Excited‐State Triple Proton Transfer of 7‐Hydroxyquinoline along a Hydrogen‐Bonded Alcohol Chain: Vibrationally Assisted Proton Tunneling

Kwon, Oh‐Hoon; Lee, Young-Shin; Yoo, Byung‐Kuk; Jang, Du‐Jeon

doi:10.1002/anie.200503209

Cited by 93 publications

(235 citation statements)

References 36 publications

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“…An intermediate, which has a large dipole moment, will manifest its presence in changes of the rates with polarity (9,13,16). At a given polarity, the determination of kinetic isotope effect (KIE) is also a powerful measure of the degree of concertedness, if truly dominant (19)(20)(21)(22). The results of such experiments, ''proton inventory'' (19)(20)(21)(22), are unique for concerted double-protontransfer reactions.…”

mentioning

confidence: 99%

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Kwon

Zewail

2007

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

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159

149

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The dynamics of excited-state double proton transfer of model DNA base pairs, 7-azaindole dimers, is reported using femtosecond fluorescence spectroscopy. To elucidate the nature of the transfer in the condensed phase, here we examine variation of solvent polarity and viscosity, solute concentration, and isotopic fractionation. The rate of proton transfer is found to be significantly dependent on polarity and on the isotopic composition in the pair. Consistent with a stepwise mechanism, the results support the presence of an ionic intermediate species which forms on the femtosecond time scale and decays to the final tautomeric form on the picosecond time scale. We discuss the results in relation to the molecular motions involved and comment on recent claims of concerted transfer in the condensed phase. The nonconcerted mechanism is in agreement with previous isolated-molecule femtosecond dynamics and is also consistent with the most-recent high-level theoretical study on the same pair.7-azaindole ͉ femtochemistry ͉ reaction dynamics ͉ tautomerization S ince the work by Watson and Crick on the determination of DNA structure in 1953 (1), proton-translocating tautomerization of base pairs has been suggested as a cause of mutations (2). Model systems with similar pair structures can be studied by photoinducing proton transfer (3, 4), and such studies are helpful for understanding the dynamics of mutagenesis (5).In this regard, the dimers of 7-azaindole (7-AI) molecules are prototypical because they are structurally similar to the H-bonded adenine-thymine (A-T) pair with two bonds; the guanine-cytosine (G-C) base pair has three H-bonds. Following the first observation of excited-state double proton transfer in 7-AI by Taylor et al. in 1969 (6), a number of researchers have studied its characterization in the dimers and also of 7-AI monomers catalyzed by various H-bonded counterparts of protic guest molecules (ref. 7 and references therein). One of the fundamental issues, with important implications, is whether the transfer process (8-17) of the two hydrogens proceeds in a concerted manner with a single transition state, or through a stepwise pathway by forming an intermediate species, as shown in Fig. 1.The first report of the dynamics of double proton transfer in 7-AI in the isolated molecule (molecular beam) was published in 1995 (8). The pair was excited with a femtosecond pulse, and another pulse, at different time delays, was invoked to observe the mass spectra. The parent ion showed a biexponential behavior, even near the zero-point energy, and the time constants of both decays changed significantly, by a factor of more than eight, with isotopic substitution. These two time constants (650 fs and 3.3 ps) were found to decrease with the excess excitation energy, as more vibrational modes (18) The mechanism has been discussed in a number of theoretical papers. Ab initio configuration interaction singles calculations showed that the electronic excitation was localized on one moiety in the 7-AI dimer, making the t...

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mentioning

confidence: 99%

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Kwon

Zewail

2007

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

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159

149

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“…It should be noted that the GSPT of 7HQ·(ROH) 2 is much slower than the ESPT or decay of the S 1 state. For example, in the case of 7HQ·(EtOH) 2 of in heptane solution, the ESPT time and the fluorescence lifetime of the keto form are 82 ps and 2.3 ns, respectively, [10] while the GSPT time is 2.0 μs. [15] Thus, any nonequilibrium effects are negligible for the GSPT of 7HQ·(ROH) 2 in the solution.…”

Section: Differences In Gspt Reaction In An Isolated Cluster and Inmentioning

confidence: 99%

“…Because of the high complexity of such systems, simple models have been extensively studied to understand the mechanism and dynamics of multiple PT in various environments at the molecular level. For example, 7-azaindole [1][2][3][4][5][6] and 7-hydroxyquinoline (7HQ) [7][8][9][10][11][12][13][14][15][16] have been investigated, both experimentally and theoretically, as biomimetic models. When excited by UV irradiation, these compounds are converted to their tautomers via multiple PT mediated by protic molecules such as H 2 O, alcohol, and NH 3 .…”

Section: Introductionmentioning

confidence: 99%

“…[7] Since then, the ESPT of 7HQ has been extensively studied in various environments such as supersonic jets, [8] alcohol solutions, [9][10][11] alcoholhydrocarbon mixed solutions, [9,10,12] and water nanopools in reverse micelles. [13] The precursor of alcohol-mediated ESPT is a cyclic H-bonded complex, 7HQ·(alcohol) 2 , in which one 7HQ and 2 alcohol molecules are connected by 3 H-bonds, as shown in Scheme 1.…”

Section: Introductionmentioning

confidence: 99%

“…The ground-state PT (GSPT) of 7HQ·(alcohol) 2 has also been studied in solution. [9,10,14,15] Park et al studied the kinetics of the GSPT for 7HQ·(ROH) 2 , where R = methyl, ethyl, isopropyl, or tert-butyl, in heptane solution to determine the rate constants and H/D kinetic isotope effect (KIE). [15] They suggested that a reorganization of the complex to a suitable configuration precedes the intrinsic PT process and that the observed GSPT kinetics is partially governed by the preceding configuration optimization process.…”

Section: Introductionmentioning

confidence: 99%

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Reaction pathway and H/D kinetic isotope effects of the triple proton transfer in a 7‐hydroxyquinoline‐methanol complex in the ground state: A computational approach

Mori

2017

J of Physical Organic Chem

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Multiple proton transfer (PT) is an essential process in long-range proton transport such as proton relay in enzymes. 7-Hydroxyquinoline (7HQ) undergoes alcohol-mediated PT in both the excited and ground states, and this system has been investigated as a biomimetic model. In the present study, the reaction pathway of triple PT in a 7HQ-methanol cluster, 7HQ·(MeOH) 2 , in the ground state has been investigated using density functional theory calculations to clarify the reaction mechanism and the origin of the experimentally observed kinetic isotope effect (KIE). The PT takes place in an asynchronous concerted fashion, in which the oxygen atom in 7HQ first accepts a proton from the directly hydrogen-bonded MeOH. The rate constants and primary H/D KIEs have been estimated with canonical variational transition state theory in combination with the small curvature tunneling approximation. The tunneling effect on the PT rate is significant, and the KIE is much greater than 1 at room temperature. The rule of the geometric mean for the KIEs breaks down because of the asynchronicity in the motions of 3 protons and tunneling effect. In addition, the rate constant is smaller, the KIE is larger, and the activation energy is higher compared with the experimental values in heptane solution, suggesting that the PT dynamics in solution is governed by not only the intrinsic PT process but also thermal fluctuation of the solute and solvent molecules, which plays an important role in the configurational change of the 7HQ·(MeOH) 2 complex.

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Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

Hsieh

Jiang

Chou

2010

Hydrogen Bonding and Transfer in the Excited State

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Proton transfer represents one of the most fundamental processes involved in chemical reactions as well as in living systems [1]. Vast numbers of scientists have been participating in relevant research, leading to thousands upon thousands of publications and numerous books on the topic. Accordingly, various types of proton transfer reaction, depending on, for example, the reaction in the ground or excited state, adiabatic versus non-adiabatic, strong and weak hydrogen bonding, acidity (proton donor) and basicity (proton acceptor), have been identified [2][3][4][5][6][7][8]. At this time, proton transfer reactions seem to have the potential for unlimited extensions and aspects in terms of fundamentals and applications. Thus, to cover the broad spectrum of proton transfer research within a limited number of pages seems to be an unachievable task. Rather than trying to achieve the impossible, this chapter focuses mainly on areas related to excited-state proton transfer (ESPT) [9] with pre-existing intramolecular hydrogen bonds. Under this constraint, the photoinduced process involving excited-state proton dissociation and/or protonation in bulk solvents or clusters, also a currently active topic that has received considerable attention [10][11][12][13][14], unfortunately cannot be addressed in this chapter.One particular focus of this chapter is bifunctional molecules possessing a proton donor group and a proton acceptor group, such that ESPT takes place via self-associated hydrogen-bonded (HB) dimers and/or solvent bridge relay. In spite of numerous explorations and elegant research on guest-molecule-assisted ESPT reactions, the results of which have provided great insight into the fundamentals as well as perspectives in applications,

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Excited‐State Triple Proton Transfer of 7‐Hydroxyquinoline along a Hydrogen‐Bonded Alcohol Chain: Vibrationally Assisted Proton Tunneling

Cited by 93 publications

References 36 publications

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Reaction pathway and H/D kinetic isotope effects of the triple proton transfer in a 7‐hydroxyquinoline‐methanol complex in the ground state: A computational approach

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

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