Vibrational polarizability and hyperpolarizability of p-nitroaniline

Within the double harmonic oscillator approximation scheme based on an RHF/6–31G electronic structure, the vibrational contributions to the polarizability, and first and second hyperpolarizability tensors of p-nitroaniline have been evaluated for different optical processes by adopting the infinite frequency or enhanced approximation. The static vibrational longitudinal first hyperpolarizability reaches 1352 au whereas the corresponding electronic counterpart is 1264 au. The [α2]0,0 and [μβ]0,0 contributions to the vibrational longitudinal second hyperpolarizability are, in the static limit, equal to 44750 and 61112 au, respectively, in comparison with the 70874 au electronic value. These vibrational contributions remain important when considering the different dynamic optical processes with the exception of second- and third-harmonic generation. The main contributions to both the first and second vibrational hyperpolarizabilities are due to vibrational normal modes which describe the oscillation between aromatic-like and quinoid-like structures.