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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.

Single crystals and polycrystalline samples of lithium formate monohydrate (HCO2Li·H2O) were X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 200 or 295 K. Two different radical species were observed. The overall dominating species is the ˙CO2− radical trapped in the crystal matrix at an orientation not very different from that of the parent CO2 fragment in the unirradiated matrix. The g- and 13C hyperfine coupling tensors of the ˙CO2− radical were determined. The large linewidth (about 1.5 mT) of the polycrystalline EPR spectrum is due to extensive hyperfine couplings with lithium ions and protons in the environment. Four lithium couplings and four proton couplings associated with the ˙CO2− radical were measured, and all couplings were assigned to specific matrix nuclei. The spectra yield evidence for a second radical in low relative abundance. One small lithium hyperfine interaction detected was ascribed to this radical. Spectral simulations of the EPR and ENDOR spectra support the conclusions made.

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one α-coupling, and three β-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1–H bond with respect to the N–C1–C2 plane is estimated to be 13–15° (bending angle θ ≈ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions.

Nitrogen monoxide (NO) introduced into partially and fully lithium ion (Li+)-exchanged A-type zeolite was studied by X-band EPR measurements. Two types of NO species, NO monoradical and NO–NO biradical, were detected for the partially ion-exchanged zeolite, while the latter was less detectable for the fully ion-exchanged one. The EPR parameters of the NO monoradical provided evidence that the electrostatic field associated with Li+ ions in A-type zeolites is weaker than that with Na+ ions. It was found that the molecular distance of NO biradical formed in partially Li+-exchanged zeolite was shorter than that in sodium ion-exchanged A-type zeolite.

An EPR study was carried out to investigate both the structure and molecular dynamics of triethylamine (Et3N) and tripropylamine (Pr3N) radical cations generated in γ-irradiated AlPO4-5 from neutral mother molecules incorporated as an organic template. Both Et3N+• and Pr3N+• radical cations were stable at 300 K and showed strongly temperature-dependent EPR spectral line-shapes in the temperature range from 77 to 300 K. The spectral change was successfully explained by assuming a two-site exchange model in which two inequivalent β-hydrogens interchange their positions with each other. The experimental spectra were quantitatively analyzed by a simulation method using the exchange rate as a variable parameter. From an Arrhenius plot of the rate constants, activation energies of 9.1 and 11.4 kJ mol−1 were evaluated for Et3N+• and Pr3N+• in AlPO4-5, respectively.

The nature of quartet state nitrogen atoms formed in γ-irradiated NaN3 powder was investigated by EPR, ENDOR and ENDOR-induced EPR (EIE). The ENDOR spectrum observed at 110 K clearly showed two sets of 14N signal pairs centered at aN/2 (half the 14N-hyperfine splitting) and at 3aN/2, strongly suggesting the existence of quartet state 14N atoms (4S3/2). At 160 K, the EPR spectrum exhibited a clear fine structure accompanied by a hyperfine coupling due to 14N. The zero-field splitting, D, gradually increased with decreasing temperature from 170 to 40 K, and then became nearly constant. The temperature dependence of D was explained in terms of a local lattice vibration coupled with spin states of 14N atoms. At below 95 K, a new fine structure with larger D was observed. The observed two D-values were attributed to 14N atoms located at two different interstitial sites in the monoclinic NaN3 structure.

An EPR study was carried out to elucidate the zeolite cage effect on the stability and molecular dynamics of [(CH3)3N]•+ and [(CH3)3NCH2]•+ radical cations generated in γ-irradiated Al-offretite, SAPO-37 and SAPO-42, which contained (CH3)4N+ ions as organic template. The [(CH3)3N]•+ radical cation was stable at room temperature when it was generated in relatively small size cages such as the sodalite cages of SAPO-37 and the β-cages of SAPO-42. On the other hand, the [(CH3)3NCH2]•+ radical cation was stable not only in the sodalite cages of SAPO-37, but also in relatively large cages such as gmelinite cages or the main channels of Al-offretite and the α-cages of SAPO-42. Strongly temperature dependent EPR spectra were observed for the [(CH3)3NCH2]•+ radical cation stabilized in Al-offretite and SAPO-42. They were successfully analyzed by a spectral simulation method assuming a three-site exchange for the methyl protons. The evaluated exchange rates were in the order SAPO-37 (sodalite cages)

The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (aiso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with ∣A‖∣

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N–CH2]+• radical formed in a γ-irradiated Al-offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed 1H hyperfine (hf) splitting was a1=0.46 mT for three magnetically equivalent hydrogens, one of each methyl group at 110 K and a1′=0.15 mT for three of each methyl group at 300 K; the hf splittings due to two equivalent CH2 hydrogens and the central 14N were a2=2.24 and a3=0.35 mT, respectively, independent of the temperature. This result suggests that rotation of the methyl group around the N–CH3 bond is responsible for the temperature dependent line shapes. Assuming a three-site jump model in which three methyl protons interchange their positions with each other, the EPR line shapes were successfully analyzed by a simulation method using the jump rate as a variable parameter in the temperature range from 140 to 270 K. The rate was found to increase from 7.0×106 s−1 (140 K) to 4.1×108 s−1 (270 K) with temperature. From an Arrhenius plot of the rate constants, an activation energy of 8.1 kJ mol−1 was evaluated for the methyl group rotation of [(CH3)3N–CH2]+• in Al-offretite.

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B1g and 2B2g states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B2g-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

A combination of EPR lineshapes, ENDOR and ESEEM was performed to investigate possible Jahn–Teller effects in mono- and perdeuterated benzene radical cation in a polycrystalline CFCl3 matrix. Replacement of one proton by a deuteron was utilized to exclude significantly fast quantum pseudorotation and overall rotary motion below 77 K. Clear evidence was obtained for static Jahn–Teller distortion at temperatures up to 30 K, with major spin densities on two para positions (1 and 4) in agreement with a b2g(χs+) localized orbital. From ENDOR measurements of C6H5D+, the isotropic and dipolar coupling constants for protons occupying both high and low spin density positions were accurately measured. Additional ESEEM experiments and simulations confirmed the results, indicating further that deuteron-isotope substitution does not disturb appreciably the fundamental dynamics of the benzene radical benzene. The hyperfine tensors of C6D6+ in the Jahn–Teller distorted configuration obtained by ESEEM were analogous to those of fully protonated compounds, but the components were scaled by the magnetic moment ratio of deuterons to protons.

The diffusion and bonding of NOx (x=1, 2) and Cu2+ species in zeolites are reviewed, based mainly on our own research. The molecular motion of adsorbed NO2 has been examined with EPR in several zeolite samples and analyzed using the slow-motional EPR theory. In X- and Y-type zeolites the broadening of the spectra with temperature could be analyzed by simulations using a rotational diffusion model in agreement with earlier results in Vycor glass and Cu-metal. For the diffusion of NO2 in Na-mordenite and Na-ZSM-5 the broadening of the spectra with increased temperature could be better simulated with the Heisenberg type of spin exchange model. The exchange was attributed to the interaction between NO2 molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na+, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of H2O molecules adsorbed on the zeolite because of weaker interaction between NO2 and the surface. A more detailed slow-motional analysis indicated that at each temperature a distribution of diffusion rates occurs. NO adsorbed on Na-A zeolite was found to be present as monomer at low pressure ( 100 mbar). The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NO–NO species depends on Na+ for its stability. The detailed structure of a Cu2+ complex with water molecules in Cu-ZSM-5 zeolite was characterised with ENDOR at 4 K. The complex has an axial distorted octahedral structure with two water molecules at a longer distance in axial position and four molecules in the equatorial positions. The Cu2+ complexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with 1H, 2H and 14N hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

Piperidine forms small aggregates in most Freon matrices at cryogenic temperatures; upon X-irradiation, hydrogen–proton transfer between piperidine radical cations and piperidine molecules occurs.

The electronic structures of the azetidine radical cation and the neutral azetidin-1-yl radical have been investigated by EPR spectroscopy and theoretical ab initio UHF, MP2 and SDCl calculations. The radical cation was assigned a structure with a planar ring according to the theoretical and experimental results. Ab initio UHF calculations predicted the neutral azetidin-1-yl radical to have a puckered ring structure, with puckering angles 12.0 and 15.4° on the STO-3G and 6-31G* levels, respectively. This result was supported by the observed nonequivalent values of the experimental β-proton hyperfine coupling constants, suggesting that the neutral radical has a puckered structure at low temperature in a solid halocarbon matrix. Calculated SDCl values for the isotropic and anisotropic hyperfine coupling constants of the azetidine cation and the azetidin-1-yl radical were found to be in qualitative agreement with the experimental data.

The dynamics of the methylcyclohexane and 1,1-dimethylcyclohexane radical cations in a solid perfluoromethylcyclohexane matrix at low temperature have been studied by electron paramagnetic resonance (EPR) spectroscopy. The reversible variations of the experimental EPR linewidth, observed for both cations in the temperature region 4–173 K, have been reproduced through simulations employing a dynamical model for the molecular motion. It was assumed that an interconversion between two energetically equivalent mirror images of the molecular framework occurred. The related activation energy has been determined to be 0.2 and 0.3 kcal mol–1 for the methylcyclohexane and 1,1-dimethycyclohexane radical cations, respectively.

Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose to obtain information about possible ring opening and other fragmentation processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDOR and EIE spectroscopy, two H-abstraction radicals present in a 3∶1 relative ratio in the irradiated crystals were identified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra the relative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III, respectively. Radical III is proposed to be of the allyl radical type –CHCH–˙CH– formed formally by a concerted H2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal and structure is at variance with observations from aqueous solution studies of succinimide, where the CO–CH2 bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT) calculations, predicting equivalent hyperfine interactions to the two α-type protons at C2 and C4 in excellent agreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that both couplings originate from the same radical species. Possible mechanisms for the formation of radical III are discussed.

The formation of silver–ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after γ-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)2 stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The ‘gas-phase’ geometries of Ag(C2H4) and Ag(C2H4)2 complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)2 complex of D2h symmetry are in very good agreement with those obtained experimentally.

The ENDOR technique has been used to study 133Cs hyperfine couplings of SO3– radical ions in X-irradiated Cs2S2O6 single crystals. The orientational dependent parameters for two couplings were obtained from the ENDOR spectra and assigned with the aid of ENDOR-induced EPR. The larger coupling, a(I)= 30.55 MHz, was attributed to the Cs+(2)S(2)O3˙– interaction and the smaller coupling, a(II)= 9.59 MHz, to the crystallographically different product Cs+(2)S(1)O3˙–. The difference in distances was estimated from the experimentally measured hf splittings using a point-dipole approximation, and was found to be in agreement with crystallographic data. A temperature dependence of hyperfine couplings was observed and possible explanations are discussed in terms of the displacements induced by the piezoelectric and pyroelectric properties of the Cs2S2O6 crystal.

X-irradiated crystals of L-ascorbic acid have been examined by e.s.r. spectroscopy in both the X and Q bands at room temperature. Three types of radicals have been observed. The forbidden transitions and 13C hyperfine splitting in its natural abundance have been detected. In order to determine the structure of all three radicals, the experimental results have been supported by semiempirical MNDO and INDO calcultions. Also, a method to calculate the dipolar hyperfine coupling tensor from a proton interacting with spin densities localised on two atoms has been used. The results are compared with the experimentally determined dipolar coupling tensors for distant hydroxy-group protons, interacting with spin densities localised on carbon and oxygen atoms.

Single crystals of L-ascorbic acid have been irradiated and investigated at 77 K. The signals due to triplet radical pairs were observed. Three radical pairs, separated within the pair by 5.07, 6.28 and 8.46 A were interpreted. The radicals which form the pairs are of the same type as those studied previously at room temperature as free radicals and published elsewhere. The location of the three types of radical pairs in the crystal was found by the comparison of the experimental results with crystal data. There are some signals arising from other radical pairs. It was not possible to follow the lines over a sufficient range of orientations to make an interpretation.

Rotational isomers of n-butane cations with partly deuterated methyl groups have been studied in CH3CCl3, CF2ClCFCl2 and CFCl3 matrices at 4.2 K and temperatures above 77 K. It is shown that the abundance ratio between different isomers depends on the temperature and the matrix. Temperature-induced oscillations of the partially deuterated methyl groups decrease the pertinent proton hyperfine splittings. The experimental data for the spin density of the n-butane radical cation are in excellent agreement with INDO calculations based on optimized geometries obtained from MNDO and ab initio calculations.

The equilibrium geometries of the propane cation in its three lowest electronic states, 2B1, 2B2 and 2A1(C2v symmetry assumed), have been calculated by the UHF method within the ab initio MO-LCAO-SCF approximation. The 2B1 state is predicted to be lowest in energy and the 2B2 and 2A1 states to be of almost the same energy, 12 kcal mol–1 above the 2B1 state. The isotropic hyperfine coupling constants have been calculated and compared with experimental data. The calculations confirm the e.s.r. identification of the 2B1 state, obtained in an SF6 matrix. The previous experimental assignment, 2B2, for the state obtained in Freon matrices was found to be ambiguous and an assignment to the 2A1 state can not be ruled out. Dipolar hyperfine coupling constants have been calculated for the 2B1, 2B2 and 2A1 states of the propane cation and for the 2A1 ground state of the cyclopropane cation. In the latter case, comparison with experimental data has been made with reasonable agreement. The equilibrium geometry agrees well with previous calculations. Complementary experimental data for the propane and cyclopropane cations are reported.

Single crystals of pyridoxine hydrochloride have been irradiated and investigated at room temperature. Two radicals of different stability have been observed. Hyperfine couplings to six protons were fully characterised for each radical. The trapping sites for both radicals were found by comparison of experimental results with crystal data. The stable radical is formed by loss of hydrogen at C6, as observed earlier in γ-irradiated pyridoxine hydrochloride. The unstable radical is formed by loss of hydrogen at C8.