Search

The previously reported gas-phase lithium cation basicity (LCB) scale (Taft et al. Pure Appl. Chem. 1990, 62, 17) was revised on the basis of recent experimental and theoretical (G2 calculations) results. A new anchoring based on the experimental LCA value for H2O is suggested (all earlier reported values of LCB should be reduced by 2.6 kcal/mol). New LCBs for 28 compounds were measured using FT-ICR, and a revised LCB scale now extended to 205 compounds is given. Correlations between gas-phase basicities toward lithium cation and proton were examined. Though a general trend is discernible, fair correlations are obtained provided that separate lines are drawn for homogeneous families. The differences in slopes are traced back to the different sensitivities to structural effects. Large deviations are explained by either a different attachment center for Li⁺ and H⁺ or a chelation effect toward Li⁺. G2 and G2(MP2) calculations of LCBs for a wide selection of 37 compounds and density functional theory (B3LYP/6-311+G**) calculations of LCBs of 63 compounds of different classes were carried out. In most cases the performed calculations adequately describe the gas-phase lithium cation basicities of a wide variety of bases of different chemical origin and LCB range. The results of the calculations were also used for explaining the largest deviations from correlation lines between gas-phase lithium cation and proton basicities.

The energies of combustion of 2-methylpropanamide, 2,2-dimethylpropanamide, and 1-adamantyl carboxamide have been determined using a static bomb calorimeter. The vapour pressures have been measured over a temperature range of about 15 K by the Knudsen-effusion technique. From the experimental results the following quantities for the three compounds, at 298.15 K, have been derived: CompoundΔfHmo(cr)ΔsubHmoΔfHmo(g)kJ·mol−1kJ·mol−1kJ·mol−12-Methylpropanamide−368.6 ± 0.986.0 ± 0.2−282.6 ± 0.92,2-Dimethylpropanamide−399.7 ± 1.386.6 ± 0.4−313.1 ± 1.41-Adamantyl carboxamide−427.0 ± 2.4108.0 ± 0.5−319.0 ± 2.5

The injection of SiC particles (150 µm size) into laser surface melted commercial purity titanium, Ti–6Al–4V (wt-%) alloy, and Ti–2·5Cu (wt-%) alloy has been investigated using 1·75 kW laser power, 5 mm beam diameter, 0·15 g s-lpowder flowrate and traverse speeds ranging from 7 to 20 mm -l. Partial dissolution of SiC occurred and fine dendrites of TiC nucleated at the particle/matrix interfaces and also within the matrix. Silicon enrichment of the matrix and a eutectic constituent were observed. The microhardness of the melted zone was increased to 600–650 HV (500 g).MST/964

Solvent effects on the wavenumber of the maximum of the longest wavelength electronic absorption band of all-trans-β-carotene were determined in 34 solvents. Together with results from previous studies, a data set for 51 solvents, mostly non-hydrogen bond donors, was constructed. This information was analyzed in terms of reaction field models and also showed its value for correlation purposes when used either alone or in combination with standard empirical solvent polarity–polarizability scales. © 1998 John Wiley & Sons, Ltd.

Because of the current trend in manufacturing to pack a greater number of electronic components into a given volume, the thermal failure of microelectronic systems is becoming an increasingly difficult design problem. This paper describes heat-transfer research into the important case of air cooling by forced convection of an array of electronic modules on a printed circuit board (PCB). Because of the interrupted nature of the airflow over the surface and the complex three-dimensional nature of the heat flow path, theoretical analysis is difficult and heat-transfer data for design are relatively scarce.A research rig was designed and constructed which provided control of module power dissipation and continuously monitored the temperature of individual packages. An automatic data-acquisition system was successfully developed to record the large amount of experimental data that was generated. Realistic finite element models of the electronic package, leads and substrate were developed and used to reveal the temperature distribution. These models were also used, in conjunction with the experimental data, to determine heat-transfer coefficients. Correlations were established that related the value of the heat-transfer coefficient to distance from the leading edge of the PCB. It is intended that these correlations, together with the thermal analysis contained in the paper, will provide useful data for designers.

The gas-phase basicity and proton affinity of pentacene have been determined by means of Fourier transform ion cyclotron resonance (FT−ICR) mass spectrometry. AM1 semiempirical calculations for neutral and protonated linear polyacenes ranging from benzene to pentaicosacene were carried out. The influence of successive annellations (up to 25 rings) on structures, heats of formation, and proton affinities was investigated. Extrapolated values for an infinite linear polyacene have been derived. AM1-calculated electronic structures (HOMO and LUMO energies) for the studied linear polyacenes have been compared with the available experimental ionization potentials and electron affinities.

The standard molar enthalpy of formation of gaseous 1-adamantyl chloride (1Cl,g) was determined experimentally by means of thermochemical techniques. Ab initio quantum-chemical calculations at the G2(MP2) level were performed on adamantyl cation (1⁺) and on 1Cl. Combination of these data with other experimental and computational results for adamantane (1H), isobutane (2H), tert-butyl cation (2⁺), and tert-butyl chloride (2Cl) provided purely experimental and purely computational, independent values for the standard enthalpy and Gibbs energy changes for reactions 1 and 2: where X = Cl. This information was used to determine a value of 162.0 ± 2.0 kcal mol^-1 for the standard molar enthalpy of formation of gaseous 1⁺, ΔfH°m(1⁺,g).