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An endofulleropyrrolidine, with H2 as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H2 has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H2 guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H2 guest. The ortho-H2/para-H2 interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic ortho-H2 to the diamagnetic para-H2 causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride.

The photochemical interconversion of the two allotropes of the hydrogen molecule [para-H(2) (pH(2)) and ortho-H(2) (oH(2))] incarcerated inside the fullerene C(70) (pH(2)@C(70) and oH(2)@C(70), respectively) is reported. Photoexcitation of H(2)@C(70) generates a fullerene triplet state that serves as a spin catalyst for pH(2)/oH(2) conversion. This method provides a means of changing the pH(2)/oH(2) ratio inside C(70) by simply irradiating H(2)@C(70) at different temperatures, since the equilibrium ratio is temperature-dependent and the electronic triplet state of the fullerene produced by absorption of the photon serves as an "on-off" spin catalyst. However, under comparable conditions, no photolytic pH(2)/oH(2) interconversion was observed for H(2)@C(60), which was rationalized by the significantly shorter triplet lifetime of H(2)@C(60) relative to H(2)@C(70).

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N-X-N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (14N-X-14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N-X-15N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical (14N-O-15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N-O-15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N-O-15N)# and (14N-O-15Nr)# (products of chemical reaction with r#) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N-X-N) biradical is estimated to be 4-6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N-X-N). The para-nitroxyl biradical (14N-P-15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls.

The adsorption characteristics (binding distributions, location, interactions, supramolecular structures, kinetics) of two cyclic ketones, 2-phenylcyclododecanone (112) and 2-phenylcyclopentadecano...

Photochemical, magnetic resonance, adsorption isotherms, and surface area measurements have been integrated to investigate the surface coverage dependence of the supramolecular structure and dynamics of two isomeric ketones, oMeDBK (molecular cross section similar to that of o-xylene) and pMeDBK (molecular cross section similar to that of p-xylene), adsorbed on three forms of zeolites with the MFI structure:silicalite, ZSM-5, and LZ-105. For each zeolite, the two isomeric ketones display striking qualitative differences in the experimental responses as a function of surface coverage. These differences are assigned to coverage-dependent changes in the supramolecular structural, dynamic, and binding characteristics of the adsorbed ketones and of the adsorbed reactive intermediates produced by photolysis of the ketones. In the case of pMeDBK, this ketone is adsorbed into the cages and channels of the internal surface. The external surface consists of two sites for the binding of oMeDBK: pores or “holes”, an...

Monodisperse silicalite crystals (170 μm × 40 μm × 40 μm) have been synthesized and the 29Si spin−lattice relaxation rates have been measured by high-resolution solid-state NMR. Loading of 5 wt % p-xylene into the silicalite channel system reduces the 29Si spin−lattice relaxation time compared to that of empty silicalite. The relaxation time is unaffected when p-xylene was replaced by p-xylene-d10 of the same loading. This indicates that dipole−dipole coupling between protons of guest molecules and silicon atoms does not contribute significantly to the 29Si spin-relaxation time. A novel method employing aromatic endoperoxide is demonstrated for controlled addition of molecular oxygen into silicalite samples. The results showed that interaction with a paramagnetic oxygen molecule is not the only relaxation mechanism for 29Si nuclei, and that zeolite structural changes caused by loaded organic guest molecules also play an important role in determining the 29Si spin−lattice relaxation time.

The product distributions produced by the photoinduced dichlorination of n-dodecane (nD) and the photoinduced monochlorination of 1 -chlorododecane (1CD) adsorbed on two pentasil zeolites (silicalite and LZ- 105) have been investigated. The results are explained in terms of a supramolecular model for which the mobile and diffusing chlorination reagents (CWC12) enter the zeolite particle from the external surface and diffuse preferentially along the linear channels of the zeolite internal surface that contain immobile adsorbed nD (or 1CD) molecules. The model assumes that the outermost layer of adsorbed substrates is attacked preferentially, that the attack occurs at the proximal end of adsorbed nD molecules closest to the extemal surface, and that, after the first chlorination, the substrate molecules in an inner layer are protected from chlorination by "blocking" molecules parked in the outer layer. The model describes each substrate molecule adsorbed on the internal surface in terms of supramolecular isomeric structures that are capable of characterizing the specific void space sites occupied by the substrate. A detailed analysis of the results allows the conclusion that the compensating cations tend to be preferentially located in the zigzag channels rather than in the linear channels or intersections and that the variation of selectivity of chlorination with experimental conditions results from redistribution of the isomeric supramolecular structures.

(1)H NMR of two H(2)@C(60) nitroxide derivatives has been characterized indirectly by reducing to their corresponding hydroxylamines. Nuclear spin relaxation of the endohedral H(2) and external protons of the H(2)@C(60) nitroxide and its corresponding hydroxylamine were measured and analyzed. The observed spectra are consistent with negligible scalar coupling between the unpaired electron and the endo-H(2). An unexpectedly large bimolecular relaxivity induced in the hydroxylamine by the corresponding nitroxide can be explained by rapid hydrogen atom transfer between the two species.

Computer aided analysis of electron paramagnetic resonance (EPR) spectra of tetrachloro-dibenzodioxin labelled with a nitroxide radical (TCDD-T) was employed to investigate the adsorption of dioxin vapours onto two carbons, the activated carbon (AC) NORIT GL50 and a carbon directly taken from a waste incinerator (WI) plant, termed EVN.2, and a low-polar micelle-templated mesoporous silica, termed lpMTS. Several experimental conditions were varied to optimize the adsorption process and to mimic dioxin adsorption occurring in a WI plant. Solvent vapours did not perturb dioxin adsorption. EPR analysis suggested that the nitroxide labelled-dioxin interacted (dipole–dipole interactions) with oxygenated groups, mainly C–O–C groups of the carbon and siloxane groups of lpMTS. The kinetics of adsorption was analyzed at different flow rates. The adsorption was large for NORIT and lpMTS already after few seconds, but a complete adsorption was achieved with a slow kinetics, whereas the EVN.2 sample showed a negligible adsorption even after 1 min. The results are interpreted in terms of the surface characterization (SBET, pore volume and pore size distribution) of the adsorbents. The EPR method described in this study can provide useful and unique information on the adsorption process of dioxins onto microporous carbons and mesoporous MTSs. Moreover, this method may be applied to different adsorbates and porous adsorbents.

Bisadduct isomers of a H(2)@C(60) derivative with nitroxide addends have been synthesized, isolated and characterized. The (1)H NMRs of endohedral H(2) of the major isomers show well-separated chemical shifts, which could be useful for structural assignment and identification of the purity of the C(60) bisadduct isomers.

Irradiation of solutions of aromatic aldehydes or ketones generally results in photoinduced hydrogen abstraction and yields mixtures of both asymmetric and symmetric coupling products of the radicals resulting from hydrogen abstraction. In contrast, irradiation of hydrocarbon—faujasite zeolite slurries of aromatic aldehydes or ketones results in production of radical pairs by intermolecular hydrogen abstraction, which is followed by the formation of good yields of the asymmetric geminate coupling products. The mechanistic basis of this difference in chemoselectivity is proposed to result from a combination of strong preferential adsorption of the carbonyl compounds to the internal surface of the zeolite and inhibition of the diffusional motion of the geminate radical pair produced by photolysis by the “spectator” hydrocarbon molecules that occupy the internal surface but which do not participate chemically in the reactions.

The primary photochemical process of 2,2,12-trimethylcyclododecanone ( 1a ) and 2,2,12,12-tetramethylcyclododecanone ( 1b ) is exclusively C 1 C 2 α cleavage to form biradicals 2a and 2b which then disproportionate to form enals. This contrasts with the strong preference for γ-hydrogen abstraction for the parent cyclododecanone. The preference of α cleavage over γ-hydrogen abstraction for the methyl-substituted large ring cycloalkanones is explained by the faster rate of α cleavage due to stabilization of the intermediate biradical by methyl substitution. No nuclear polarization can be observed in the photochemically induced dynamic nuclear polarization (photo-CIDNP) of 1a in the absence of radical scavengers. Measurement of the 13 C isotopic content of the photoproducts shows that the major enal product is slightly enriched in 13 C in the absence of radical scavengers through isotopomer separation via the magnetic isotope effect. However, the recovered 1a is not enriched in 13 C. The addition of radical scavengers, such as molecular oxygen, CBrCl 3 or CCl 4 , causes dramatic nuclear polarization and an increase in the 13 C enrichment of the enal product. The presence of radical scavengers opens up a competitive pathway which serves as the spin (or isotopomer) sorting process required for both the generation of T 0 S biradical CIDNP and isotope enrichment. In the absence of radical scavengers, the observation of the slight 13 C enrichment in the enal product and the absence of nuclear polarization result from the slow decarbonylation process which is fast enough to serve as an isotopomer sorting process, but not fast enough to serve as a spin sorting process.

Over the past two decades, the photolytic reactions of dibenzyl ketones sorbed on zeolites have been investigated. The reported results are consistent with a supramolecular model that takes into account the physical and chemical nature of the structure of the zeolites and their effect on the reactive radical intermediates produced by photolysis of adsorbed molecules. The model incorporates various phenomena such as surface coverage, external and internal sorption, surface diffusion, radical sieving, and the resulting product distributions. This account reports direct evidence for the validation of the model through FT-IR spectroscopy and through a new method for "titrating" the binding sites via EPR spectroscopy. It is shown that it is possible to adjust and modulate the photolytic product distribution by varying the parameters of the system. The effects of co-adsorbed spectator molecules with different polarities, namely water, pyridine, and benzene, on the photolysis of o-methyldibenzyl ketone and dibenzyl ketone sorbed on MFI zeolites is examined. This study provides insights into a displacement mechanism caused by spectator molecules and further demonstrates how the product distribution of photolysis of sorbed ketones can be controlled. The kinetics of persistent radicals formed by photolysis of ketones sorbed on zeolites is directly monitored over time by EPR, providing a measure of the lifetime of these reactive organic intermediates. Finally, measurement of Langmuir isotherms was employed to provide classical evidence for the model.

The interconversion of ortho-hydrogen (oH(2)) and para-hydrogen (pH(2)), the two nuclear spin isomers of dihydrogen, requires a paramagnetic spin catalyst such as a nitroxide. We report the design and demonstration of spin catalysis of the interconversion of oH(2) and pH(2) incarcerated in an endofullerene based on a reversible nitroxide/hydroxylamine system. The system is an example of a reversible magnetic spin catalysis switch that can increase the rate of interconversion of the nuclear spin isomers of H(2) by a factor of approximately 10(4).

2 H NMR, EPR, computational and product analyses of the photolysis of 2,4-diphenylpentan-3-one (DPP) adsorbed on MFI size/shape selective zeolites are consistent with supra- molecular structural changes as a function of surface coverage that provide a novel method for the generation of persistent diffusing organic free radicals. The MFI topology zeolites (e.g. silicalite and ZSM-5) are of technical significance in both size/shape selective catalysis and in molecular sieve separations. 1 The technically attractive properties of MFI zeolites originate from supramolecular, structural (substrates as guests, zeolite crystal as host) and dynamic (shape/size dependent supramolecular molecular dif- fusion) characteristics, i.e. substrates possess 'effective' molec- ular diameters comparable to the size/shape of the holes on the external surface of the zeolite crystal and/or the channels and intersections on the internal surface. 2 The siting and diffusion of a substrate are vital in determining the product selectivity in catalysis and the resolution efficiency in separations. 3 Understanding the role of diffusion and siting requires the uncoupling of both supramolecular, structural and dynamic features from the effects of activation at active sites and rearrangements of reactive intermediates. The effects of activation are pronounced at high temperatures with zeolites possessing strongly acidic sites ( e.g. HZSM-5). We report a supramolecular photochemical investigation 4 at room temperature in which the activation and diffusional/siting effects of zeolite catalysis are uncoupled. Activation of the substrate towards reaction is provided by photochemical methods which (i) excite adsorbed substrates whose sitings and reaction products are directly determined by 2H NMR analysis and (ii) produce persistent 5,6 adsorbed carbon-centred radical intermediates whose structure and mobility are directly deter- mined by EPR spectroscopy. Computational methods are employed for (i) analysis of the plausibility of the siting and diffusional processes on the external and internal surfaces, (ii) rationalization of the variation of product distribution as a function of supramolecular structure, and (iii) simulation of the EPR spectra of the persistent supramolecular radical species formed upon photoexcitation of the adsorbed substrates. In the molecular (solution) photochemistry (eqns. (1)-(3)) of 2,4-diphenylpentan-3-one (DPP) a-cleavage from T1 produces, after rapid loss of CO, a-methylbenzyl radicals (MB·) (eqn. (1)), which undergo random radical combination to produce 2,3-diphenylbutane (DPB) (ca. 95%) and disproportionates to styrene (S) and ethylbenzene (EB) (ca. 5%). The rates of the radical-radical reactions in eqns. (2) and (3) are close to diffusion controlled 5,8 and, in the absence of radical scavengers, determine the lifetime of the 'molecular' radicals MB ·. Results of product analysis and EPR measurements as a function of coverage (DPP/(Na)ZSM-5 (w/w), Si/Al = 80, average particle size ca. 0.1 m) showed that (i) the dominant products depend on the coverage, with disproportionation to form S and EB being favored by low coverage ( < 0.3%) and combination to form DPB being increasingly favored as the coverage increases,9 and (ii) the maximum EPR intensity of the signal of the persistent MB·radicals is insensitive to loading. For example, the ratio of disproportionation to combination of MB·radicals in solution is typically of the order of 0.1; at 0.3% loading of DPP the ratio is 3.1.

Magnetic resonance, surface area measurements and computational techniques have been integrated to elucidate the supramolecular photochemistry of two isomeric ketones adsorbed on two MFI zeolites (silicalite and ZSM-5) and to demonstrate that common factors proportional to the available external surface area operate to determine the measured parameters in each case.

Photolyse de l'o-methylbenzyl benzyl cetone et de la p-methylbenzyl benzyl cetone en presence de Na-ZSM-5 Na-ZSM-11 et LZ-105. Obtention de divers diaryl-1,2 ethanes. Action du tetramethyl-2,2,6,6 piperidinoxyle