Photophysics of 7-Azaindole, Its Doubly-H-Bonded Base-Pair, and Corresponding Proton-Transfer-Tautomer Dimeric Species, via Defining Experimental and Theoretical Results

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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.

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

confidence: 85%

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

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“…The method executed in a previous set of papers (2)(3)(4)(5)(6) and in the companion paper (1) allows us to propose an alternative mechanism for the double-proton transfer of the classic 7AI dimer (see below). The method used is based on the following theoretical levels of calculation.…”

Section: Theoretical Methodsmentioning

confidence: 99%

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: A model for the secondary evolution of the classic C _2h dimer and comparison of four mechanisms

Catalán

Pérez

Valle

et al. 2002

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A mechanism is proposed for the formation in gas phase, during a short time, of the delicately symmetrical coplanar C 2h classic 7-azaindole (7AI) doubly hydrogen-bonded dimer. Of the five card-pack or otherwise random geometry structures most likely to be formed in the supersonic jet expansion molecular beam, none would be an obvious precursor to the C 2h dimer. One unstable dimer with dipole-dipole, van der Waals, and plane-to-plane hydrogen bonding is shown to be capable of unhinging about the hydrogen-bond pair as an axis, from 0°to 90°to 180°, yielding a deep minimum for the C 2h structure with its delicate geometry and symmetry. This relaxation mechanism is feasible in the 3-s interval between the nozzle escape and the first laser pulse interception of the molecular beam. In the second part of the paper four published mechanisms are compared for concerted vs. two-step biprotonic phototransfer for the 7AI dimers. The dependence of the latter two models on H-atom instead of proton-transfer as an intermediate step negates the mechanism in a singlet (,*) electronic state by the valency repulsion, in the 3-electron orbital that would be generated. The concerted mechanism for biprotonic phototransfer is reaffirmed by the analysis of the quantum mechanical conditions set on the biprotonic transfer in the photo-excited molecular 7AI pair. M olecular pairs in valency, H-bonding, or close van der Waals interaction upon excitation are subject to in-phase perturbation by the photon electromagnetic field, in most cases the electric-dipole component. The resultant excitation is perforce coherent. In a doubly H-bonded pair such as the classic C 2h dimer of 7-azaindole (7AI), the coherent excitation induces several simultaneity responses. The molecular pair exhibits (i) excitonic state splitting, (ii) a shift of electron density within the framework of each molecule, resulting in synchronous acid-base changes at opposite ends of the molecule, and (iii) intermolecular acceptance vs. ejection with respective coordinated ejection vs. acceptance of protons in the partner molecule. These simultaneity principles were presented in the companion paper (1). In the research on (7AI) 2 photo-induced biprotonic transfer, the final molecular (7AI) product appears as the classic C 2h 7AI tautomer pair. This is a structure of snowflake-like symmetric delicacy. It is coplanar, centrosymmetric, and is in the C 2h symmetry point group, the most symmetric of the set of dimers studied. Upon adiabatic expansion by means of a supersonic nozzle with carrier gas, cooling the warm reservoir vapor to 7 K or less, dimerization can occur. In the preceding paper (1) we have shown calculations of six stable dimers with significant binding energies; five of them are either card-pack structures or rather random geometry dimers easily formable upon supersonic expansion.Although the calculations show the C 2h dimer to be the most stable of the six dimer structures considered, it seems most unlikely that the spatial orientation requirements of this rather ...

“…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 two different interpretations for femtosecond dynamics of the 7-azaindole dimer in solution triggered very intense debates, and the mechanism of the double proton transfer has been a central issue of the discussion (17,(24)(25)(26)(27)(28)(29)(30)(31)(32)(33). Detailed theoretical calculations for the excited-state potential energy surface along the proton transfer coordinate have been performed to examine the existence of a local minimum corresponding to the intermediate species (24,30,(34)(35)(36).…”

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

145

115

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...