A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline

Abstract

The proton transfer processes of 10-hydroxybenzo[h]quinoline (HBQ) and its cyano derivatives (4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) in ethyl acetate. The optimized geometric structures and infrared spectra show that the intramolecular hydrogen bond of enol form structure is weakened when the cyano group substitutes on the pyridine ring and that is strengthened when the cyano group substitutes on the phenol ring in the S0 state. Electron spectra show that the substitution of cyano group also has a significant effect on electron spectra. The reason for the change in the electron spectra can be obtained from the analysis of frontier molecular orbitals (MOs). The calculated potential barriers for proton transfer of these four molecules are 6.83, 7.83, 4.87 and 2.79 kcal/mol respectively in the S0 state, illustrating that the presence, position and number of cyano groups affect the proton transfer processes to some extent. After vertical excitation to the S1 state, the proton transfer processes of these four molecules become barrierless because of the promotion of charge transfer. Through the analysis of Mulliken charge, it can be found that the cyano group, an electron-withdrawing group, can change the strength of hydrogen bond by withdrawing the electronic charge from hydroxyl oxygen or neighboring nitrogen atom of its ring and thus affects the barrier in the S0 state.