Machio Iwasaki

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...

Second-order perturbation treatment of the general spin hamiltonian in an arbitrary coordinate system

Abstract A second-order perturbation treatment is given for the general spin Hamiltonian in an arbitrary coordinate system. A general solution is obtained within a limit of high-field approximation. It is not necessary that all of the tensors be coaxial or that the g and A tensors be symmetric. The only restriction is that the effective hyperfine Hamiltonian including the nuclear Zeeman term be much greater than the quadrupole Hamiltonian. The nuclear Zeeman term can be greater or smaller than the hyperfine term. A convenient method is given for obtaining solutions without introducing transformation matrices explicitly for the spin operators. The transition probability is given to the first order and supplementarily to the second order for some important cases.

ENDOR studies of the superhyperfine couplings of hydrogen‐bonded protons. I. Carboxyl radical anions in irradiated L‐alanine single crystals

The carboxyl radical anion trapped in an irradiated single crystal of L ‐alanine was studied. ENDOR techniques were used to clarify the origin of the transferred proton in this anion. The hyperfine tensor of the transferred proton and the superhyperfine tensors of the hydrogen‐bonded protons in the neighboring NH3+ groups were determined. The results show that the transferred proton forms a normal covalent bond with the carboxyl oxygen atom and is located outside the COO plane. The direction of the O–H bond formed by the transferred proton is approximately parallel to one of the three hydrogen bonds in the undamaged crystal. This suggests that this proton is transferred across this particular hydrogen bridge. In addition, the large positive isotropic coupling of the transferred proton was accounted for by the structure of the protonated anion.

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.

ENDOR studies of the superhyperfine couplings of hydrogen‐bonded protons. IV. Carboxyl radical anions in irradiated glycine and α‐amino isobutyric acid

The carboxyl radical anions trapped in irradiated single crystals of α‐glycine and α‐amino isobutyric acid have been studied by ENDOR. To elucidate the controlling factors for the selective proton transfer from the neighboring NH3 groups to the anion across the hydrogen bridge, the superhyperfine coupling tensors of the protons in the neighboring NH3 groups have been determined. The results suggest that two of the three amino protons hydrogen bonded to the COO group of the anion are located in the middle of the hydrogen bridge or jumping between the nitrogen and oxygen atoms. The latter interpretation seems to be more probable. The remaining one is located in the neighboring NH3 group. The following controlling factors suggested in our previous paper were confirmed by comparing the results with the reported crystallographic data: firstly, the protons participating in the hydrogen bond, which is formed in the direction of the unpaired electron orbital of the anion, are easily transferred to the anion side;...

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.

Pairwise Trapping of Radicals in Irradiated n‐Hydrocarbons and Related Compounds as Studied by Electron Spin Resonance

The ESR signals due to the radical pairs were found in n‐hydrocarbons γ irradiated at 77°K. Besides the spectra due to the ΔMs = 2 transition, ΔMs = 1 spectra were also found on the tail of the predominant spectra of isolated radicals. The ΔMs = 1 spectra of the radical pairs show nearly the same hyperfine splitting as that of the ΔMs = 2 spectra. From the analysis of the ΔMs = 1 spectra, the separation d‖ due to the zero‐field splitting was determined to be 292 G from which the distance of the paired radicals was estimated to be 5.75 A. The radical pairs are probably in the form of two alkyl radicals on the adjacent hydrocarbon chains. The value of 5.75 A is consistent with the intercarbon distance between the adjacent chains aligned along the direction of the crystalline b axis. A glassy hydrocarbon such as squalane also gave the ΔMs = 2 spectrum but did not give the hyperfine lines on the tail of the ΔMs = 1 spectrum as is the case of high polymers. In addition, the effect of aditives and substituents ...