A comparative study of first hyperpolarizabilities of the acidic and basic forms of weak organic acids in water

Abstract

Dynamic and static first hyperpolarizabilities (β) of a series of benzoic and cinnamic acids in water have been investigated. These acids exist in a mixture of acidic and basic forms in solution. The hyperpolarizability of the basic form is higher than that of the corresponding acid form which is attributed to the red-shift of the absorption maximum of the basic form relative to that of the acid form. A ZINDO-correction vector (CV) method including solvent effects through a self-consistent reaction field (SCRF) for computing β reproduce measured values within experimental errors. Static ab initio hyperpolarizabilities within SCRF are slightly lower than the ZINDO/CV/SCRF β's. Semiemperical as well as ab initio gas phase calculations show the correct experimental trend in β but their magnitudes are much smaller than the measured values.