Proton transfer reaction in 2-hydroxy-3-formyl benzoic acid at room temperature and 77 K and some study on AM1 potential surfaces

Proton transfer processes in 2-hydroxy-3-formyl benzoic acid (2HFBA) have been studied by means of absorption and emission spectroscopy in some non-polar and weakly polar solvents at room temperature and 77 K in relation to 4-hydroxy-3-formyl benzoic acid (4HFBA). The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (10,500 cm−1) due to the formation of enol tautomer of 2HFBA in non-polar solvents. In 1,4-dioxane and tetrahydrofuran, 2HFBA forms hydrogen bonded complex while both tautomer and hydrogen bonded complex have been detected in benzene. Unlike 4HFBA, 2HFBA forms hydrogen bonded complex in presence of a base, like triethyl amine in non-polar solvents due to the rupture of intramolecular bond. Moreover, 2HFBA shows phosphorescence even in absence of base on lowering the temperature to 77 K. The energetics of the ground and excited state proton transfer in 2HFBA have been studied at the AM1 level of approximation. The ground singlet is predicted to have a large activation barrier on the proton transfer path while the barrier height is much lower on the corresponding excited singlet surface.