Keichi Nunome

Structures and reactions of radical cations of some prototype alkanes in low temperature solids as studied by ESR spectroscopy

The structures and reactions of radical cations of prototype alkanes such as ethane, propane, isobutane, neopentane, etc. produced in SF6 and in other halocarbon matrices irradiated at 4 K have been extensively studied by ESR. The geometrical structures as well as the unpaired electron orbitals of these cations are unequivocally characterized based on the simple symmetry considerations as well as the INDO MO calculations. The ESR hyperfine coupling constants determined for these cations as well as their deuterated homologs give conclusive evidence for the interpretations. The unpaired electron in linear alkane cations is delocalized over the in‐plane C–H and C–C σ bonds forming delocalized σ radicals, whereas that in highly branched alkane cations is more confined to one of the C–C bonds forming localized σ radicals. The H1s spin density, giving a large hyperfine coupling in the linear alkane cations, is largely contributed from a direct participation of the C–H bond in the unpaired electron orbital, wher...

Pairwise trapping of radicals in single crystals of n‐decane irradiated at 1.5 and 4.2°K

The electron spin resonance spectra of single crystals of n‐decane‐d22, n‐decane‐h22, and their mixtures irradiated and measured at 1.5 and 4.2°K have been studied to shed light on the mechanisms of alkyl radical formation in saturated hydrocarbons. From the analyses of low temperature spectra and of the spectral changes caused by thermal annealing, it is suggested that almost all alkyl radicals are inherently formed in pairs and that the initial radical pair formation is fairly random especially for distant radical pairs, although there is some preference in the short range reactions for forming very close radical pairs with separation shorter than 6 A. The predominant radical pairs in deuterated crystals irradiated at 1.5 and 4.2°K have separations ranging 6–25 A. The spatial distribution of the generation of paired radicals depends upon irradiation temperature. Radical site migration takes place even at 35°K. There is a remarkable mass effect on the spatial distribution of the paired radicals. The deut...

ESR spectra and structures of radical cations of some branched alkanes: β‐proton couplings in C–C σ cations

The systematic ESR studies of radical cations of five kinds of methyl substituted butanes show that the unpaired electron is rather confined to one of the C–C σ bonds in contrast to the linear alkane cations, in which the unpaired electron delocalizes over the entire σ molecular chain. Analyses of the origin of the trans C–Hβ proton couplings indicate that spin transfer due to hyperconjugation plays an important role in these branched alkane cations. It is also shown that the hyperconjugative effect in these branched alkane cations is two times higher than that in the neutral alkyl π radicals. The large difference of the front and back lobe interactions in the β‐proton couplings in these C–C σ radicals is attributable to the bent structure of the radical carbon atom. The front lobe interaction is about one‐half of that of the back lobe interaction as is the case of vinyl radicals.

ESR studies of irradiated methane and ethane at 4. 2 K and mechanism of pairwise trapping of radicals in irradiated alkanes

The spatial distributions of trapped hydrogen atoms and methyl radicals in CH4 and CD4 irradiated at 4.2 K have been studied by ESR. Most of the hydrogen atoms are trapped within ∼16 A from each parent methyl radical forming distributed radical pairs. This indicates that the thermalization distance of hydrogen atoms produced by 4.2 K radiolysis is ≲16 A in methane. Evidence has been obtained for the formation of a small amount of ĊH3...ĊH3 pairs, which are supposedly formed from hot hydrogen atom reactions. No evidence has been obtained for the formation of a pair of hydrogen atoms. There has been also obtained the experimental evidence that thermal hydrogen atoms unreactive with methane can abstract a hydrogen atom from ethane at 10–20 K. In the light of these results, the following conclusions are deduced: A part of hydrogen atoms produced in radiolysis of alkanes undergo hot abstraction to form very close alkyl radical pairs whereas the rest is thermalized and immobilized at the place close to each par...

ESR studies of local concentrations of radicals in polyethylene irradiated at 1.5, 4.2 and 77 K

Abstract The microwave power saturation behaviour of alkyl radicals in polyethylene irradiated at 1.5, 4.2 and 77 K suggests that the local concentrations of radicals in spurs is considerably higher in the samples irradiated at 1.5 and 4.2 K. Upon annealing the samples at 77 K, the local concentrations became nearly the same as those of the sample irradiated at 77 K. The decrease of the local concentrations at 77 K was larger than that of the total concentrations suggesting an expansion of the initial spur at 77 K.

The fate of primary cations in radiolysis of alkanes as studied by ESR

Abstract The structures and reactions of alkane cations (RH+) have been studied by ESR to elucidate the fate of primary cations in radiolysis of alkanes. Radical cations of prototype alkanes such as C2H6, C3H8, iso-C4H10 and neo-C5H12 etc. as well as their partially deuterated analogues were stabilized in irradiated frozen matrices such as SF6, CFCl2CF2Cl and CFCl3 having a higher ionization potential than that of these alkanes contained as dilute solutes. RH+ in SF6 and in CFCl2CF2Cl converts into alkyl radicals by deprotonation probably through bimolecular reactions, whereas RH+ in CFCl3 unimolecularily decomposes into olefinic cations by H2 and/or CH4 elimination reactions. It is further found that the electronic structures of propane and isobutane cations in halocarbon matrices are different from those in SF6 and the difference is drastically reflected in the site preference of their deprotonation reactions. The results are discussed in relation to the mechanisms of pairwise formation of alkyl radicals in low temperature radiolysis of neat alkanes and its suppression by addition of electron scavengers.

Electron spin resonance studies of structures and reactions of radical cations of a series of cycloalkanes in low-temperature matrices

Radical cations of a series of cycloalkanes (C3–C8) produced radiolytically at 4.2 K in a variety of halogenated matrices have been characterized by e.s.r. spectroscopy as σ-delocalized radicals, in which the unpaired electron occupies the molecular orbital lying in the molecular (equatorial) plane. Thus the protons in the equatorial C—H bonds participating in the MO give rise to large hyperfine couplings as compared with the axial protons. Among these, radical cations of C3, C4, C6 and C8 are Jahn–Teller (J.T.) active and exhibit static distortions at 4.2 K, giving non-equivalent proton couplings. Regardless of the matrices used, the cation distorts to the same direction along the J.T.-active ring-deformation mode giving essentially the same geometry and singly occupied molecular orbital (SOMO). The distorted structure is dynamically averaged at elevated temperatures, giving equivalent couplings. The temperture changes of the spectra of c-C3H+6 and c-C6H+12 are accounted for by a three-site jumping (or tunneling) model using a modified Bloch treatment. The site-jumping process may be pseudorotation in the J.T. potential trough. However, the onset temperature of the jumping process and thus the activation energy differ from matrix to matrix, so that the observed distortion may be a matrix-assisted static J.T. distortion. The results may be also accounted for in terms of a static distortion due to matrix effects and an averaging of the matrix field by reorientation of the cations around the normal to the molecular plane. However, the latter explanation seems less probable because the onset of the averaging process of c C6H+12 at temperatures as low as ≲4.2 K does not appear to be ascribable to molecular reorientation. Most of these cations undergo deprotonation to form cycloalkyl radicals in SF6 and in CFCl2CF2Cl at ≳100 K.

Static distortion of 2E molecular cations of cyclobutane at 4.2 K giving 2B2 with C2v symmetry and its dynamic average at >77 K: ESR evidence

Etude par spectrometrie RPE de la distorsion de la structure du cation cyclobutane c-C 4 H 8 + qui est une espece presentant un effet Jahn Teller tres important

ENDOR studies of the superhyperfine couplings of hydrogen‐bonded protons. III. Protonated radical anions in irradiated L‐valine hydrochloride monohydrate

The so‐called carboxyl radical anion trapped in irradiated single crystals of L‐valine hydrochloride monohydrate has been studied by ENDOR. The spectra exhibited two OHβ proton couplings, indicating that the anion is protonated to form a radical of the type R–Ċ(OH)2. The coupling tensors and their orientations clearly indicate that the amino proton in the neighboring NH3 group is transferred to the anion through the intermolecular hydrogen bond. Combining the results with those previously obtained for L‐alanine and succinic acid, the variation of the dipolar tensor elements with the isotropic values has been examined for the OHβ proton couplings in this type of radical. It is then shown that the anisotropic coupling is approximately independent of the dihedral angle of the O–Hβ bond. The dipolar tensors were also calculated as a function of the dihedral angle, and the results are in good agreement with the observations.

ENDOR studies of methyl radicals in irradiated single crystals of CH3COOLi⋅2H2O

The methyl radicals trapped in single crystals of CH3COOLi⋅2H2O irradiated at 77 °K have been studied by ENDOR. The proton hyperfine coupling tensor was determined to be (−21.30, −20.57, −21.61) G. The absolute sign of the coupling was determined in referring to the nuclear dipole–dipole interaction. The deviation from axial symmetry indicates that the methyl radical undergoes hindered oscillation about the second axis in the radical plane in addition to rapid reorientation about the C3 axis. The superhyperfine coupling tensors were determined for all of the seven neighboring protons within 5 A from the radical carbon atom. The nearest neighboring CH3 and water protons gave the tensors (3.07. 0.24, 0.26) and (2.01, −1.16, −1.00) G, respectively. The dipolar terms of these neighboring protons and others lead to the conclusion that the methyl radical is trapped in practically the same position with the same orientations as those of the methyl group in the undamaged molecule. The precise analysis suggests th...