Solvent-dependent structural dynamics of 2(1H )-pyridinone in light absorbing S₄ (ππ*) state

Abstract

The B‐band resonance Raman spectra of 2(1H)‐pyridinone (NHP) in water and acetonitrile were obtained, and their intensity patterns were found to be significantly different. To explore the underlying excited state tautomeric reaction mechanisms of NHP in water and acetonitrile, the vibrational analysis was carried out for NHP, 2(1D)‐pyridinone (NDP), NHP–(H2O)n (n = 1, 2) clusters, and NDP–(D2O)n (n = 1, 2) clusters on the basis of the FT‐Raman experiments, the B3LYP/6‐311++G(d,p) computations using PCM solvent model, and the normal mode analysis. Good agreements between experimental and theoretically predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands in both the FT‐Raman and the resonance Raman spectra. The results indicated that most of the B‐band resonance Raman spectra in H2O was assignable to the fundamental, overtones, and combination bands of about ten vibration modes of ring‐type NHP–(H2O)2 cluster, while most of the B‐band resonance Raman spectra in CH3CN was assigned to the fundamental, overtones, and combination bands of about eight vibration modes of linear‐type NHP–CH3CN. The solvent effect of the excited state enol‐keto tautomeric reaction mechanisms was explored on the basis of the significant difference in the short‐time structural dynamics of NHP in H2O and CH3CN. The inter‐molecular and intra‐molecular ESPT reaction mechanisms were proposed respectively to explain the Franck–Condon region structural dynamics of NHP in H2O and CH3CN.Copyright © 2015 John Wiley & Sons, Ltd.

UV and Resonance Raman Spectroscopic and Theoretical Studies on the Solvent-Dependent Ground and Excited-State Thione → Thiol Tautomerization of 4,6-Dimethyl-2-mercaptopyrimidine (DMMP)

“…In contrast, this transformation was significantly slower in water, as shown in Figure B, where approximately one-third of the thione DMMP tautomer remained intact after 40 min of photolysis. The results indicate that the intermolecular H-bonding interactions greatly contribute the excited-state proton transfer process, similar to a previously reported solvent-assistant ESPT reaction. − More interestingly, three new bands appeared at 304, 256, and 220 nm in cyclohexane along with the characteristic thiol bands at 271 and 236 nm after 5 min of photolysis. Upon continued exposure of this sample to photolysis, an additional three new absorption bands were observed at ∼320, 250, and 202 nm, with the band at ∼320 nm being the most intense after 20 min.…”

“…Thus, the calculated Δν 7–8 , Δν 12–15 , Δν 15–16 , Δν 16–17 , and Δν 17–18 bands are 48, 154, 94, 29, and 25 cm –1 , respectively, which correlate with the experimental values of 50, 159, 75, 41, and 34 cm –1 . Consequently, the solid-state form of DMMP should be assigned to the thione dimer with NH···S bonds. ,, …”

“…Notably, the assistance of a single water molecule for proton transfer decreases the energy barrier by half; in solution, these calculated activation barriers are 32.0 and 14.8 kcal/mol, respectively. Thus, the barrier for hydrogen transfer between 2-pyrimidinethiol and 2­(1 H )-pyrimidinethione in the ground state generally exceeds 30 kcal/mol, indicating that tautomerization is challenging in the ground state. , However, with the aid of a few water molecules the transition-state energies for some similar compounds have been reported to decrease sharply to almost half for direct proton transfer in the ground state, due to the role of hydrogen bonding in both the thione–thiol tautomerization reaction and molecular dynamics. − Recently, the solvent-assisted excited-state intra- and/or intermolecular proton transfer (ESIPT) mechanism has received growing attention because of its crucial role in the activity of photoacids and photobases, − including photosensitive proteins − and metal complexes . Interestingly, it has also been considered among some of the primary factors enabling the evolution of life due to its role in nucleic acid base photochemistry. − In addition, photoinduced proton transfer, which is commonly accompanied by a pronounced redistribution of electron density, results in large shifts in the Stokes emission, giving rise to the possibility of bioluminescence color tuning and selective molecular photoswitching and information storage. − …”

“…Consequently, the solid-state form of DMMP should be assigned to the thione dimer with NH•••S bonds 17,56,57. 3.2.…”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Zhang and co‐workers studied the solvent‐dependent structural dynamics of 2(1H)‐pyridinone in the light absorbing S 4 (ππ*) excited state. Good agreement between experimental and theoretical predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands in both the FT‐Raman and the resonance Raman spectra …”