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.