Unified approach to absorption intensities in vibrational spectroscopies via dynamic electromagnetic shieldings at the nuclei of a molecule

The intensity of the radiation absorbed by a vibrating molecule can berationalized in terms of non quantized electric Lorentz forces acting on the chargednuclei. These forces depend on the local electric field.A general expression for the effective electric field at the nuclei of a molecule inthe presence of an external electromagnetic radiation, represented as amonochromatic wave, is discussed, showing that the molecular response can bedescribed within a general framework by frequency-dependent nuclear electric andelectromagnetic shielding and hypershielding tensors.It is found that the electric and electromagnetic hypershieldings areproportional to the geometrical derivatives of the dynamic dipole polarisability andof the optical rotatory power, respectively.Since intensities in Raman spectroscopy and vibrational Raman optical activity areusually interpreted in terms of these derivatives, the results demonstrate theequivalence of either description.