Dynamic Metastable Characteristics of Carbon Cages Embedded with Er2C2

Novel endohedral metallofullerenes (EMFs), namely, Er2C2@C2v(5)-C80, Er2C2@Cs(6)-C82, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88, had been experimentally synthesized, and the unique structures and many fascinating properties had also been widely explored. Nevertheless, the position of the Er atoms inside the cage shows a severe disorder within the stable EMF monomer, which is difficult to understand and explain from the experimental point of view. In this work, based on the density functional theoretical calculations, the Er2C2@Cs(6)-C82 has 73 directional isomers and 2 Er atoms that are far beyond from Er-Er single bonding and tend to be close to the cage side (marked as "shell"), and the core (Er2C2 units) takes on a butterfly shape as generally revealed. The energy difference between any two of the isomers is in the range of 0.05 to 25.6 kcal/mol, indicating a relatively easy thermodynamic transition between the isomers. The other five Er carbide cluster EMFs (Er2C2@C2v(5)-C80, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88) are also studied in the same way, and 30, 37, 39, and 43 most stable Er-oriented sites inside the cage, respectively, are obtained. In addition, the shape of the Er2C2 gradually changed from butterfly to linear. Moreover, the electronic structure and molecular orbital analyses show that it is easy for Er2C2@C80-88 to form a charge transfer state of [Er2C2]4+@[C80-88]4- via the dynamic core-shell coordination equilibrium. Er2C2 with a steep drop in chemical stability is restricted to forming varying degrees of metastable states in the shell, determined by the shell size, to ensure the overall stability. The lowest unoccupied molecular orbital energy level of these EMFs is increased by 0.5-1.1 eV compared with fullerenes C80-88, potentially providing favorable conditions for suitable energy level matching with EMF as an electron acceptor used in organic solar cell devices.

Small Amount Makes a Big Difference: Critical (n - 1)d Valence Orbitals of Heavy Alkaline Earth Metals inside Cage Clusters

Heavy alkaline earth metals (Aes) are usually considered to engage in chemical bonding by donating the two electrons on ns atomic orbitals (AOs). In this work, a series of typical endohedrally doped cage clusters Ae@cage (Ae = Ca, Sr, Ba; cage = C₃₂, C₇₄, C₉₄, B₄₀, Si₂₀, Sn₁₂, Au₁₆) were thoroughly investigated by means of density functional theory calculations. We found that their occupied molecular orbitals have ∼1 to 14% contributions from Ae-(n - 1)d AOs due to electron back-donation from the cage. Though the amount is small, it is hard to ignore: with the d orbitals, all these endohedral clusters exhibit obviously shortened Ae-cage distances, greatly enhanced encapsulation stabilities, changed highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, and much lowered Ae valences far from ideal +2. Evidently, the valence orbitals of Ca/Sr/Ba in these systems should include both ns and (n - 1)d. By disclosing the critical role of unnoticed metal orbitals, our work provides completely new insights into the cluster field.

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Revisit of the Saito-Dresselhaus-Dresselhaus C2 ingestion model: on the mechanism of atomic-carbon-participated fullerene growth

We introduce a mechanistic study based on a controversial fullerene bottom-up growth model proposed by R. Saito, G. Dresselhaus, and M. S. Dresselhaus. The so-called SDD C₂ addition model has been dismissed as chemically inadmissible but here we prove that it is feasible via successive atomic-carbon-participated addition and migration reactions. Kinetic calculations on the formation of isolated pentagon rule (IPR)-obeying C₇₀ and Y₃N@C₈₀ are carried out by employing the SDD model for the first time. A stepwise mechanism is proposed with a considerably low barrier of ca. 2 eV which is about 3 eV lower than a conventional isomerization-containing fullerene growth pathway.

The inner-induced effects of YCN in C76 on the structures and nonlinear optical properties

Very recently, an unprecedented novel monometallic cluster of fullerenes entrapping a yttrium cyanide (YCN) cluster inside a popular C82 cage YCN@Cs(6)-C82 was synthesized and characterized. Inspired by this investigation, four non-IPR YCN@C1(17459)-C76, YCN@C2v(19138)-C76, YCN@C2(17646)-C76, and YCN@C1(17894)-C76 (1, 2, 3, and 4) containing a pair of adjacent pentagons are designed to explore the encapsulated molecular effect on their interaction energies and nonlinear optical properties. The interaction energy (E int) values of 1, 2, 3, and 4 are -481.35 (1), -477.91 (2), -482.04 (3), -482.69 (4) kcal mol(-1), respectively, which shows that the E int value of 4 is the largest. Furthermore, the electron-transfer is mainly from the YCN to C76 cage. When YCN is encapsulated into C76 cage, we can find that the α0 values of the four molecules are very close, ranging from 6.50 × 10(2) to 6.65 × 10(2) au. Significantly, the first hyperpolarizabilities are in relation to the encapsulated molecular: 1.63 × 10(3) (1) > 8.03 × 10(2) (2) > 7.76 × 10(2) (4) > 4.86 × 10(2) au (3), the results show that the βtot value of 1 is the largest. Besides this, the encapsulation of the YCN to C76 cage brings some distinctive changes in its UV-vis spectra along with its other electronic properties that might be used by the experimentalists to develop the potential nonlinear optical nanomaterials based on endohedral metallofullerenes.

Theoretical study of the structures and first hyperpolarizabilities of C60Cl n and Li@C60Cl n (n = 4, 6, 8, 10)

We recently reported (Song Y-D et al., 2016, J Mol Model 22:50) that doping with Li greatly affects the static first hyperpolarizability of C60Cl2. In this work, with a view to creating nonlinear optical materials with high thermodynamic stability and wide transparent regions, a series of Li@C60Cl n (n = 4, 6, 8, 10) were designed. The structures, electrostatic potentials, electronic structures, absorption spectra, and linear and nonlinear optical properties of C60Cl n and Li@C60Cl n were systematically investigated using density functional theory (DFT) methods. The results of our calculations indicated that the stability of these molecules decreases in the order Li@C60Cl10 > Li@C60Cl8 > Li@C60Cl6 > Li@C60Cl4. It is clear that the number of Cl atoms greatly influences the stability of Li@C60Cl n . Li@C60Cl n showed greater thermodynamic stability than Li@C60Cl2. We also investigated the first hyperpolarizabilities of Li@C60Cl n and found them to be 2973, 3640, 4307, and 2627 au for n = 4, 6, 8, and 10, respectively-higher than that of Li@C60Cl2. Finally, we noted that the transparent region could be modulated by increasing the number of Cl atoms: Li@C60Cl n possess wider transparent regions than that of Li@C60Cl2. We therefore believe that this study has highlighted an effective method for designing nonlinear optical materials with high thermodynamic stability and wide transparent regions.

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

Density functional theory combined with statistical mechanics calculations indicate that Sc₃N@C_2v(39718)–C₈₂ and Sc₃N@C_s(39715)–C₈₂ linked by a single Stone–Wales transformation can be obtained at the fullerene formation temperature region.

Theoretical Insight into the Ambiguous Endohedral Metallofullerene Er3C74: Covalent Interactions among Three Lanthanide Atoms

All of C74-based endohedral metallofullerenes (EMFs) are found to be monometallofullerenes with the same D3h(14246)-C74 cage so far. An opening question is whether other C74 cages could survive during the production of some novel C74-EMFs. Theoretically, we studied the trimetallic endohedral fullerene Er3C74, the existence of which had been proven without any further characterizations. Two thermodynamically stable Er3C74 isomers were obtained, both of which could be expressed as Er3@C74, meaning that previously synthesized Er3C74 is indeed an endohedral trierbium fullerene. Besides the isomer with well-known D3h(14246)-C74 cage which obeys isolated pentagon rule (IPR), another one possesses the C1(13771)-C74 cage with two adjacent pentagons. Notably, it is the first time an endohedral metallofullerene containing the C1(13771)-C74 cage has been reported. Frontier orbitals analysis, bonding analysis in terms of quantum theory of atoms-in-molecule (QTAIM) and Mayer bond order, together with two-dimensional maps of electron localization function (ELF) and Laplacian of electron density of Er3@D3h(14246)-C74 and Er3@C1(13771)-C74 show obvious covalent interactions not only between metallic atoms and carbon cage but also among three erbium atoms. Finally, simulated IR spectra of Er3@D3h(14246)-C74 and Er3@C1(13771)-C74 were simulated, which should be useful to distinguish those two isomers.

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 (13)C NMR spectra of those three stable structures are introduced to assist experimental identification and characterization in future.