Electron spin resonance study of the anaerobic photolysis of alkylcobaloximes; photochemical electron transfer

The anaerobic irradiation of alkyl(pyridinato)cobaloximes gives in the first instance the photochemical electron transfer reaction from the solvent or from the equatorial ligands for the methyl- and benzyl-(pyridinato)cobaloximes and homolytic cleavage for the isopropyl, isobutyl, n-pentyl and cyclohexyl compounds.     

Electron spin resonance studies of the anaerobic and aerobic photolysis of alkylcobaloximes

Aerobic and anaerobic photolysis of methyl(pyridine)cobaloxime, benzyl(pyridine)cobaloxime and analogous compounds in CHCl₃ results only in an electron transfer reaction from an equatorial ligand producing photo-reduction of Co^III to Co^II, the complex retaining its axial ligands.If after the anaerobic photolysis of benzyl(pyridine)cobaloxime the oxygen is introduced without any further photolysis we obtain an ESR spectrum of nitroxide, arising from the attack of a benzyl radical on the dimethylglyoxime equatorial ligand.For the other complexes, homolytic cleavage of the CoC bond occurs and in the presence of oxygen gives rise to the superoxide cobalt complex adduct Py(Co^IIIO₂^?.During photolysis of methyl(pyridine)cobaloxime in isopropanol homolytic cleavage of the CoC bond occurs in preference to electron transfer reaction from the equatorial ligands.The anaerobic photolysis of benzyl(pyridine)cobaloxime in isopropanol or in water at 113–133 K results in an electron transfer reaction. However, at 170 K we observe the formation of the Co^II complex arising from CoC bond cleavage.A mechanism for photo-induced insertion of oxygen in the CoC bond is proposed.     

Electron spin resonance study of polyglycols

Polyethylene glycols and polypropylene glycols were irradiated with gamma-rays from a ⁶⁰Co source at 77°K. Electron spin resonance spectra of the free radicals produced were recorded. It was found that the rupture in the polymer chain takes place at *C? C and C? O bonds. The radicals identified were ? O? CH₂ in polyethylene glycols and mainly in polypropylene glycols. Thermal decay of these radicals followed a second-order law, and the decay constant calculated was k = 2.32 × 10⁵ cm.³ mole^?1 sec.^?1 A complete discussion of the results is presented.     

Electron spin resonance study of radicals generated in cellulose/N-methylmorpholine solutions after flash photolysis at 77 K

The processes of radical formation in N-methylmorpholine-N-oxide monohydrate (NMMO) and cellulose/NMMO solutions were studied by ESR at 77 K under high-power UV (lambda = 248 nm) excimer laser flash photolysis. Radicals mainly generated were attributed to the nitroxide type radicals -CH2-NO*-CH2- and -CH2-NO*-CH3 at the first step and methyl *CH3 and formyl *CHO radicals at the second step of the photoreaction. Kinetic studies of these radicals revealed that formation and recombination rates of the radicals depend on the cellulose concentration in cellulose/NMMO solutions and the concentration of additional ingredients, e.g. Fe(II) and propyl gallate. Even at frozen state temperature, acceleration or quenching of radical reaction processes was found. The proposed scheme of UV light-induced NMMO degradation during irradiation based on ESR data correlates well with independently obtained results based on high-performance liquid chromatography (HPLC). The analysis of degradation products by HPLC, e.g. aminoethanol and acetaldehyde, supports the assumption concerning a radical-initiated ring opening of NMMO.     

Electron spin resonance study on plasma-induced surface radicals of poly(ethylene naphthalate)

We have undertaken Ar plasma irradiation on poly(ethylene naphthalate) (PEN) powder, and the radicals formed were studied by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated PEN shows a five-line spectrum separated with nonbinomial intensity distribution, indicating that the spectrum is an outline of multicomponent spectra. The systematic computer simulation of the observed ESR spectra disclosed that the spectra consist of two types of radicals in structural term: the major radicals formed were assigned to dihydronaphthalenyl-type radicals generated by a nearly random addition of a hydrogen atom to the naphthalene ring, and immobilized dangling bond sites at the surface crosslinked moiety.     

Electron spin resonance study of the radiation-induced polymerization of N-vinylcarbazole

When crystals of N-vinylcarbazole are γ-irradiated at 77°K., the ESR spectrum observed before warming consists of three peaks attributed to a radical–cation with the unpaired spin associated mainly with the nitrogen atom. Above 90°K. polymerization occurs, initiated by the cation, and the spectrum changes to that of an alkyl type of radical, ?N? ?H? CH₂, trapped in the polymer. Single crystals were used for a detailed analysis of the nuclear hyperfine parameters of the observed radicals.     

Electron spin resonance study of irradiated monomer and polymer of 3,3-bis(chloromethyl)oxetane

ESR spectra were studied for irradiated 3,3-bis(chloromethyl)oxetane (BCMO) single crystals and poly-BCMO obtained by polymerizing BCMO single crystals in solid state. In both cases, the spectra consist of the superposition of two-line and threeline components at ?196°C. On warming the sample, only the former survived. From the examination of hyperfine coupling tensors, it is concluded that the two-line spectrum is due to the free radicals for the monomer and ? C(CH₂Cl)₂? ?H? O? for the polymer sample, respectively. The orientation of the free radicals gives information about the mode of ring opening in the solid-state polymerization of BCMO.     

Electron spin resonance study of polymerization of butadiene with tris(acetylacetonato)titanium and triethylaluminum

ESR spectra of homogeneous catalyst derived from tris(acetylacetonato)titanium(III) and triethylaluminum were observed at several temperatures from ?78°C, to +25°C, at molar ratios of aluminum to titanium of 1–108. At ?78°C, this catalyst yields a violet complex which shows an ESR signal with a g value of 1.959 and is associated with the first intermediate. At ?40°C to ?30°C, this signal decreases, and two signals with g values of 1.947 and 1.960 are observed. The latter two signals diminish at ?5°C to +10°C, while two kinds of new signals with g values of 1.965 and 1.969 appear overlapping each other. The structures of the species corresponding to these five signals are discussed on the basis of the ESR spectra, the intensity change, and the unpaired spin distribution. A new signal with a g value of 1.978 is found in the presence of butadiene at 25°C at Al/Ti > 8 and is assigned to a growing end of polybutadiene with this catalyst. The polymer yield increases remarkedly at Al/Ti molar ratios greater than 10. The microstructure of the resulting polymer consists almost completely of 1,2 units. The structure of the growing end is proposed to be a titanium (III) species containing two 1-substituted allyl groups, by comparison with the structure ascribed to the growing end of polybutadiene with n-butyl titanate-triethylaluminum catalyst.     

Electron spin resonance of manganese(II) complexes with proteins

Analysis of the spectra arising from complexes of Mn²⁺ with carbonic anhydrase B(CA) and β-galactosidase (β-Gal) indicates that the dominant anisotropic term in the spin hamiltonian is a quadratic zero field splitting (ZFS) interaction of an approximately rhombic symmetry, characterized by a considerable extent of distribution in the ZFS parameters.     

Electron spin resonance study of radiation degradation of polytetrafluoroethylene

Electron spin resonance measurements were performed at different stages of radiation degradation of polytetrafluoroethylene. It was found that under radiation, peroxy radicals were transformed into chain-end fluorocarbon radicals, which in turn became into chain-side radicals. On the basis of these data, a scheme for the degradation mechanism is given.     

Electron spin resonance study of new N-alkoxyphosphoramidyl and N-alkylphosphoramidyl radicals

The title radicals have been generated and studied by esr spectroscopy. The N-alkoxyphosphoramidyl radicals are formed by hydrogen abstraction from the corresponding protic parent compounds while the N-alkylphosphoramidyls can be generated by photolysis of cyclopropane solution of the corresponding N-bromo derivatives. Both types of radical exhibit esr data characteristic of a π electronic configuration.     

Electron spin resonance study of poly-3,3-bis-(chloromethyl)oxetane irradiated with ultraviolet light

When poly-3,3-bis(chloromethyl)oxetane has been irradiated at ?196°C in a nitrogen atmosphere with ultraviolet light, a triplet spectrum is observed. After warming the sample, both a doublet and a singlet ESR spectra are observed. These spectra are attributed to and ? CH₂? O, respectively. The formation mechanism of these free radicals is discussed. It is concluded that the main process of radical formation is the dissociation of chemical bonds from the excited state of the polymer produced through the energy absorption by irregular groups acting as sensitizers. In the presence of oxygen, the radical yield at ?196°C is greater than that in nitrogen atmosphere. This is attributed to the extra absorption of light by the charge transfer complexes of polymers with oxygen molecules. It is also proposed that participation of a charge transfer complex in photooxidation of ether is important in the primary radical formation step. When a polymer sample irradiated in vacuum with ultraviolet light is treated at ?78°C for a few minutes in the presence of air, peroxy radicals form. This shows that oxygen molecules diffuse very easily into this polymer, even at this low temperature.