Small carbides of third-row main group elements: structure and bonding in C3X compounds (X = K-Br).

The molecular structures of third-row main group tricarbides C(3)X (X = K-Br) have been studied by quantum chemical methods. It is found that less electronegative elements (K, Ca, Ga, Ge) favor either fan or rhombic structures (resulting from side interactions with either linear or triangular C(3) units), whereas the more electronegative elements (As, Se, Br) favor linear or three-membered ring structures (resulting from σ-type interactions with either linear or triangular C(3) units). The predicted global minima are of fan type for C(3)K, rhombic for C(3)Ca, C(3)Ga, and C(3)Ge, linear for C(3)As and C(3)Se, and a three-membered ring for C(3)Br. In order to aid in their possible experimental identification the molecular geometries, vibrational frequencies, IR intensities, and dipole moments have been provided. The nature of the interactions has been characterized through an analysis of the electronic charge density. In addition, the relative stability of the different isomers has been also rationalized in terms of an energy decomposition analysis.