Ab initio structure of the (Na(2)[CAl(4)])(2) dimer. Next step toward solid materials containing tetracoordinate...

We performed ab initio calculations on the (Na(2)[CAl(4)])(2) dimer in order to test if the two CAl(4)(2-) groups react to form the more stable dimeric structure, or if the two CAl(4)(2-) groups remain separated in a true dimeric structure. Working at the B3LYP/6-311+G* level of theory (previously found to be satisfactory in our earlier calculations with CAl(4)(-) and Na[CAl(4)](-)), we established that structures with the C-C bond are higher in energy than the structures with two isolated structural CAl(4)(-) units separated by more than 5 A with their structural and electronic integrity preserved. However, alternative structures involving reaction between two CAl(4)(2-) groups forming a C(2)Al(8)(4-) cluster without the C-C bond are higher in energy, but they are still competitive with the true dimeric structure. While we found alternative structures of Na(4)C(2)Al(8) with the energy comparable to that of the true dimeric structure, we hope that the solid ionic salt with the pentaatomic tetracoordinate planar carbon [CAl(4)](2)(-) building block can be synthesized.