Electronic excitation and ionization behavior of N-hydroxypyridine-2(1H)-thione and its deprotonated anion in a polarizable medium studied using quantum chemical computations
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N-Hydroxypyridine-2(1H)-thione (N-HPT) is an important photochemical generator of hydroxyl radicals; however, it has been pointed out that N-HPT is not a specific precursor of hydroxyl radical. Photoionization of N-HPT competes with photochemical N–O bond cleavage in neutral aqueous solution. The possibility of a competitive reaction could be critical for studies using N-HPT as the radical precursor; therefore, the detailed behaviors of electronic excitation and ionization of N-HPT and its deprotonated anion, which is the dominant tautomer under neutral pH conditions, are studied using quantum chemical methods with the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM). The detailed assignment of the UV–Vis spectra of N-HPT is provided, and the origin of the observed negative solvatochromism is found to be the charge transfer excitation between the sulfur and the pyridine ring. The photochemical N–O bond cleavage occurs via the conical intersections between the lowest π → π* and π → σ* states and between the π → σ* and ground state, when N-HPT dissociates into PyS· and ·OH radicals. The calculated ionization potentials of N-HPT and the deprotonated N-HPT anion are 5.75 and 4.67 eV in PCM water. This demonstrates that the charge transfer excitation energy between N-HPT and liquid water becomes significantly lower for the deprotonated anion in comparison with the neutral molecule. Even under mild photochemical conditions, photoinduced ionization of N-HPT may occur in neutral aqueous solution.
KeywordsExcited state Ionized state Solvent effect Charge transfer Photochemical bond cleavage Hydroxyl radical precursor
This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), Japan, and MEXT (Ministry of Education Culture, Sports, Science and Technology, Japan) program “Elements Strategy Initiative to Form Core Research Center.” Some of the computations were performed at the Research Center for Computational Science, Okazaki, Japan.
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