On the additivity of bond dipole moments. Stark effect studies of the rotationally resolved electronic spectra of aniline, benzonitrile, and aminobenzonitrile

We study the influence of an applied electric field on the fully resolved electronic spectra of aniline (AN), benzonitrile (BN), and 4-aminobenzonitrile (ABN) in the gas phase. Using these data, we test the commonly held but rarely proven assumption that the total dipole moment of a polyatomic molecule is the vector sum of bond dipole moments, localized in different parts of the molecule. We find that μa(ABN)≈μa(AN)+μa(BN) in the excited S1 state, but not in the ground S0 state. Possible reasons for this non-additivity are discussed.