Saturation of the electron-withdrawing capability of the NO2 group in nitroaromatic anions: spectroscopic and quantum-chemical evidence.
The electronic (UV-vis) and resonance Raman (RR) spectra of the anionic species derived from 4-nitrophenol (pNP) and 4-nitroaniline (pNA) are reported. The interpretation of the electronic transitions in the visible, near-UV region was supported by quantum-mechanical calculations, allowing a consistent analysis of the enhancement patterns observed in the RR spectra, which show substantial differences in relation to those observed for the neutral species. The removal of the proton of the donor groups (OH and NH2) leads to additional charge density at the oxygen atoms of the electron-withdrawing group (NO2). On the other hand, when the RR spectra of [pNP]- and [pNA]- are compared, a drastic difference concerning the enhancement of modes related to the NO2 moiety is noticed. In particular, in the case of [pNA]-, the RR enhancement pattern involving the NO2 moiety is at variance with those observed for nitroaromatics in general, because the corresponding normal modes involve a more complex composition. Such results are in accordance with the quantum-chemical calculations, which indicate, in the case of [pNA]-, a saturation of the charge density at the N-O bonds, even in the ground state (i.e., the charge density is very similar in the ground and excited-state, which precludes large geometric variations of the NO2 moiety in the two electronic states). Conversely, in this case the most enhanced bands have significant participation of ring modes, which suggests that the charge in the excited-state is now much more distributed in the molecule as a whole.
Ando
Rômulo A
RA
Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, São Paulo 05508-000, SP, Brazil.
Borin
Antônio C
AC
Santos
Paulo S
PS