Excited state dipole moments of nitrobenzene derivatives

Abstract

The excited state dipole moments of a series of m- and p-nitrobenzene derivatives of type A–Ph–D have been evaluated from absorption spectra measurements (in the vapour and in solution) by employing various solvent shift equations, and by semiempirical MO methods (MIM and PPP). The µ_e values calculated from different solvent shift equations differ appreciably but they all give the same relative order for the moments and can be used for qualitative comparison of µ_e for a series of closely related compounds. MIM and PPP calculations, in contrast to the experimental findings, give comparable values of µ_e with the same order of magnitude for all the compounds. They also show that generally µ_e(calc) is much greater than µ_e(exp). Results suggest that the excited state dipole moments of the title compounds can be evaluated for qualitative purposes by means of a simplified equation through the measurement of the frequency shift obtained from the gas-phase and solution (apolar solvent) spectra.