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Five Regioisomers of Dimethyl Dodecahedrane Derivatives: A Hybrid DFT B3LYP Study 时间:2017-09-27 15:33 来源:未知作者:admin 点击: 次 Abstract:The hybrid density functional (B3LYP/6-31G(d, p)) method was used to understand why 1,2-dimethyl dodecahedrane has not been reported yet. From the direct dimethyl substitution of the dodecahedrane cage, we could have five C20H18(CH3)2 derivatives which were geometrically optimized without constraints. The results suggest that 1,16-dimethyl dodecahedrane derivative is the most stable, whereas the 1,2-dimethyl derivative is the most unstable; this implies that the distortion due to steric hindrance interactions would be a crucial effect on the relative energies of the dimethyl dodecahedranes. It would be disadvantaged thermodynamically for 1,2-dimethyl derivative that, in the experimental results, was not synthesized yet. The LUMO of each of the derivatives was equivalently delocalized over the void within the cage, implying that dimethyl derivatives are able to encapsulate atoms at the center of the cage. The HOMO was limitedly delocalized on the cage. The characteristics of the HOMO of derivatives show three patterns, implying that each derivative might undergo one of three entirely different sets of characteristic chemical reactions with electrophilic reagents.
1. Introduction
The geometry of fullerenes obeys the isolated-pentagon rule. However, this rule cannot be satisfied for the smallest fullerene C20, which contains twelve 5-membered rings (MRs) and zero 6-MRs. Therefore, the C20 fullerene would be highly reactive owing to the fusion of strained 5-MRs. Furthermore, this strain could be removed through reactions of C20 that form sp3 bonds. By using density functional theory method, the structures and electronic states of alkyl radicals (R, , –4) added to C20 fullerenes were investigated [1]. Also it was proposed that thiyl radical-promoted polycyclization to form dodecahedrane would be exergonic [2].
Density functional calculations of the reactions CX3 radical + C20H20 (X = H, F, Cl, and Br) based on two pathways (H-displacement and H-abstraction from C20H20) showed that CH3 radical prefers H-abstraction from C20H20 while F-displacement is favorable for the reaction of CF3 radical with the fullerene. Exothermic characters of H-abstraction with the lower potential barrier indicate that the H-abstraction would dominate the reaction + C20H20 while endothermic characters with high potential barrier heights for two reaction pathways of C20H20 with indicate unfavorable thermodynamically and kinetically pathways for these reactions [3].
Using the PBE0/cc-pVDZ quantum chemical method, dipole moments, polarizabilities, the first hyperpolarizabilities for 1,2-diethynyldodecahedrane C20H18(CCH)2 and diethynylmethanododecahedrane C20H18C(CCH)2 including two neighboring ethynyl groups in vacuum and in tetrahydrofuran (THF) were reported [4].
A dodecahedrane derivative, namely, the symmetric 1,16-dimethyl derivative, has been synthesized experimentally [5–8]. Since then, the synthesis of methyl-substituted dodecahedrane was also confirmed by X-ray analysis. Moreover, Pólya’s theorem was applied to the enumeration of substituted dodecahedrane isomers [9]. The total synthesis of dodecahedrane was confirmed by the single-peak nature of the 1H and 13C NMR spectra [10], as well as the X-ray spectra [11, 12]. Methyl dodecahedrane, 1,4-dimethyl (), 1,6-dimethyl (), 1,16-dimethyl () dodecahedranes, and trimethyl dodecahedrane have subsequently been reported [13–15]. However, at present no 1,2-dimethyl () and 1,7-dimethyl () dodecahedranes have been reported, yet.
The structures of dodecahedrane and its derivatives were studied at the level of molecular mechanics calculations [16, 17], INDO calculations [18], and hybrid density functional B3LYP calculations [19]. Recently the relative energies, electronic properties, and atomic structures of disubstituted C20H18X2 regioisomers (X = F, Cl, Br, or OH) at the level of B3LYP/6-31G(d,p) have been determined [20]. Here the predominant electronic configurations of the regioisomers with all substituents X which are higher electron negativity than carbon are not so different, but the HOMO and LUMO maps for five regioisomers are not similar, implying that different exohedral complexes may undergo a distinct set of characteristic chemical reactions.
However, to the best of our knowledge, no the first principle calculations of the full optimization of the five dimethyl dodecahedranes, C20H18(CH3)2 derivatives using B3LYP/6-31G(d,p), have been reported. Such calculations would be required in order to understand, in the point of energetics, why only three derivatives have been synthesized. Accordingly, in this study we determine the relative energies, electronic properties, and atomic structures of five derivatives with the same spin state, following the methods of Hwang et al. 2012 [20]. Also it is interesting to see the effect of the methyl substituents on the frontier orbitals, because carbon atoms have the same electronegativity in the substituents and the cage.