Base triggered enhancement of first hyperpolarizability of a keto-enol tautomer

This work describes the base triggered enhancement of first hyperpolarizability of a tautomeric organic molecule, namely, benzoylacetanilide (BA). We have used the hyper-Rayleigh scattering technique to measure the first hyperpolarizability (β) of BA which exists in the pure keto form in water and as a keto-enol tautomer in ethanol. Its anion exists in equilibrium with the keto and enol forms at pH 11 in aqueous solution. The β value of the anion form is 709 × 10(-30) esu, whereas that of the enol is 232 × 10(-30) esu and of the keto is 88 × 10(-30) esu. There is an enhancement of β by ~8 times for the anion and ~3 times for the enol compared to the keto form. All these are achieved by altering the equilibrium between the three forms of BA by simple means. MP2 calculations reproduce the experimental trend, but the computed β values are much lower than the measured values. DFT calculations with the standard B3LYP functional could not predict the right order in the β values. The difference between experimental and calculated values is, perhaps, due to the fact that electron correlation effects are important in computing optical nonlinearities of large organic molecules and MP2 and B3LYP calculations done here for different forms of BA could not account for such effects adequately.