Resonance Raman studies of benzene derivatives with strong conjugation: nitrile substitution

Raman spectra in resonance with the La state of benzene derivatives exhibit activity in both the fundamental and two-quantum overtones of modes that are of e2g symmetry in benzene. The Raman spectra can be used to test proposals of Petruska that result in the conclusion that 1,2,3-trisubstituted benzenes should have La transitions with very small allowed transition moments. We find that nitrile substitution results in absorption and resonance Raman spectra that show deviations from Petruskaʹs rules. The 1,2,3-tricyano derivative exhibits Raman spectra that demonstrate allowed transition character.