Hydrogen-abstraction reactions of fully hydrogenated fullerene cages with the amino radical: a density functional study

<p>We have applied density functional theory calculations to study the reactions of NH₂ + C<i>_n</i>H<i>_n</i> (<i>n</i> = 20, 40, 50, 60, 70 and 80). Due to the hard curvature in C₂₀ cage, the NH₂• + C₂₀H₂₀ → NH₃ + C₂₀H₁₉• reaction is nearly thermoneutral with a high potential barrier height. For the C<i>_n</i>H<i>_n</i> fulleranes with <i>n</i> > 20 the transition states appear earlier on the reaction paths, as can be anticipated for exothermic reactions. Using the spherical excess parameter, we distinguished different curvatures on the surfaces of fullerane cages. The reaction enthalpies ΔH°₂₉₈ and potential barrier heights ΔE^TS of the considered reactions indicate good correlation with the values of <i>ϕ_i</i> parameter, showing an upward trend with the curvature increasing at carbon sites. We have also investigated the H-abstraction of the chemical derivatives of the C₂₀H₂₀ cage (C₂₀H₁₉–CH₃, C₂₀H₁₉–CH₂CH₃ and C₂₀H₁₉–CH₂CH₂CH₃) in comparison to the corresponding isolated alkanes (CH₄, C₂H₆ and C₃H₈). Overall, it could be inferred that the H-abstraction from the primary and secondary C–H bonds of isolated alkanes could occur more easily than fullarane derivatives.</p

A Computational Investigation of the Electronic Properties of Partially Hydrogenated Fullerenes C₆₀H_n (<i>n</i> = 18, 20, 24, 36 and 48)

<div><p>A DFT study is performed to investigate the electronic and magnetic properties of partially hydrogenated fullerenes C₆₀H_n (n = 18, 20, 24, 36, and 48) based on NMR parameters, NICS, and NPA analysis. A few peaks appear in the ¹³C NMR pattern of each C₆₀H_n, showing the electrostatic environments of C nuclei are divided into a few layers that are related to three local structures around C atoms. Since, hexagons and pentagons in the zone of hydrogenated carbons are puckered, the range of isotropic chemical shifts (δ_iso) for each group of carbons are caused by different curvatures at the corresponding sites. Partially hydrogenation of fullerenes results in isolated cyclohexatriene fragments with almost planarity surface, in some cases, which yields ¹³C δ_iso of 130.7–132.4 ppm. Moreover, cyclohexatriene pole ring with the calculated NICS value of −8.9 ppm at its center, similar to that of benzene (−8.0), can be considered as benzenoid fragment. Magnetic properties are also examined inside the compounds, indicating that NICS values are strongly negative in the middle of all the cages, except for C₆₀H₂₄ and C₆₀H₄₈. The predicted NICS values may be also useful for identification of the hydrogenated fullerenes through their endohedral ³He NMR chemical shifts.</p></div