Modified solvatochromic equations for better estimation of ground and excited state dipole moments of p-aminobenzoicacid (PABA): Accounting for real shape over hypothetical spherical solvent shell

Abstract

Abstract Solvatochromic equations are critically dependent on the choice and values of the Onsager cavity radius, hence incorrect choice of the cavity radius could lead to erroneous estimates of dipole moment. The equations approximate a molecule as a point dipole residing in a spherical solvent cavity. However, for elongated molecules having elliptical shapes, such approximation is rather an over simplification. The problem of deducing the Onsager radius of long chain molecules in solvatochromic equations is re-examined from the point of view of solvent cavity volume. Consequently, we improved the equations by incorporating the volume of a cavity which is more appropriate to represent the real shape and size of the molecule of interest. The ground and excited state dipole moments of our model system, PABA, were calculated according to improved Lippert-Mataga, Bakhshiev’s, Bilot-Kawski and McRae’s equations. In this work, ground-sate dipole moment determined using the improved Lippert’s equation remarkably yields quite close result with a values estimated by electric deflection methods Solvation of PABA has also been studied theoretically by integral equation formalism polarizable continuum model (IEF-PCM) with density functional theory (DFT) methods to obtain absorption energies, ground-state dipole moments, volume of cavity. The calculations predicted solvent effects on absorption band maxima and the molar volume which are in agreement with the experimental data.