Density functional study on the structures and energies of the Ti2C2 cluster

The structures and energies of the binary dititanium dicarbide, Ti2C2, in t he lowest singlet, tripler, and quintet states have been investigated by de nsity functional theory using the hybrid B3LYP functionals. Geometries and frequencies for a number of isomeric structures are presented at the B3LYP level. A rhombic structure with a transannular C-C bond (VI) is found to be the global Ti2C2 minimum. The other four-membered planar ring structures ( VII and VIII) derived from the two-fold addition of carbon to the Ti-Ti or the TI-C bond of the cyclic (C-2v) Ti2C are characterized as higher energy local minima. A linear structure (I) with terminal titanium atoms is found to be a higher energy minimum in its singlet, tripler, and quintet potentia l energy surfaces (PES) and has a bonding characteristic of a cumulene-like valence structure. Besides, a unique nonplanar C-2v structure, which has n ot been obtained in the isovalence electronic Si2C2 and SiC3 systems, has b een identified as a minimum. Cyclic structures are energetically favored ov er the linear structures. A comparison with the tetra-atomic group IVA sili con-carbon clusters is given, where appropriate.