinetically Labile Metal Complexes

Pairs of enantiomeric molecules (??,??) of a substrate of unknown, but incomplete enantiomeric purity may be coupled to each other through labile coordination to a metal center M. This results in the formation of an equilibrium mixture of diastereoisomeric complexes, viz. meso-L_nM????(=m) and a pair of enantiomers L_nM???? and L_nM????(= e), where L is an achiral auxiliary ligand. General expressions are presented for determining the ratio of enantiomers [??]₀/[??]₀ from experimental parameters, which may be obtained from NMR measurements. Effects of diastereoselectivity are specifically considered. Limiting cases (nearly pure or nearly racemic substrates, very high or no diastereoselectivity, very large or no excess of free substrate) are discussed. The cases of additional diastereoisomerism owing to different coordination geometries and of the formation of complexes with more than two substrate molecules per metal center are also investigated. The results presented can not only be used for determining the enantiomeric excess but also for designing optimal strategies for the successive resolution of partly enriched samples.     

Coulombic and ring-shaped potentials treated in a unified way via a nonbijective canonical transformation

This paper is concerned with the three-dimensional potentialV _q =² (2a ₀/r–q a ₀ ²/r ² sin² ) ₀ which comprises as particular cases the ring-shaped potential (q = 1) and the Coulomb potential (q = 0). The Schrödinger equation for the potentialV _q is transformed via a nonbijective canonical transformation, viz., the Kustaanheimo-Stiefel transformation, into a coupled pair of Schrödinger equations for two-dimensional harmonic oscillators with inverse-square potentials. As a consequence, the discrete spectrum for the potentialV _q is obtained in a straightforward way. A special attention is paid to the caseq = 0. In particular, the coupled pair of Schrödinger equations for two-dimensional harmonic oscillators is tackled in the situations where the spectrum for the potentialV ₀ is discrete, continuous, or reduced to the zero point. Finally, some group-theoretical questions about the potentialV _q are mentioned as well as a connection, via the Kustaanheimo-Stiefel and the Levi-Civita transformations, between the quantum-mechanical problems for the potentialV _q and the Sommerfeld and Kratzer potentials.     

Cobalt-catalyzed dimerization of alpha-olefins to give linear alpha-olefin products

[Cp*P(OMe)3CoCH2CH3]+ [BarF]-, generated by the addition of HBArF to Cp*P(OMe)3Co(ethene), catalyzes the oligomerization of 1-hexene to give dimers and trimers. When a deficit of the acid is used, linear alpha-olefin dimers are produced at the expense of trimeric products: e.g., 1-butene, 1-hexene, and 1-octene give 1-octene, 1-dodecene, and 1-hexadecene, respectively.     

Ring H/hydroxyl H scrambling before loss of water from salicylic acid molecular ions

Scrambling before formation of the [M — 18] ion from salicylic acid is considerably reduced in comparison with scrambling before formation of the [M — 17] ion from benzoic acid.     

A dynamic dilution system-based evaluation of the procedure adopted for determining ozone precursor volatile compounds

A dynamic dilution system was created to evaluate the performance and the reliability of ozone precursor volatile organic compound (VOC) sampling (TO-Can canisters) and analysis (thermal desorption/gas chromatography/flame ionisation detection) techniques used by the Laboratoire Interrégional de Chimie du Grand Est (LIC). Different atmospheres of VOCs were generated at concentrations between 0.8 and 25 ppb, with temperatures of 0, 10, 20 and 30 °C, and with relative humidities of 0, 30, 50, 70 and 90%. These conditions are generally representative of those commonly observed in ambient air in the eastern France. This dynamic dilution allows the simulation of a wide range of scenarios (concentrations, temperatures and relative humidities). After assessing the capacity and performance of the system, it was applied in order to evaluate the recoveries and stabilities of VOCs from canisters used for the collection and analysis of two mixtures of VOCs. The first mixture contained six alkanes (ethane, propane, butane, pentane, hexane and heptane), and the second contained five alkenes (ethene, propene, butene, 1-pentene and 1-hexene), five aromatics (benzene, toluene, ethylbenzene, m-xylene and o-xylene), acetylene, and 1,3-butadiene. No significant losses of alkanes from the canisters were observed after 21 days of storage, and good recoveries of alkanes from the canisters (>80%) were obtained regardless of the concentration, the temperature and the relative humidity. However, losses of certain aromatics were noted at low relative humidity.     

Equation of state of simple dense fluids. Thermal pressure coefficients and effective hard sphere diameters

The thermal pressure coefficient of a fluid may be simply related to the hard sphere term in a van der Waals type of equation of state. Experimental data for liquid argon and simulation results for a Lennard-Jones 12-6 fluid have been used to give information about the temperature dependence of the hard sphere diameter. The implications of this behaviour have been briefly discussed.     

In situ determination of the thermodynamic surface properties of chemically modified surfaces on a local scale: an attempt with the atomic force microscope

We have monitored deflection-distance curves with an atomic force microscope (AFM) in contact mode, with a silicon nitride tip, on chemically modified silicon wafers, in the air. The wafers were modified on their surface by grafting self-assembled monolayers (SAMs) of different functional groups such as methyl, ester, amine, or methyl fluoride. A chemically modified surface with a functionalized hydroxyl group was also considered. Qualitative analysis allowed us to compare adhesive forces versus chemical features and surface energy. The systematic calibration procedure of the AFM measurements was performed to produce quantitative data. Our results show that the experimentally determined adhesive force or thermodynamic work of adhesion increases linearly with the total surface energy determined with contact angles measured with different liquids. The influence of capillary condensation of atmospheric water vapor at the tip-sample interface on the measured forces is discussed. Quantitative assessment values were used to determine in situ the SAM-tip thermodynamic work of adhesion on a local scale, which have been found to be in good agreement with quoted values. Finally, the determination of the surface energy of the silicon wafer deduced from the thermodynamic work of adhesion is also proposed and compared with the theoretical value.