Introduction: Molecular modeling can be used to predict the UV/VIS spectra of molecules. In this exercise two common indicators will be constructed, in both the protonated and deprotonated forms. The spectra of these will be generated and compared with experimental results.
Molecular Modeling Component: The protonated and deprotonated forms of both phenolphthalein and methyl red are constructed using a molecular modeling program. The structures and corresponding spectra shown in this document were generated using CAChe. The molecular structures were geometry optimized first with molecular mechanics and then the semiempirical AM1, and the spectral data were generated using ZINDO.
Protonated Phenol Red
Deprotonated Phenol Red
The spectra predicted from the ZINDO calculations show a significant increase in lmax for the deprotonation of both the phenolphthalein and phenol red.
Experimental Component: Students will prepare buffer solutions of pH 4 and pH 10. Acidic and basic solutions of both indicators are prepared and the UV/VIS spectra measured experimentally.
Results: The trend in lmax predicted by the molecular modeling is the same as that seen experimentally.