Masaru Shiotani

Gamma-radiation induced cross-links in ethylene—propylene rubber studied by CP—MAS NMR

Abstract A new technique of 13 C-NMR, the CP-MAS method, was applied to study a chemistry of cross-links induced by ψ-irradiation of ethylene-propylene rubber. The chemical species of cross-linking points were specified with their relative concentrations by the analysis of the CP-MAS spectra obtained before and after irradiation. It was found that the short branches were also formed by the irradiation. A comparison was made between the cross-links detected by the CP-MAS method and those obtained by the Charlesby-Pinner analysis of the gelation caused by the ψ-irradiation. The conventional 13 C-NMR of the cross-linked and swollen EPR provided us an information on the sol parts of the sample but little information on the cross-links in the gel parts.

ESR Study of Molecular Order and Motion in a Nematic Liquid Crystal

The molecular order and the motion of a spin probe, vanadyl acetylacetonate (VAA), incorporated in a nematic liquid crystal N-(p-methoxybenzylidene)-p-n-butylaniline (MBBA) were explored by the ESR technique. By using the anisotropy of the magnetic parameters, the degree of the order of VAA in various phases was evaluated. An abrupt increase at the solid-liquid crystalline transition and a monotonical decrease at the clearing point were observed. The latter suggests that a short range order still remains in the isotropic phase near the clearing point. In the isotropic phase, the correlation time was evaluated using Kivelsons formalism to be in the range of 10-9 to 10-10 sec. A higher order was found in the nematic phase upon increasing the magnetic field strength. An analysis for the ordering due to the magnetic field indicates that a small magnitude of order survives even at the zero field limit.

Formation of methylpropargyl radicals from tetramethylcyclobutadiene radical cations: an esr study

Abstract Exposure of dilute solutions of CH 3 CCCH 3 in CC1 2 FCC1F 2 and related solvents to ionizing radiation at 77 K produced the radical cation (CH 3 CCCH 3 ) + . On annealing, these were initiallycpnverted into the corresponding dimer cation, c -C 4 (CH 3 ) 4 + , which subsequently decomposed to give the propargyl radical . This novel result is supported by studies using CH 3 CCCD 3 which gave, finally, the radicals and in a ratio of approximately 9:1. The absence of the radicals and indicates that the cyclobutadiene ring is strongly distorted (D 2h ) and that the initial dimers do not undergo equilibration via symmetrical dimers under our experimental conditions. These results are compared to liquid-phase data for c -C 4 (CH 3 ) 4 + radicals and explanations for the differences in behaviour are proposed.

Spin trapping of carboxylic esters γ-irradiated in solid and liquid phases

Abstract The radicals formed in the liquid and solid state radiolysis of esters of acetic acid CH3-COOR (R =-CH3, -CH2-CH3, -CH(CH3)2, -C(CH3)3) were investigated by the spin trapping technique using 2,4,6-tri-t-butylnitrosobenzene. After irradiation in the solid phase ( - 196°C), the nitroxide spin-adducts of the following radicals could be identified: H·, ·CH2-COOR, CH3-COOR·, R = alkyl. Radiolysis in the liquid state (-78°C) resulted in the nitroxide spin-adducts of alkoxy radicals. Furthermore, anilino-type spin-adducts were detected. Based on these results a mechanism for the primary reactions in the radiolysis of these esters is proposed.

EPR spectra and structure of the radical cations of fluorinated benzenes

EPR spectra for the radical cations of a series of fluorinated benzenes, generated by irradiation with γ-rays in halocarbon solid matrices, have been observed at low temperatures. The spectra consist of a hyperfine structure with axially symmetric anisotropy mainly due to fluorine nuclei. The observed spectra have been analysed by simulation. Ab initio calculations have been conducted for the cation radicals to obtain their optimized geometries. The results reveal that an unambiguous deformation in geometry is brought about by cationization in each case. INDO calculations have been performed for the optimized geometries of these radical cations to calculate the hyperfine couplings. The calculated couplings strongly support the observed ones. The symmetry of the SOMO for the radical cations resembles that of the HOMO of their neutral mother molecules. The deformed geometries of these radical cations suggests that in the process of releasing an electron from an HOMO, those chemical bonds with bonding nature in the HOMO become elongated and those bonds with antibonding nature become shortened. It is concluded that the structure and symmetry of the SOMO of these radical cations are affected not only by the number of substitutions by fluorine but also by the position of substitution.

The radical cation of hexafluoro-1,3-butadiene: An ESR study

Abstract Two distinct radical cations have been observed by ESR in a γ-irradiated solid solution of hexafluoro-1,3-butadiene in freon. INDO calculations of ES

2-methyl tetrahydrofuran radicals formed by γ-radiolysis in the liquid phase

Abstract Identification of the radical species formed by γ-radiolysis of MTHF in the liquid phase was made by analysis of the ESR spectra observed from the spin-adducts of the radicals. A new secondary MTHF radical was found in addition to the tertiary radical.

Application of high resolution NMR spectroscopy on the solid state to the degradation of polyethylene by high dosages of γ-radiation

Abstract High resolution (HR) NMR spectroscopy of solid polymer by the cross-polarization (CP) and magic angle spinning (MAS) method was applied to the study of the degradation of polyethylene (PE) by γ-irradiation. No change in the HR NMR spectra was observed in high density polyethylene irradiated up to 300 M rad. Above 300 M rad, deviation of chemical shifts of the main peak corresponding to methylene carbons occurred to higher field. Fractions of the methylene in the gauche form were quantitatively estimated from the observed deviations of the main peaks. Several new peaks, including methyl and carbonyl peaks, were detected after irradiation. The relative concentrations of the methyl and carbonyl groups produced by irradiation were estimated from the observed intensities of the corresponding peaks. Chemical reactions resulting in the formation of the methyl and carbonyl groups are discussed on the basis of the observed changes in the relative concentrations of these groups.

Electron spin resonance spectra and structure of the radical cations of 1,3-dichloropropane and other dichloroalkanes

Exposure of a dilute solution of 1,3-dichloropropane in Freon to X-rays at 77 K gave e.s.r. spectra composed of many lines, which are attributable to hyperfine coupling to two equivalent chlorine nuclei and two equivalent protons in the corresponding cation. Similar experiments with methylsubstituted compounds revealed that the two equivalent protons can be assigned to one of the, protons in each terminal methylene group. CNDO/2 calculations for the radical cation suggest that reasonable values for the coupling constants of these two protons are obtained for a specific nonplanar cyclic structure having a Cl–Cl three-electron bond (σ12σ21). E.s.r. spectra were also studied for the radical cations of other dichloroalkanes, Cl(CH2)nCl. For n= 1, 2, 4, and 5, these also gave spectra indicating a SOMO on two equivalent chlorine nuclei.

Extra coupling with sodium of an amino radical adsorbed on the zeolite

Abstract ESR spectra were observed from amino radicals adsorbed on the pre-heat-treated zeolite sorbing ammonia molecules. The extra coupling with the sodium 23 Na in the zeolite was observed in the spectrum from the amino-radicals.