Spectroscopy of doubly hydrogen-bonded 7-azaindole. Reinvestigation of the excited state reaction

Bulska, Hanna; Grabowska, Agnieszka; Pakuła, B.; Sepioł, Jerzy; Waluk, Jacek; Wild, Urs P.

doi:10.1016/0022-2313(84)90042-5

Cited by 44 publications

(96 citation statements)

References 25 publications

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“…17). Both Zewail's group (4-6) and Castleman's group (7,8) use a femtosecond laser pulse at 305-310 nm, bracketing (32,258 cm Ϫ1 ) exactly the dipoleallowed molecular exciton band at 32,290 cm Ϫ1 , yet they do not recognize the simultaneity of the two-molecule excitation implied by that frequency in their mechanism of ''single-step'' hydrogen transfer. Second, the molecular orbital changes (HOMO͞LUMO; highest occupied molecular orbital͞lowest unoccupied molecular orbital) upon excitation at the pyridino-N site and at the pyrrolo-N site (3) offer dramatic indication of the simultaneity (concertedness) of the biprotonic transfer driving force in the dimer.…”

Section: The Three Defining Simultaneity Principles Of Photo-induced mentioning

confidence: 99%

“…This dimer certainly seems to be the main stable one observed in equilibrium conditions at 298 K in solution studies, and the essential one by which to observe biprotonic transfer dimer f luorescence. Previously, a ''nonreactive'' (non-proton-transfer) dimer (15,16) and larger oligomeric forms of 7AI have been suggested (31)(32)(33). Fuke and Kaya (16) demonstrated that the relative amount of reactive and nonreactive dimers depends on the stagnation pressure of the carrier gas.…”

Section: Dimerization Modes Of 7ai In Adiabatic Nozzle Supercooled Momentioning

confidence: 99%

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The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

Catalán

Pérez

Valle

et al. 2002

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

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Six stable dimer models for 7-azaindole (including the classic C2h doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2 ab initio method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (i) intermolecular coherent excitation molecular exciton simultaneity, (ii) intramolecular acid-base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (iii) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic C2h dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.T he photoinduced biprotonic transfer in the classic 7-azaindole (7AI) hydrogen-bonded C 2h dimer (refs. 1 and 2 and references therein) seemingly has revealed contrasting mechanisms according to the experimental techniques applied. Electronic spectroscopic observations and quantum theoretical calculations require a concerted (3) one-step simultaneous intermolecular transfer of the two protons involved in the hydrogen bonding of the 7AI base pair. In contrast, femtosecond laser-pulsed photo-excitation followed by femtosecond laserpulsed ionization and then time-of-flight (TOF) mass spectrometry (4-6), or with giant laser pulse-induced coulomb explosion (7, 8) followed by TOF mass spectrometry of the resultant ions (and molecular fragments), has suggested to these researchers a two-step neutral H-atom transfer, based on kinetic analysis of the ion appearance and isotopic effects. We shall return to the question of neutral H-atom transfer vs. H ϩ (proton) transfer at the end of the companion paper (9).In the first part of this paper, we give an outline summary of the spectroscopic demands on simultaneity of the biprotonic transfer for the concerted transfer mechanism, and then in the second part we present an analysis of the structures and protontransfer potential functions and the energies of formation for the variety of 7AI stable dimers that could form at supersonic velocities in the adiabatic-nozzle supercooled molecular beam. This presentation is suggested as a pathway to resolving the conflict between the spectroscopic and the kinetic results. Background of 7AI Dimer Biprotonic TransferFor photo-induced biprotonic transfer we emphasize three strict requirements of simultaneity of action that must exist.

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Section: The Three Defining Simultaneity Principles Of Photo-induced mentioning

confidence: 99%

Section: Dimerization Modes Of 7ai In Adiabatic Nozzle Supercooled Momentioning

confidence: 99%

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

Catalán

Pérez

Valle

et al. 2002

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

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“…The doubly hydrogen-bonded dimer of 7-azaindole is a prototypical system showing such a reaction in the photoexcited state. This dimer has served as one of the most central model systems, and hence its photochemistry and photophysics have been extensively studied for more than three decades (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation ( Fig.…”

mentioning

confidence: 99%

“…The reactive nature of the 7-azaindole dimer in hydrocarbon solvent is manifested by its steady-state fluorescence spectrum, which shows significantly red-shifted fluorescence attributed to the tautomer fluorescence. The steady-state fluorescence spectra that were examined under various conditions provided information about photochemical properties of the relevant excited states (5,6,9,10). The proton transfer time was first reported as Ͻ5 ps in 1979 by picosecond fluorescence spectroscopy (14), and then it was determined as 1.4 ps by measuring the rise of the tautomer fluorescence with femtosecond time resolution (15).…”

mentioning

confidence: 99%

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

Takeuchi

Tahara

2007

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

145

115

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The dynamics and mechanism of the double proton transfer reaction of the 7-azaindole dimer was investigated in solution by excitation wavelength dependence in steady-state and femtosecond time-resolved fluorescence spectroscopy. Femtosecond measurements in the UV region revealed that the dynamics of the dimer fluorescence exhibits remarkable change as the excitation wavelength was scanned from 280 to 313 nm. The fluorescence showed a biexponential decay (0.2 and 1.1 ps) with 280-nm excitation, whereas it exhibited a single exponential decay (1.1 ps) with 313-nm excitation (the red-edge of the dimer absorption). This observation clearly indicates that the 0.2-ps component is irrelevant to the proton transfer. In the visible region, we found that the tautomer fluorescence rises in accordance with the decay of the dimer fluorescence with a common time constant of 1.1 ps. This finding unambiguously denies the appearance of any intermediate species in between the dimer and tautomer excited states, indicating that the double proton transfer reaction is essentially a single-step process. We conclude that the double proton transfer of the 7-azaindole dimer in solution proceeds in the concerted manner from the lowest excited state with the 1.1-ps time constant. On the basis of the experimental data obtained, we discuss the long-lasting concerted versus step-wise controversy for the double proton transfer mechanism in solution.femtosecond spectroscopy ͉ fluorescence ͉ ultrafast ͉ photochemistry E xcited-state proton transfer has been receiving a great deal of attention, because it plays crucial roles in photochemistry and photobiology (1-3). The doubly hydrogen-bonded dimer of 7-azaindole is a prototypical system showing such a reaction in the photoexcited state. This dimer has served as one of the most central model systems, and hence its photochemistry and photophysics have been extensively studied for more than three decades (4-18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation (Fig. 1) (4-7, 9, 10). The reactive nature of the 7-azaindole dimer in hydrocarbon solvent is manifested by its steady-state fluorescence spectrum, which shows significantly red-shifted fluorescence attributed to the tautomer fluorescence. The steady-state fluorescence spectra that were examined under various conditions provided information about photochemical properties of the relevant excited states (5, 6, 9, 10). The proton transfer time was first reported as Ͻ5 ps in 1979 by picosecond fluorescence spectroscopy (14), and then it was determined as 1.4 ps by measuring the rise of the tautomer fluorescence with femtosecond time resolution (15).A fundamental question relevant to this ultrafast double proton transfer is the cooperativity of the motion of the two protons. So far, it has been intensively argued whether the two protons move in a concerted or step-wise way. The head-on arguments on this mechanism were first made on the basis of time-resolved data for th...

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“…Since then, numerous studies have focused on the mechanisms of ESDPT in 7AI 2 in solution and matrix [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and in the gas phase [35][36][37][38][39][40][41][42][43][44][45][46][47] by using various spectroscopic techniques. Theoretical studies have been also carried out to investigate the hydrogen-bonded structures, potential energy curves (or surface), and molecular dynamics of 7AI 2 .…”

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confidence: 99%

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

Sekiya¹,

Sakota²

2006

Bulletin of the Chemical Society of Japan

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The 7-azaindole dimer (7AI 2 ) is a prototype of double hydrogen-bonded molecules. 7AI 2 has been considered as a model DNA base pair and has attracted much attention to the mechanism of the excited-state double-proton transfer (ESDPT). Two ESDPT mechanisms, stepwise and concerted mechanisms, have been proposed so far. Great efforts have been devoted to clarify the mechanism of ESDPT using experimental and theoretical methods. However, the reaction mechanism had been controversial for more than a decade. We provide the resolution of the two mechanisms on the basis of new data obtained from electronic spectroscopy and picosecond time-resolved spectroscopy in the gas phase. The initial state of the ESDPT reaction has been well characterized by investigating the exciton resonance interaction with UV-UV hole-burning spectroscopy for various 7AI 2 isotopomers. The lowest-excited state of 7AI 2 has been classified into the weak coupling case of the exciton theory. We have concluded that the ESDPT reaction in 7AI 2 occurs via the concerted mechanism on the basis of the results of picosecond time-resolved experiments and the H/D kinetic isotope effect on ESDPT studied by measuring the vibronic-state selective dispersed fluorescence spectra. ESDPT of 7AI 2 has a ''dynamic cooperative'' nature that may arise from the coupling of the two moving protons with the reorganization of electrons. We have provided a new paradigm of ESDPT, where two quantum effects, the exciton resonance interaction and the proton tunneling, are concerned with the ESDPT reaction.Proton-transfer reactions are one of the most basic reactions, and they are important in physics, chemistry, and biology. [1][2][3][4][5][6] Spectroscopic studies on various proton-transfer systems combining with theoretical studies have provided rich information about the hydrogen-bonded structure and proton-transfer dynamics. In particular, a very detailed picture has been obtained for intramolecular single-proton transfer in polyatomic molecules, with laser spectroscopic measurements in the gas phase combining with theoretical studies. For example, a proton transfers by quantum mechanical tunneling mechanism when a potential energy barrier exists on a potential energy surface. The proton tunneling was clearly observed in the electronic and vibrational spectra of several polyatomic molecules. [6][7][8][9][10] Spectroscopic data and theoretical studies showed that the proton tunneling has a multi-dimensional nature, i.e., the motion of the moving proton couples with the coordinates of the heavy atoms that constitute the molecule. Such a multi-dimensional nature and quantum mechanical tunneling are important properties that characterize proton-transfer reactions. Now much effort is devoted to obtain a more complete picture for singleproton transfer reactions and also a new picture for more complicated multiproton-transfer reactions. There are many examples of multiproton-transfer reactions such as proton relay systems in enzymes, in hydrogen-bonded water complexes, and prot...

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Spectroscopy of doubly hydrogen-bonded 7-azaindole. Reinvestigation of the excited state reaction

Cited by 44 publications

References 25 publications

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

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

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

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