Vibrational signatures of gaseous Meisenheimer complexes bonded at carbon and nitrogen

- Anionic complexes of 1,3,5-trinitrobenzene are characterized in the gas phase.
- IRMPD spectroscopy points to a covalent Meisenheimer complex structure.
- Quantum chemical calculations are employed to assign experimental vibrations.
- Carbon versus oxygen attack by enolate anions is discussed.

Anionic adducts of 1,3,5-trinitrobenzene (TNB) with deprotonated pyrrolidine (Pyr), imidazole (Im), acetone (Ac) and acetylacetone (Acac) have been delivered into the gas phase by electrospray ionization. The so-formed ions, TNB-Nu− (Nu = Pyr, Im, Ac, Acac), have been interrogated by IRMPD spectroscopy in the fingerprint range. DFT calculations at B3LYP/6-311++G(d,p) level have been performed for a survey of candidate structures. All adducts conform to anionic σ-complexes (Meisenheimer complexes). The symmetric stretching modes of the nitro groups yield a dominant vibrational signature at 1200-1250 cm−1, the red-shift with respect to the degenerate frequency of 1367 cm−1 in neutral TNB reflecting the extent of negative charge delocalization. The enol complexes TNB-Ac− and TNB-Acac− are largely represented by C-bonded species.

235