Publisher Summary This chapter discusses thermal reactions in aqueous colloidal systems. It aims is to analyze the reactions from a mechanistic point of view, and therefore discussion is focused on organic reactions in aqueous micelles. The structure of micelles or other colloidal droplets is considered only to the extent needed to understand reactivity. A key feature of micelles and similar colloidal aggregates is that they can incorporate solutes. For example an ionic micelle can bind a non-ionic solute and also, by virtue of its charge, attract counterions. It may affect reaction rates and equilibria by bringing reactants together or keeping them apart, but because the micelle can exert a medium effect it is necessary to separate the “medium” and “concentration” effects of the micelle. It reviews that the overall subject of reaction in submicroscopic aggregates has expanded so rapidly and in so many different directions that interest on specific areas has to be focused. The general principles, which govern the effects of normal, aqueous, micelles on reaction rates and equilibria are considered and some specific reactions and the relation of micellar effects to mechanism are discussed.

Organic Reactivity in Aqueous Micelles and Similar Assemblies

Issues associated with the exposure of patients to strong, static magnetic fields during magnetic resonance imaging (MRI) are reviewed and discussed. The history of human exposure to magnetic fields is reviewed, and the contradictory nature of the literature regarding effects on human health is described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with magnetic field exposure and no evidence for hazards associated with cumulative exposure to these fields. The very high degree of patient safety in strong magnetic fields is attributed to the small value of the magnetic susceptibility of human tissues and to the lack of ferromagnetic components in these tissues. The wide range of susceptibility values between magnetic materials and human tissues is shown to lead to qualitatively differing behaviors of these materials when they are exposed to magnetic fields. Mathematical expressions are provided for the calculation of forces and torques.

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Safety of Strong, Static Magnetic Fields

The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Effects of weak magnetic fields on free radical recombination reactions

Organic photochemistry in organized media

A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution

Publisher Summary This chapter focuses on the magnetic field and magnetic isotope effects on the products of organic reactions. The chapter discusses reactions of radical pairs and examines the spin states of radical pair systems as archetypes. The crucial condition for observation of magnetic field effects is a competition between two processes: one of the processes must be magnetic field dependent and the other is magnetic field independent. As for magnetic field effects, magnetic isotope effects are generally very small or negligible in homogeneous solution, but may become significant in microheterogeneous environments, such as micelles. The chapter illustrates an example of a magnetic isotope effect involving 17 O, occurring in the thermolysis of endoperoxides of aromatic compounds. The decomposition of certain endoperoxides produces singlet molecular oxygen and the yield of singlet oxygen is magnetic field dependent. Such effects have been observed in many different kinds of chemical systems, and are readily interpreted in terms of the well-established principles of chemically induced magnetic polarization.

Magnetic Field and Magnetic Isotope Effects on the Products of Organic Reactions

On montre que le couplage spin-orbite est renforce dans les diradicaux ayant un groupe acyle terminal. Effet de la longueur de la chaine et de la conformation

Matthew B. Zimmt

Papers

Magnetic Field and Magnetic Isotope Effects on the Products of Organic Reactions

Nanosecond Flash Photolysis Studies of Intersystem Crossing Rate Constants in Biradicals: Structural Effects Brought About by Spin-Orbit Coupling

Triplet Energy Transfer as a Probe of Surface Diffusion Rates: A Time-Resolved Diffuse Reflectance Transient Absorption Spectroscopy Study

Dynamics of micellized radical pairs. Measurement of micellar exit rates of benzylic radicals by time-resolved flash CIDNP and optical spectroscopy

Ketone photochemistry on solid silica. A diffuse reflectance laser flash photolysis study