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On the competition between linear and cyclic isomers in second-row dicarbides.

Second-row dicarbides C(2)X (X = Na-Cl) are investigated with quantum mechanical techniques. The cyclic-linear competition in these systems is studied, and the bonding scheme for these compounds is discussed in terms of the topological analysis of the electronic density. C(2)Na, C(2)Mg, C(2)Al, and C(2)Si are found to prefer a C(2)(v)-symmetric arrangement corresponding to a T-shape structure. On the other hand, for C(2)P, C(2)S, and C(2)Cl the linear isomer is predicted to be the ground state. A detailed analysis of the variation of the electronic energy and orbital energies with the geometry has been carried out. A simple theoretical model, taking into account the main interactions between the valence orbitals of both fragments, the X atom and the C(2) molecule, allows an interpretation of the main features of these compounds.