Studies on the nonbonded interactions of divalent organic selenium

We report here the characteristics of the weak nonbonded interactions formed by a divalent selenium atom; i.e., Se center dot center dot center dot N, Se center dot center dot center dot O, Se center dot center dot center dot F, Se center dot center dot center dot Cl, and Se center dot center dot center dot Br interactions. Seven series of 2-substituted benzeneselenenyl compounds [2-C6H4(SeY)CH2X; X = NRR', OH, F, Cl, etc.; Y=Cl, Br, CN, etc.], some of which were labeled with an NMR active stable isotope, were synthesized. The intramolecular Se center dot center dot center dot X interactions were characterized by H-1, N-15, O-17, F-19, and Se-77 NMR spectroscopy in solution as well as by quantum chemical calculation and natural bond orbital (NBO) analysis. It was found that the strength of the Se center dot center dot center dot X interactions decreases with decreasing the basicity of the X atom, i.e., in the order of X = N, O, F, Cl, Br. The trend demonstrated the importance of the hypervalent nature of selenium for formation of the Se center dot center dot center dot X interactions. Convenient correlations were proposed for estimation of the strength of Se center dot center dot center dot X interaction from the Se-77 NMR chemical shift or the nonbonded Se center dot center dot center dot X atomic distance.