1. INTRODUCTION
The concept of atoms or molecules encapsulation in cavities of a host molecule has gained a significant interest in pharmaceutical, food, and material research studies.1–8 Hydrocarbon cages and fullerenes are suitable candidates for encapsulation of atoms or small molecules. In the past several experimental9–17 and theoretical18,19,28–35,20–27 accounts on such fullerene based encapsulated systems are reported. However, experimental studies on atoms or molecules entrapped fully hydrogenated fullerenes are rare. For example, He@C20H20 is the only synthesized encapsulated system of C20H20 (Dodecahedrane) reported thus far.36 Due to the lack of experimental studies, the knowledge progression of such C20H20based encapsulation systems have mostly been via theoretical studies.37–41
The first ab initio study on C20H20 based encapsulation systems belongs to Disch and Schulman who studied X@C20H20 (X = H+, He, Li+, Be, Be+, Be2+, Na+, Mg2+) species at Hartree-Fock level.38 According to their analysis He, Be, Be+, and Na+encapsulated C20H20 are metastable with respect to the corresponding fragments.38 Moran et al. carried out a comprehensive analysis of geometries, stabilities, and energetics of X@C20H20 (X = H, He, Ne, Ar, Li, Li+, Be, Be+, Be2+, Na, Na+, Mg, Mg+, Mg2+) using B3LYP/6-311+G(d,p) in 2002.41 They found that X = Be, Be+, and Be2+ encapsulated complexes possess C5v symmetry, where X is located close to an interior pentagonal face of C20H20. All the other species bear Ihminima.41 Furthermore, they reported the He, Ne, Li, Li+, Li, Na, Na+, Be, Be+, Be2+, Mg, Mg+, and Mg2+ form more stable exohedral complexes with C20H20 compared to the corresponding endohedral species.41
Interestingly, Li, Na, and Mg encapsulated C20H20 species (M@C20H20; M = Li, Na, Mg) can be recognized as “superalkalis” owing to their lower first ionization energies (IE1s).41 The rather diffuse nature of the highest occupied molecular orbital (HOMO) is responsible for their lower ionization potentials. Moreover, it may engender the superior electrical conductivity of Li@C20H20 predicted by Wang et al.40 The existing expanded electron cloud of the ground state of each M@C20H20 (M = Li, Na, Mg) is nearly spherical resembling an atomic s-orbital. In that sense these molecules are similar to the previously studied“solvated electron precursors” (SEPs), where an SEP is a complex that consists of M(L)n q+ core (M = metal, L = ligand) with one or few diffuse electrons.42–47Interestingly, SEPs populate atomic p-, d-, f-, and g-type orbitals in excited states. The exact Aufbau order introduced for the M(NH3)4 (M = Li, Na) SEPs is 1s, 1p, 1d, 2s, 2p.42 The implementation of an augmented basis set on terminal H-atoms is critically important for the correct representation of their excited states.43,44Specifically, the M:cc-pVTZ, N/O:cc-pVTZ, H:d-aug-cc-pVTZ combination is proven to describe the excited electronic states of SEPs accurately and efficiently.43,44
Similar to the SEPs the M@C20H20 (M = Li, Na, Mg+) populate higher angular momentum p-, d-, f-shaped orbitals in low-lying excited electronic states, hence can be recognized as “superatoms” . The main goal of the present work is to analyze their ground and excited states adopting high-levelab initio calculations. By means of highly accurate electron propagator theory calculations their exact Aufbau models and basis set effects on excitation energies are analyzed. The computational approach and the results of this work are discussed under sections 2 and 3, respectively. Main findings of the study are summarized in section 4.