Electron spin resonance heating temperature

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

Evidence for such stable free radicals has been obtained from electron spin resonance measurements. A sample of the 77-23 PVC-styrene, which had been exposed to 0.8 megarad of gamma-radiation at room temperature, displayed resonance peaks comparable to 3 X 10 8 mole per gram of free radicals (compared with a diphenyl picrylhydrazyl standard). When an identical sample was heated for 10 minutes at 75°C. following irradiation, the free radical population had fallen below detection limits. Heating evidently destroyed or decreased the free radical content by reaction or termination. 

The electron spin resonance of pyropolymers has been reviewed by Singer (44). Broadly speaking, heat treatments up to 650° C increase the number of free radicals present but above this temperature the number of unpaired spins decreases. This has been related to the increase in size of the crosslinked system (45) but it has been shown that this effect is almost entirely due to the effect of oxygen on the polyene/aro-matic n bonding system (43). 

In the process of photocatalysis, the electrons and holes produced on photoirradiated Ti02 powders are trapped at the particle surface to form unpaired-electron species (step (4) in Fig.D.3). Photocatalytic reactions are actually the reactions of these radicals with reactant molecules at the Ti02 surface. Electron spin resonance (ESR) spectroscopy has been used for the detection of the photoproduced radicals on Ti02 at low temperatures such as 77 K. It has been reported that photoproduced electrons are trapped at various different sites titanium atoms on the surface or inside the particles, or oxygen molecules adsorbed on the surface. On the other hand, photoproduced holes are trapped at lattice OAygen atoms near the particle surface or at surface hydroxyl groups. We analyzed these radical species for several Ti02 photocatalysts that are commercially available, and found that the differences in the photoproduced radicals resulted from different heat-treatment conditions and the reactivity with several molecules.17)... 

The partial reduction of dication 30 to give radical cation 31 can be monitored in toluene at room temperature by electron spin resonance, as the 1 2 3 2 1 quintet, as described by Banister and co-workers <1993JCD1421>. Further reduction to give the diradical 32 results in a 1 1 1 triplet overlapping with a quintet. Upon stronger heating, rearrangement to diradical 33 was observed. 

The sample heated at 605 °C showed the maximum activity for O/P conversion, and this sample also showed the maximum area under the electron spin resonance line and the minimum width, which means the maximum amount of localized paramagnetic centres. The samples heated at higher temperatures showed enhanced activities for H/D exchange and decreasing activities for O/P conversion. The ESR lines were broadened, which indicates exchange interactions between the paramagnetic centres. 

Direct observation of transition-state selectivity has been observed from the low-temperature cyclization of dienes inside H-mordenite and H-ZSM-5 (9). By using electron spin resonance (ESR) spectroscopy, it has been possible to explore radical formation upon the sorption of dienes on H-mordenite and H-ZSM-5. From the analysis obtained, it was found that the dienes are not very reactive for oligomerization inside H-mordenite channels. Heating H-mordenite with presorbed 1,4-pentadiene or 1,5-hexadiene yields selective cyclization of molecules via cycloalkenie radicals inside the H-mordenite channel. However, in the smaller pores of H-ZSM-5 (although die nature of both acid and redox sites in both zeolites are the same) no eyclo-olefinie radicals are formed as shown by the ESR spectrum. These experiments illustrate the reality of transition-state selectivity inside the pores of zeolites. 

Thermal dissociation of the weak F-F bond in F2 [D°(F2) = 153.97 kJ mol-1] (287) can be brought about equally well by heating to temperatures above roughly 120°C, so that a similar mechanism could operate in those reactions where purely thermal activation is used. The N-F bond in NF3 is considerably stronger [Z)°(NF2-F) = 238.5 kJ mol-1], and for it to be involved in the thermal activation process considerably higher temperatures would be required. There is strong electron spin resonance (ESR) evidence for the intermediate formation of the NF3+ radical cation (45, 134, 211). 

Calorimetrically the initial heat was not obtained on rehydration. Therefore, calcination at high temperatures produced irreversible modifications to these catalysts. An electron spin resonance (ESR) study of NO adsorption showed that the number of strong Lewis acid sites increased with the dehydration temperature, particularly when dehydroxylation occurred above 773 K, whereas IR studies showed that the total number of both Lewis and... 

Arrhenius plots demonstrating the effect of temperature on lettuce seed aging rate (squares) and on molecular mobility calculated using the Adam-Gibbs model and heat capacity measurements (open circles) and from rotational motion using electron spin resonance measurements. Data are from Walters et al., (2004) (aging rate), Walters, (2004) (glass relaxation rates) and Buitink et al., (2000) (ESR measurements). 

T1he long-lived free radicals produced by y-radiolysis of dry proteins have been studied (10, 20, 23) by electron spin resonance (ESR). It was shown (11, 22, 35) that when proteins are irradiated at 77°K., ESR centers are formed ( primary radicals), which on heating to room temperature are irreversibly changed to give spectra which are identical with those found after radiolysis at room temperature ( secondary radicals). It was inferred (23) from the ESR spectra that on irradiation of proteins, two main types of radicals are formed at room temperature. In the one, the unpaired spin is localized at the cysteine sulfur, while in the other it is mainly associated with glycine residues. 

Electron-spin resonance has been measured at 9400 and 51.7 Me. for a variety of charcoals heated to various temperatures. A very sharp resonance line has been observed by proper heat treatment and subsequent evacuation of the charcoal. Oxygen and nitric oxide adsorption at room temperature decrease the absorption intensity and widen the absorption band. Nitrogen and hydrogen have no effect at room temperature on the electron-spin resonance of the charcoal. 

When sodium azide is irradiated with UV light at 298°K, no appreciable optical absorption is detected at wavelengths greater than 500 nm. However, if after this irradiation the crystal is heated to 600°K, a new broad optical absorption with a maximum around 520 nm is produced [26,67]. The band has been attributed to colloidal sodium. The insensitivity of the optical absorption to temperature supports the assignment of the band to colloids. Neither the position of the maxima or the band-width is affected by temperature. The identification is supported by the characteristic electron spin resonance of colloids [26]. The colloidal particle diameter was estimated to be 15 A [26]. 

The microwave absorption of the YBa2Cu307-5 high temperature superconductor has been studied by electron spin resonance (ESR). The responses are related to the development of the superconducting phase by examination of samples after each of three heating stages in the sample preparation process. An apparently axially symmetric g 2 signal (g > 2.24,... 

Taking a different route to materials characterization, Hayes and Schurr use an experimental archaeological approach to develop a method for classification of archaeological maize and bone. They prepare comparative sets of maize and bone heated to different temperatures for differing periods of time and in conditions that include or exclude oxygen. Using Electron Spin Resonance (ESR), they are able to determine the maximum heating temperature... 

To better understand the mechanism of the dimethylbutadiene-MA copolymerization, low-temperature (-196 to -60°C) initiated polymerizations were studied with y rays and spectral changes observed by electron spin resonance.The study shows that liquid-phase copolymerization of MA and dimethylbutadiene takes place with the formation of a copolymer with regular alternation of monomer units, even while heating a glassy monomer mixture exposed to y radiation at -196°C. From an analysis of the electron paramagnetic resonance spectra and calorimetric data, it was clearly established that the mechanisi of copolymerization for this system consists of the stepwise addition of monomer molecules to a growing radical. This is contrary to a CTC homopolymerization model proposed earlier.It is assumed that the cause of the almost exclusive addition of monomers to foreign radicals... 

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