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

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.