Structural and energetic properties of OC-BX₃ complexes: unrealized potential for bond-stretch isomerism

We have explored the structural and energetic properties of OC-BX3 (X = F, Cl, or Br) complexes using computations and low-temperature infrared spectroscopy. Quantum-chemical calculations have provided equilibrium structures, binding energies, vibrational frequencies, and B-C potential energy curves. The OC-BF3 system is a weak, long-bonded complex with a single minimum on the B-C potential (R(B-C) = 2.865 Å). For the remaining two complexes, OC-BCl3 and OC-BBr3, computations predict two stable minima on their B-C potential curves. The BCl3 system is a weak complex with a long bond (R(B-C) = 3.358 Å), but it exhibits a secondary, meta-stable minimum with a short bond length of 1.659 Å. For OC-BBr3, the system is a weak complex with a relatively short bond of 1.604 Å (according to wB97X-D/aug-cc-pVTZ), but also has a secondary minimum at R(B-C) = 3.483 Å. This long-bond structure is the global minimum according to CCSD/aug-cc-pVTZ. In addition, the long-bond forms of both OC-BCl3 and OC-BBr3 were observed in matrix-isolation IR experiments. The measured CO stretching frequencies were 2145 cm-1 and 2143 cm-1, respectively. No signals due to the short-bond forms of OC-BCl3 and OC-BBr3 were observed.