Isotope Effect in the UV Photolysis of Hydrogenated and Perdeuterated Fulleranes
Fulleranes, the hydrogenated fullerenes C60H36 and C60D36 have been synthesized in n-hexane where they show an absorption maximum at 217 nm in their electronic absorption spectra. Similarly C70H38 and C70D38 show an absorption maximum in n-hexane at 214 nm. The interstellar light extinction curve shows a “bump” just at 217 nm which is attributed to hydrogenated interstellar carbon dust. Remarkably also fulleranes which can be considered an analogous of hydrogenated carbon dust show a maximum of absorption just at 217 nm. The width of this absorption appears to be consistent with the observed widths of the UV bump. The photolysis of fulleranes with monochromatic light at 254 nm causes a shift of the absorption maximum from 217 nm to longer wavelengths. This band shift of the absorption maximum has already been observed in the photolysis or in the thermal processing of hydrogenated carbon dust. The band shift is due to the release of molecular hydrogen, a process which causes the aromatization of the carbon dust. C60H36 and C60D36 are photolyzed at a rate kC60H36 = 2.45 × 10−3 s−1 while kC60D36 = 2.27 × 10−3 s−1. This implies an isotope effect so that C60H36 is photolyzed 1.08 times faster than C60D36. In the interstellar medium this implies a deuterium enrichment of the hydrogenated interstellar carbon dust. The presence of a measurable isotope effect suggests that the photolysis of C60H36 and C60D36 involves the rupture of the C–H and C–D bond with release of H2. The hydrogenated C70 fullerenes C70H38 and C70D38 have been photolyzed at 254 nm under Ar or He flow. The photolysis rate constant have been measured: k = 1.54 × 10−3 s−1 for C70H38 while the photolysis of C70D38 occurs at a rate of 1.17 × 10−3 s−1. A remarkable isotope effect in the photolysis of the two molecules has been determined kH/kD = 1.32. The photolysis mechanism and products of C70H38 and C70D38 have been discussed and based on the isotope effect, the rate determining step involves, as for other fulleranes, the activation of the C–H and C–D bond. Fullerane C60H18 and its deuterated analogous C60D18 were synthesized in n-hexane solution by a reduction reaction of C60 under the action of HCl or DCl on Zn dust. The resulting solutions were subjected to UV irradiation at 254 nm from a low pressure mercury lamp under He. It was found that at 212 nm the photolysis rate constant of C60H18 molecule Hk212 = 8.68 × 10−4 s−1 was significantly higher than that of its deuterated analogous C60D18:Dk212 = 5.93 × 10−4 s−1. Similarly, at 256 nm it was confirmed the result that C60D18 was photolyzed more slowly than C60H18. In fact, also in this case Hk256 = 6.83 × 10−4 s−1 is significantly higher than that of its deuterated analogous C60D18:Dk212 = 3.74 × 10−4 s−1. Kinetic isotope effect involving the C–H and C–D bond activation has been advocated to explain the differences in photodecomposition speed of C60H18 in comparison to C60D18.
KeywordsElectronic Absorption Spectrum Isotope Effect Interstellar Medium Kinetic Isotope Effect Asymptotic Giant Branch
The present research work has been supported by grant AYA2007-64748 of the Spanish Ministerio de Ciencia e Innovacion.
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