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 methods. In this regard, R-bearing molecules XO (where X = Cl, Br, and I) were subjected to direct interaction with NH within dimeric and trimeric forms in the form of NH···XO/XO···NH and NH···XO···NH complexes, respectively. As confirmed by electrostatic potential analysis, the studied R-bearing molecules XO 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 XO molecules to engage in unconventional R and well-established R-hole site-based interactions with Lewis bases. This was confirmed by negative interaction ( ) 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 values were recorded for the NH···XO···NH trimers over the NH···XO and XO···NH dimers, outlining the higher preference of the XO 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.