Effects of Dispersion and Charge-Transfer Interactions on Structures of Heavy Chalcogenide Compounds: A Quantum Chemical Case Study for (Et 2 Bi) 2 Te.

The reasons for the unusually small Bi-Te-Bi bond angle of 86.6° observed in the crystal strucure of (Et 2 Bi) 2 Te are investigated by quantum chemical calculations. With the help of coupled cluster theory at the CCSD(T) level it is demonstrated that the structure of an isolated monomer should have a bond angle larger than 90°, despite a Bi-Bi distance in good agreement with the value of 4.09 Å found in the crystal structure. The discrepancy is resolved by a lengthening of the Bi-Te bond in the crystal, which is shown to be caused by partial electron transfer from neighbouring molecules to the Bi-Te σ* orbital. Through symmetry-adapted perturbation theory at the DFT-SAPT level it is shown that London dispersion interactions are highly important for the packing of molecules in the solid state and, in turn, for the small Bi-Te-Bi bond angle.