The Ground and Excited State Hydrogen Transfer Potential Energy Surface in 7-Azaindole

Chaban, Galina M.; Gordon, Mark S.

doi:10.1021/jp9837838

Cited by 88 publications

(89 citation statements)

References 18 publications

(37 reference statements)

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“…It is expected that only one of the two moieties in the dimer becomes excited by the pump laser pulse (25). If this were the case, the single exponential decay times observed in our experiments for five to nine waters could result from selftautomerization of the excited 7-Aza moiety by means of a water molecule (26,27) acting as a catalytic proton bridge (28). Further experiments are under way to further elucidate this behavior.…”

Section: Resultsmentioning

confidence: 90%

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Folmer

Wisniewski

Hurley

et al. 1999

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

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Presented here are femtosecond pump-probe studies on the watersolvated 7-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392-398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer. C luster science studies have long been utilized to yield unique perspectives of microscopic properties related to the bulk condensed phase (1). This approach involves the investigation of a broad spectrum of clusters ranging from isolated species to the study of fully solvated species, thus illustrating the progression from the gas phase to the condensed phase. In reference to our specific experiments as an example, we have been able to study the excited state double-proton transfer (ESDPT) of the 7-azaindole (7-Aza) dimer under conditions ranging from an isolated dimer to a state of solvation where the hydrogen-bonded dimer is clustered with as many as nine water molecules. As the number of water molecules on the nonreactive dimer increases, we begin to see evidence that the dimer molecule is behaving more as it would in a fully solvated condensed-phase environment. In support of these findings, it has been reported that clusters with as little as six waters begin to show liquid-like properties (2-4).The model DNA base pair 7-Aza has proven to be an interesting and enlightening species for study in both the gas and condensed phases. Of utmost interest is the double-proton transfer that the 7-Aza dimer undergoes on excitation to the S 1 state. This ESDPT was first observed in solution by Kasha and coworkers (5). Later, Kaya and coworkers performed extensive supersonic jet spectroscopic studies on the 7-Aza monomer, dimer, and solvated forms of these species (6-9). The first direct determination of the rates of the double-proton transfer was made in the gas phase by Zewail and c...

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Section: Resultsmentioning

confidence: 90%

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Folmer

Wisniewski

Hurley

et al. 1999

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

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“…This and related experiments confirm the simultaneity requirement of concerted doubleproton PT in the S 1 excited state of 7-AI. Ab initio calculations on aquo-complexes of 7-AI have been carried out by Shukla and Mishra (21) and independently by Chaban and Gordon (22).…”

Section: Biprotonic Transfer In Solutionmentioning

confidence: 99%

Resolution of concerted versus sequential mechanisms in photo-induced double-proton transfer reaction in 7-azaindole H-bonded dimer

Catalán

Valle

Kasha

1999

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

118

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The experimental and theoretical bases for a synchronous or concerted double-proton transfer in centrosymmetric H-bonded electronically excited molecular dimers are presented. The prototype model is the 7-azaindole dimer. New research offers confirmation of a concerted mechanism for excited-state biprotonic transfer. Recent femtosecond photoionization and coulombic explosion techniques have given rise to time-of-f light MS observations suggesting sequential two-step biprotonic transfer for the same dimer. We interpret the overall species observed in the time-of-f light experiments as explicable without conf lict with the concerted mechanism of proton transfer.The doubly H-bonded dimer of 7-azaindole (7-AI) has been studied exhaustively as a model prototype for DNA base-pair tautomerization, as it is recognized to undergo a biprotonic transfer (1) on photoexcitation. A central issue in doubleproton transfer (PT) reactions is whether a sequential (stepwise) or a concerted mechanism is applicable at the two proton-donor, proton-acceptor sites (2). A stepwise mechanism requires a reaction potential energy curve having an intermediate minimum between the potential minimum for the normal tautomer species, and that of the PT tautomer species, so that a finite lifetime for the transient intermediate species could be observed (3). Recent femtosecond MS results on 7-AI dimer produced in adiabatic expansion (4, 5) had been reported in supercooled molecular beams produced by adiabatic jet expansion, claiming to have established by fast transient kinetics a two-step PT mechanism for photo-excited 7-AI, with apparent theoretical calculation corroboration (6). Another laboratory (7) has made a claim of arresting the intermediate involving a one-PT in 7-AI dimer via a coulomb explosion technique, also in supercooled molecular beam experiments. We believe these results have been misinterpreted, and because they already are being accepted in some quarters (2, 8) as proven, we present refined calculations and experimental results to verify the concerted biprotonic transfer mechanism for 7-AI. Both of the jet expansion molecular beam experiments use severely invasive techniques (photoionization, coulomb explosion) to produce cationic species as required in the time-of-flight (TOF)-MS detection. We show that these laser photoionization procedures introduce a major perturbation on the electronic states of the molecular systems involved, and in effect, to create the cationic molecular species assumed to be an intermediate.The development of femtosecond laser techniques now permits dynamic chemical events to be clocked at the picosecond and subpicosecond time scale. Consequently, a novel duality of criteria for molecular reaction mechanisms seems to have arisen: (a) the classical requirement with demonstration of an intermediate valley in a reaction potential energy curve, or (b) a (sub)picosecond observation of passage through an intermediate molecular configuration. Both of these could satisfy the Bridgman operational criteri...

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“…15, which is inconsistent with the fact that 7AI becomes acidic in the S 1 state upon π-π* electronic excitation, 37 and other high level electronic structure calculations. 2 This study shows that the ESDPT in the 1:1 7-azaindole:water complex occurs asynchronously to make an H3O + -like moiety at the TS.…”

Section: Resultsmentioning

confidence: 69%

“…In the trajectory of the AIMD simulations, the O-H proton moves first to the 6-membered ring followed by movement of the N-H proton from the 5-membered ring to water. However, the TS structures obtained by Chaban et al 2 and Kina et al 15 suggest that the N-H proton from the 5-membered ring moves first, and the water proton follows. The AIMD trajectory was far from the intrinsic reaction coordinate that might be one of the high-energy reaction paths, thus further investigation is required.…”

Section: Introductionmentioning

confidence: 97%

“…1 7AI has an N-H bond and a heteroaromatic N atom as hydrogen bond donor and acceptor site, respectively, and the normal form becomes unstable with respect to the tautomer form in the S1 state. 2,3 The monomer species of 7AI is capable of excited state double proton transfer (ESDPT) from the five-membered ring (donor site) to the six-membered ring (acceptor site) in a variety of environments, including a gas phase as well as solvent clusters when assisted by solvent molecules. Huang et al 4 studied the 1:1 7AI:water complex in the first excited state in cold beams and reported a fluorescence lifetime of 8 ns, which implies that the tautomerization rate constant of ESDPT cannot be larger than approximately 10 7 s -1…”

Section: Introductionmentioning

confidence: 99%

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Multiconfiguration Molecular Mechanics Studies for the Potential Energy Surfaces of the Excited State Double Proton Transfer in the 1:1 7-Azaindole:H_2OComplex

Han¹,

Kim

2010

Bulletin of the Korean Chemical Society

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The multiconfiguration molecular mechanics (MCMM) algorithm was used to generate potential and vibrationally adiabatic energy surfaces for excited-state tautomerization in the 1:1 7-azaindole:H2O complex. Electronic structures and energies for reactant, product, transition state were computed at the CIS/6-31G(d,p) level of theory. The potential and vibrationally adiabatic energies along the reaction coordinate were generated step by step by using 16 high-level Shepard points, which were computed at the CIS/6-31G(d,p) level. This study shows that the MCMM method was applied successfully to make quite reasonable potential and adiabatic energy curves for the excited-state double proton transfer reaction. No stable intermediates are present in the potential energy curve along the reaction coordinate of the excited-state double proton transfer in the 1:1 7-azaindole:H2O complex, indicating that these two protons are transferred concertedly. The change in the bond distances along the reaction coordinate shows that two protons move very asynchronously to make an H3O + -like moiety at the transition state.

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The Ground and Excited State Hydrogen Transfer Potential Energy Surface in 7-Azaindole

Cited by 88 publications

References 18 publications

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Resolution of concerted versus sequential mechanisms in photo-induced double-proton transfer reaction in 7-azaindole H-bonded dimer

Multiconfiguration Molecular Mechanics Studies for the Potential Energy Surfaces of the Excited State Double Proton Transfer in the 1:1 7-Azaindole:H_2OComplex

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The Ground and Excited State Hydrogen Transfer Potential Energy Surface in 7-Azaindole

Cited by 88 publications

References 18 publications

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Resolution of concerted versus sequential mechanisms in photo-induced double-proton transfer reaction in 7-azaindole H-bonded dimer

Multiconfiguration Molecular Mechanics Studies for the Potential Energy Surfaces of the Excited State Double Proton Transfer in the 1:1 7-Azaindole:H2OComplex

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Multiconfiguration Molecular Mechanics Studies for the Potential Energy Surfaces of the Excited State Double Proton Transfer in the 1:1 7-Azaindole:H_2OComplex