J. K. S. Wan

Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounds

Electron spin resonance emission was observed from some photochemically produced semiquinone radicals in liquid isopropanol. It is proposed that the dynamic electron polarization is due to the optically spin polarized triplets of the parent quinones and their subsequent hydrogen abstraction reaction with retention of polarization in the resultant semiquinone radicals. Theoretical calculations have been carried out using typical zero field parameters for aromatic triplet molecules and the established assumption that the intersystem crossing rates to the three sublevels are unequal. Calculations reveal significant spin polarization of the triplets randomly oriented in an arbitrary magnetic field. Further treatment shows that when the triplet depolarization via relaxation is comparable to the chemical hydrogen abstraction rate, the resultant radical retains the polarization and is thus in the emissive mode.

Microwaves and chemistry: The catalysis of an exciting marriage

The potential of microwave power as a tool to facilitate chemical reactions has not whetted the chemist’s appetite in the past and the phenomenon and uses of microwaves have remained in the comer of spectroscopists and engineers for a long time. The possibility of microwaves initiating chemical changes has nevertheless excited our imagination for the past ten years. We will present the original development of the concept of microwave catalysis/sensitization in chemistry and the coming of age of the techniques as an enabling technology in the industrial world. A number of demonstrated applications ranging from hydrocarbon oxidations to environmental technology will be illustrated, as well as the most recently developed technique and applications of the microwave-induced acoustic phenomenon.

Applications of high power micro wave catalysis in chemistry

Several applications of microwave induced catalysis in chemical reactions are discussed including the synthesis of higher aliphatic and aromatic hydrocarbons from methane, the oxidation of hydrocarbons with water as oxidant, and the destruction of SO2 and NO. A model describing the concept of the technology is briefly introdueed with a discussion of several of the parameters affecting the interactions of the microwaves with the catalysts and the chemical reagents in the process.

Interaction of butylated hydroxytoluene (BHT) with phospholipid bilayer membranes: Effect on 22Na permeability and membrane fluidity

Abstract Butylated hydroxytoluene (BHT) is a small lipophilic molecule that is widely used as a food preservative. The interaction of this chemical with phospholipid bilayer membranes was monitored by measuring changes in 22Na transport and hydrocarbon chain motion. Vesicles composed of saturated phospholipids display a marked increase in 22Na permeability in the temperature region of the phase transition temperature of the component phospholipid. BHT greatly reduces this permeability increase. When a series of structural analogues was examined, there was found to be a poor correlation between lipid solubility and the capacity to decrease 22Na transport. However, the presence of a hydroxyl group appeared to be an important structural requirement for the permeability change. In a parallel set of experiments, a spin labelled fatty acid ester was incorporated into similar vesicles and the mobility of the label used as a measure of lipid hydrocarbon chain motion. The phase transition temperature of a phospholipid is associated with a marked increase in fatty acyl chain motion. BHT lowers the temperature at which the lipid chains display significant motional freedom (i.e. melt). Since this change in membrane fluidity cannot account for the capacity of BHT to reduce 22Na permeability some additional perturbation must be occurring. It is proposed that this additional perturbation involves an alteration at the interface.

Free radical induced oxidation of alkoxyphenols : some insights into the processes of photoyellowing of papers

The photoyellowing of lignin-rich papers has been demonstrated to depend on the formation of phenoxyl radical intermediates and their ultimate conversion into various products, including quinones. Molecular oxygen has also been observed as a necessary adjunct to this process, although the mechanism is not understood. This work demonstrates the requirement for the reaction of O2 with active radical intermediates (in processes analogous to autoxidation reactions) in order for the photoyellowing of the phenolic moieties to occur. Photo-oxidations of a variety of alkoxyphenols and their reactions with model alkyl, alkoxy and alkylperoxy radicals are studied by CIDEP.

An esr study of the reactions of decacarbonyldimanganese, trimethyltinpentacarbonylmanganese and decacarbonyldirhenium with quinones and α-diketones

Abstract The thermal and photochemical reactions of Mn2(CO)10, Me3SnMn(CO)5, and Me2Sn[Mn(CO)5]2 with various quinones and α-diketones were studied by ESR. The results clearly show that with p-quinones the predominant manganese radical adducts are formed via complexation of the metal with the quinonoid π-electron systems. However, a different reaction with o-quinones and α-diketones leads to the formation of manganese radical adducts mainly via metal chelation to the carbonyl oxygens. The photolysis of Re2(CO)10 with o-quinones also yields the analogous rhenium radical adducts.

Magnetic field and concentration dependence of CIDNP in some quinone photolyses: Further evidence for an Overhauser mechanism

A study has been made of the magnetic field dependence of chemically induced nuclear spin polarization (CIDNP) of fluorine nuclei in the photolysis of tetrafluoro‐1,4‐benzoquinone and tetrafluoro‐1,4‐hydroquinone in benzene. Also studied was the benzoquinone concentration dependence of proton CIDNP in the photolysis of benzoquinone in CDCl3. These studies are compared with the theory of an Overhauser mechanism of CIDNP in which the photoexcited triplet mechanism produces electron‐spin‐polarized semiquinone radicals whose polarization is transferred to the nuclear spins by electron–nuclear cross relaxation. The results support the conclusion, previously reached for the tetrafluoro‐1,4‐benzoquinone case on the basis of other evidence, that this Overhauser mechanism contributes to CIDNP in these systems.

A mechanistic study of the microwave induced catalytic decompositions of organic halides

A series of model organic haliae compounds were decomposed by the technique of microwave catalysis. The reactions were 100% efficient which were followed and characterized by gas chromatography, mass spectrometry, and direct electron spin resonance detection. The results clearly establish the primary free radical mechanisms of the decomposition by breaking selectively the C-X bond, leading to an organic fragment/radical and a stable form of solid metal halides. Many of the non-halogenated organic products are of economic and chemical value and can be recovered in the microwave processing.

An esr study of the photochemical reactions of group vib metal carbonyls with phenanthroquinone and the radical ligand exchange with group va organometals

Abstract Photolyses of Group VIB metal hexacarbonyls with phenanthroquinone in toluene solution led to the formation of some stable metal carbonyl-quinone radical complexes. These parent radical complexes underwent further ligand exchange reactions between the carbonyl and Group VA organometals. The ESR observations provide some insights into the reaction mechanisms as wellas the dynamics and the probable exchange sites in the thermal subtitution reactions.

Microwave Induced Catalytic Reactions of Carbon Dioxide and Water: Mimicry of Photosynthesis

In an attempt to emulate the highly successful photosynthetic recycling of CO2 to form energetically useful fuels in nature this study investigates the microwave induced reaction of carbon dioxide and water in a continuous flow system using a supported nickel catalyst and 2.45 GHz microwave radiation with an average incident power of 2.2 kW. The major reaction products were methane, ethane, methanol, acetone, C3 and C4 alcohols. The yields of methane, C3 and C4 alcohols reached maximum values after 30 s of irradiation, while the yields of ethane, methanol and acetone were proportional to the irradiation time within the investigated range.