Electronic properties of multifurcated bent hydrogen bonds CH3⋯Y and CH2⋯Y

Abstract

H-bonding angle ∠YHX has an important effect on the electronic properties of the H-bond Y⋯HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H₃CZ (Z = F, Cl, Br), H₂CO and H₂CF₂ with the proton acceptors Cl⁻ and Br⁻ at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7–12 kcal mol⁻¹, and large blue shifts, Δr(HC) = −0.0025 – −0.006 Å and Δv(HC) = 30–90 cm⁻¹. The natural bond orbital analysis shows that the blue shifts of these H-bonds Y⋯H_nCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) → σ*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to σ*(CH), but to σ*(CZ) of the donor; intramolecular hyperconjugation n(Z) → σ*(CH), in that the electron density in σ*(CH) comes back to n(Z) of the donor such that the occupancy in σ*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y⋯H₂CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y⋯H₃CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.