INTERACTION ENERGIES ASSOCIATED WITH SHORT INTERMOLECULAR CONTACTS OF C-H BONDS .1. ABINITIO COMPUTATIONAL STUDY OF C-H=ANION INTERACTIONS, C-H=X- (X- = I3-, IBR2-, ICL2-)

The nature of the C-H...anion contact interactions found for organic charge-transfer salts was investigated by performing SCF-MO and MP2 level calculations on the model systems H3C-H...Y-I-Y- (Y = I, Br, Cl). The binding energies of the H3C-H...Y-I-Y- systems are estimated to be 1.1, 1.3, ad 1.6 kcal/mol for Y = I, Br, and Cl, respectively. The binding energy increase, observed when Y varies from I to Br to Cl, is consistent with the expected hydrogen-bonding abilities of the halogen atoms. The C-H bond prefers to make a short contact with the terminal halogen atoms of Y-I-Y-; this tendency increases as Y changes from I to Br to Cl, and the C-H...Y-I-Y- interaction energies do not strongly depend upon the C-H...Y contact angle.