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Charge density topological study of bonding in lithium clusters

Part I: Planar Li n clusters (n=4, 5, 6)

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Abstract

The topological behaviour of the electron density (ρ) derived from correlated wavefunctions is analyzed for Li2, Li4(D 2h ), Li5(C2v ), and Li6(D 3h ) planar clusters considered in their optimal geometry. The topology ofρ of Li2 shows an unusual maximum located at the midpoint of the Li-Li equilibrium distance. The occurrence of maxima ofρ at positions other than nuclei (characteristic also for planar Li4, Li5, and Li6 clusters) implies the existence of molecular subspaces (bounded by zero-flux surfaces in the gradient ofρ at each point of the surface) which do not enclose a nucleus but still satisfy the virial theorem. This result provides a generalization of Bader's quantum theory of atoms in molecules to systems in which electrons behave partially as mobile metallic electrons. Maxima ofρ preferentially occur within the triangles (two in Li4, two in Li5 and three in Li6), while the number of maxima at the Li-Li midpoint is minimized: they are present only when the existence of a maximum within a triangle is not allowed because of the non suitable formal valence of the Li atoms involved. All the cluster atoms are bonded to “attractors” associated with the unusualρ maxima, but they are not directly bonded to each other. The cluster stability is found to be dependent on the number and kind ofρ maxima. The topological analysis clearly differentiates between Li atoms which occupy different coordination positions within the cluster in terms of their local and average properties. In particular, the degree ofsp hybridization is markedly different for Li atoms with two, three or four nearest neighbors. This implies that a unique definition of a reference valence state for atoms in clusters is impossible. As a consequence, the use of standard electron density difference maps for the description of the charge accumulation and depletion process which ensues the chemical bonding, appears rather questionable.

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Authors and Affiliations

  1. Centro CNR “Relazioni fra Struttura e Reattivita' Chimica”, via Golgi 19, I-20133, Milano, Italy

    Carlo Gatti

  2. Dipartimento di Chimica Inorganica e Metallorganica, Centro CNR, Universita' di Milano, via Venezian 21, I-20133, Milano, Italy

    Piercarlo Fantucci & Gianfranco Pacchioni

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  1. Carlo Gatti
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  2. Piercarlo Fantucci
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  3. Gianfranco Pacchioni
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Dedicated to Professor J. Koutecký on the occasion of his 65th birthday

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Gatti, C., Fantucci, P. & Pacchioni, G. Charge density topological study of bonding in lithium clusters. Theoret. Chim. Acta 72, 433–458 (1987). https://doi.org/10.1007/BF01192234

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