Experimental and theoretical studies on the nature of weak nonbonded interactions between divalent selenium and halogen atoms

To investigate the nature of weak nonbonded selenium(...)halogen interactions ((SeX)-X-... interactions; X = F Cl, and Br), three types of model compounds [2-(CH2X)C6H4SeY (1-3), 3-(CH2X)-2-C10H6-SeY (4-6), and 2-XC6H4CH2SeY (7-9); Y = CN, Cl, Br, SeAr, and Me] were synthesized, and their Se-77 NMR spectroscopic behaviors were analyzed in CDCl3. The gradual upfield shifts of Se-77 NMR absorptions observed for series 1-3 and 4-6 suggested that the strength of (SeX)-X-... interaction decreases in the order of (SeF)-F-... > (SeCl)-Cl-... > (SeBr)-Br-.... The quantum chemical calculations at the B3LYP/631H level using the polarizable continuum model (PCM) revealed that the most stable conformer for 1-3 is the one with an intramolecular short (SeX)-X-... atomic contact in CHCl3 (epsilon = 4.9) and also that the n(x) --> sigma*(se-y) orbital interaction (E-Se...X) can reasonably explain the order of strength for the (SeX)-X-... interactions. On the other hand, the Se-77 NMR absorptions observed for series 7-9 did not shift significantly from the reference compounds (C6H5CH2SeY), indicating the absence of the (SeX)-X-... interaction for 7-9 presumably due to attenuation of basicity for the halogen atom that is substituted directly to the aromatic ring. These observations suggested that the n(x) --> sigma*(se-y) orbital interaction is a dominant factor for formation of weak (SeX)-X-... interactions. Electron correlation was also suggested to be important for the stability.