Theoretical study of the excited-state double proton transfer in the (3-methyl-7-azaindole)-(7-azaindole) heterodimer (original) (raw)
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Journal of Photochemistry and Photobiology A: Chemistry, 2010
The rule of the geometric mean in rates and kinetic isotope effects has long been used as a criterion for identifying the reaction mechanism, e.g., stepwise vs. concerted, for double proton transfer reactions. Potential energy surfaces of double proton transfers for excited state tautomerization in a 1:1 7-azaindole:H 2 O complex were generated at the MRPT2//CASSCF(10,9)/6-31G(d,p) level. Variational transition state theory, including multidimensional tunneling approximation, was used to calculate rates and kinetic isotope effects. No intermediates were present along the reaction coordinate. Two protons in the excited state tautomerization were transferred concertedly, albeit asynchronously. Positions of the variational transition states depend very much on the isotopic substitution. The asynchronicity of two protons in flight breaks the underlying assumption of the rule of the geometric mean so that the relation, k HH /k HD ≈ k HD /k DD , is no longer valid in the excited state double proton transfer. Breakdown of the geometric mean rule does not necessarily entail that the reaction mechanism is stepwise; therefore this rule should be used very carefully as a criterion for identification of the mechanism.
The Journal of Physical Chemistry, 1993
Theoretical Golden Rule treatment of the dynamics of triplet and singlet excited-state intramolecular proton transfer (ESIPT) in 2-(2'-hydroxyphenyl)benzoxazole (HBO) and its derivative 2-(2'-hydroxy4'-methylphenyl)benzoxazole (MHBO) is presented. These compounds express typical and very similar non-Arrhenius temperature behavior of the rate of triplet keto-enol equilibration BI = ~K -E + kE-K despite the experimental evidence of different reaction regimes and keto-enol energy gaps. In the method applied, all modes of the transferred atom (stretching, bending, and twisting) are taken into account, including an effective intramolecular promoting mode coupled to the H-motion, which is mainly responsible for the temperature dependence of the rate constant. Input data for the dynamic calculations is the standard output of the AM1 (structural and force field) results for both tautomers; the electronic coupling integral J i s an adjustable parameter. The calculated rate 01 is only slightly sensitive to the two experimentally suggested arrangements of the enol and keto triplet states (hE = EK -EE = 0 and 9.9 kJ/mol for HBO and MHBO, respectively), as experimentally observed, and is in good agreement with the experimental results for both isotopes. The study of the singlet ESIFT in these compounds suggests that the primary reason for the rapidity of the process, as well as for the distinctions between the triplet and singlet transfer, is in electronic factors, mainly the electronic coupling.
Excited State Proton Transfer in 3-Methyl-7-Azaindole Dimer. Symmetry Control
The Journal of Physical Chemistry A, 2006
The concerted double proton transfer undergone by the C 2h dimer of 7-azaindole upon electronic excitation has also been reported to occur in 3-methyl-7-azaindole monocrystals and in dimers of this compound under free-jet conditions. However, the results obtained in this work for the 3-methyl-7-azaindole dimer formed in a 10-4 M solution of the compound in 2-methylbutane suggest that the dimer produces no fluorescent signal consistent with a double proton transfer in the liquid phase or in a matrix. In this paper, the spectroscopic behavior of the doubly hydrogen bonded dimer of 3-methyl-7-azaindole is shown to provide a prominent example of molecular symmetry control over the spectroscopy of a substance. This interpretation opens up a new, interesting research avenue for exploring the ability of molecular symmetry to switch between protontransfer mechanisms. It should be noted that symmetry changes in the 3-methyl-7-azaindole dimer are caused by an out-of-phase internal rotation of the two methyl groups
Excited-state triple proton transfer (ESTPT) reactions in 7-azaindole (7AI) complexed with two water, with one water and one methanol, and with two methanol molecules were investigated by dynamics simulations in the first excited state computed with the second order algebraic-diagrammatic construction (ADC (2)) method. The results show that photoexcitation may trigger ultrafast an asynchronous concerted proton transfer via two solvent molecules along an intermolecular hydrogen-bonded network. The probability of occurrence of ESTPT ranges from 32% for 7AI(H 2 O-MeOH) to 64% for 7AI(MeOH) 2 . The average time for completing the ESTPT varies between 58 and 85 fs depending on the complex. The proton transfer (rather than hydrogen transfer) nature of the reaction was suggested by the nonexistence of crossings between the * and * states.
Excited-state double proton transfer in heterodimers of 1-azacarbacole
Chemical Physics Letters, 1982
In hctcrodlmers o! I-azac~rbuole \rlth 7-azmndolc cwdcd-stJte double proton ~mnsfcr occurs. The resulting klUlOllleI SIIO\\S rl lluorcscence band WIIII a maximum at 5 IO nm TIIC ground-state assocldhon constanl for thC formalIon of I-mcarb,xolc/7-azaindole lwtcrodlmers In 3-mclhylpcntanc at 20°C. .IS dctcrmrncd from tluorexencc mIenshy data. is KC] = 3500 * 5011 W'. 20000 cm-l, their green emrssion has been called F, Ruorescence. The tautomeric forms Tee, Ttt. and T,, are formed from the excited dtmers m an adiabatic reaction on the excited state surface (D& + T&, Di', + Til, DE,-f T~I) mvolving a double proton
Molecules
Excited state intramolecular proton transfer (ESIPT) is a photoinduced process strongly associated to hydrogen bonding within a molecular framework. In this manuscript, we computed potential energy data using Time Dependent Density Functional Theory (TDDFT) for triphenylsubstituted heterocycles, which evidenced an energetically favorable proton transfer on the excited state (i.e., ESIPT) but not on the ground state. Moreover, we describe how changes on heterocyclic functionalities, based on imidazole, oxazole, and thiazole systems, affect the ESIPT process that converts an enolic species to a ketonic one through photon-induced proton transfer. Structural and photophysical data were obtained theoretically by means of density functional theory (DFT) calculations and contrasted for the three heterocyclics. Different functionals were used, but B3LYP was the one that adequately predicted absorption data. It was observed that the intramolecular hydrogen bond is strengthened in the excited state, supporting the occurrence of ESIPT. Finally, it was observed that, with the formation of the excited state, there is a decrease in electronic density at the oxygen atom that acts as proton donor, while there is a substantial increase in the corresponding density at the nitrogen atom that serves as proton acceptor, thus, indicating that proton transfer is indeed favored after photon absorption.
Proceedings of The National Academy of Sciences, 1999
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.