The dihydrogen bond in X3C–H⋯H–M complexes (X = F, Cl, Br; M = Li, Na, K). A correlated quantum chemical ab initio and density functional theory study

Abstract

Quantum chemical calculations were performed on nine dihydrogen-bonded complexes with haloform (F₃CH, Cl₃CH and Br₃CH) as a proton donor and alkali metal hydride (HLi, HNa and HK) as a proton acceptor. MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p) results show that the stabilization energies of these complexes are large and comparable to the stabilization energies of standard H-bonded complexes. Elongation and weakening (red shift) of the CH, HNa and HK bonds upon complexation were found while contraction and strengthening (blue shift) was observed in HLi. The H⋯H bond was found to be ionic and its ionicity is larger than that of the H⋯Y bond in standard and improper H-bonds. The calculated free energy (ΔG) revealed that only potassium hydride complexes (F₃CH⋯HK, Cl₃CH⋯HK and Br₃CH⋯HK) are stable under standard conditions (T = 298.150 K and p = 101.325 N m⁻²) in the gas phase. To elucidate the role of the electrostatic contribution, the optimization of the proton donor and proton acceptor molecules in the electric field of a partner was performed. The HLi bond is contracted in the electric field of the haloform while the HM (M = Na, K) bonds are elongated and the electrostatic field itself is sufficient explanation of these phenomena. Natural bond order (NBO) and natural resonance theory (NRT) analyses were performed. The NBO analysis revealed that significant electron density was transferred from the σ bonding orbital of a proton acceptor to the antibonding σ*(CH) orbital of the proton donor. Symmetry adapted perturbation theory (SAPT) was utilized to decompose the total interaction energy into physically correct contributions.