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

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, OCH 3 , OH, N(CH 3 ) 2 , and O − ) on the extent of the charge transfer to the electron‐withdrawing NO 2 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 ν s (NO 2 ) and ν (C = C) sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH 3 ) 2 that the symmetric stretching of the NO 2 , ν s (NO 2 ), 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.