The Electrostatic Potential Source Function (EPSF): a valuable tool to study enantioseparations involving sigma-holes as recognition sites

Enantioseparation on chiral stationary phase (CSP) is based on the formation of transient diastereomeric selectand-selector complexes in a chiral environment generated by a chiral selector. Diverse short-range directional interactions, including hydrogen bonds, pai-pai, dipole-dipole, and van der Waals interactions, underlie complex formation and promote the enantioseparation [1]. In this context, recently, our groups discovered that sigma-hole interactions can be involved in HPLC enantioseparations, enlarging the range of interactions which are active in this environment [2]. Computational tools and studies in silico have greatly contributed to the understanding of sigma-hole interactions [3]. In particular, sigma-hole being a region of electronic charge depletion, electrostatic potentials (EPs) have been widely used as an indicator of the anisotropy of the molecular charge distribution. While it is well acknowledged that sigma-holes originate from the cylindrical symmetry of the sigma-bond and of the electron sharing along its axis, nothing is quantitatively known about the role played by the various moieties of a molecule in producing such holes. The Bader-Gatti source function [4] for the electron density (ED) was thus extended to the EP, to yield a rigorous measure of how the various pieces of a molecule determine the extent of sigma-holes as measured by the EP value at their associated EP maxima on the 0.002 au ED isosurface. A FORTRAN package was developed for such a purpose [2c]. A first application of the EPSF tool to the chalcogen bonds in HPLC enantioseparations of fluorinated 4,4'-bipyridines has been reported [2c]. Here, the EPSF method is presented in detail and new examples are discussed which concern halogenated and seleno-substituted atropisomeric 4,4'-bipyridines as sigma-hole bond donors. These compounds were enantioseparated on polysaccharide-based CSPs and used as test probes to study recognition mechanisms involving sigma-hole sites. References [1] Scriba, G. K. E., J. Chromatogr. A 2016, 1467, 56-78. [2] (a) Peluso, P., Mamane, V., Aubert, E., Dessì, A., Dallocchio, R., Dore, A., Pale, P., Cossu, S., J. Chromatogr. A 2016, 1467, 228-238; (b) Dallocchio, R., Dessì, A., Solinas, M., Arras, A., Cossu, S., Aubert, E., Mamane, V., Peluso, P., J. Chromatogr. A 2018, 1563, 71-81; c) Peluso, P., Gatti, C., Dessì, A., Dallocchio, R., Weiss, R., Aubert, E., Pale, P., Cossu, S., Mamane, V., J. Chromatogr. A 2018, 1567, 119-129. [3] Kolá?, M. H., Hobza, P., Chem. Rev. 2016, 116, 5155-5187. [4] Bader, R. F. W., Gatti, C., Chem. Phys. Lett. 1998, 287, 233-238. Acknowledgement: This work has been supported by Università Ca' Foscari Venezia, Italy (DSMN, ADIR funds). C. G. acknowledges funding from Danmarks Grundforskningsfond (award No. DNRF93).