Theoretical study of association of acetylene, benzene, and carbonyl molecules “squeezed” inside fullerene cages

Abstract

The equilibrium geometries, normal mode frequencies, magnetic shielding constants, and energetic characteristics of model endohedral 2(C2H2)@C70, (C4H4)@C70, 2(C6H6)C84, (C12H12)@C84, 6(CO)@C84, and (C6O6)@C84 clusters, mimicking the structure and properties of the guest molecules under “ultrahigh” pressures inside the fullerene cages, were calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. According to the calculations, all the structures under consideration correspond to local minima of the corresponding potential energy surfaces. The 2(C2H2)@C70 isomer with two separated endohedral acetylene molecules turns out to be considerably less favorable than the (C4H4)@C70 isomer with endohedral cyclobutadiene. The 2(C6H6)C84 isomer with two separated endohedral benzene rings is almost 30 kcal/mol less favorable than the (C12H12)@C84) isomer with distorted endohedral prismane. The 6(CO)@C84 isomer with six separated carbonyl molecules is almost 45 kcal/mol less favorable than the (C6O6)@C84 isomer in which the carbonyls are associated to form a more compact cyclic hexamer C6O6. The barriers separating the “associated” and “dissociated” (consisting of monomers) isomers are estimated at 10–15 kcal/mol. Calculations show that, in extremely tight endoclusters, the increase in compression and strain energy caused by the repulsion of the electronic shells of guest molecules and wall atoms is accompanied by a sharp energetic stabilization of compact associated isomers (including those poorly stable or unstable in the free state) as compared with the dissociated isomers, on the one hand, and by a sharp decrease in activation barriers, on the other hand. Both factors should favor the realization of association processes unlikely or impossible under common conditions.