Unconventional\nRadical and Radical-Hole Site-Based\nInteractions in Halogen-Bearing Dimers and Trimers: A Comparative\nStudy

Radical (R<sup>•</sup>) and R<sup>•</sup>-hole site-based\ninteractions are comparatively studied, for the first time, using <i>ab initio</i> methods. In this regard, R<sup>•</sup>-bearing\nmolecules <sup>•</sup>XO<sub>3</sub> (where X = Cl, Br, and\nI) were subjected to direct interaction with NH<sub>3</sub> within\ndimeric and trimeric forms in the form of NH<sub>3</sub>···<sup>•</sup>XO<sub>3</sub>/<sup>•</sup>XO<sub>3</sub>···NH<sub>3</sub> and NH<sub>3</sub>···<sup>•</sup>XO<sub>3</sub>···NH<sub>3</sub> complexes, respectively.\nAs confirmed by electrostatic potential analysis, the studied R<sup>•</sup>-bearing molecules <sup>•</sup>XO<sub>3</sub> had the outstanding potentiality to interact as Lewis acid centers\nvia two positive sites dubbed as R<sup>•</sup> and R<sup>•</sup>-hole sites. Such an observation proposed the potentiality of the\nconsidered <sup>•</sup>XO<sub>3</sub> molecules to engage in\nunconventional R<sup>•</sup> and well-established R<sup>•</sup>-hole site-based interactions with Lewis bases. This was confirmed\nby negative interaction (<i>E</i><sub>int</sub>) energies,\nranging from −4.93 to −19.89 kcal/mol, with higher favorability\nfor R<sup>•</sup> site-based interactions over the R<sup>•</sup>-hole site-based ones. MP2 energetic features furnished higher preferability\nfor the R<sup>•</sup> site-based interactions than the R<sup>•</sup>-hole site-based ones in the case of chlorine- and\nbromine-bearing complexes, and the reverse was true for the iodine-bearing\ncomplexes. Moreover, elevated <i>E</i><sub>int</sub> values\nwere recorded for the NH<sub>3</sub>···<sup>•</sup>XO<sub>3</sub>···NH<sub>3</sub> trimers over the NH<sub>3</sub>···<sup>•</sup>XO<sub>3</sub> and <sup>•</sup>XO<sub>3</sub>···NH<sub>3</sub> dimers,\noutlining the higher preference of the <sup>•</sup>XO<sub>3</sub> molecules to engage in R<sup>•</sup> and R<sup>•</sup>-hole site-based interactions in the trimeric form over the dimeric\none. These results might be considered a requisite linchpin for numerous\nforthcoming supramolecular chemistry and crystal engineering studies.