(b) Complete characterization of H-bonded complexes of substituted phenols and water
In the gas phase, the primary interaction of the water molecules with the weak acids is that they are bound to the substrate by means of H-bond and the water molecule acts as H-bond acceptor. The question that the cluster research community has been debating over is what will be the minimum size of the cluster when the proton transfers from the acid to the base. The question that we were addressing is not quite the same but is somewhat related. What is the dependence of the binding strength of the complexes on the nature of the substrate, viz. the pKa of the acid? In this context, we have shown that the H-bond strength depends linearly on the pKa of the acid; i.e. lower the pKa stronger the H bond and vice versa. We have completely characterized the 1:1 complexes of four para substituted phenols whose pKa varies from 7 to 10 in the ground state and 3 to 7 in the excited state. The correlation was established using two parameters viz. the red shift in the band origin of the complex relative to the monomer and the red shift in the OH stretching frequency. The former was monitored using REMPI spectroscopy and the later using the IR-UV double resonance spectroscopy. The observed results were corroborated using the ab-initio calculations at the MP2 and DFT level to determine the global minima and their corresponding IR spectra.