Probing the Hybrid σ/π-Holes in sp²-Hybridized Chalcogen Bond Donors and their Complexes with Ammonia

sp- and $s{p}^2$ -hybridized chalcogen bond (ChB) donor molecules, an ammonia acceptor molecule, and the corresponding 1:1 donor-acceptor complexes formed between them are computed with density functional theory. Adding an allelic carbon linker into the backbone of the donor moiety generally reduces the number of σ/π-hole regions from two to one. This compositional modification to the interactive region of ChB donors results in a strip-like area of localized electropositive potential along the extension of the carbon-chalcogen bond. The presence of these regions gives rise to a reduction in ChB directionality and, thus, relatively flat potential energy surfaces. Remedies to this behavior are provided through additional hydrogen bond interactions between the amino hydrogen atoms and the electron-accepting strips on the sides of the ChB donors. These highly tunable ChB donors, and by extension their donor-ammonia complexes, are interrogated to differentiate subtle differences in the chemicophysical properties with and without electron-withdrawing substituents. The structures, electronics, and energetics exhibited by the ChB donors before and after complexation are discussed and utilized to identify the rational design guidelines developed herein.