Electronic excitation and ionization behavior of N-hydroxypyridine-2(1H)-thione and its deprotonated anion in a polarizable medium studied using quantum chemical computations
- 162 Downloads
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.
- 2.Blatt J, Taylor SR, Kontoghiorghes GJ (1989) Comparison of activity of deferoxamine with that of oral iron chelators against human neuroblastoma cell lines. Cancer Res 49:2925–2927Google Scholar
- 17.Nakatsuji H (1992) Electronic structures of ground, excited, ionized, and anion states studied by the SAC/SAC-CI theory. Acta Chim Acad Sci Hung 129:719–776Google Scholar
- 23.Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2010) Gaussian 09, revision B. 01. Gaussian, WallingfordGoogle Scholar
- 25.Mennucci B, Cancès E, Tomasi J (1997) Evaluation of solvent effects in isotropic and anisotropic dielectrics and in ionic solutions with a unified integral equation method: theoretical bases, computational implementation, and numerical applications. J Phys Chem B 101:10506–10517CrossRefGoogle Scholar