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Dynamics and magnetic resonance properties of Sc(3)C(2)@C(80) and its monoanion.

Taubert, Stefan and Straka, Michal and Pennanen, Teemu O and Sundholm, Dage and Vaara, Juha (2008) Dynamics and magnetic resonance properties of Sc(3)C(2)@C(80) and its monoanion. Physical chemistry chemical physics : PCCP, 10 (47). pp. 7158-68. ISSN 1463-9076

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Abstract

We report density functional theory (DFT) studies on the endohedral scandium carbide fullerene Sc(3)C(2)@C(80) and its monoanion [Sc(3)C(2)@C(80)](-). The system consisting of a Sc(3)C(2) moiety inside the I(h) C(80) 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 Sc(3) plane. In contrast to the equilibrium geometry optimisations using large basis sets that predict a trigonal bipyramidal structure, a planar Sc(3)C(2)-moiety is preferred during the finite-temperature simulation. In the molecular dynamics picture, Sc(3)C(2)@C(80) is best described as an equilibrium between the two static minimum structures. Calculations of the vibrational frequencies show that the earlier predicted C(2) 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 C(2) 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 Sc(3)C(2)@C(80) 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. (13)C nuclear magnetic resonance (NMR) chemical shifts have been calculated for the closed-shell monoanion of Sc(3)C(2)@C(80). The (13)C shifts were also calculated for Sc(2)C(2)@C(84), 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 (13)C NMR spectrum of Sc(2)C(2)@C(84). The neutral Sc(3)C(2)@C(80) system with one unpaired electron is further characterised by calculating the hyperfine coupling constants, the g tensor, as well as paramagnetic NMR (pNMR) (13)C 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 Sc(3)C(2) 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 (13)C shifts, implying the potential of this method in structural assignment.

Item Type:Article
Subjects:Physical Science > Nanomagnetics
ID Code:414
Deposited By:Lesley Tobin
Deposited On:04 Dec 2008 15:58
Last Modified:04 Dec 2008 15:58

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