Dynamics and magnetic resonance properties of Sc3C2@C80 and its monoanion

Abstract

We report density functional theory (DFT) studies on the endohedral scandium carbide fullerene Sc₃C₂@C₈₀ and its monoanion [Sc₃C₂@C₈₀]⁻. The system consisting of a Sc₃C₂ moiety inside the I_h C₈₀ fullerene has been studied by using first principles molecular dynamics simulations at the DFT level. On the picosecond time scale, the triangle defined by the scandium atoms is seen to jump between orientations along the equatorial six-membered ring belt of the cage. The confined carbide unit, in turn, is engaged in a flipping motion through the Sc₃ plane. In contrast to the equilibrium geometry optimisations using large basis sets that predict a trigonal bipyramidal structure, a planar Sc₃C₂-moiety is preferred during the finite-temperature simulation. In the molecular dynamics picture, Sc₃C₂@C₈₀ is best described as an equilibrium between the two static minimum structures. Calculations of the vibrational frequencies show that the earlier predicted C₂ and C_2v symmetric isomers are in fact saddle points, with one imaginary normal mode frequency that is related to the flipping motion of the confined carbon dimer. Reoptimisation revealed two new minimum energy structures where the C₂ unit is tilted with respect to its orientation in the earlier suggested higher-symmetry structures. The nature of the bonding in the static structures of the two isomers of Sc₃C₂@C₈₀ has been investigated using the electron localisation function and natural population analysis. Some increased electron pair localisation is detected on the six-membered rings closest to the Sc atoms. ¹³C nuclear magnetic resonance (NMR) chemical shifts have been calculated for the closed-shell monoanion of Sc₃C₂@C₈₀. The ¹³C shifts were also calculated for Sc₂C₂@C₈₄, for further comparison to experimentally measured spectra. The confined carbon atoms are strongly deshielded in these metallofullerenes, implying an incorrect earlier interpretation of the experimental ¹³C NMR spectrum of Sc₂C₂@C₈₄. The neutral Sc₃C₂@C₈₀ system with one unpaired electron is further characterised by calculating the hyperfine coupling constants, the g tensor, as well as paramagnetic NMR (pNMR) ¹³C shifts for both static isomers. The chemical shifts of the confined carbon atoms and the hyperfine coupling constants of all the confined atoms are strongly dependent on the conformation of the Sc₃C₂ moiety. Consequently, dynamical effects are expected to be important in the modelling of the magnetic properties of endohedral scandium carbide fullerenes. The two low-lying isomers have rather different pNMR ¹³C shifts, implying the potential of this method in structural assignment.