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High-resolution solid-state 13C NMR spectra of long-chained n-alkanes included in the nano-channel of MCM-41 have been observed. Each spectrum has two sets of peaks: one from very flexible molecules and the other from molecules with no significant flexibility. These molecules are characterized by carbon spin lattice relaxation times, T1, of 10 s, respectively. This phenomenon is ascribed to phase separation of the alkanes in the nano-channel, i.e., the amorphous and crystalline phases, respectively. The mechanism of this phase separation and the dynamics of these molecules in the nano-channel are discussed.

Mesoporous silica, MCM-41, was packed in a pyrex column and used as a flow reactor for a solution system. The reactant solution, containing xanthone and xanthene, was irradiated with UV light while flowing in the column reactor under a magnetic field. When MCM-41 with a pore diameter of 2.5 nm was employed, a large increase in the `cage reaction' yield was observed in comparison with that for MCM-41, 3.9 nm. About 25% of this nanopore effect was canceled by the magnetic field of 500 mT.

Proton matrix ENDOR spectroscopy has been used to probe the proton environment of the stable tyrosine D+ radical present in Photosystem II membranes. ENDOR spectra in randomly oriented and oriented membranes are reported. Simulations of the spectra are given using both the point dipole approximation and the spin distribution over the tyrosine radical, which were found to provide reasonably good fits. The following results are reported: (1) Five groups of protons can be detected in the matrix ENDOR spectra of D+, that have ENDOR peak separations less than 2 MHz, and are located at distances between 3.5 and 6.7 A from the center of the tyrosine ring. These protons are directed approximately parallel to the PS II membrane plane. (2) Three pairs of ENDOR lines with separations of 2.9, 4.1 and 4.7 MHz, are also reported, together with a signal from the hydrogen bonded to the terminal oxygen in the tyrosine molecule. These protons can be exchanged by treatment in a deuterated buffer. From the orientation dependence of these ENDOR lines, it is concluded that they correspond to protons with distance vectors from the ring center that are perpendicular to the PS II membrane plane. We suggest that this specific organization of protons around the tyrosyl D+ radical contributes to the particular orientation of the β-CH2 group in the radical, and may also play a role in the unusual stability of the D+ radical form in the PS II membrane.

Magnetic field effects on the recombination luminescence of geminate pairs were observed in several X-irradiated systems. In decane solution of pyrene, the magnetic field dependence was sharp, i.e. it saturates at a relatively low magnetic field. In squalane solution, on the other hand, the magnetic field dependence is dull, and the fluorescence continues to increase even at 300 mT. In the case of pyrene-doped ethylene-propylene rubber (EP rubber), the magnetic field dependence becomes sharp again through the viscosity of the medium is much higher than that of squalane. In the presence of a quencher, the magnetic field dependence for squalane solution become sharp, and is similar to that for decane solution without a quencher. These observations were analyzed theoretically with the aid of calculations for simple model systems. It has been shown that the time profile of recombination as well as the initial spin state of the geminate pair can be obtained through curve fitting of the observed magnetic field dependence of the fluorescence.

The propane-cation radical of C s symmetry with one of the C—C bonds elongated, was detected by ESR in a frozen C 3 F 8 matrix at 4 K; this asymmetric structure has been predicted theoretically to be the most stable form. The singly occupied molecular orbital is 10a′ which spreads over the entire molecular plane, analogously to the 4b 1 orbital of the propane cation with C 2v symmetry in SF 6 . In the C s cation, however, the unpaired electron density in the elongated C—C σ-bond is much higher than that in the other part of the molecule. Above 77 K, interconversion between the two equivalent deformed structures makes the hfc's of the two inplane protons equal to each other. The averaged value is approximated one half that of the c 2v cation. The role of matrix molecules in the electronic structure and the dynamics of the propane-cation is discussed.

The process of radiation damage for long-chain n-alkanes was investigated to elucidate that for polyethylene. Chain-end alkyl radicals were preferentially formed not only through primary C-H scission but also through a hydrogen atom reaction in odd-n-alkane, as was shown by analysis of the ESR spectra for isotopic mixtures of tridecane-h28/tridecane-d28 and nonadecane-h40/nonadecane-d40. To elucidate the mode of alkyl radical migration in solids through intermolecular hydrogen atom abstraction, D/H exchange was detected in mixed crystals of eicosane-d42/eicosane-h42. The chain-end region was found to be more reactive than the inner region for radical site migration. In addition, formation of a microscopic amorphous island by destruction of the crystal lattice was also detected. The influences of molecular packing were large in all cases.

Quenching of excimer and monomer flourescences induced by X-ray and UV-light as well as that of ODESR amplitude was observed with freon 113 as the quencher. The Stern-Volmer plots of these results show a bend at low quencher concentrations. Analysis of these plots give some precise information on the kinetic processes of the delayed geminate pairs.

The role of alkyl substitution on the electronic structure of cyclohexane radical cation was investigated with low-temperature matrix ESR and semiempirical MO calculations. It was found that the unpaired electron, which is originally delocalized throughout the cyclohexane ring, tends to be confined to one of the CC σ-bonds in the ring at the position of the alkyl substitution. When the alkyl chain is longer (NC⩾4), the unpaired electron tends to flow out from the ring to the side chain. Based on these findings, the difference between the alkyl-substitution effects on the electronic structures of an aliphatic and an aromatic Jahn-Teller active cyclic cation is discussed.

It is shown that the monomer fluorescence and the excimer fluorescence from a dilute pyrene solution in squalane at room temperature are mainly from “delayed” geminate pairs (Py−, S+) and (Py−, Py+), respectively, from the ODESR spectra observed at the monomer and the excimer fluorescence. The recombination time of the former pair is found to be much shorter than that of the latter from analysis of these ODESR amplitudes. This indicates that the hole (S+) hops between the solvent molecules towards the solute anion.

It was found that γ-radiolysis of n-alkanes (Cn=6–8) and cyclohexane in CFCl2CF2Cl and CF3CCl3 in the liquid phase produces corresponding monochlorinated alkanes with a high G-values (⩾ 600). This chlorination occured selectively at the positions C2 and C3, and the yield of 1-chlorohexane was

A single crystal ESR study of the title compound shows that the oxy radical RO· is selectively formed in one of the two OH groups attached to the same carbon atom. In relation to this observation, selective formation of radicals in some symmetrical molecules is discussed. It is suggested that a slight deviation from exact symmetry causes highly selective reactions in the crystalline media. Irradiated single crystals offer us a well defined chemical reaction field in which selective reactions take place. Studies from such a view may be of considerable interest in relation to enzymatic activity.

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.

A set of new peaks in the high-resolution solid-state 13 C NMR spectrum of γ-irradiated crystalline n -alkanes have been assigned to intact alkanes in the glassy region formed by corruption of the crystal structure. The saturation recovery curve for the interior methylene carbon showed that the flexibility of the crystalline region near this glassy region is also increased. This defect may profoundly affect successive radiation damage to the alkane crystal.

To characterize the radiation damages produced at 4 K in polyethylene and related hydrocarbons, a series of studies have been carried out by using electron spin resonance(ESR) spectroscopy. It has been found that the local concentration of radiation induced paramagnetic species is considerably higher in the irradiation at 4 K than that at 77 K. The causes of this irradiation temperature effect on the spatial distribution of radicals have been extensively studied. It is suggested that the suppression of hydrogen atom migration and the reactivity of hydrogen atoms at 4 K play an important role in the low temperature radiation damages of saturated hydrocarbons. In the present paper the results previously and newly obtained are surveyed and discussed as a whole.

The ESR spectra of X-irradiated (at 1.5 and 4.2 K) single crystals of n -decane- d 22 , n -decane- h 22 and their mixtures have been measured at 1.5 and 4.2 K. It is concluded that almost all alkyl radicals are inherently formed in pairs in a fairly random fashion and that the spacial distribution of the radicals depends on the irradiation temperatures. In the crystal of n -decane- d 22 , a 1,3-carbon intrachain radical pair is identified together with a number of very close interchain pairs which are formed between almost all kinds of the first neighboring chains. The crystal of n -decane- h 22 gives considerably larger spacial distribution of radicals, indicating a remarkable mass effect on the radical pair formation.

The paper describes the observation of deuterium substitution effects in the static distortion of Jahn-Teller active species at 4 K. In partially deuterated ethane radical cations a CH bond preferentially occupies the in-plane upright position if H is in the methyl group, whereas a CD bond occupies essentially the same upright position when CH3 is replaced by CD3. The activation energy for pseudorotation is estimated to be 250 cal/mol for CH3CH3+ and is higher than 500 cal/mol with partial deuteration of a CH3.

The radical cations of (CH 3 ) 2 CCH(CH 3 ) as well as CH 3 CHCH2 exhibit a large CH proton coupling which may not be explained by the planar structure. Twisting arond the CC bond is suggested, because a hyperconjugative effect is expected to give a large β coupling for the CH proton in the twisted structure.

Radiolysis of CH 4 , CH 3 CH 3 , CH 3 CD 3 , C 3 H 8 , iso -C 4 H 10 , and CH 2 CH 2 in Xe matrices at 4.2 K gave trapped H(D) atoms with a comparable yield of radicals formed from scission of a C-H(D) bond through energy transfer from Xe to the hydrocarbon additives. H tr and >.;CH 2 CD 3 are preferentially formed from CH 3 CD 3 with the H tr /D tr ratio of 2.1 and the >.;CH 2 CD 3 /CH 3 >.;CD 2 ratio of 2.3 H tr and CH 2 >.;CH are formed from CH 2 CH 2 by a rapture of olefinic C-H bond. Upon annealing at around 45 K, H tr is detrapped and mobilized to react with C 2 H 6 and C 2 H 4 giving >.;C 2 H 5 by H abstraction and addition, respectively. Preferential abstraction of H from CH 3 CD 3 took place at around 45 K showing a large isotope effect with k H / k D > 9. The reactions of thermal H atoms produced by radiolysis of CH 4 with CO to form H>.;CO have also been observed at around 45 and 18 K in Xe and CH 4 matrices, respectively. These results suggest that methane may not be an exceptional compound to produce hydrogen atoms in the radiolysis of neat alkane at low temperature. Absence of trapped H atoms in irradiated neat alkanes except CH 4 may imply that all the H atoms produced react with alkanes except CH 4 . The reactivity of thermal H atoms at cryogenic temperature suggests that not only hot but also thermal H atom reactions are involved in the solid phase radiolysis below 77 K.

Direct evidence has been obtained for the radiolytic production of D atoms in neopentane-d12 irradiated at 4.2 K by observing ESR signals from trapped D atoms (Dtr) together with neopentyl-d11 radicals (R). The drastically low Dtr yield ( D tr /∽ = 1 9 ) as compared with the high Htr yield (1/1) in the radiolysis of methane suggests that the hydrogen atoms more easily react with neopentane-d12 at 4.2 K and that the mean excess kinetic energy of the D atoms radiolytically produced is as low as to exhibit a marked dependence on the bond dissociation energies. On the other hand, the D atoms scavengeable by a dilute solute of C2H4 (1 mol%) to give ©H2CH2D radicals at 77 K are estimated to be 35% of the total radical yield. This indicates that the low Dtr yield does not always mean the low production of D atoms and that only a minor fraction of scavengeable D atoms is trapped at 4.2 K. The interpretation is further confirmed by the photolysis of HI in neopentane-d12. The low trapping yield ( H tr /∽ = 1 10 ) in neopentane-d12 in contrast to the high yeild (25/1) in CH4 is parallel to the radiolysis.

Trichlorofluoromethane has become the most common choice of matrix for the radiolytic preparation of radical cations for study by ESR spectroscopy. In most studies, no cation other than that due to the substrate is detected by ESR spectroscopy. However, under certain circumstances, a well-defined species is detected, which is thought to be the dimer cation (CFCl3)+2.

It is found that an oxygen-centred n π radical of HCOOCH 3 is produced radiolytically in CFCl 3 at 4.2 K without forming a σ* complex with a matrix molecule. This cation converts into the carbon-centred radical cation HC + (OH)OCH 2 by an intramolecular hydrogen-atom transfer upon warming to 77 K. This is clear experimental evidence for a McLafferty rearrangement of ester radical cations.

The photoinduced and thermal reactions of σ radical cations of methyl-substituted propanes and butanes in the condensed phases have been characterized by ESR. Photoelimination of CH4 giving olefinic π radical cations commonly takes place except for H2 elimination from isobutane radical cations. The thermal reactions of methyl-substituted butane radical cations in CFCl2CF2Cl were found to be deprotonation from the CH3 group to give primary alkyl radicals.

A 4 K matrix ESR study shows that the molecular radical cations of isopropyl formate and acetate, produced radiolytically in halocarbon matrices at 4.2 K, undergo spontaneous rearrangement due to a selective intramolecular hydrogen shift from the tertiary CH bond in the isopropyl group to the carbonyl oxygen atom giving RC + (OH)OC(CH 3 ) 2 , where R = H or CH 3 . The radical cation of tert-butyl acetate undergoes further fragmentation at the ester CO bond following a similar rearrangement to give an isobutene radical cation in CFCl 3 .

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a 2 , in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D 3h to C 2v at 4 K in CFCl 3 , and the SOMO is likely to be 6b 2 , in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C 6 F 12 as well as in CFCl 2 CF 2 Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.

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.

ESR observation is described of a new type of reaction of alkane radical cations in solid matrices; the cyclobutane radical cation in CFCl 3 and in SF 6 suffers photoinduced ring opening followed by 1,3-hydride ion transfer to rearrange into the but-1-ene radical cation, which further photoisomerizes into the cis-but-2-ene radical cation. The results are compared with thermal ring opening reported for the cyclopropane radical cation to give the trimethylene radical cation CH 2 CH 2 CH 2 + in CFCl 2 CF 2 Cl. A possibility of hydride ion transfer to form CH 2 CH 2 CH 3 is discussed as an alternative interpretation of the spectrum assigned to the trimethylene radical cation.

It has been found that the yields of trapped alkyl radicals in linear alkanes irradiated at 77 K exhibit an alternation depending upon whether the carbon number is even or odd. The alternation becomes more prominent in a series of higher alkanes starting from n-C11H24 to n-C19H40, for which the crystal structures as well as the ESR spectral features are known to exhibit a regular alternation. The results suggest that the radiation chemical events are affected by the molecular packing, that is, the crystal structures at the chain end and/or the interchain structures.

The ESR spectrum of the radical cation of 3-methylpentane (3-MP) in CFCl2CF2Cl has been obtained. The spectrum is almost identical to that for the 2-methylbutane radical cation. From the spectral analysis it is concluded that 3-MP+ has a GT skeletal conformation and the unpaired electron is confined to the C3-C4 bond giving rise to large hyperfine couplings for the trans C-Hβ protons attached to the C2,C5 and C6 atoms, where C6, is on the side of the 3-methyl carbon atom. The results are totally consistent with concepts previously established from studies on the methyl-substituted propane and butane radical cations.

The paper describes the direct and conclusive evidence that thermal H atoms abstract a hydrogen atom from the CH bond at cryogenic temperatures (10–30 K). It has been observed by ESR that thermal H atoms are trapped in CH 4 containing 0.5 mol% of C 2 H 6 when X-irradiated at 4.2 K and that upon warming to 10–20 K the trapped H atoms are freed and react selectively with C 2 H 6 forming C 2 H 5 radicals. The difference in the CH bond dissociation energies between CH 4 and C 2 H 6 must be responsible for this selectivity. The reaction of thermal H atoms at cryogenic temperatures is further confirmed by the 4.2 K photolysis of HI(0.15 mol%) in xenon matrices containing C 2 H 6 (0.75%). The H atoms trapped immediately after the photolysis are freed at 20–30 K and react with C 2 H 6 forming C 2 H 5 radicals. The tunnel effect must play an important role in the reactions at cryogenic temperatures.