title: The origins of the directionality of noncovalent intermolecular interactions.
abstract: The recent σ-hole concept emphasizes the contribution of electrostatic attraction to noncovalent bonds, and implies that the electrostatic force has an angular dependency. Here a set of clusters, which includes hydrogen bonding, halogen bonding, chalcogen bonding, and pnicogen bonding systems, is investigated to probe the magnitude of covalency and its contribution to the directionality in noncovalent bonding. The study is based on the block-localized wavefunction (BLW) method that decomposes the binding energy into the steric and the charge transfer (CT) (hyperconjugation) contributions. One unique feature of the BLW method is its capability to derive optimal geometries with only steric effect taken into account, while excluding the CT interaction. The results reveal that the overall steric energy exhibits angular dependency notably in halogen bonding, chalcogen bonding, and pnicogen bonding systems. Turning on the CT interactions further shortens the intermolecular distances. This bond shortening enhances the Pauli repulsion, which in turn offsets the electrostatic attraction, such that in the final sum, the contribution of the steric effect to bonding is diminished, leaving the CT to dominate the binding energy. In several other systems particularly hydrogen bonding systems, the steric effect nevertheless still plays the major role whereas the CT interaction is minor. However, in all cases, the CT exhibits strong directionality, suggesting that the linearity or near linearity of noncovalent bonds is largely governed by the charge-transfer interaction whose magnitude determines the covalency in noncovalent bonds.
authors.affiliation: Department of Chemistry, School of Science, China University of Petroleum (East China), Changjiangxi Road 66, 266580, Tsingtao, China.
authors.forename: Changwei
authors.initials: C
authors.lastname: Wang
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, Michigan, 49008.
authors.forename: Liangyu
authors.initials: L
authors.lastname: Guan
authors.affiliation: Institute of Chemistry and Lise Meitner Minerva Center for Computational Quantum Chemistry, the Hebrew University, Jerusalem, 91904, Israel.
authors.forename: David
authors.initials: D
authors.lastname: Danovich
authors.affiliation: Institute of Chemistry and Lise Meitner Minerva Center for Computational Quantum Chemistry, the Hebrew University, Jerusalem, 91904, Israel.
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, Michigan, 49008.
authors.forename: Yirong
authors.initials: Y
authors.lastname: Mo
keywords: block-localized wavefunction; directionality; energy decomposition analysis; noncovalent bond; σ-hole