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Room temperature fluorine electron nuclear double resonance (ENDOR) has been successfully observed for several superstable fluorocarbon radicals ·C(C2F4R)(i-C3F7)2 in solution. Three radicals were employed in which CF3, F, and O-c-C6F10SO3C2F5 were introduced as R, and all the hyperfine couplings (hfcs) obtained by ENDOR were assigned with the help of ESR simulation and ab initio MO calculation. In case of ·C(i-C3F7)3 large 13C and considerable β-fluorine couplings suggest the nonplaner arrangement for the central and three carbons at the β-position, in spite of the fact that all the methyl fluorine show the same hfc. Therefore, a rapid puckering motion along the C3 axis together with the methyl rotation should average the hfc’s of the 18 fluorine nuclei to give the same value. When one of the CF3 groups is substituted with an F nucleus, the five CF3 groups give two hfc values, suggesting some dynamics still exists for the molecular frame. When a large group, O-c-C6F10SO3C2F5, is substituted for CF3, all the five CF3 groups become nonequivalent and the ENDOR signal becomes intensive and sharp even at 290 K, indicating that the molecular frame becomes rigid. The relation between the ENDOR spectra of these systems and the intramolecular dynamics is discussed.

Real-time ESR observations have been made on the aqueous solutions of di-tert-butyl nitroxide and 2,2,6,6-tetramethylpiperidine-1-oxyl-4-ol flowing in a quartz column of 0.81 mm ⌀ packed with well-dried MCM-41. In both systems, the ESR spectrum is composed of two signals, the major signal (>98% of the total) is a very broad one assigned to the radical in the nanochannel of MCM-41, and the minor signal is a sharp one due to the radical in the bulk space between the MCM-41 particles. Although the spin probes are highly condensed deep in the cylindrical nanospace of MCM-41, they are transported downstream in a rather short time. The analysis of these observations led us to conclude that the aqueous solution is transported through the nanochannel of MCM-41 at a small rate but still much larger than that predicted by the conventional law. The same type of experiment was made with ethanol solutions of the same spin probes, whose ESR spectra also show different shapes in the two spaces at a high concentration bu...

A few alcoholic solutions of di-tert-butyl nitroxide (DTBN), a spin probe, at a high concentration were condensed on several silica materials, such as MCM-41, two types of SBA-15, and fumed silica, at various amounts in vacuum. At a very low solution dose the electron spin resonance (ESR) spectrum is that of an immobilized nitroxide radical. With increasing solution dose, the spectrum is gradually sharpened and a well-separated three-line spectrum is observed at the dose that is estimated to fill the surface with a monomolecular layer. Thus, the DTBN molecule can make rapid tumbling motion on this solvent layer. With a further increase in the solution dose the ESR spectrum is modified in different ways from system to system: the line width increases approximately linearly with respect to the solution dose for the SBA-15 and fumed silica systems, but it remains almost constant for the MCM-41 system until the solution dose exceeds the total volume of a nanochannel. The line width increase with respect to the solution dose is small for the SBA-15 system but large for the fumed silica system. These results have been interpreted geometrically with the structures of these silica materials and a condensation model for the alcohols on these surfaces. In relation to the present results, a model of the collective molecular flow of the alcohol solutions through the nanochannel of MCM-41 is given.

The solution of di-tert-butylnitroxide (DTBN, a spin probe) in a low molecular weight alcohol was condensed in the MCM-41 nanochannel with changing its volume successively. When the solution volume is much smaller than that of nanochannel, the broad ESR spectrum of immobilized DTBN is observed. The spectrum becomes sharp abruptly when the solution volume exceeds a critical value that is necessary to cover the whole surface of the nanochannel as the monomolecular layer. With a further increase in the solution volume, little change is observed in the line width. From these observations a model is derived for the physicochemical state of alcohol: (1) When the dose is below the critical value, alcohol molecules are adsorbed on the surface sporadically as isolated molecules or as clusters and have strong interactions with the silica wall. (2) At the critical value the alcohol molecules form the monomolecular layer on the surface. They interact with each other by forming a network structure and become free fro...

ESR spectra were observed for the alcoholic solutions of spin probes at a very high concentration, 30 mM, both in the clear solution and encapsulated in the nanochannel of MCM-41. At a high temperature, the clear solution shows a very broad ESR spectrum, whose line width is mainly due to the spin exchange induced by collision between the solute spin probes. On the other hand, the solution encapsulated in the nanochannel of MCM-41 shows a much sharper spectrum. This sharp line width indicates that collision between the solute molecules is quenched in the nanochannel. The effect is more prominent for the MCM-41 system with smaller channel diameter. In the suspension of MCM-41, the ESR spectrum is composed of the two parts, the sharp one from the solution in the nanochannel and the broad one from the bulk phase. Thus, the quenching of collision in the nanochannel is independent of the coexistence of the solution out of the nanochannel. When the system is cooled down at a temperature 10-20 degree above the melting point, the ESR spectrum of the solution in the nanochannel changes to the glassy signal. Upon warming the system, the spectrum is sharpened gradually and the glassy pattern disappears at around 293 K. These observations have been explained with a model that the solvent molecules form a liquid-crystalline like structure in the nanochannel with hydrogen bonding which prevents the solute from making translational diffusion in the nanochannel. The unexpected rapid flow of the alcoholic solution through the nanochannel and a large cage effect in a photoreaction of the solute in it, both of which were reported by the present authors, are consistent with this model.

The effect of MCM-41 on the ESR spectrum of an aqueous spin probe solution was observed. The sharp ESR spectrum turns into a rather broad characteristic one leaving a sharper signal as the minor component, immediately after the addition of MCM-41 powder to the system. This observation indicates that MCM-41 traps the solute molecule into the nanochannel by letting the solvent water form a rather stable molecular cage, since the ESR line shape indicates that the nitroxide radical undergoes anisotropic rotation without being adsorbed on the wall. Thermodynamic parameters for this process are estimated from the temperature dependencies of the trapping efficiency. This process is explained in terms of the surface enthalpy of the liquid specifically intensified in the nanospace.

A spin-probe ESR study has been made on the dynamics of 2-propanol and water molecules in the nanochannel of MCM-41 at various temperatures. In the former system, 2-propanol is separated into two phases: one with molecules immobilized in the ESR time scale and the other with mobile ones, even at temperatures more than 40 degrees higher than the bulk melting point. In the case of water, on the other hand, only the "immobilized" water was detected at a temperature as high as 313 K. At higher temperature, spin-probe molecule undergoes anisotropic rotational diffusion to reduce resistance from the solvent molecules in the nanochannel. These results are explained in relation to the intermolecular network intensified in the nanochannel. Static as well as dynamic structures of these solutions have been discussed.

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.

The physical states of long-chain n-alkane molecules in the nanochannel of MCM-41, a mesoporous silica, were studied by observing the ESR spectra of alkyl radicals formed by radiolysis. The ESR spectra for the systems with n-decane, n-undecane, n-dodecane, or n-tridecane, which were confined in the MCM-41 nanochannel and irradiated with γ-rays at 77 K, were almost identical. Since the ESR spectra for the irradiated n-alkane crystals, which are due to the three types of radicals, R I (*CH 2 CH 2 R), R I I (CH 3 *CHCH 2 R'), and R I I I (RCH 2 *CHCH 2 R"), change alternatively as the carbon number changes consecutively, the above results suggest that these n-alkanes are solidified in a similar way and cannot form regular crystals in the nanochannel. A model is proposed for the molecular packing from the dynamics, the relative yields of these three radicals, which do not deviate largely from those for n-undecane and n-tridecane crystals, and their temperature dependencies.

The ESR spectrum of a spin-probe in 2-propanol, which was condensed into the nano-channel of MCM-41 from its vapor, showed three well-separated nitrogen hyperfine lines. This observation indicates ...

D/H isotope-substitution effects on the Jahn–Teller (J–T) distortion of cation radicals of partially deuteriated benzenes; benzene-d5, benzene-1,2,4,5-d4, benzene-1,4-d2, and benzene-1,3,5-d3, were investigated by electron spin resonance (EPR) using frozen haloalkanes and porous silicas as matrices. Quantum-chemical calculations using the density functional theory (DFT) method were also performed. The way of distortion of the cation radicals of selectively deuteriated benzenes is unique relative to the deuteriation sites in all the matrices used. The ground state of the distorted cation is 2B2g of D2h symmetry, regardless of the way of isotope labeling. The undeuteriated C–H bonds occupy the specific positions in the distorted form where the higher spin density appears, in agreement with the result of theoretical calculations. The role of the matrix to stabilize the distorted form is also discussed.

A spin-probe electron spin resonance (ESR) study was made on the alcoholic solution flowing through a quartz column packed with MCM-41 to clarify the dynamics of the liquid molecules in the nanochannel. The ESR spectra of a few hydrophobic spin probes showed that they undergo rotational diffusion preferentially along the longest molecular axis, indicating that the nanochannel is effectively narrowed further for these radicals by the influence of the solvent. Since almost identical ESR spectra were observed for the static samples, which were prepared in vacuo by introducing the solutions into the quartz tube with the MCM-41 powder and sealing off, the solution in the above-mentioned experiment should really flow through the nanochannel of MCM-41. Although a laminar flow is expected from the classical theory, the calculated flow rate is almost zero. In addition, the duration for the spin-probe molecules to flow through the column was basically not dependent on their affinity to the silica surface. To explain all these phenomena, we propose a model that the liquid molecules flow collectively by slipping on the surface of the nanochannel.

13C NMR spectra were observed for the mixed solutions composed of 2-propanol and cyclohexane in contact with a mesoporous silica, MCM-41, to study the distribution and dynamics of these molecules in the nanochannel of MCM-41. From the analysis of the spectral and relaxation data, we deduced that a high concentration gradient exists in the radial distribution of these liquid molecules in the nanochannel: alcohol molecules prefer the region near the surface and cyclohexane molecules the central portion of the nanochannel. Further evidence for this new model have been obtained by the spin-probe ESR observations. In addition, we obtained a model for the movement of solution molecules in the nanochannel: molecules “flow or diffuse collectively” by slipping on the channel surface.

Photochemical reduction of xanthone in an alcoholic solution flowing in a column packed with MCM-41, a mesoporous silica, was dependent on the magnetic field. The free radical intermediates were detected by the spin trapping method and the yield of spin adducts was also dependent on the magnetic field. In addition, a radical-quenching experiment was made to analyze the bulk concentrations of the free radical intermediates as functions of the magnetic field. Although these magnetic field dependencies were different from those observed in the micelle, we concluded that these observations are explained with the “relaxation mechanism” of the radical pair model: i.e. the intermediate radical pair in a cage has a life span comparable to or longer than the spin lattice relaxation times of these radical intermediates. The mechanism and characteristics of this cage effect in the nanotube, which is opened at the both ends, are discussed.

The photoreaction of xanthone in an alcoholic solution flowing through a column reactor filled with the mesoporous silica MCM-41 was performed. It was found that the product yields depend greatly on both the pore size of the filler and the nature of the solvent. The product yields of this reaction, which involves a pair of radicals as an intermediate, were dependent on the magnetic field as well. These results were interpreted with a model wherein the lifetime of the radical pair was elongated by several hundred times or more in the mesopore, or nanotube, compared with that in the bulk solution. This unusual phenomenon was attributed to the collective movement of the solvent molecules in the nanotube, which would thereby inhibit the translational diffusion of the solute molecules.

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.

A layered complex with the chemical composition of Cu2(OH)3C12H25OSO3 was obtained through direct precipitation from a sodium dodecyl sulfate micellar solution of Cu(II) at a nearly neutral condition (pH approximately equals 6.8). From the XRD, 13C CPMAS NMR, and the spin probe ESR observations, a modified botallackite structure with a monolayer alkyl phase was proposed for the complex. Magnetic susceptibility measurements for the complex showed that there exist at least two exchange couplings: one is positive, and its magnitude is larger than that of the other, which is negative (J approximately equals -8 K). Although the cupric ions of the complex are paramagnetic, most of those are ESR silent and only the ESR signals from defects could be observed even at 298 K. A spin clustering model has been employed to explain the magnetic properties of the complex.

It has been found that the benzene cation radical (Bz+) is produced by radiolysis of a phenyl-substituted mesoporous silicate MCM-41 (Ph-MCM), and it shows a static Jahn–Teller distortion even at 77 K. The distorted structure deduced from the electron paramagnetic resonance (EPR) spectrum is almost equal to that reported for the cation radical observed in CFCl3 at 4.2 K, the singly occupied molecular orbital (SOMO) of which has been assigned to Ψs. On the other hand, Bz+ produced by radiolysis of benzene molecules adsorbed on Ph-MCM as well as on MCM-41 give dynamically averaged EPR spectra at 77 K, although the dynamics are frozen at 4.2 K. On radiolysis of '‘as-synthesized’' phenyl-MCM, not Bz+ but C6H5+–Si–R is generated. The trapping sites of Bz+ in the mesoporous silicates and the mechanism of stabilization of the distorted form are discussed.

ESR spectra of the ethylene radical cation were detected at cryogenic temperatures in SF6, C2F6 and C3F8. From the unusually small hyperfine couplings estimated for1H and13C, it has been shown that the ethylene radical cation has a non-planar structure with a torsional angle in the range of 8°–23°. Upon annealing the sample at a temperature above 93 K, the ethylene radical cation in SF6 changed into a monofluoroethyl radical through charge recombination with fluoride anion or SF 6 − .

The magnetic-field and the microwave effects on the photoreactions of anthraquinone-2-sulfonate and anthraquinone-2-carboxylic acid with xanthene were observed in an SDS micellar solution. The reaction products were analyzed by high performance liquid chromatography. With increasing the magnetic-field strength from the residual value to 400 mT, the yield of dixanthenyl, which is an “escape product”, increased considerably. Upon irradiating the system with a microwave (9400 MHz, 1.0 kW), however, the yield decreased almost to the minimum value at around 334 mT, where the intermediate radical pair becomes resonant with the microwave field. Therefore, we can detect the ESR spectrum of the transient radical pair by the product yield (product-yield-detected ESR or PYESR). In the present study, we observed a clear reversed microwave effect at the center of the PYESR spectrum, whose cause was attributed to “spin locking” of the radical pair by an intense microwave field. The microwave effects for the two systems were, different from each other in several respects including the extent of this spin locking effect in the product yield. The PYESR spectra were calculated by using a quantum mechanical method to discuss the relation between the microwave effect and the radical pair dynamics, the latter of which was also observed in the present study by the “pulse-PYESR” method.

The photochemical reduction of xanthone (XO; X = C13H8O) by xanthene (XH2) in a sodium dodecyl sulfate micelle was controlled by a resonant microwave pulse in a magnetic field. The product yields w...

A large number of papers have been published on the magnetic field effect (MFE) in various kinds of chemical which were detected mainly by the transient absorption meth0d~9~ and product analy~is.~,~ The radical pair model of CIDNP9$Io has been proposed for the mechanism of this magnetic field dependence and was confirmed by several techniques. For the photoreduction of quinones in sodium dodecyl sulfate (SDS) micellar solutions, we found that the ESR transitions of both the SDS radical and the semiquinone radical affect the yield of a product that is produced only from the SDS radical (i.e. spin adduct).” This is direct evidence of the RP model for this kind of magnetic field dependence. As another evidence of the RP model, Closs et a1.I2 and McLauchlan et al.I3 have explained a new-type of CIDEP found by Sakaguchi et al.I4 for a similar system with the interaction in the RP. Because with our method the ESR spectrum of the RP is traced by the change of the final-product yield, we call this method product-yield-detected ESR,l5%l6 or PYESR in short. Since these spin-dependent phenomena are deeply concerned with the relation between the chemical bond and the spin correlation of the two unpaired electrons from which a new bond is created,I7 these are undoubtedly an important theme of the basic chemistry. In the PYESR technique, we usually use the spin trapping meth~d’~.’~ to separate the escape product as a spin adduct from the cage product, and also to determine the yield by ESR with which in-situ observation is easy. Since PYESR is also possible by using the HPLC method,20 other techniques such as NMR should be potentially applicable. As a similar technique to detect the ESR of a spin pair,21 the optically-detected ESR technique has been well-known in solid-state and subsequently applied to solution chemi~try.~~-~~ These studies have been successful in elucidating the structure of ionic species in the irradiated sy~tems,~~,~~,~~ the reaction mechanism of the photosynthetic system,27 and the dynamics of an ion pair in a photolytically ionized system.28 The main difference with our method is that we observe the yield of a final product and the

published in Advance ACS Abstrucrs, April 1, 1995. 0022-365419512099-6452$09.00/0 large electrostatic repulsion is expected between micelles and the rate is small and difficult to detect. There are many studies on the radical-pair-decay kinetics in micelle^'^-^' made using the transient optical absorption method. However, using the optical absorption technique, it is impossible to detect the difference between the free radicals with and without the radical- pair-interaction. Therefore analysis of the decay kinetics of the radical pair in those previous reports has been made with a very simplified mode1.I7-*' Experimental Section Sodium dodecyl sulfate (SDS) was purchased from Nakarai Chemicals (Kyoto) as the purest grade and used without further purification. Guaranteed grade anthraquinone from Wako Pure Chemicals (Tokyo) was recrystallized from ethanol. Dibro- monitrosobenzenesulfonate (DBNBS) was synthesized following literature procedure.35 The concentration of anthraquinone was adjusted by monitoring the optical absorption spectrum. The temperature of the system was kept constant by using a controller (Shinko Technos MCD-130, Osaka) with N2 gas flow. The details of the apparatus were given el~ewhere.~*,~~ The well degassed SDS micellar solution containing anthraquinone and DBNBS (spin trap) was supplied to the ESR flat cell (10

ESR saturation phenomenon was observed for various alkane cation radicals derived from cyclohexane, propane and norbornane in frozen halocarbon matrices at 4 K. In addition to spectral diffusion by spin-spin interaction between the radicals, which is a common mechanism of ESR relaxation in the solid state, isotropic hyperfine modulation by the dynamics between the equivalent distorted structures was found to be very important for those centers which show Jahn-Teller (JT) or pseudo-JT effects. The effect of annealing of the sample at 77 K is also discussed.

Three pulse sequences have been employed for laser and microwave-pulse irradiation in the pulse mode product-yield-detected ESR technique to detect the dynamical feature of transient radical pair. In this method, the decrease in spin adduct yield as functions of delay times for irradiation/termination of the microwave pulse after the laser pulse are simulated with the Runge-Kutta method for a detailed reaction model to obtain the rate constants of cage product formation, escape of the radical pair, as well as the hydrogen abstraction reaction. As the representative reaction systems, photoreduction of anthraquinone, anthraquinone-2-sulfonate, and benzophenone in SDS micellar solution were employed. Effects of cyclohexene, added as a hydrogen donor, NaCl and Ethanol, added to change the microviscosity of micelle, on these reaction rate constants have been tested.

Product analysis was made for the photoreduction of anthraquinone in SDS micellar solution under the various magnetic fields with a simultaneous microwave irradiation. It has been confirmed that control of a chemical reaction is possible by inducing the ESR transitions of the intermediate radical pair.

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.

The spatial inhomogenuity of a linear low-density polyethylene rod shaped with a metallic mold was clearly imaged by the solvent-probe NMR imaging technique. Images with T2 contrast were obtained using the T2-flash sequence, which reflects the spatially resolved mobility of the solvent molecules absorbed in the polymer. The NMR image was very much dependent on the thermal history both during and after the molding process. These results were interpreted with the aid of the nuclear relaxation parameters of the matrix polyethylene determined by the high-resolution solid state NMR technique. It is suggested that the solvent-probe NMR imaging technique provides valuable information concerning the micro structure (domain structure) and its spatial distribution of a crystalline polymer.

It was found that norbornane radical cation distorts from the C 2υ symmetry to a lower one in several frozen matrices at cryogenic temperatures. The mode of distortion was discussed with the concept of «pseudo Jahn-Teller effect». This interpretation was extended to the distorted structures of other alkane radical cations which have no degenerated highest occupied molecular orbitals (non-Jahn-Teller cation). Effects of the matrices and thermal annealing on the distorted structure were also analyzed

A pulse mode PYESR (Product-Yield-Detected ESR) spectrometer was developed by using an excimer laser and a TWT microwave amplifier as the components. The effect of ESR transition of the transient radical pair on the yield of the spin adduct of one of the component radicals was observed at a high microwave power of up to 16 W without microwave heating of the solvent. The spin adduct yield of the SDS radical in the photoreduction of anthraquinone in SDS micellar solution initiated by a laser pulse of 308 nm was modulated by up to 45% by a microwave pulse of 10 μs with a peak power of 16 W, which is resonant on the ESR transition of the transient anthrasemiquinone radical. The ESR spectrum of the radical pair was obtained by tracing this modulation as a function of the magnetic field, under which the photoreaction occurred. From the dependence of the product yield on the microwave pulse duration, we could obtain a time profile of the appearance of the transient radical pair.

The ODESR spectrum and magnetic field dependence on recombination fluorescence were observed for X-ray irradiated pyrene-doped polystyrene at temperatures of 242—348 K. The ODESR intensity as a function of the pyrene concentration, 0.1—8.9 wt%, showed an unusual minimum at about 1.0%. Two phases were separated in the magnetic field dependence of the fluorescence: one was sharp and saturates at fields of over 50 mT, while the other was broad with a dip at around 60—150 mT. The cause of this dip was naturally attributed to the ST−1 level crossing. The sharp magnetic field effect also showed a minimum at around a concentration of 1.0 wt%. These novel findings have been interpreted using a recombination model modified from the previous one for pyrene-doped ethylene–propylene rubber and polyethylene. The essential points of the present model are: (1) although electron hopping within the polystyrene molecule is rapid, electron transfer at the last step of recombination between the polystyrene anion and the pyre...

It was found that norbornane radical cation distorts from the C 2υ symmetry to a lower one in several frozen matrices at cryogenic temperatures. The mode of distortion was discussed with the concept of «pseudo Jahn-Teller effect». This interpretation was extended to the distorted structures of other alkane radical cations which have no degenerated highest occupied molecular orbitals (non-Jahn-Teller cation). Effects of the matrices and thermal annealing on the distorted structure were also analyzed

A large number of papers have been published on the magnetic field effect (MFE) in various kinds of chemical which were detected mainly by the transient absorption meth0d~9~ and product analy~is.~,~ The radical pair model of CIDNP9$Io has been proposed for the mechanism of this magnetic field dependence and was confirmed by several techniques. For the photoreduction of quinones in sodium dodecyl sulfate (SDS) micellar solutions, we found that the ESR transitions of both the SDS radical and the semiquinone radical affect the yield of a product that is produced only from the SDS radical (i.e. spin adduct).” This is direct evidence of the RP model for this kind of magnetic field dependence. As another evidence of the RP model, Closs et a1.I2 and McLauchlan et al.I3 have explained a new-type of CIDEP found by Sakaguchi et al.I4 for a similar system with the interaction in the RP. Because with our method the ESR spectrum of the RP is traced by the change of the final-product yield, we call this method product-yield-detected ESR,l5%l6 or PYESR in short. Since these spin-dependent phenomena are deeply concerned with the relation between the chemical bond and the spin correlation of the two unpaired electrons from which a new bond is created,I7 these are undoubtedly an important theme of the basic chemistry. In the PYESR technique, we usually use the spin trapping meth~d’~.’~ to separate the escape product as a spin adduct from the cage product, and also to determine the yield by ESR with which in-situ observation is easy. Since PYESR is also possible by using the HPLC method,20 other techniques such as NMR should be potentially applicable. As a similar technique to detect the ESR of a spin pair,21 the optically-detected ESR technique has been well-known in solid-state and subsequently applied to solution chemi~try.~~-~~ These studies have been successful in elucidating the structure of ionic species in the irradiated sy~tems,~~,~~,~~ the reaction mechanism of the photosynthetic system,27 and the dynamics of an ion pair in a photolytically ionized system.28 The main difference with our method is that we observe the yield of a final product and the

Radical cations which have the fully extended structure (trans-trans conformation) but have lost the symmetry of the mother molecules were detected by electron spin resonance for n-pentane, 3-methylpentane, and 3,3-dimethylpentane in frozen matricesat 4 K. It was indicated, with the aid of an INDO calculation, that one of the two inner C-C bonds in the main chain is elongated and the unpaired electron tends to be confined in this bond. Selective bond elongation was also shown for the trans-gauche conformers of 3-methylpentane and 3,3-dimethylpentane cations

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.

Propane cation radicals with CS symmetry were found by electron spin resonance in C3F8 and C2F6 at 4 K. They have one C–C bond elongated considerably, and the unpaired electron is in the 10a’ orbital of CS symmetry, which is derived from the 4b1 orbital of C2V symmetry by the deformation. The highest spin density appears in the elongated C–C bond, and the in‐plane methyl proton at the other end has the largest hyperfine coupling (hfc) of about 9 mT. The degree of deformation from the C2V form is dependent on the matrices. From the general consideration on the structure of propane cation, it is deduced that the cation in SF6, which has been considered to have C2V symmetry, also has CS symmetry with a smaller distortion, and that distortion to CS symmetry is an intrinsic nature of the propane cation.

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.

The fluorescence detected ESR (ODESR) spectrum and X-induced and UV-excited fluorescence spectra were successfully obtained for ethylene propylene rubber doped with pyrene under various conditions. From the analysis of the ODESR intensity and its line-width, it was found that charge hopping between the solute pyrenes is the main mechanism for geminate recombination at the pyrene content larger than 0.3 wt%. At elevated temperatures, the excimer fluorescence relative to the monomer fluorescence induced by X-ray excitation is considerably stronger than that by UV-excitation, especially at high concentrations. On the other hand, at lower temperatures below ca. 280 K this difference could not be observed even at high concentration of 3.0 wt%. These observations indicate that the dimer cation formation, from which the excimer fluorescence is efficiently emitted, is accelerated by the hopping of cation-radical center to the clustering pyrenes at high temperatures and high concentrations. While only the electron hopping takes place at low temperatures, thus whether a charge recombination results in excimer formation or not is determined by the cationic trapping site, i.e. if several pyrenes are clustering there, excimer may be formed regardless of the excitation source. Although rapid translational motion is restricted in ethylene propylene rubber, the effective formation of excimer or dimer cation at high temperatures indicates that rotational diffusion and small distance translational diffusion are allowed within the fluorescence lifetime at those temperatures.

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.

The fluorescence detected ESR (ODESR) spectrum of a squalane solution of p-terphenyl was observed at various temperatures, concentrations, and microwave powers. Analysis of the ODESR amplitude, the line shape, and the area under the spectrum which is normalized by the fluorescence intensity led us to conclude that two geminate pairs (one with a terphenyl anion and a hole of the solvent squalane and the other with a terphenyl anion and its cation) contribute to the ODESR spectrum. It was also concluded that the former pair has a much shorter recombination time than that of the latter. It is suggested that though the hole center hops among the solvent squalanes, the rate is much less than that for the hole in aromatic solvents.

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.