Endohedral complex of fullerene C₆₀ with tetrahedrane, C₄H₄@C₆₀

Abstract

B3LYP/6–31G(d) hybrid HF/DFT calculations were carried out to determine the structural and electronic properties of the endohedral complex of C₆₀ with tetrahedrane C₄H₄. It was demonstrated that C₄H₄ was seated in the center of the C₆₀ cage and existed in a molecular form inside the fullerene. The formation of this complex was endothermic with inclusion energy of 141.05 kcal/mol. C₄H₄ endohedral doping slightly perturbed the molecular orbitals of C₆₀. The calculated HOMO–LUMO gaps, the electron affinity (EA) and the ionizational potential (IP) indicated that C₄H₄@C₆₀ seemed to be more kinetically reactive than C₆₀. The IR active modes and harmonic vibrational frequencies of C₄H₄@C₆₀ were also discussed.

Introduction

Since lanthanum atom was firstly placed into the fullerene C₆₀ cage [1], endohedral fullerenes, the spherical carbon molecules incorporating single atoms [2], [3], [4], [5] and noble gases [6], [7], [8] or small molecules and clusters [9], [10], [11] inside the framework have attracted great interest in their physical and/or chemical properties such as pseudoatom behavior, magnetism, nonlinear optical behaviors, and superconductivity. However, the production rate of the endohedral fullerenes is quite low compared with the ordinary fullerenes. The preparation of endohedral fullerenes has so far relied on hard to control physical processes, such as co-vaporization of carbon and metal atoms [2] or high-pressure/high-temperature treatment with noble gases [7], which yield only limited quantities (e.g., only a few milligrams) of a pure product after laborious isolation procedures. Due to the extreme difficulties in producing macroscopic quantities and isolating pure samples, theoretical studies are helpful tools in investigating and predicting the structural and electronic properties of endohedral fullerenes. For example, theoretical calculations on lanthanum metallofullerene La@C₈₂ [12], [13] suggested that the La atom donated its three valence electrons to C₈₂ to form an endohedral complex La³⁺C₈₂³⁻, which was confirmed by electron paramagnetic resonance (EPR) studies [14]. Gauge-independent atomic orbital (GIAO) and the nucleus-independent chemical shift (NICS) calculations for the encapsulated hydrogen in H₂@C₆₀ and its derivatives interpreted well the experimental data [11]. In N@C₆₀, P@C₆₀ and C@C₆₀, the N, P and C atoms retain their atomic character when trapped in the cage [4], [15], [16], [17], while towards encaged small molecules, C₆₀ acted as a polarizable sphere that stabilized the polar molecules and destabilized the nonpolar ones [18], [19].

Tetrahedrane C₄H₄ (THD) is a theoretically interesting molecule, and the problem of its synthesis was recognized more than half a century ago but only its derivatives had been obtained [20], [21]. As a result of the enormous angular strain [22], theoretical works predicted its stability in the absence of other reactants [23], while in the derivatives the lability of THD is circumvented by means of spatial shielding of the tetrahedrane framework by four bulky groups [21]. Since the fullerene cage may act as a partial Faraday cage that shields the atom on molecule trapped inside from the majority of the field applied [4], [24], it can be viewed as a nearly ideal “container” or “trap” for any highly reactive complex. Here we study the endohedral complexes of fullerenes C₆₀ with THD. A question of major interest is whether the fullerene cage can stablize the trapped tetrahedrane. On the other hand, as tetrahedrane can isomerizes to cyclobutadiene [25], we also consider the inclusion of D_2h cyclobutadiene in the fullerene cage. We hope that the present study will encourage further theoretical and experimental analysis of the system.