The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution

Takeuchi, Satoshi; Tahara, Tahei

doi:10.1073/pnas.0610141104

Cited by 146 publications

(124 citation statements)

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“…The lowest excited triplet 1 3 A ′ π-π * has an L a character as it has been assumed in the experimental study as well as predicted in the earlier theoretical studies. Phosphorescence emission of azaindole in the argon matrix shows a sharp S 0 ← T 1 0-0 band at 3 eV.…”

Section: Sac-cisupporting

confidence: 58%

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

Arulmozhiraja

Coote

Hasegawa

2015

The Journal of Chemical Physics

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Articles you may be interested inElectronic spectra and photodissociation of vinyl chloride: A symmetry-adapted cluster configuration interaction study J. Chem. Phys. 124, 034312 (2006) Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study Electronic structures of azaindole were studied using symmetry-adapted cluster configuration interaction theory utilizing Dunning's cc-pVTZ basis set augmented with appropriate Rydberg spd functions on carbon and nitrogen atoms. The results obtained in the present study show good agreement with the available experimental values. Importantly, and contrary to previous theoretical studies, the excitation energy calculated for the important n-π * state agrees well with the experimental value. A recent study by Pratt and co-workers concluded that significant mixing of π-π * and n-π * states leads to major change in the magnitude and direction of the dipole moment of the upper state vibrational level in the 0,0 + 280 cm −1 band in the S 1 ← S 0 transition when compared to that of the zero-point level of the S 1 state. The present study, however, shows that all the four lowest lying excited states,n-π * , and π-σ * , cross each other in one way or another, and hence, significant state mixing between them is likely. The upper state vibrational level in the 0,0 + 280 cm −1 band in the S 1 ← S 0 transition benefits from this four-state mixing and this can explain the change in magnitude and direction of the dipole moment of the S 1 excited vibrational level. This multistate mixing, and especially the involvement of π-σ * state in mixing, could also provide a route for hydrogen atom detachment reactions. The electronic spectra of benzimidazole, a closely related system, were also investigated in the present study. C 2015 AIP Publishing LLC. [http://dx

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Section: Sac-cisupporting

confidence: 58%

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

Arulmozhiraja

Coote

Hasegawa

2015

The Journal of Chemical Physics

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“…Electronic excitation reduces pK a , reshapes the energy surface, and lowers the tautomerization barrier, thus leading to much faster proton transfer rates than in the ground state (15). For example, photo-initiated ESPT in 7-azaindole/alcohol complexes occurs within ∼100 ps with negligible barrier (<1 kcal/mol) (17), and the double-proton transfer in 7-azaindole dimer proceeds with a 1.1-ps time constant (18).…”

Section: Resultsmentioning

confidence: 99%

“…However, they still suffer from certain limitations, including the difficulty of interpreting variations in electronic spectra, the inclusion of pH indicators, and questions about reactive species that may be created by the capacitive discharge. The more recent studies to capture proton transfer dynamics have used ultrafast photo-excitation to induce tautomerization on the excited electronic state (15)(16)(17)(18)(19)(20). This has proven a fruitful test bed for dynamics, but experiments in roomtemperature aqueous solution would more accurately report on physiological tautomerism.…”

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confidence: 99%

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Peng

Baiz

Tokmakoff

2013

Proc. Natl. Acad. Sci. U.S.A.

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We provide a systematic characterization of the nanosecond groundstate lactam-lactim tautomerization of pyridone derivatives in aqueous solution under ambient conditions using temperature-jump transient 2D IR spectroscopy. Although electronic excited-state tautomerization has been widely studied, experimental work on the ground electronic state, most relevant to chemistry and biology, is lacking. Using 2D IR spectroscopy, lactam and lactim tautomers of 6-chloro-2-pyridone and 2-chloro-4-pyridone are unambiguously identified by their unique cross-peak patterns. Monitoring the correlated exponential relaxation of these signals in response to a laser temperature jump provides a direct measurement of the nanosecond tautomerization kinetics. By studying the temperature, concentration, solvent, and pH dependence, we extract a thermodynamic and kinetic characterization and conclude that the tautomerization proceeds through a two-state concerted mechanism. We find that the intramolecular proton transfer is mediated by bridging water molecules and the reaction barrier is dictated by the release of a proton from pyridone, as would be expected for an efficient Grothusstype proton transfer mechanism.keto-enol tautomerism | multidimensional | time-resolved spectroscopy | ultrafast T automerism of aromatic heterocycles has been extensively studied owing to its importance in biochemical processes such as enzyme catalysis (1), ligand binding (2), and spontaneous mutagenesis (3). To characterize the tautomeric equilibria, researchers have used techniques such as UV absorption (4), circular dichroism (5), X-ray crystallography (3), NMR (6), Raman (7), and IR absorption spectroscopy (8). However, these methods face several challenges to characterizing thermodynamic and kinetic data for tautomeric equilibria and exchange processes. For example, electronic spectra are broad and featureless, which complicates spectral interpretation, particularly for systems with multiple tautomers. Although NMR provides excellent structural resolution, it can only directly monitor chemical exchange in real time on millisecond and longer time scales, rather than the picosecond to nanosecond time scales expected for proton transfer under physiological conditions. From the theoretical point of view, quantum mechanical calculations have been performed extensively to characterize the relative stability of tautomeric systems, their activation barriers, and the reaction mechanisms (9, 10); nevertheless, experimental validation of these predictions is scarce. To provide a characterization under ambient aqueous conditions, we need a technique with both the structural sensitivity to unambiguously identify various tautomers and the time resolution to probe the rapid proton transfer dynamics. To this end, we demonstrate the capability of 2D IR spectroscopy coupled with a nanosecond temperature-jump (Tjump) laser to reveal the nonequilibrium lactam-lactim tautomerization kinetics of pyridone derivatives.Time-resolved studies of tautomerism have a long and storie...

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“…It is interesting to note the recent experimental study of Takeuchi and Tahara 20 for the double-proton-transfer in the 7AI dimer in solution, in which the 7AI dimer converts to the tautomer configuration in the excited state. They investigated this process by excitation wavelength dependence in steadystate and femtosecond time-resolved fluorescence spectroscopy and concluded that it proceeds in a concerted manner.…”

Section: Iiia Energetics and Dynamics Of Tautomerization In The 7aimentioning

confidence: 99%

Ab Initio QM/MM Molecular Dynamics Study on the Excited-State Hydrogen Transfer of 7-Azaindole in Water Solution

et al. 2008

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Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI−(H2O)n; n = 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI−H2O and 7AI−(H2O)2 clusters show an asynchronous hydrogen transfer at t ∼ 50 fs after the photoexcitation. While the ESHT mechanism for 7AI−H2O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI−(H2O)2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S1 state, 7AI and (H2O)n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located. Disciplines Chemistry CommentsReprinted (adapted) ReceiVed: May 17, 2008; ReVised Manuscript ReceiVed: July 16, 2008 Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI-(H 2 O) n ; n ) 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI-H 2 O and 7AI-(H 2 O) 2 clusters show an asynchronous hydrogen transfer at t ∼ 50 fs after the photoexcitation. While the ESHT mechanism for 7AI-H 2 O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI-(H 2 O) 2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S 1 state, 7AI and (H 2 O) n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located.

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The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution

Cited by 146 publications

References 47 publications

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

Electronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Ab Initio QM/MM Molecular Dynamics Study on the Excited-State Hydrogen Transfer of 7-Azaindole in Water Solution

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