Experimentally and computationally determining the theoretical aqueous pKa of three UV-absorbing benzophenone derivatives

Sarah-Beth Christine Loder

2012

UV-absorbers are common pharmaceuticals used in many cosmetics today. Water insolubility makes them easy to incorporate into oil-based lotions, providing protection from harmful UVA and UVB light. The acid dissociation constant, pKa, is one value that can be used to understand the properties and intermolecular interactions of a molecule. This value can be determined by several methodologies, both in lab and on the computer. pKa's are generally reported in aqueous solution, although it cannot directly be determined for water-insoluble molecules. Experimentally using a combination of binary solvent systems and spectrophotometric titrations, the acidity constant was found using the Benesi-Hildebrandt method. Computationally, the structures of the protonated and deprotonated species were optimized using density functional theory methods, B3LYP/6-31+G(d,p), and implicit solvation, CPCM model. These computations yield pKa values unrealistically high, but may be brought into a more accurate range with the addition of multiple explicit waters for stabilization. Experimental results produced pKa's of 13.7 for oxybenzone, 12.8 for dioxybenzone and 13.7 for sulisobenzone with standard errors under 0.05.