Halide adducts of 1,3,5-trinitrobenzene: vibrational signatures and role of anion-pi interactions

The fluoride and chloride complexes with 1,3,5-trinitrobenzene (TNB-F - and TNB-Cl-) are obtained as gaseous species by electrospray ionization and are assayed by IR multiple photon dissociation spectroscopy along with quantum chemical calculations. The stable structure is quite different for the two complexes. A strongly covalent σ-complex (Meisenheimer complex) holding fluorine on a tetrahedral carbon at a formerly unsubstituted ring position is the stable form of TNB-F- while a σ-complex with fluorine bound to a nitro-substituted carbon is 78 kJ/mol higher in energy. The TNB-Cl- complex presents an off-center binding of chlorine placed above a CH group of a scantily perturbed TNB unit. This structure conforms to a weak σ-binding motif. The observed vibrational features support two diverse binding motifs and previous findings about TNB-I- and TNB-Br- show the transition to occur between the fluoride and chloride adducts while both I- and Br- complexes are consistent with a weak σ-binding motif. Implication of these results is expected in the design of halide ion transporters. © 2013 Elsevier B.V. All rights reserved.