(63) Charged fullerene clusters
Date:
Contributors: Huber, S. E., Gatchell, M., Zettergren, H., & Mauracher, A.
Venue: The 2018 Chemistry Research Symposium, ChRS2018, Kasetsart Univesity Kamphaeng Saen Campus, Thailand, May 26-27, 2018
Abstract: Here we present how basic considerations of electrostatics and dispersion allow one to understand and predict the stability of multiply charged van-der-Waals (vdW) oligomers at low temperatures, which we exemplify for the case of doubly charged fullerene clusters. Dicationic fullerene clusters have been produced via charge transfer reactions from highly charged Xe cations [1]. Due to the long-range interaction between neutral fullerene clusters and the highly charged Xe cations the ionization process is very gentle and the smallest dication observed was the fullerene pentamer. We use two simple approaches that reduce the complex many-body systems to their essentials. In our crudest ansatz, the geometrical structures of the fullerenes are entirely omitted leaving behind chargeable and polarizable point particles located on the corners of regular polyhedra (polygons in the simplest cases), to which we hence refer as the Point Charges on Polyhedra (PCP) model. In a second step, replacement of the fullerenes by ideally conducting metal spheres reintroduces the spatial extent of the cluster constituents. This second model is referred to as the Charged Sphere (CS) model. In both models solving the complex many-body problem of the underlying quantum-chemical system, i.e. the electronic Schrödinger equation, is replaced by simple classical force field evaluations. This is done by using a two-body potential for the vdW interactions derived from Density Functional Theory (DFT) in combination with fundamental electrostatic expressions. As electrostatic and dispersive forces and associated energies are reasonably described by analytical functions, there is no need for expensive electronic structure calculations in our models. Instead, just the knowledge of the cluster geometry and simple energy evaluations are required to predict the stability of the multiply charged clusters. For the DFT calculations, to which we compare the PCP and CS models, we applied a decomposition ansatz of the total energy which allowed us to obtain very accurate results concerning both the electrostatic and the dispersive interaction. This is a prerequisite to handle the delicate energy balance.
We explore the thermodynamic stability of small fullerene clusters as well as the kinetics of fragmentation reactions (C60)n2+ → (C60)(n-1)+ + C60+ [2]. We show that the analysis of the stability of doubly charged fullerene clusters can be effectively cut down to the interplay between classical electrostatics and dispersion interactions. The development and application of the heuristic PCP and CS approaches results in excellent agreement with DFT calculations. Based on these results, we can explain why the pentamer is the smallest doubly charged fullerene cluster ever observed. Moreover, the existence of kinetic barriers also for smaller systems may in principle stabilize doubly charged fullerene clusters down to the dimer. However, for cluster sizes below the pentamer this would require highly efficient means of active cooling.
References:
[1] B. Manil, L. Maunoury, B. A. Huber, J. Jensen, H. T. Schmidt, H. Zettergren, H. Cederquist, S. Tomita and P. Hvelplund; Phys. Rev. Lett. 91, 215504 (2003). [2] S. E. Huber, M. Gatchell, H. Zettergren and A. Mauracher; Carbon 109, 843 (2016).