Lactim–lactam tautomerism is favoured over enol–keto tautomerism in 2-hydroxy-5-(4-fluorophenyl)nicotinic acid: Experimental and quantum chemical approach

A model compound, 2-hydroxy-5-(4-fluorophenyl)nicotinic acid (HFPNA) has been synthesized and its ground and excited-state properties towards tautomerisation via possible two ways of proton transfer process have been elaborately examined by means of steady-state absorption, emission and time-resolved emission spectroscopy and quantum chemical calculations. The theme issue of the present contribution is to illustrate a direct competition between two potential excited-state intramolecular proton transfer (ESIPT) pathways within the same molecule. By a direct comparison of spectral characteristics of HFPNA with those of salicylic acid (SA) and 2-hydroxypyridine (2HP) under similar experimental conditions it has been demonstrated that lactim–lactam tautomerisation dominates over enol–keto tautomerisation in HFPNA. Experimental evidences and structural calculation at Density Functional Theory (DFT) (B3LYP/6-31G**) and Hartree–Fock (6-31G**) levels predict the existence of both lactim and lactam-forms in the ground state. The ground and excited-state theoretical potential energy surfaces (PES) of HFPNA along both the ways of proton transfer coordinate at DFT level reveal that PES of HFPNA resemble well with that of 2HP while there are prominent differences from that of SA. We also report on the possibility of application of the studied molecule as a sensor of medium-pH following its sensitive response towards pH of the medium.