A new molecule 9,10-dihydroxybenzo[h] quinoline (i.e. 9-10-HBQ) is focused in the present work about its excited state proton transfer (ESPT) mechanism. Though comparing potential energy barriers, it is found that the ultrafast ESPT process could occur in the $S_1$ state without potential energy barrier along with hydrogen bond $O_3$-$H_4···N_5$ forming 9-10-HBQ-PT1 structure, subsequently, the second proton transfers via another intramolecular hydrogen bonded wire $O_1$-$H_2···N_3$ with a low potential energy barrier (about 7.69 kcal/mol) in the $S_1$ state forming 9-10-HBQ-PT2 configuration. After completing excited state dynamical process, the $S_1$-state could turn back to $S_0$ state with occurring reversed ground state proton transfer forming initial 9-10-HBQ structure.

The reference [J. Org. Chem. 2015, 80, 10817.10828] has reported that the Quinone Methides (QMs) that are of significant medicinal benefits in biological field can be generated via photo-deamination reaction. In this study, the density functional theory (DFT) and time-dependent DFT (TDDFT) methods have been carried out to investigate the reactive mechanisms of free amines compounds 1 and 3. The excited state hydrogen bonding strengthening mechanism has been demonstrated by analyses of frontier molecular orbitals (MOs). Potential energy surfaces (PESs) calculations have been performed for illustrating excited state intramolecular proton transfer (ESIPT) and photo-deamination reactions of forms 1 and 3. The reactive sites of QM1 and QM3 molecules can be accurately predicted via analyses of ESP.

Sulfonamide antibiotics are an important class of organic pollutant in the aquatic environments. To understand the hydrolysis behavior of sulfonamides, the hydrolysis mechanisms of a typical sulfonamide sulfachloropyridazine (SCP) were investigated using the density functional theory (DFT) at the B3LYP/6-31+G (d, p) level. SCP hydrolysis resembles nucleophilic substitution by water molecule attacking sulfonyl group (pathway 1) and heterocyclic aromatic ring (pathway 2) respectively. Due to the electrophilic center sulfur atom in pathway 1 carrying much larger positive charge than the carbon atom in the pathway 2, the sulfonyl group can be easily attacked by water molecule, and thus the pathway 1 can be dominant. By comparing the hydrolysis energy barrier of different forms of SCP, it was found that the SCP hydrolysis in neutral and once-protonated state are much more energetically favorable to proceed than the double protonated form. In addition, the hydrolysis path is not found for the dissociated anionic SCP. As the pH values in solution decreases, the corresponding neutral and once-protonated SCP increases, then the hydrolysis rate becomes faster, which is consistent with the experimental observations that the hydrolytic degradation rate at pH=4 is much faster than those of pH=7 and 9.