A critical review of the chemical kinetics of SF4, SF5, and S2F10 in the gas phase

The gas phase chemical kinetics of SF₄, SF₅, and S₂F₁₀ are reviewed with particular emphasis on relevance to the general problem of the dielectric breakdown of SF₆. Specific reaction systems treated are SF₄ + F₂, SF₅ + SF₅, and the pyrolysis of S₂F₁₀. Computer modeling calculations were carried out to arrive at the best estimates of rate parameters. Based on the results of these calculations, sets of recommended rate parameters are provided. The major discrepancies and problems in establishing the kinetic data base are described. Thermochemical consequences of different model calculations are given.     

Modeling Studies of the Formation and Destruction of NO in Pulsed Barrier Discharges in Nitrogen and Air

This paper presents the results of modeling studies on the formation and destruction of NO in pulsed barrier discharges in nitrogen and air. The goals of this work are to identify the major processes involved in the formation and destruction of NO in air discharges, to distinguish between oxidative and reductive paths for NO destruction, to explore the potential importance of excited state reactions, to evaluate the role of water in such systems, and to identify the final products in the absence of heterogeneous processes. In all cases, the systems were modeled with 100×10^–4% (100 ppm) of added NO, with and without 3% added water. The focus in all of this work is chemistry in the post-pulse regime.     

Dihydrobenzoxazines and tetrahydroquinoxalines by a tandem reduction‐reductive amination reaction

A tandem reduction‐reductive amination reaction has been applied to the synthesis of 3,4‐dihydro‐2H‐1,4‐benzoxazines and 1‐acetyl‐1,2,3,4‐tetrahydroquinoxalines. The nitroketones required for the benzoxazine ring closures were prepared by (A) alkylation of the anion derived from 2‐nitrophenol with an allylic halide or (B) nucleophilic aromatic substitution of an allylic alkoxide on 2‐fluoro‐1‐nitrobenzene followed by ozonolysis. Precursors for the quinoxalines were prepared by alkylation of the anion of 2‐nitroacetanilide with an allylic halide followed by ozonolysis. Catalytic hydrogenation of the nitroketones using 5% palladium‐on‐carbon in methanol then gave the target heterocycles by a reduction‐reductive amination sequence. The N‐methyl derivatives for both ring systems were easily prepared by adding 5‐10 equivalents of aqueous formaldehyde prior to the reduction. The dihydrobenzoxazines were isolated in high yield following purification by chromatographic methods; tetrahydroquinoxalines were isolated in a similar manner and possessed differentiated functionality on the two nitrogens.     

Diastereoselective synthesis of substituted tetrahydroquinoline-4-carboxylic esters by a tandem reduction-reductive amination reaction

A diastereoselective synthesis of 1-methyl-2-alkyl- and 2-alkyl-1,2,3,4-tetrahydroquinoline-4-carboxylic esters has been developed from methyl (2-nitrophenyl)acetate (1). The method involves alkylation of 1 with an allylic halide, ozonolysis of the double bond, and catalytic hydrogenation. The final hydrogenation initiates a tandem sequence involving (1) reduction of the aromatic nitro group, (2) condensation of the aniline or hydroxylamine(8) nitrogen with the side chain carbonyl, (3) reduction of the resulting nitrogen intermediate, and (4) reductive amination of the tetrahydroquinoline with formaldehyde produced in the ozonolysis to give a methyl (+/-)-1-methyl-2-alkyl-1,2,3,4-tetrahydroquinoline-4-carboxylate. Removal of the formaldehyde prior to hydrogenation gives the simple (+/-)-2-alkyl derivatives. The products are isolated in high yield as single diastereomers having the C-2 alkyl group cis to the C-4 carboxylic ester. The reaction has been extended to the synthesis of tricyclic structures with similar high diastereoselection.     

Mechanisms of decomposition of mixtures of ethyl acetate and isopropyl bromide subjected to pulsed infrared laser irradiation

The infrared laser induced decomposition of mixtures of ethyl acetate and isopropyl bromide has been studied. The ratio of the yields of products ethylene and propylene, arising from the unimolecular decomposition reactions: ethyl acetate → ethylene + acetic acid, and isopropyl bromide → propylene + hydrogen bromide, were measured as a function of the ratio of ethyl acetate to isopropyl bromide and pressure of added helium. The results indicate clearly that in these systems non-equilibrium behavior is found up to the highest pressures used (about one atmosphere). A two level kinetic model is suggested which qualitatively explains the observations.     

OVID and SUPER: Two overlap programs for drug design

Summary Two programs, OVID and SUPER, for exploring the similarity of molecules with respect to their action at a receptor are described. OVID accepts two molecules as input and optimizes the three-dimensional overlap of specified atoms in one molecule with specified atoms in the second molecule. The result is expressed as a percent of the theoretical maximum. OVID gives a quantitative measure of the extent of a guessed correspondence between two molecules based on volume overlap of selected atoms. The Achilles' heel of OVID is that the correspondence between the two molecules has to be guessed. We realized that it would be better to systematically examine all possible correspondences of two structures to minimize the chance of overlooking a superior correspondence. We created SUPER to satisfy this need. SUPER accepts two molecules as input and finds the top twenty correspondences of their surfaces and charge distributions, giving a quantitative measure of the extent of each correspondence. An instructive example of the application of OVID and SUPER to the design of leukotriene D₄ receptor antagonists is described. SUPER appears to be a practical brainstorming tool for the medicinal chemist trying to understand how molecules whose structures may not resemble one another in an obvious way can bind to the same site.     

Ion-ion reactions in the gas phase: Proton transfer reactions of protonated pyridine with multiply charged oligonucleotide anions

Isolated triply and doubly charged anions of the single-stranded deoxynucleotide 5′-d(AAAA)-3′ were allowed to undergo ion-ion proton transfer reactions with protonated pyridine cations within a quadrupole ion trap mass spectrometer. Sufficiently high ion number densities and spatial overlap of the oppositely charged ion clouds could be achieved to yield readily measurable rates. Three general observations were made: (1) the ion-ion reaction rate constants were estimated to be 10^{? (7 ? 8)} cm³ ion^?1 s^?1; (2) the ion-ion reaction rates were found to be dependent on the reactant ion number density, which could be controlled by both the reactant ion number and the pseudopotential well depth, and (3) very little fragmentation, if any, was observed, as might normally be expected with highly exothermic proton transfer reactions.     

The adsorption of lysozymes: A model system

Hen egg white lysozyme was adsorbed onto clean borosilicate glass and n-pentyl silane-treated glass surfaces. Both modified (reductively methylated) and native lysozyme were studied. Variable angle X-ray photoelectron spectroscopy (VA-XPS) suggested differences in the nature of the adsorbed layer depending on substrate properties, as well as on degree of methylation of the protein. Adsorbed film thickness (as measured in the dehydrated state by XPS) ranged from 14 Å on hydrophilic glass to 25 Å on the hydrophobic surface. Degree of surface coverage ranged from 45% on the hydrophobic to 69% on the hydrophilic surface. The results suggest that lysozyme unfolds to a greater extent and covers more surface on the hydrophilic glass, possibly due to strong electrostatic interactions at the pH 7.4 conditions used in the study. An analysis of the surface structure of native hen lysozyme by molecular graphics has also been performed, suggesting that adsorption on hydrophobic surfaces should occur via the hydrophobic patch opposite the enzyme active site cleft. A comparison with human lysozyme has also been made using total internal reflection fluorescence (TIRF) spectroscopy to measure protein adsorption on model surfaces. The two proteins have significantly different interfacial properties.     

Stopped-flow study of the gas-phase reaction of ozone with organic sulfides: Dimethyl sulfide

The autoinhibiting reaction of ozone with dimethyl sulfide (DMS), has been studied at 296°K and 1.1 kPa (8 torr) as a function of the concentrations of both reactants. The major products of the reaction are H₂CO, H₂O, CO, and SO₂. The specific rate of primary attack of O₃ on DMS is immeasurably slow. It is suggested that the rapid overall rate observed for this reaction is due to a chain reaction initiated by the very slow primary reaction. It is concluded that reaction (1) cannot be important under atmospheric conditions and that the major loss process for DMS in the atmosphere is probably reaction with photochemically generated free radicals.