Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho-Methylanthrone

Gamarnik, Alla; Bise, Ryan T.; Pang, Lilis; Jenks, William S.

doi:10.1021/ja00146a011

Cited by 41 publications

(59 citation statements)

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“…Interestingly, a similar shift was observed in o-methylanthrone but not in a m-and p-dimethyl-substituted compound. 5 On the basis of the fact that a 10 cm -1 red shift was observed in the CdO stretch of the o-methyl compound, it was speculated that this shift may be associated with an intramolecular C-H‚‚‚O bond. 20,21 Similarly, a 5 cm -1 red shift was also observed in the CdO stretch of DMT as compared to that of tetralone with bands occurring at 1678 and 1685 cm -1 , respectively.…”

Section: Resultsmentioning

confidence: 99%

“…Low-temperature phosphorescence measurements of optically dilute samples with excitation at 340 nm were taken between 15 and 88 K as described previously. 5 Accumulation of the photoproduct at 14 K in ethanol and in methyltetrahydrofuran (MTHF) was accomplished by irradiation with a 250 W mercury lamp until a yellow discoloration assigned to the trapped photoenol was readily evident. Evidence for the photoproduct was obtained in MTHF by FTIR and in EtOH by fluorescence spectroscopy.…”

Section: Methodsmentioning

confidence: 99%

“…1 Large isotope effects and nonlinear Arrhenius plots have been observed, [2][3][4][5] and some theoretical studies have followed. 2,6 It has been proposed that tunneling occurs from n,π* excited states when the transferring hydrogen is favorably positioned relative to the singly occupied n-orbital.…”

Section: Introductionmentioning

confidence: 99%

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Deuterium Tunneling in Triplet 5,8-Dimethyl-1-tetralone by Phosphorescence Detection between 80 and 15 K

Johnson

Gamarnik

1996

J. Phys. Chem.

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Phosphorescence measurements with 5,8-dimethyl-1-tetralone (DMT) and 5,8-dimethyl-1-tetralone-d 8 (DMTd 8 ) in methylcyclohexane were carried out between 15 and 80 K to analyze their triplet-state hydrogen-and deuterium-transfer reactions. As expected for a decay process displaying a very large isotope effect, emission could be observed only for the deuterated compound. Phosphorescence intensities and decay measurements in the case of DMT-d 8 were analyzed to determine the rates of deuterium transfer which were used to build an Arrhenius plot where tunneling can be readily identified. A temperature-independent triplet state reaction rate (k D (QMT) ) (5 ( 1) × 10 2 s -1 ) below 40 K was assigned to quantum mechanical tunneling. Detection of the photoproduct after irradiation at 77 and 15 K was accomplished by fluorescence and by FTIR spectroscopy. Results obtained in this study are in excellent agreement with laser flash photolysis results reported by Al-Soufi et al. (Al-Soufi, W.; Grellmann, K. H. J. Phys. Chem. 1991, 95, 2022-2026.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Deuterium Tunneling in Triplet 5,8-Dimethyl-1-tetralone by Phosphorescence Detection between 80 and 15 K

Johnson

Gamarnik

1996

J. Phys. Chem.

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“…Notably, these complications are further exacerbated in the case of long‐lived excited states that are capable of delocalizing through the crystal by exciton delocalization mechanisms 17. Until recently, the simplest way to determine the absolute rates of excited state reactions in the solid state was by taking advantage of changes in emission yields and lifetimes caused by competing chemical decay pathways, which we developed and implemented for the study of hydrogen atom transfer by quantum mechanical tunneling 18–21…”

Section: Introductionmentioning

confidence: 99%

Photochemical reaction mechanisms and kinetics with molecular nanocrystals: surface quenching of triplet benzophenone nanocrystals

Simoncelli

Kuzmanich

Gard

2010

J of Physical Organic Chem

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Organic molecular nanocrystals suspended in water are useful when studying reactions that occur in the solid state because they retain not only the reactive and supramolecular properties of bulk crystals, but are also amenable to transmission spectroscopy. Having previously studied the triplet state of benzophenone nanocrystals by laser flash photolysis transmission spectroscopy, we now report nanosecond experiments in the presence of several possible quenchers: anionic and cationic surfactants, dissolved oxygen, and as a function of solvent deuteration (H2O and D2O). After finding these to have no effect, several anionic quenchers (I−, Br−, and N 3−) were tested by Stern–Volmer analysis. Significant correlation between the quenching rates in solution and in nanocrystals suggests that the electronic excitation is accessible to quenchers at the surface. Copyright © 2010 John Wiley & Sons, Ltd.

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“…Nuclear quantum effects often play an important role, as evidenced experimentally by the kinetic isotope effect (KIE), the ratio of rate constants for the hydrogen and deuterium forms of the reducing agent. Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13)(14)(15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).…”

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Colossal kinetic isotope effects in proton-coupled electron transfer

Huynh

Meyer

2004

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

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P roton-coupled electron transfer (PCET) reactions are at the heart of important biological processes, including photosynthesis and respiration (1-4). These reactions avoid high-energy intermediates through the concerted transfer of an electron and a proton. PCET is distinguished from H-atom transfer (HAT) in that the transferring electron and proton come from different sites in the reducing agent rather than from a single chemical bond (5-10). Nuclear quantum effects often play an important role, as evidenced experimentally by the kinetic isotope effect (KIE), the ratio of rate constants for the hydrogen and deuterium forms of the reducing agent. Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13-15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).In earlier work, we described a PCET reaction with a k 1 (H 2 O)͞k 1 (D 2 O) KIE of 175 at room temperature based on the reduction of benzoquinone (Q) by an osmium complex containing a phosphorus-hydrogen bond (18). We report here values of 198 and 455 for PCET reactions from sulfur and nitrogenhydrogen bonds based on an extended analysis of reactions reported earlier in preliminary accounts. 3.0 ϫ 10 Ϫ4 M to 8.0 ϫ 10 Ϫ2 M. The temperature of solutions during the kinetic studies was maintained to within Ϯ0.1°C with use of Lauda RM6 and Thermo Neslab RTE 7 circulating water baths and monitored with an Omega HH-51 thermocouple probe. All rate constants cited in this work are reported as the averages of at least three or more independent experiments.Cyclic voltammetric experiments were measured with the use of Princeton Applied Research (PAR) models 263A and 273 potentiostats, and bulk electrolyses were performed with a PAR model 173 potentiostat͞galvanostat. All measurements were conducted in a three-compartment cell in 1:1 (vol͞vol) CH 3 CN͞ H 2 O with 1.0 M NH 4 PF 6 as the supporting electrolyte. A glassy carbon working electrode was used for aqueous measurements. All potentials are referenced to the saturated sodium chloride͞ calomel electrode (SSCE, 0.236 V vs. the normal hydrogen electrode) at room temperature and are uncorrected for junction potentials. In all cases, the auxiliary electrode was a platinum wire. The solution in the working compartment was deoxygenated by N 2 or argon bubbling. All redox couple potentials are the average of four independent measurements and are Ϯ2 mV. Results and DiscussionThe kinetics of the reactions shown in Eqs. 1-3 were studied in 1:1 (vol͞vol) CH 3 CN͞H 2 O mixtures at 25.0 Ϯ 0.1°C in 1.0 M NH 4 PF 6 ͞HPF 6 mixtures. The details of the reaction in Eq. 2 with phosphorus as the proton-donor atom were reported in ref. 18, and preliminary accounts with S and N were reported in refs. 19 and 20. The structures of the Os(V) nitrogen-based electronproton acceptor and Os(IV) sulfur-ba...

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Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho-Methylanthrone

Cited by 41 publications

References 6 publications

Deuterium Tunneling in Triplet 5,8-Dimethyl-1-tetralone by Phosphorescence Detection between 80 and 15 K

Deuterium Tunneling in Triplet 5,8-Dimethyl-1-tetralone by Phosphorescence Detection between 80 and 15 K

Photochemical reaction mechanisms and kinetics with molecular nanocrystals: surface quenching of triplet benzophenone nanocrystals

Colossal kinetic isotope effects in proton-coupled electron transfer

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