Abstract
Relative concentrations of six isomeric Eu@C—one based on the IPR C cage (i.e., obeying the isolated-pentagon rule, IPR), two cages with a pentagon–pentagon junction (symmetries and ), a cage with one heptagon, a cage with two heptagons, and a cage with two pentagon–pentagon fusions—are DFT computed using the Gibbs energy in a broad temperature interval. It is shown that the two non-IPR isomers with one pentagon–pentagon junction prevail at any relevant temperature and exhibit comparable populations. The IPR-satisfying structure is disfavored by both energy and entropy.
Highlights
C72 has a special position among medium-size fullerenes [1,2,3] as it has not been isolated in the pristine form
One can expect that if the encapsulate is relatively free, at sufficiently high temperatures, its behavior in different cages will bring about the same contribution to the partition functions
The four C72 cages originally selected [10] for Ca@C72 are treated here: the IPR D6d cage (a), two non-IPR cages (b) and (c) with one pentagon–pentagon junction each with the symmetries C2 and
Summary
C72 has a special position among medium-size fullerenes [1,2,3] as it has not been isolated in the pristine form. Ca@C72 computations that two non-IPR (i.e., IPR-violating) structures with one pentagon–pentagon junction and the C2 and C2v symmetries (code numbers [1,2,3,10,11] 10612 and 11188, respectively) produce Ca-endohedrals with lower energy than encapsulation in the IPR cage. The inter-isomeric separation potential energies were further improved by the geometry optimizations at the B3LYP/6-311+G*∼SDD level.
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