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A proton transfer competition of 1,8-di(pyridin-2-yl)-9H-carbazole in excited-state intramolecular proton transfer through on-the-fly dynamic simulation
(2025-10-01)
Inporn, Wutthana
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Kungwan, Nawee
;
Daengngern, Rathawat
Excited-state intramolecular proton transfer (ESIPT) of 1,8-di(pyridin-2-yl)-9H-carbazole (DPyCz) was investigated using static and excited-state dynamic simulations. Three-distinct configurations, each involving a different pyridyl moiety forming an intramolecular hydrogen bond, were identified to explore the competition in ESIPT. Additionally, slower PT, occurring at ∼200 fs, was found to depend on the competition between the pyridyl nitrogen atoms as acceptors. In contrast, DPyCz-B, which features only a single acceptor site, facilitated twice faster PT than that of DPyCz-A. Thus, our findings suggested that the ESIPT process of DPyCz is possibly governed by the interplay of intramolecular hydrogen bonding and excited-state charge redistribution.
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Experimental and Theoretical Exploration of Ultrafast Excited State Double Proton Transfer in 2,5-Bis(2-benzimidazolyl)hydroquinone
(2025-07-01)
Prommin, Chanatkran
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Chaihan, Komsun
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Daengngern, Rathawat
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Mori, Seiji
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Akutsu-Suyama, Kazuhiro
The electronic properties and excited-state intramolecular double proton transfer of 2,5-bis(2-benzimidazolyl)hydroquinone (bis-HBI) in a nonpolar solvent were investigated using a combined experimental and theoretical approach. Bis-HBI was successfully synthesized and its characterization was confirmed through <sup>1</sup>H NMR and FT-IR. Three distinct emission peaks of bis-HBI were observed at 484, 597, and 730 nm, which were theoretically assigned to the di-enol (EE), mono-keto (EK), and di-keto (KK) species, respectively. The emission peaks at longer wavelengths (597 and 730 nm) are attributed to tautomerization upon photoexcitation and are assigned to the mono-keto and di-keto species, which result from multiple proton transfers. These species exhibit kinetically and thermodynamically favorable behaviors. On-the-fly dynamics simulations reveal that the double proton transfer process occurs ultrafast, within 433 fs. Additionally, both backward and forward proton transfers are observed during the first and second proton transfers, indicating tautomeric equilibria between the three species—EE, EK, and KK—on the excited-state surface. This is consistent with the potential energy surface along the proton transfer coordinate.
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A theoretical investigation into the demethylation mechanism of dimethylsulfide over the W3O6 cluster
(2025-07-01)
Aziz, Hafiz Aji
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Kungwan, Nawee
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Gleeson, Duangkamol
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Saelee, Tinnakorn
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Daengngern, Rathawat
Understanding how dimethyl sulfide (DMS) breaks down to form value-added products such as methanol on transition metal oxide catalysts is important for improving desulfurization processes. In this study, the reaction mechanism over a tungsten oxide cluster (W₃O₆) is elucidated using density functional theory (DFT) at the M06-L/LANL2DZ/aug-cc-pVTZ level of theory. Two competing mechanistic pathways were discovered over the W₃O₆ cluster: Pathway A) direct demethylation followed by methanol desorption and Pathway B) a water-assisted concerted demethylation pathway. Pathway A involves sequential steps with a moderate demethylation barrier (49.43 kcal/mol), but a significantly higher barrier (68.78 kcal/mol) for subsequent methanol formation, imposing a kinetic bottleneck. Remarkably, Pathway B, mediated by an explicit water molecule, facilitates a lower-barrier concerted transformation (56.19 kcal/mol), effectively bypassing the high-energy intermediate. Kinetic modeling via Transition State Theory and the Energetic Span Model reveal that despite the very low turnover frequency (TOF = 9.29 × 10<sup>−30</sup> s⁻¹), the water-assisted pathway is energetically superior. These findings highlight the important role of water in helping the reaction proceed and offer insight for designing better catalysts for sulfur removal from DMS to methanol over the W₃O₆ cluster.
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Tautomeric enhancement of 2-(1H-pyrazol-5-yl)pyridine in photoinduced proton transfer by water-assisted molecules
(2024-09-20)
Salaeh, Rusrina
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Inporn, Wutthana
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Chansen, Warinthon
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Kungwan, Nawee
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Daengngern, Rathawat
Theoretical insight of excited state proton transfer (ESPT) in 2-(1H-pyrazol-5-yl)pyridine abbreviated as PPP interacting with water wires: PPP(H<inf>2</inf>O)<inf>n</inf> where n = 1–3 has been presented in both static and dynamics studies. Explicit water molecules placed around PPP have been simulated to elucidate the intermolecular hydrogen bonding interactions between them. Hydrogen bond strengthening in the excited state (S<inf>1</inf>) has been verified by shorter bond distances and redshift of IR vibrational spectra involving the proton transfer (PT) process. Furthermore, on-the-fly excited state dynamics simulations of all complexes have been performed to provide detailed information on the PT mechanism. The dynamic results show that one water molecule added to the neighboring PPP as an intermolecular hydrogen bonding bridge can promote double excited state intermolecular proton transfer up to 36% compared with its intrinsic intramolecular hydrogen bond of the PPP system. Meanwhile adding a second or third water molecule could decrease the probabilities of PT. Especially in PPP(H<inf>2</inf>O)<inf>3</inf>, a rearrangement of water molecules is observed that PT occurs via one water molecule. Hence, the models with explicit water molecules interacting with PPP as intermolecular hydrogen bonding bridge are a great representative role played by water molecules of the PT process at the molecular level.
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Dynamics simulation of excited-state proton transfer reactions of 8-hydroxyquinoline with water clusters: A TD-DFT study
(2023-08-01)
Kerdpol, Khanittha
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Daengngern, Rathawat
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Kungwan, Nawee
Detailed pictures of the excited-state proton transfer reactions of 8-hydroxyquinoline (8HQ) and its small clusters of water have been systematically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods at B3LYP/TZVP and TD-B3LYP/TZVP level of theory, respectively. Intramolecular hydrogen bond (intraHB) and intermolecular hydrogen bond (interHB) interactions between 8HQ and water cluster become stronger in the first excited state (S<inf>1</inf>), confirmed by hydrogen bond distances and topological analysis. In addition, TD-B3LYP dynamics simulations have been elucidated and revealed that excited-state intramolecular proton transfer (ESIntraPT) and multiply excited-state intermolecular proton transfer (ESInterPT) reactions of these complexes can take place in the ultrafast time scale of femtoseconds. Increasing the number of water molecules may give rise to a barrier energy that is anti-correlated to the probability of tautomer species from phototautomerization, especially in 8HQ-W<inf>1</inf> and 8HQ-W<inf>1+1</inf>. Moreover, water molecules initiate multiple proton transfer occurring through two- and three-step ESInterPT namely excited-state double proton transfer (ESDPT) and excited-state triple proton transfer (ESTPT) in which the interHBs are formed by rearrangement of water molecules of 8HQ-W<inf>2</inf>. This circumstance takes a longer time (up to 470 fs) compared to that of ESIntraPT. Thus, the simulated results from this study could provide insight into ESIntraPT and ESInterPT of phototautomerization and the significant photodynamics which could not be found from the experimental aspects of 8HQ and its solvent clusters.
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Effect of water microsolvation on the excited-state proton transfer of 3-hydroxyflavone enclosed in -cyclodextrin
(2021-02-02)
Kerdpol, Khanittha
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Daengngern, Rathawat
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Sattayanon, Chanchai
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Namuangruk, Supawadee
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Rungrotmongkol, Thanyada
The effect of microsolvation on excited-state proton transfer (ESPT) reaction of 3-hydroxyflavone (3HF) and its inclusion complex with -cyclodextrin (-CD) was studied using computational approaches. From molecular dynamics simulations, two possible inclusion complexes formed by the chromone ring (C-ring, Form I) and the phenyl ring (P-ring, Form II) of 3HF insertion to -CD were observed. Form II is likely more stable because of lower fluctuation of 3HF inside the hydrophobic cavity and lower water accessibility to the encapsulated 3HF. Next, the conformation analysis of these models in the ground (S0) and the first excited (S1) states was carried out by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, respectively, to reveal the photophysical properties of 3HF influenced by the -CD. The results show that the intermolecular hydrogen bonding (interHB) between 3HF and -CD, and intramolecular hydrogen bonding (intraHB) within 3HF are strengthened in the S1 state confirmed by the shorter interHB and intraHB distances and the red-shift of O-H vibrational modes involving in the ESPT process. The simulated absorption and emission spectra are in good agreement with the experimental data. Significantly, in the S1 state, the keto form of 3HF is stabilized by -CD, explaining the increased quantum yield of keto emission of 3HF when complexing with -CD in the experiment. In the other word, ESPT of 3HF is more favorable in the -CD hydrophobic cavity than in aqueous solution.
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Local structure elucidation and reaction mechanism of light naphtha aromatization over Ga embedded H-ZSM-5 zeolite: Combined DFT and experimental study
(2020-10-15)
Wongnongwa, Yutthana
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Kidkhunthod, Pinit
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Sukkha, Usa
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Pengpanich, Sitthiphong
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Thavornprasert, Kaew arpha
Local structures and mechanisms for n-pentane aromatization on Ga embedded H-ZSM-5 zeolite (Ga/ZSM-5) were elucidated using Synchrotron-based X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations to understand the role of Ga/ZSM-5 zeolite in aromatics synthesis. XAS data suggests that Ga ligates with four oxygen or four hydrogen atoms. Catalytic tests results suggest that conversion by Ga/ZSM-5 catalyst cannot occur via C6–C8 non-aromatic intermediates, while the availability of Ga metal sites promotes the aromatization of C2 and C3 species. Therefore, conversion of n-pentane to benzene or toluene comprises four steps, i) cracking, ii) GaH<inf>2</inf> activation, iii) cyclization, and iv) dehydrogenation. Our model predicts the key intermediate in n-pentane aromatization on Ga/ZSM-5 zeolite to be a five-membered Ga-C4 ring structure. The ring undergoes expansion to form a seven-membered Ga-C6 ring. Moreover, we discuss thermodynamics and kinetic results for the benzene and toluene formation pathways. Our results provide new finding for the role of Ga/ZSM-5 zeolites in n-pentane aromatization processes.
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Nuclear quantum and H/D isotope effects on three-centered bonding diborane: Path integral molecular dynamics simulations
(2020-05-15)
Daengngern, Rathawat
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Kobayashi, Osamu
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Kungwan, Nawee
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Ngaojampa, Chanisorn
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Tachikawa, Masanori
Nuclear quantum and H/D isotope effects of bridging and terminal hydrogen atoms of diborane (B<inf>2</inf>H<inf>6</inf>) molecules were systematically studied by classical ab initio molecular dynamics (CLMD) and ab initio path integral molecular dynamics (PIMD) simulations with BHandHLYP/6-31++G** level of theory at room temperature (298.15 K). Calculated results clearly show that H/D isotope effect appears in the distribution of hydrogen (deuterium) of B<inf>2</inf>H<inf>6</inf> (B<inf>2</inf>D<inf>6</inf>). Geometry of B<inf>2</inf>H<inf>6</inf> also plays a significant role in the nuclear quantum effect proved by PIMD simulations, but slightly deviated from its equilibrium structure when simulated via CLMD simulation. The bond lengths between boron atoms R (B1 … B2) and the bridging hydrogen atoms R<inf>HH</inf> (H<inf>B</inf>1 … H<inf>B</inf>2) of the B<inf>2</inf>H<inf>6</inf> molecule obtained from PIMD simulations are slightly longer than those of the deuterated form of the diborane (B<inf>2</inf>D<inf>6</inf>) molecule. The principal component analysis (PCA) was also employed to distinguish the important modes of bridging hydrogen as related to the nuclear quantum and H/D isotope effects. The highest level of contribution obtained from PCA of PIMD simulations is bending, while various mixed vibrations with less contribution were also found. Therefore, the nuclear quantum and H/D isotope effects need to be taken into account for a better understanding of diborane geometry.
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Excited-state intramolecular proton transfer reactions of 2,5-bis(2′-benzoxazolyl)hydroquinone and its water cluster exhibiting single and double proton transfer: A TD-DFT dynamics simulation
(2019-07-15)
Daengngern, Rathawat
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Salaeh, Rusrina
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Saelee, Tinnakorn
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Kerdpol, Khanittha
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Kungwan, Nawee
Detailed pictures of the excited-state intramolecular proton transfer (ESIPT)of 2,5-bis(2′-benzoxazolyl)hydroquinone (BHQ)and its water cluster have been investigated by dynamics simulations on the first lowest-excited energy using time-dependent density functional theory (TD-DFT). We focused on the structural, photophysical and dynamic properties of BHQ in the absence and presence of water molecules through intermolecular hydrogen bonds (interHBs). Our dynamics simulations reveal three possible mechanisms of the ESIPT processes: i)no proton transfer (No PT); ii)single PT (SPT); and iii)double PT (DPT), that could take place within the PT time of 160 fs via intrinsic intramolecular hydrogen bonds (intraHBs). The ESIPT mechanism of isolated BHQ elucidates that back PT is likely to be found at 64% rather than the SPT (32%)and DPT (4%), which is in good agreement with the experiments of dual fluorescence from di-enol and mono-keto emissions. Notably, the results from BHQ with water (BHQ-(H<inf>2</inf>O)<inf>2</inf>)reveal that the participation of water might produce a remarkable effect on promoting the SPT process up to 60% and DPT up to 7 times when compared to conditions of no water. The simulated probability of PT is well related to possible PT mechanisms regarding different tautomers in the fluorescence spectra found in previous experiments. The existence of di-keto tautomer arose from the DPT of BHQ and its water cluster and was not observed in the UV/Vis spectrum.
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Nitric oxide oxidation on warped nanographene (C80H30): a DFT study
(2019-01-01)
Roongcharoen, Thantip
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Kungwan, Nawee
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Daengngern, Rathawat
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Sattayanon, Chanchai
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Namuangruk, Supawadee
The possible use of the recently synthesized warped nanographene C<inf>80</inf>H<inf>30</inf> for NO oxidation by O<inf>2</inf> molecule has been investigated using density functional theory. The reaction starts with the adsorption and dissociation of O<inf>2</inf> molecule on the central pentagon of C<inf>80</inf>H<inf>30</inf> with the activation energies of 24.2–26.6 kcal/mol depending on the active sites. Then, the dissociated O atoms readily oxidize NO to NO<inf>2</inf> twice. The first NO oxidation occurs with barrierless, while the second NO oxidation requires a small energy barrier of 16.0 kcal/mol. The low activation energy barrier pathway indicates high catalytic activity of this nanographene for NO oxidation. Charge analysis reveals that such high catalytic activity of nanographene is attributed to the charge transfer from the saddle-shaped C<inf>80</inf>H<inf>30</inf> to the dissociated O atoms which makes it reactive to NO molecule. Desorption of NO<inf>2</inf> product, which is the rate-limiting step of NO oxidation in some catalysts, is easily occurred in this nanographene (less than 2 kcal/mol), indicating the prevention of catalyst poisoning. This study suggests that C<inf>80</inf>H<inf>30</inf> nanographene is a promising catalyst for NO removal in ambient condition.

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