Conformation of 2,2-dimethylpropane encapsulated in fullerenes

Abstract

It is known that nano objects are capable of considerably affecting physicochemical characteristics of molecules encapsulated therein [1]. In particular, it has recently been shown that the potential energy minimum for internal rotation of ethane in a nanotube, in contrast to the free molecule, corresponds not to the staggered but to the eclipsed conformation [2]. The present communication reports on the conformational preference of 2,2-dimethylpropane (neopentane) molecule encapsulated in fullerenes C60 and C80 according to DFT PBE/3z calculations (PRIRODA software package [3]). An isolated 2,2-dimethylpropane molecule is characterized by an experimental barrier to restricted rotation of 4.3 kcal/mol, the staggered conformer being preferred [4]. The calculation data for free neopentane molecule also indicate relative preference of the staggered conformer, but the calculated ΔH value is lower by 0.5–1.3 kcal/mol than the experimental one (see table). The staggered conformer was also found to be more favorable for the system neopentane–fullerene C60, where the shortest distance from the neopentane hydrogen atoms to the fullerene sphere is 1.8–1.9 A, while the transition state is represented by the eclipsed structure. However, unlike free neopentane molecule, the calculated barrier to restricted rotation in the encapsulated molecule is three times higher, and the C–C bond in both conformers is considerably shorter. Correspondingly, the distance between the eclipsed hydrogen and carbon atoms also shortens. In the system neopentane–fullerene C80 where the internal fullerene cavity is larger, the distance between the neopentane hydrogen atoms and fullerene sphere is longer (2.1–2.2 A), and the potential barrier to restricted rotation increases to a lesser extent (by a factor of 2.2 to 2.4); also, the difference between the C–C and H · · · C distances in the free and encapsulated molecules becomes smaller (see table). The examined systems are characterized by an appreciable negative or positive charge on the encapsulated neopentane molecule both in the ground and in the transition states, though the entire fullerene–neoISSN 1070-4280, Russian Journal of Organic Chemistry, 2014, Vol. 50, No. 6, pp. 921–922. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.V. Kuznetsov, 2014, published in Zhurnal Organicheskoi Khimii, 2014, Vol. 50, No. 6, pp. 935–936.