Pei Zhao

Dimetallic sulfide endohedral metallofullerene Sc2S@C76: Density functional theory characterization

In terms of density functional theory combined with statistic mechanics computations, we investigated a dimetallic sulfide endohedral fullerene Sc2S@C76 which has been synthesized without any characterization in experiments. Our theoretical study reveals that Sc2S@Td(19151)‐C76 which satisfies the isolated‐pentagon rule (IPR) possesses the lowest energy, followed by three non‐IPR structures (Sc2S@C2v(19138)‐C76, Sc2S@Cs (17490)‐C76, and Sc2S@C1(17459)‐C76). To clarify the relative stabilities of those isomers at high temperatures, enthalpy–entropy interplay has been taken into consideration. Calculation results indicate that three species Sc2S@Td(19151)‐C76, Sc2S@C2v(19138)‐C76, and Sc2S@C1(17459)‐C76 have noticeable molar fractions at the fullerene‐formation temperature region (500–3000K), and the Sc2S@C1(17459)‐C76 with one pentagon pair becomes the most predominant isomer above 1800 K, suggesting that the unexpected non‐IPR structure is thermodynamically favorable at elevated temperatures. In addition, the structural characteristics, electron features, UV‐vis‐NIR adsorptions, and 13C NMR spectra of those three stable structures are introduced to assist experimental identification and characterization in future.

Single Step Stone-Wales Transformation Linking Two Thermodynamically Stable Sc2O@C78 Isomers.

Among the very recently reported dimetallic oxide fullerenes Sc2O@C2n (n = 35-47), a representative Sc2O@C78 still lacks of further characterizations. Herein, a systematical investigation on Sc2O@C78 has been performed by density functional theory combined with statistical thermodynamic studies. Two isolated pentagon rule (IPR) satisfying isomers, Sc2O@D3h(24109)-C78 and Sc2O@C2v(24107)-C78, are disclosed to possess prominent thermodynamic stabilities at the temperature region of fullerene formation. Significantly, these two structures are related by a single Stone-Wales transformation. Moreover, bonding critical points, bond orders, and delocalization indices have been analyzed to uncover covalent interactions in both isomers. In addition, (13)C NMR spectra and UV-vis-NIR adsorptions of the two stable structures are introduced to assist experimental identification and characterization in the future.

Theoretical Insight into Sc2O@C84: Interplay between Small Cluster and Large Carbon Cage

Very recently, a series of endohedral fullerenes Sc2O@C2n (n = 35-47) were facilely produced. However, only two of them have been further characterized so far. Theoretically, we studied another discandium oxide endohedral fullerene without any characterizations, Sc2O@C84, which is the second most-abundant species in terms of relative heights of all mass spectrum peaks. Two thermodynamically stable isomers with isolated pentagon rule-obeying cages were found, namely, Sc2O@C2v(51575)-C84 and Sc2O@C1(51580)-C84. This is the first case that an endohedral fullerene containing the C2v(51575)-C84 cage acts as the lowest-energy isomer, and it is the first report of a clusterfullerene containing the C1(51580)-C84 cage. The endohedral Sc2O cluster can keep its ideal structure after encapsulation, while both C84 cages have deformed dramatically. Orbital analysis suggests that nucleophilic and oxidization reactions of both isomers should take place on the cage, while regioselectivity of Sc2O@C2v(51575)-C84 and Sc2O@C1(51580)-C84 is different due to their different characteristics of the highest occupied orbital distribution. Two-dimensional electron localization function and Laplacian of electron density maps unambiguously indicate strong electrostatic interactions exist between one scandium atom and the oxygen one. Meanwhile, overlaps of occupied metal atom orbitals and the cage ones along with Mayer bond order analysis identify that covalent interactions between a scandium atom and each C84 cage cannot be neglected. At last, (13)C NMR, UV-vis-NIR, and IR spectra of both Sc2O@C84 isomers were simulated theoretically. Because of their structural difference, all spectra between two isomers are significantly divergent. Consequently, these spectra are helpful to distinguish Sc2O@C2v(51575)-C84 and Sc2O@C1(51580)-C84 in further experimental characterizations.

Regioselectivity of Sc2C2@C3v(8)-C82: Role of the Sumanene-Type Hexagon in Diels-Alder Reaction.

Recently, several experiments have demonstrated high chemical reactivity of the sumanene-type hexagon in Sc2C2@C82. To further uncover its reactivity, the Diels-Alder reaction to all the nonequivalent C-C bonds of C82 and Sc2C2@C82 has been investigated by density functional theory calculations. For the free fullerene, the [5,6] bond 7 is the thermodynamically most favored, whereas the addition on the [6,6] bond 3 has the lowest activation energy. Diels-Alder reaction has no preference for addition sites in the sumanene-type hexagon. However, in the case of the endohedral fullerene, the [6,6] bond 19 in the special hexagon becomes the most reactive site according to both kinetic and thermodynamic considerations. Further analyses reveal that bond 19 in Sc2C2@C82 exhibits the shortest bond length and third largest π-orbital axis vector. In addition, the LUMOs of bond 19 are also symmetry-allowed to interact with butadiene.

Theoretical Insight into Sc2C76: Carbide Clusterfullerene Sc2C2@C74 versus Dimetallofullerene Sc2@C76

In terms of density functional theory in combination with a statistical thermodynamic method, we have investigated the Sc2C76 species including dimetallofullerenes Sc2@C76 and carbide clusterfullerenes Sc2C2@C74. Two dimetallofullerenes, Sc2@Cs(17490)-C76 and Sc2@Td(19151)-C76, possess the lowest relative energies but exhibit poor thermodynamic stability within the fullerene-formation region (500-3000 K). In contrast, four carbide clusterfullerene isomers, Sc2C2@D3h(14246)-C74, Sc2C2@C2v(14239)-C74, Sc2C2@C2(13333)-C74, and Sc2C2@C1(13334)-C74, have excellent thermodynamic stability when considering the temperature effect. The Sc2C2@D3h(14246)-C74 isomer, which satisfies the isolated-pentagon rule (IPR), was characterized by its crystallographic structure; however, the other three non-IPR structures with two pairs of pentagon adjacencies are predicted for the first time. In particular, Sc2C2@C2(13333)-C74 and Sc2C2@C1(13334)-C74 are linked by a single Stone-Wales transformation. Meanwhile, bonding critical points and Mayer bond orders in the four isomers were analyzed to disclose the unique interactions between the inner clusters and cages. Additionally, the structural characteristics, 13C and 45Sc NMR chemical shifts, and IR spectra of the four stable isomers are introduced to assist experimental identification and characterization in the future.

Sc3N@Cs(39715)–C82: a missing isomer linked to Sc3N@C2v(39718)–C82 by a single step Stone–Wales transformation

The trimetallic nitride template endohedral metallofullerene Sc3N@C82 has been investigated by density functional theory (DFT) combined with statistical thermodynamics methods. Both Sc3N@C2v(39718)–C82 and Sc3N@Cs(39715)–C82 which satisfy the isolated pentagon rule (IPR) are proved to possess considerable thermodynamic stability in the temperature region of fullerene formation. It should be noted that Sc3N@Cs(39715)–C82 is reported for the first time, while Sc3N@C2v(39718)–C82 was characterized by single-crystal X-ray diffraction analysis in 2015. Interestingly, a single Stone–Wales transformation links these two isomers. Meanwhile, their electronic structures and frontier molecular orbitals were analyzed to disclose their electronic properties. Whats more, the UV-vis-NIR and 13C NMR spectra of Sc3N@Cs(39715)–C82 and Sc3N@C2v(39718)–C82 were simulated to assist identification and characterization in future experiment.

Theoretical Insights into Monometallofullerene Th@C76: Strong Covalent Interaction between Thorium and the Carbon Cage

Th@C76 has been studied by density functional theory combined with statistical mechanics calculations. The results reveal that Th@ T d(19151)-C76 satisfying the isolated pentagon rule possesses the lowest energy. Nevertheless, considering the enthalpy-entropy interplay, Th@ C1(17418)-C76 with one pair of adjacent pentagons is thermodynamically favorable at elevated temperatures, which is reported for the first time. The bonding critical points in both isomers were analyzed to disclose covalent interactions between the inner Th and cages. In addition, the Wiberg bond orders of M-C bonding in different endohedral metallofullerenes (EMFs) were investigated to prove stronger covalent interactions of Th-C in Th-based EMFs.