The CH···HC interaction in biphenyl is a delocalized, molecular‐wide and entirely non‐classical interaction: Results from FALDI analysis

• Published in: Journal of computational chemistry Vol. 42; no. 10; pp. 706 - 718
• Main Authors: Bates, Thomas G., Lange, Jurgens H., Cukrowski, Ignacy
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 15.04.2021; Wiley Subscription Services, Inc
• Subjects: atomic interaction line; bond path; Bonding strength; chemical boding; Decomposition; density bridge; Electron density; FALDI; intramolecular interaction; Qualitative analysis; Structural integrity; intramolecular interaction; electron density; bond path; atomic interaction line; chemical boding; density bridge; FALDI
• ISSN: 0192-8651; 1096-987X
• DOI: 10.1002/jcc.26491

In this study we aim to determine the origin of the electron density describing a CH···HC interaction in planar and twisted conformers of biphenyl. In order to achieve this, the fragment, atomic, localized, delocalized, intra‐ and inter‐atomic (FALDI) decomposition scheme was utilized to decompose the density in the inter‐nuclear region between the ortho‐hydrogens in both conformers. Importantly, the structural integrity, hence also topological properties, were fully preserved as no 'artificial' partitioning of molecules was implemented. FALDI‐based qualitative and quantitative analysis revealed that the majority of electron density arises from two, non‐classical and non‐local effects: strong overlap of ortho CH σ‐bonds, and long‐range electron delocalization between the phenyl rings and ortho carbons and hydrogens. These effects resulted in a delocalized electron channel, that is, a density bridge or a bond path in a QTAIM terminology, linking the H‐atoms in the planar conformer. The same effects and phenomena are present in both conformers of biphenyl. We show that the CH···HC interaction is a molecular‐wide event due to large and long‐range electron delocalization, and caution against approaches that investigate CH···HC interactions without fully taking into account the remainder of the molecule. The FALDI scheme is used to show that the majority of atoms in biphenyl contribute electron density to the H,H inter‐nuclear region to form a channel of delocalized density, that is, the Bader's bond path. This illustrates how a molecular system, here a planar biphenyl, is minimizing its energy when being disturbed from its equilibrium twisted structure.