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 ab initio methods. In this regard, R • -bearing molecules • XO 3 (where X = Cl, Br, and I) were subjected to direct interaction with NH 3 within dimeric and trimeric forms in the form of NH 3 ··· • XO 3 / • XO 3 ···NH 3 and NH 3 ··· • XO 3 ···NH 3 complexes, respectively. As confirmed by electrostatic potential analysis, the studied R • -bearing molecules • XO 3 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 3 molecules to engage in unconventional R • and well-established R • -hole site-based interactions with Lewis bases. This was confirmed by negative interaction ( E 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 E int values were recorded for the NH 3 ··· • XO 3 ···NH 3 trimers over the NH 3 ··· • XO 3 and • XO 3 ···NH 3 dimers, outlining the higher preference of the • XO 3 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.