A. E. Stiegman

Morphology and pore structure of silica xerogels made at low pH

Abstract The pore structure and morphology of sol–gel derived silica xerogels made with high concentrations of acid catalyst are reported. In particular, a range of acid concentrations between 0.01 M, corresponding to a pH near the point-of-zero-charge (PZC) of silica and 1.03 M were investigated. Consistent with previous studies, the gelation time of the sols decreased with increasing acidity in this range. Nitrogen physisorption measurements of the dried xerogels revealed that the pore volumes and surface areas increased dramatically with increasing acid concentration. The microstructure of the samples made under high acid conditions, analyzed by atomic force microscopy (AFM), showed globular structures of ∼20 nm diameter which pack in such a way as to create mesoporous regions between them. This morphology is quite similar to that observed for materials with similar gelation times, but made at pH values higher than the PZC. This supports the generally held view that the pore structure and morphology are dictated by the relative rates of condensation vs. hydrolysis.

Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors

For the Multiangle Imaging SpectroRadiometer (MISR), currently under development for the Earth Observing System, we plan to use deployable diffuse reflectance panels to provide a flight calibration of its nine cameras. Near-lambertian reflectance characteristics are desirable to facilitate flat-field camera intercomparisons and to allow each camera to be calibrated under the same illumination levels. Panel spatial and spectral uniformity and stability with time are also required. Spectralon™, a commercially available diffuse reflectance material made from polytetrafluoroethylene (PTFE), has been baselined in the MISR design. To assess the suitability ofthis material, a series of environmental exposure tests were performed. No degradation of the optical properties was apparent following proton bombardment, and stability through UV illumination was satisfactory, provided simple cleaning and handling procedures were implemented. One surprise during the testing, however, was a buildup of several thousand volts of static charge, which developed while simulating a rare pass through an auroral storm. Such a potential for charge buildup is not unique to PTFE, but exists for many thermal control paints used to cover spacecraft exteriors. Further testing of the charged Spectralon failed to produce arcing to the metallic housing frame, and models indicate that charge neutralization will occur after passage through the storm. For these reasons we intend to fly Spectralon as per our original plan.

Photochemical disproportionation of metalmetal bonded carbonyl dimers

Mecanismes de reactions radicalaires dans la majorite des reactions de dismutation relatives a Mn 2 (CO) 10 , Re 2 (CO) 10 , Co 2 (CO) 8 , Cp 2 Mo 2 (CO) 6 , Cp 2 Fe 2 (CO) 4 , Cp 2 Ni 2 (CO) 2

Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material

A detailed chemical analysis was carried out on Spectralon, a highly Lambertian, diffuse reflectance material. Results of this investigation unambiguously identified the presence of an organic (hydrocarbon) impurity intrinsic to the commercial material. This impurity could be removed by a vacuum bake-out procedure and was identified as the cause of optical changes (degradation) that occur in the material when exposed to UV light. It was found that when this impurity was removed, the Spectralon material was photochemically stable and maintained its reflectance properties even after extensive solar UV exposure.

Correspondence on Microwave Effects in Organic Synthesis

Angewandte Chemie recently published an unusual Essay by Prof. C. Oliver Kappe and co-workers entitled, “Microwave Effects in Organic Synthesis—Myth or Reality?” The major point of discussion is an Edge Article we published last year in the RSC journal Chemical Science entitled, “On the Rational Design of Microwave-Actuated Organic Reactions.” The Angewandte Essay is unusual in its inclusion of 38 pages of experimental details in the Supporting Information, 30 of which describe unpublished data from original experiments pertaining to our work. These data are interpreted as conflicting with our report. We reject their alternative interpretations of our experiments, and we discuss here how the new data from their experiments in fact align with our conclusions. In our article, we provided an example of a microwave (MW)-actuated reaction system, which we define as one for which MW irradiation provides a synthetic advantage over conventional heating at the same temperature. We designed an extreme reaction system—an ionic benzyl-transfer reagent in a nonpolar aromatic solvent—and subjected it to extreme conditions: constant MW irradiation at high power (300 W). Our design of this chemical system was predicated on the concept of selective heat storage in the domains existing around microwave-absorbing solutes in nonabsorbing media at fixed (and typically high) applied microwave powers. The design rationale aligns with an understanding of selective microwave heating obtained from fundamental dielectric relaxation studies carried out by Richert and Huang. In such a system, the selective absorption of microwave energy by the solute will result in localized energy in the solvation domain (i.e. effective temperature) that is higher than the surrounding medium. The solute then acts as a molecular radiator, transferring thermal energy to the bulk solution. If convective heat transfer out of the domains is slow compared to the build-up of thermal energy within the solute domains, then the thermal energy of the solute will exceed what would be predicted based on the measured temperature of the bulk solution. As an extension of that work, one can hypothesize that a microwave-absorbing reactant in a nonabsorbing medium can potentially realize product formation at rates in excess of what would be expected from conventional heating at the same measured bulk temperature. Indeed, we observed reactivity enhancements under MW heating compared to conventional oil-bath heating in a benzylation of [D10]pxylene (Figure 1).

On the rational design of microwave-actuated organic reactions

Microwave-actuated organic reactions are defined herein as chemical reaction systems for which microwave irradiation provides a clear benefit over conventional heating to the same temperature. This study is focused on a rationally designed, microwave-actuated reaction (thermal Friedel–Crafts benzylation), in which a microwave-absorbing ionic solute reacts in a non-polar and largely microwave-transparent solvent. Steady-state microwave irradiation (ssMWi) induces observed levels of reactivity from the solute that cannot be duplicated by conventional heating of the homogeneous solution to similar temperatures. This observation is qualitatively consistent with the Arrhenius [k = Ae(−Ea/RT)] relationship between rate and molecular collisions (k ∝ A at constant T). A new paradigm for designing microwave-actuated organic reactions for microwave-assisted organic synthesis emerges from this study.

Reactivity of seventeen- and nineteen-valence electron complexes in organometallic chemistry

Abstract Organometallic chemistry has traditionally been the domain of diamagnetic complexes and the powerful 16- and 18-electron rule.1 This rule, as set forth by Tolman,1(a) states that “… diamagnetic organometallic complexes of transition metals may exist in a significant concentration at moderate temperature only if the metals valence shell contains 16 or 18 electrons.” Furthermore, the rule states that “organometallic reactions, including catalytic ones, proceed by elementary steps involving only intermediates with 16 or 18 metal valence electrons.” Although the rule is enormously useful for predicting the stability and reactivity of diamagnetic complexes, the range of organometallic chemistry has expanded in recent years to include numerous paramagnetic 17-valence electron complexes, which exist as both stable complexes and short-lived intermediates.2,3,4 These species, essentially metal-centered radicals, are typified by complexes such as Mn(CO)5, Mn(CO)3(PR3)2, Co(CO)4, and CpMo(CO)3 (Cp=η5−C5H5).

Stable efficient solid-state white-light-emitting phosphor with a high scotopic/photopic ratio fabricated from fused CdSe-silica nanocomposites.

1 ] Two approaches to the development of solid-state lighting have been successfully implemented in commercially available white-light light-emitting diodes (LEDs). One method is the use of a phosphor conversion diode, in which a photodiode, emitting UV light, excites a phosphor (or group of phosphors), which then emits, singularly or collectively, white light.

Parameters Affecting the Microwave-Specific Acceleration of a Chemical Reaction

Under appropriate conditions, significant microwave-specific enhancement of the reaction rate of an organic chemical reaction can be observed. Specifically, the unimolecular Claisen rearrangement of allyl p-nitrophenyl ether (ApNE) dissolved in naphthalene was studied under microwave heating and conventional convective (thermal) heating. Under constant microwave power, reaching a temperature of 185 °C, a 4-fold rate enhancement was observed in the microwave over that using convective heating; this means that the microwave reaction was proceeding at an effective temperature of 202 °C. Conversely, under constant temperature microwave conditions (200 °C), a negligible (∼1.5-fold) microwave-specific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145-175 °C and 85-155 °C cycles, where 2- and 9-fold rate enhancements, over what would be predicted by conventional thermal heating, was observed, respectively. The postulated origins of the microwave-specific effect are purely thermal and arise from selective heating of ApNE, a microwave-absorbing reactant in a nonabsorbing solvent. Under these conditions, excess heat is accumulated in the domains around the ApNE solute so that it experiences a higher effective temperature than the measured temperature of the bulk medium, resulting in an accelerated unimolecular rearrangement.

High-temperature photoluminescence in sol-gel silica containing SiC/C nanostructures

Silicon carbide and carbon nanostructures were produced by pyrolysis of organosilane or aromatic compounds in nanoporous sol-gel silica glasses. Intense photoluminescence was observed in the visible and the near infrared regions, depending on material processing. Emission bands at 2.97, 2.67, 2.53, 2.41, 2.24, 2.09, 1.93, 1.13, 1.00, and 0.85 eV were observed in samples prepared at temperatures between 870 and 1220 K. Phosphorescence emission showed two lifetime components at 300 K: a 0.03 s component and a very long component of 0.5–4 s that depends on the precursors and sample processing. These lifetimes approximately doubled at 77 K. The visible emission increased significantly as the temperature was elevated from 77 to 950 K, suggesting thermally assisted light emission from sites in the silica glasses containing SiC/C nanostructures. Surface SiC vacancy defects modeled using integrated ab initio quantum mechanics/molecular mechanics calculations suggest phosphorescence may originate from C vacancy (S...