Intermediate Q from soluble methane monooxygenase hydroxylates the mechanistic substrate probe norcarane: evidence for a stepwise reaction.

Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidation reactions because different products or product distributions result from concerted, radical, and cation based reactions. Soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidation of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compounds characteristic of radical and cationic intermediates, respectively, in addition to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-mu-oxo Fe(IV)(2) cluster-containing species compound Q, showing that it is responsible for the majority of the oxidation chemistry. The observation of products from both radical and cationic intermediates from norcarane oxidation catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramolecular rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH(-) is transferred. The estimated lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values determined from previous studies of sterically hindered sMMO probes.     

Preparation and characterization of superionic materials in a new mixed system (Cu(1−x)AgxI)–(Ag2O)–(P2O5), (0.05≤x≤0.25)

In this paper attempts have been made to prepare superionic glassy electrolytes in the mixed system 30(Cu_(1−x)Ag_xI)–46.66(Ag₂O)–23.33(P₂O₅), where x=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, using the melt quenching process. The solid samples were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy and silver ionic transport number studies. XRD analysis and FT-IR spectroscopic results have provided details regarding the various phases present in the new system and also indicated the formation of composite materials consisting of glassy and crystalline phases. The transport number measurements have indicated the formation of superionic solids having Ag⁺ ions as the mobile species in the present system.     

Spectroscopic diagnostics of chemical processes: applications of tunable optical parametric oscillators

Optical parametric oscillator (OPO) and amplifier (OPA) devices are useful for spectroscopic sensing of chemical processes in laboratory, industrial, and environmental settings. This is particularly true of nanosecond-pulsed, continuously tunable OPO/OPA systems, for which we survey a variety of instrumental strategies, together with actual spectroscopic measurements. The relative merits of OPO wavelength control by intracavity gratings and by injection seeding are considered. A major innovation comprises an OPO with a ring cavity based on periodically poled lithium niobate (PPLN) and injection-seeded by a single-mode tunable diode laser (TDL). Active cavity control by an ‘intensity dip’ method yields an optical bandwidth ≤0.005 cm^-1 (150 MHz), which compares favourably with the performance of advanced grating-tuned OPO/OPA systems. A novel adaptation of this TDL-seeded PPLN OPO employs a compact, inexpensive multimode pump laser, with which it is still possible to obtain continuously tunable single-mode signal output. Cavity ringdown (CRD) spectroscopy also figures prominently, with infrared (IR) CRD spectra from both grating-scanned and TDL-seeded OPOs reported. Finally, a tunable ultraviolet (UV) source, combining a TDL-seeded passive-cavity OPO and a sum-frequency generation stage, is developed for measurements of time-resolved IR-UV double resonance spectra of acetylene and UV laser-induced fluorescence spectra of nitric oxide. Received: 28 March 2000 / Published online: 13 September 2000     

Conversion Factors to Convert Particle Size Distributions Measured by One Method to Those Measured by Another Method

It is shown that the conversion of size distributions measured with a laser diffractometer or sedimentometer to equivalent sieve size distribution depends not only on a mean shape factor but also on the size distribution being measured. An equation is given for calculating an overall conversion factor when the cumulative sieve size distribution is of the Schuhmann form P(x)=(x/k)^m. The technique is illustrated using results from a laser diffractometer and the X-ray Sedigraph, and gives the conversion factors from diffractometer to sedigraph and vice versa.     

Electrical switching in chalcogenide liquid semiconductors

Reversible electrical switching from a higher resistance off-state to a lower resistance on-state has been observed in several chalcogenide semiconductor alloys both in the solid and liquid phases. Experiments conducted between 100 and 500°C showed that the threshold switching voltage decreases with increasing ambient temperature, and that the on-state resistance remains relatively constant up to the melting region but generally increases thereafter. The alloy Se_0.77Te_0.13S_0.10 shows a preserved on-state which can be maintained using 60 Hz ac in both the solid and liquid phases. The on-state can be reversed to the off-state by decreasing the applied field, and hence is not a conventional memory state. Below the melting region (250°C) the on-state I–V curve has a distinct knee or barrier voltage above which the current increases steeply. Above the melting range the knee becomes less clearly defined and occurs at a lower voltage. Below 345°C the above alloy has an activation energy of 0.37 eV. At this temperature an abrupt slope change or a discontuinity exists in the temperature dependence of off-state resistance, threshold field, threshold power and on-state resistance.     

Metal-Organic Frameworks for Efficient Electrochemical Reduction of Carbon Dioxide

The large concentration of carbon dioxide (CO₂) in the atmosphere can be utilized in industrial production using effective electrocatalysts such as metal-organic frameworks (MOFs). Due to good properties such as high surface area, designable functionality, and uniform constitution, MOFs are regarded as promising electrocatalysts for the carbon dioxide electrochemical reduction reaction (eCO₂RR). This review covers the importance, challenges, and mechanism of eCO₂RR, and simply discusses the progress in the synthesis methods and characterization of MOFs. The review also thoroughly discusses the advances of single metal-based MOFs, mixed metal-based MOFs, and MOF derivatives as electrocatalysts for efficient eCO₂RR.     

Electrochemical Properties of Ru Polypyridyl Phosphonates

[Ru(bpyPO₃H₂)(bpy)Cl]Cl (bpyPO₃H₂=6,6’-bipyridin-2-yl)phosphonic acid) and [Ru(tpy)(MepyPO₃H₂)Cl]Cl (MepyPO₃H₂=(pyridin-2-ylmethyl)phosphonic acid) were synthesized and characterized spectroscopically and electrochemically. Each compound was found to exhibit proton-coupled electron transfer (PCET). In the case of [Ru(bpyPO₃H₂)(bpy)Cl]Cl, a Ru(V/IV) couple was detected at 1.4 V vs. NHE. Each complex was tested for its ability to catalyze C−H bond oxidation using a variety of sacrificial oxidants, and it was found that under aqueous conditions [Ru(bpyPO₃H₂)(bpy)Cl]Cl oxidizes secondary C−H bonds using sodium periodate (NaIO₄) as the primary oxidant.