The electronic delocalization in para-substituted β-nitrostyrenes probed by resonance Raman spectroscopy and quantum-chemical calculations

Abstract

The electronic (UV-vis) and resonance Raman (RR) spectra of a series of para-substituted trans-β-nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO 2, COOH, Cl, OCH3, OH, N(CH3)2, and O-) on the extent of the charge transfer to the electron-withdrawing NO2 group directly linked to the ethylenic (C=C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the νs(NO2) and ν(C=C)sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH3)2 that the symmetric stretching of the NO2, νs(NO2), is the most substantially enhanced mode, whereas for X = O-, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd.