Recombination Reactions Of Free Radicals Research Articles

Hyperfine coupling dependence of the effects of weak magnetic fields on the recombination reactions of radicals generated from polymerisation photoinitiators

The recombination reactions of free radicals formed from the photolysis of a series of polymerisation photoinitiators were studied using time-resolved infrared spectroscopy. All molecules showed Zeeman magnetic field effects (MFEs) in the field range 0-37 mT and those molecules that produced radical pairs with average hyperfine couplings greater than 5 mT showed substantial inverted field effects at fields of less than 10 mT (so-called low field effects, LFEs). Monte Carlo simulations with full treatment of all the isotropic hyperfine couplings in the spin Hamiltonian reproduced well the observed field effects. The use of the usual analysis based on the calculated B1/2 value for the radical pair was found to be inappropriate in systems with substantial LFEs, but simple correlations between this B1/2 value and the observed field features were established.

Effects of weak magnetic fields on free radical recombination reactions

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 radical pairs. Sizeable changes in reaction yields (∽20%) are found for almost any radical pair provided the spin-correlation persists long enough for significant evolution of the electron spins under the influence of the weak applied field. The conditions necessary for observing effects as large as ∽20% are discussed.

Effects of weak magnetic fields on free radical recombination reactions

Effects of weak magnetic fields on free radical recombination reactions

Photon emission from chemically perturbed yeast cells.

Photon emission (PE) from yeast cells Saccharomyces cerevisiae strain SP-4 in normal conditions and in conditions perturbed by the addition of formaldehyde was investigated using single-photon counting equipment. PE from yeast cells, growing in a standard nutrient medium (YPG) then centrifuged and resuspended in a phosphate buffer (pH = 6.5), was measured in the presence of oxygen or argon. The solution of formaldehyde (2%) was injected into the sample. The intensity of PE increased and reached a maximum, then slowly decreased to a level which was higher than the PE level without the perturbing factor. The kinetics of PE was found to be strongly dependent upon the presence of oxygen. The model of formation and recombination of free radicals was tested. The results indicate that PE can arise during the recombination reactions of free radicals like R. + R., RO. + RO., RO2. + RO2. which are formed in the enzymatic oxidative reactions.

7345. The importance of free radical recombination reactions in CF 4/O 2 plasma etching of silicon : M Dalvie and K F Jenson, J Vac Sci Technol, A8, 1990, 1648–1653

7345. The importance of free radical recombination reactions in CF 4/O 2 plasma etching of silicon : M Dalvie and K F Jenson, J Vac Sci Technol, A8, 1990, 1648–1653

The importance of free radical recombination reactions in CF4/O2 plasma etching of silicon

The importance of free radical recombination reactions of F in CF4/O2 plasma etching of Si is explored in a combined experimental and modeling study of a single wafer etcher. The F atom concentration profiles in the reactor, as well as the silicon etch rate profiles, are strongly influenced by the recombination reactions. The surface recombination reaction is more important than the gas phase recombination reactions in determining overall system behavior. The effects of the homogeneous as well as heterogeneous recombination reaction diminishes with increasing flow rate. This is due to combined residence time and transport effects.

Combined Experimental and Modeling Study of Spatial Effects in Plasma Etching: CF 4 / O 2 Etching of Silicon

A combined experimental and modeling study of uniformity in plasma etching of silicon is presented. The model includes detailed treatment of transport (including diffusion) and chemical kinetics. Experimental measurements of spatial variations in active species concentrations are made by spatially resolved actinometry; spatial variations in etch rates are also measured. The effect of flow rate and feed composition on etch rate uniformity is examined. Model predictions are in good agreement with observations. Diffusion and free radical recombination reactions play an important role in commercial size systems. The insight gained through the model is used to point out a pitfall in the interpretation of actinometry data. The applicability of the model as a design tool is demonstrated.

The radiation crosslinking of block copolymers of butadiene and styrene

AbstractBlock copolymers of butadiene and styrene containing 21 and 61 wt.‐% styrene as well as pure polybutadiene have been investigated with respect to the extent of crosslinking produced by the irradiation with 60Co γ‐rays. The free radical yield at 77 K, G(R.), has also been measured. Addition of the styrene blocks to the butadiene chains causes a rapid decrease in both G(X), the G‐value for crosslink formation, and the free radical yield in nearly the same proportion for both, but G(X) is about twice as high as expected if crosslinks arose only from free radical recombination reactions. The effect of the styrene segments in the block copolymers on G(X) is approximately the same as in random copolymers of butadiene and styrene up to 21 wt.‐% styrene compositions.