Ronald E. Brown

Prompt and delayed fluorescence from benzophenone

Abstract Excitation of benzophenone in solution at room temperature using a pulsing nitrogen laser (3371 A, ≈ 6 nsec) leads to prompt fluorescence, phosphorescence, and E-type delayed fluorescence. The first complete spectral distribution of the benzophenone fluorescence is presented.

Exchange reactions of alkanes containing quaternary carbon atoms over supported metal catalysts

A comparison has been made of the exchange reactions with deuterium of 2,2-dimethylpropane (DMP), 2,2,3,3-tetramethylbutane (TMB) and 2,2,4,4-tetramethylpentane (TMP) over supported catalysts containing iridium, palladium, platinum or rhodium at temperatures in the neighbourhood of 400 K. Reactions were followed mass-spectrometrically and products were also examined by deuterium n.m.r. spectroscopy. Four processes or mechanisms were found to contribute to different extents with the various hydrocarbon–metal systems. These were (I) stepwise exchange, (II) methyl–group exchange, and more general multiple exchange involving(III)αγ-diadsorbed or (IV)αδ-diadsorbed intermediates. Some isomerisation of TMP was observed to occur simultaneously with exchange with the TMP–rhodium system.

Building characteristics into a shaped charge to achieve unique performance requirements

Abstract Traditionally, shaped charges have been designed and used to penetrate long line-of-sight targets. This paper presents the results of three successful design efforts in which the performance goals were quite different: the defeat of reactive appliques, the localized consolidation of jet mass, and the demonstration of ductile, tungsten jetting. Each design has required that particular attention is devoted to the nature of the liner collapse and the subsequent jetting. Finite difference hydrocodes have provided the level of detail necessary to observe the critical phenomenon. The approaches taken, the computational techniques used and comparisons between experimental results, prediction and performance goals are discussed.

Evidence for the formation of an αδ-adsorbed hydrocarbon intermediate on a rhodium catalyst

Exchange of 2,2,3,3-tetramethylbutane with deuterium over rhodium provides evidence for a mechanism involving αδ-adsorbed intermediates.

Photostationary and transient studies of the phosphorescence from the benzil-anisole system. Direct evidence for reversible exciplex formation

Photostationary (quantum yields) and transient (emission decays) studies of the benzil phosphorescence in cyclohexane in the presence of anisole indicate reversible exciplex formation (E) between the benzil triplet (3B*) and anisole with radiative decay occurring from E as well as 3B*. Values of kinetic parameters independently derived from the two experiments agree within < 20%. Both sets of experimental data were computer simulated with a common set of parameters with standard deviations < 10%. While the simulations were insensitive to the absolute values of the rate constants for exciplex formation (k1) and dissociation (k2), the ratio k1/k2 ≈ 0.3 M−1.

Vulnerability of Mortar Projectiles by Intercepting Fragmentation Warheads

A general methodology for estimating the requirements for defeating an explosive-containing mortar threat by an intercepting array of explosively generated natural and controlled fragments is discussed along with the experimental data supporting quantitative interpretation. The target response of covered TNT impacted by single fragments is predicted through numerically determined shock-to-detonation thresholds as well as empirical penetration equations. Included in the methodology is a comprehensive, deterministic endgame model that consists of an intercept model, a static and dynamic fragment model, and a hit model generating the number of effective hits for arbitrary intercept situations. Experimental data supporting the assumptions of the models are reported. The model is also useful in establishing interceptor requirements.

Mechanistic studies of the reactions of 3,3-dimethylbut-1-ene with deuterium over supported-metal catalysts

The reactions of 3,3-dimethylbut-1-ene with deuterium have been followed over a number of silica-supported metal catalysts at 254 K and the products analysed by mass spectrometry (MS) and deuterium NMR spectroscopy. The main reaction was the formation of the alkane, 2,2-dimethylbutane, in which extensive redistribution of H and D atoms had occurred in the ethyl group. It was possible to make a complete analysis of all 12 isotopic alkanes, i.e.(CH3)3CCX2CX3 where X represents H or D, in the products formed over supported Pd and Rh using both the NMR and MS results. Relatively little exchange of alkene occurred but it was most noticeable over Pd/SiO2.The results were interpreted in terms of a mechanistic model involving three types of adsorbed species: alkene, 1-alkyl and 2-alkyl. Any adsorbed alkene may either be desorbed or form one of the alkyls, both alkyls may either be desorbed as alkane product or revert to alkene. By choosing suitable parameters and solving 23 simultaneous equations, it was possible to obtain calculated distributions for the 12 isotopic products in good agreement with the experimental results. The presence of the tertiary butyl group has an influence on the relative stability of the adsorbed intermediates and also on the relative ease of activation (addition or removal) of primary and secondary H or D atoms.

Some physico-chemical characteristics that make D2O-based buffer solutions useful media for capillary electrophoresis

Capillary electrophoresis (CE) carried out in deuterium oxide based buffers can give much improvement and flexibility in analysis owing to a lowering in Joule heating and an increase in the viscosity within the capillary; moreover, as most acids are weaker in deuterium oxide than in water, pKa differences can result in enhanced resolution.

Exchange reactions of 2,2-dimethylpentane, 2,2-dimethylbutane and 2,2-dimethylpropane over Pt/SiO2 and Rh/SiO2

A comparison has been made of the exchange reactions of deuterium with 2,2-dimethylpentane (22DMP), 2,2-dimethylbutane (22DMB) and 2,2-dimethylpropane (DMP) over Pt/SiO2 to investigate the mechanisms which allow exchange to propagate across the quaternary carbon atom. The contribution of this process increases with the temperature and is augmented by an increase in the size of the alkyl group attached to the tert-butyl group. Results were also obtained with Rh/SiO2 for comparison.Deuterium NMR analyses were made on the products from the exchange of 22DMP over both metals and these proved helpful as a guide to possible mechanisms, particularly with Rh/SiO2.

Reactions of hydrocarbons over Ru/SiO2: exchange with deuterium and the onset of hydrogenolysis

Exchange reactions with deuterium of a number of hydrocarbons, chosen to provide evidence about the nature of the intermediates, have been followed mass spectrometrically over a ruthenium–silica catalyst. The selected hydrocarbons were methane, propane, 2-methylpropane (2MP), 2,2-dimethylpropane (DMP), and 2,2,3,3-tetramethylbutane (TMB). Products from the reaction of propane and 2MP were analysed by deuterium NMR spectroscopy. The extent of multiple exchange increased with temperature and mechanisms involving αα- or αδ-adsorbed intermediates were favoured.The hydrogenolyses of DMP and TMB were followed at higher temperature with emphasis on the selectivity of the reaction and the change of the pattern of products with contact time. Subsidiary experiments to learn more about the course of the reactions with TMB were carried out with 2,2,3-trimethylbutane (TriMB) and 2,3-dimethylbutane (23DMB).There was evidence for a strong hydrocarbon–ruthenium interaction leading to a sequence of events with increase of temperature, relatively easy exchange but with self-poisoning, followed by moderately easy hydrogenolysis, again subject to self-poisoning. The sequence suggested the formation in turn of reversibly adsorbed hydrocarbon intermediates, more strongly adsorbed species leading to hydrogenolysis and finally carbonaceous residues.