A theoretical study of ground and excited state proton transfer and rotamerim in salicylanilide and its 1:1 complex with methanol

The absorption and emission spectra of salicylanilide (SAN) in methanol show strong blue-shift compared with those in cyclohexane. To make clear this phenomenon, the ground (S0) and excited state (S1) proton transfer reactions in cyclohexane and in methanol solutions were investigated, respectively. In cyclohexane, the intramolecular mechanism was carried out for the isolated SAN. Whereas in methanol solution, the role of the methanol molecule in proton transfer reaction was supposed that it directly participated the reaction path, called methanol-assisted mechanism. The computational results show that barrier height for the ground state proton transfer is too high to happen, but is energetically favored in S1 for both isolated SAN and methanol-assisted system. In addition, it indicates that the hydrogen-bonding between SAN and methanol molecule can dramatically lower the barrier by methanol-assisted mechanism in S1. Herein, we also have applied a combination of the continuum dielectric and the explicit solvent model to study the spectra of SAN in methanol, which exhibit that the interaction between SAN and one methanol molecule yields a hydrogen-bonding complex responsible for the blue-shifted wavelength of absorption and emission spectra in methanol solvent.