The structures of boranecarbonyl compounds B4X6CO (X = F, Cl, Br and I) by gas-phase electron diffraction and ab initio calculations

Abstract

Gas-phase electron diffraction is a powerful technique for structural analysis of molecules in the gas phase, where they are free from packing forces that can occur in crystals. The compound B(BF₂)₃CO has been studied by gas-phase electron diffraction to compare its structure to that seen in the solid phase by low-temperature X-ray crystallography. Results show the gas-phase structure to be similar to that seen in the crystal. A model with C₃ symmetry refined to give a C–O bond length of 115.8 pm and a C–B bond distance of 150.2 pm, which compare to values of 111.7 and 152.2 pm for the solid phase. The family of borane carbonyl compounds B(BX₂)₃CO (X = F, Cl, Br or I) have all been studied by ab initio calculations to show the effects of halogen substitution and to gauge the effects of electron correlation and basis set on each structure. Compounds X = F, Cl and Br give calculated structures with C₃ symmetry in which the boron–halogen bonds lie coplanar with the C–O bond. In the case of X = I, the BI₂ groups are twisted by approximately 35° from coplanar at the DFT level as a result of the large steric interactions between iodine atoms.