Unconventional Radical and Radical-Hole Site-Based Interactions in Halogen-Bearing Dimers and Trimers: A Comparative Study

Radical (R^•) and R^•-hole site-based interactions are comparatively studied, for the first time, using <i>ab initio</i> methods. In this regard, R^•-bearing molecules ^•XO₃ (where X = Cl, Br, and I) were subjected to direct interaction with NH₃ within dimeric and trimeric forms in the form of NH₃···^•XO₃/^•XO₃···NH₃ and NH₃···^•XO₃···NH₃ complexes, respectively. As confirmed by electrostatic potential analysis, the studied R^•-bearing molecules ^•XO₃ had the outstanding potentiality to interact as Lewis acid centers via two positive sites dubbed as R^• and R^•-hole sites. Such an observation proposed the potentiality of the considered ^•XO₃ molecules to engage in unconventional R^• and well-established R^•-hole site-based interactions with Lewis bases. This was confirmed by negative interaction (<i>E</i> _int) energies, ranging from -4.93 to -19.89 kcal/mol, with higher favorability for R^• site-based interactions over the R^•-hole site-based ones. MP2 energetic features furnished higher preferability for the R^• site-based interactions than the R^•-hole site-based ones in the case of chlorine- and bromine-bearing complexes, and the reverse was true for the iodine-bearing complexes. Moreover, elevated <i>E</i> _int values were recorded for the NH₃···^•XO₃···NH₃ trimers over the NH₃···^•XO₃ and ^•XO₃···NH₃ dimers, outlining the higher preference of the ^•XO₃ molecules to engage in R^• and R^•-hole site-based interactions in the trimeric form over the dimeric one. These results might be considered a requisite linchpin for numerous forthcoming supramolecular chemistry and crystal engineering studies.