Unique Interactions Between Diborane and π Orbitals: Blue- or Red-Shifted Hydrogen Bonding?

Abstract

A new type of hydrogen-bonding interaction in the diborane (B₂H₆)···π (benzene C₆H₆, 1,3-cyclopentadiene C₅H₆, and cyclobutadiene C₄H₄) system is identified with the natural bond orbital and atoms-in-molecules analyses based on ab initio calculations. In comparison with the symmetric and asymmetric stretching vibrational modes of the bridging hydrogen atoms in free B₂H₆, the frequencies of the symmetric mode are red-shifted for B₂H₆···C₆H₆ and B₂H₆···C₅H₆ but blue-shifted for B₂H₆···C₄H₄. The frequency blue shifts of the asymmetric mode are found for all three complexes; the most significant blue shift is 14.73 cm⁻¹ for the asymmetric mode in B₂H₆···C₄H₄. In these complexes, the electron-deficient three-center two-electron bond B−H₁−B facing the π orbital is shortened, while the opposite B−H₂−B bond is elongated.