Intensities in the infra-red spectrum of benzene

The absolute intensities of the infra-red active fundamental vibrations of benzene, mono-deuterobenzene, and para-dideuterobenzene have been measured and interpreted in terms of the dipole moment derivatives: ∂μz/∂S11 = -1·43 D/Å; ∂μy/∂S18a = - 0·51 D/Å; ∂μy/∂S19a = -0·32 D/Å; ∂μy/∂S20a = -0·78 D/Å. All three spectra are consistent with these values. The ‘effective’ C—H bond moment for in-plane bending is 0·31 D and for out-of-plane bending 0·61 D. The value for the C—H bond-stretching dipole-moment derivative is 0·45 D/Å with the H atom becoming less positive as the bond stretches. In-plane distortions of the carbon skeleton which leave the C—H bond directions unchanged, produce zero dipole moment within the experimental error.