Vibrational signatures of gaseous Meisenheimer complexes bonded at carbon and nitrogen
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 inthe 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 sigma-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 representedby C-bonded species.