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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).

(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.

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).

Observations are reported of initial CIDEP polarization in the time-resolved EPR spectra of H and D atoms produced by electron radiolysis of aqueous solutions. These observations are compared with previous work on H atom CIDEP. Implications for the concepts of spur chemistry are discussed.

Analysis of the CIDNP spectrum of ethylbenzoate (BzOEt) and ethylbenzene (PhEt) produced as recombination products of the decomposition of benzoylpropionyl peroxide is reported. Mathematical expressions for the yields and CIDNP intensities of both products are derived using an extension of the radical pair model. Two methods are employed to estimate the decarboxylation rate constant, k, for the benzoyloxy radical: (a) comparison of enhanced and unenhanced intensities of the PhEt quartet, and (b) comparison of the CIDNP intensities of the quartets from BzOEt and PhEt. The two methods give estimates of k agreeing within a factor of three. The lower limit to the value of k so obtained at 130°C in o-dichlorobenzene solution is 1 × 108 s−1. This is 2 to 3 orders of magnitude larger than previous estimates and implies that the lifetime of benzoyloxy radical may be considerably shorter than usually assumed.

The photolysis of dibenzyl ketone (DBK) in micellar solution results in formation of 1-phenyl-4-methyl acetophenone (PMAP) as a minor product; quantitative integration of the 13 C NMR spectrum of PMAP produced by photolysis of DBK in micellar solution demonstrates that significant selective 13 C enrichment has occurred at three carbon atoms.

Hydrogenation de composes acetyleniques ou ethyleniques en presence de complexes de rhodium carbonyle ou de palladium