The Solvent Effect on the Electronic Spectra of Ion Pairs. Ketyls in Ethers

Garst, John F.; Walmsley, Donald; Hewitt, Carol; Richards, William R.; Zabolotny, Ernest R.

doi:10.1021/ja01057a025

Cited by 20 publications

(2 citation statements)

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“…The absorption spectrum of an ion frequently shifts to shorter wavelengths, but here a red shift is observed, suggesting that the excited state of MV 2+ interacts more strongly with OH – than the MV 2+ ground state. Spectral shifts upon ion pair formation are even seen in the absence of a significant CT interaction. − However, the correlation in Figure implies that a strong CT interaction is present, which may explain the surprisingly slow rate of ion pair dissociation that can be inferred from our measurements.…”

Section: Discussionmentioning

confidence: 75%

Ultrafast Hydrolysis of a Lewis Photoacid

2014

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This study explores the concept that electronic excitation can dramatically enhance Lewis acidity. Specifically, it is shown that photoexcitation transforms an electron-deficient organic compound of negligible Lewis acidity in its electronic ground state into a potent excited-state Lewis acid that releases a proton from a nearby water molecule in 3.1 ps. It was shown previously (Peon et al. J. Phys. Chem. A 2001, 105, 5768) that the excited state of methyl viologen (MV(2+)) is quenched rapidly in aqueous solution with the formation of an unidentified photoproduct. In this study, the quenching mechanism and the identity of the photoproduct were investigated by the femtosecond transient absorption and fluorescence upconversion techniques. Transient absorption signals at UV probe wavelengths reveal a long-lived species with a pH-dependent lifetime due to reaction with hydronium ions at a bimolecular rate of 3.1 × 10(9) M(-1) s(-1). This species is revealed to be a charge-transfer complex consisting of a ground-state MV(2+) ion and a hydroxide ion formed when a water molecule transfers a proton to the bulk solvent. Formation of a contact ion pair between MV(2+) and hydroxide shifts the absorption spectrum of the former ion by a few nm to longer wavelengths, yielding a transient absorption spectrum with a distinctive triangle wave appearance. The slight shift of this spectrum, which is in excellent agreement with steady-state difference spectra recorded for MV(2+) at high pH, is consistent with an ion pair but not with a covalent adduct (pseudobase). The long lifetime of the ion pair at neutral pH indicates that dissociation occurs many orders of magnitude more slowly than predicted by the Smoluchowski-Debye equation. Remarkably, there is no evidence of geminate recombination, suggesting that the proton that is transferred to the solvent is conducted at least several water shells away. Although the hydrolysis mechanism has yet to be fully established, evidence suggests that the strongly oxidizing excited state of MV(2+) triggers the proton-coupled oxidation of a water molecule. The observed kinetic isotope effect of 1.7 seen in D2O vs H2O is of the magnitude expected for an ultrafast concerted proton-electron transfer reaction. The ultrafast hydrolysis seen here may be a general excited-state quenching mechanism for electronically excited Lewis acids and other powerful photooxidants in aqueous solution.

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Section: Discussionmentioning

confidence: 75%

Ultrafast Hydrolysis of a Lewis Photoacid

2014

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“…Before starting the polymerization, the ketyl anion solution was introduced into the reaction apparatus through a break seal and the concentration of the ketyl anion was determined by measuring the intensity of its absorption spectrum (cmax= 12000 M-1 cm-1 ). 8 The solution was then transferred into the reaction vessel, cooled to the desired temperature, and mixed with MMA by crushing a break seal. Polymerization was stopped by opening the reaction apparatus and pouring the contents into methanol.…”

Section: Polymerizationmentioning

confidence: 99%

Sodium-Benzophenone Eetyl as an Initiator of Anionic Polymerization of Methyl Methacrylate

Ogasawara

Yoshida

Hayashi

1975

Polym J

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The anionic polymerization of methyl methacrylate in tetrahydrofuran initiated by sodium-benzophenone ketyl was studied in rigorously purified systems by means of optical absorption measurements as well as conventional polymerization techniques to elucidate the initiation process involved. It was concluded, on the basis of kinetic behavior of the ketyl anion in reacting with the monomer and the UV absorption of the product polymer, that the addition of the ketyl anion to the monomer forms a propagating carbanion and thus initiates the polymerization. The addition reaction shows an activation energy of 13 kcallmol and a rate constant of 1.2x 10-2 M-1 sec-1 at -10°C. It was also found that no termination reaction occurs, so that the propagating chains are long-lived, though the chain transfer reaction takes place very slowly. The chain transfer reaction is brought about by electron transfer from the propagating carbanion to the benzophenone present in excess in the reaction system to regenerate the initiator.

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Electron Transfer Luminescence in Solution

Zweig¹

1968

Advances in Photochemistry

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The Solvent Effect on the Electronic Spectra of Ion Pairs. Ketyls in Ethers

Cited by 20 publications

References 0 publications

Ultrafast Hydrolysis of a Lewis Photoacid

Ultrafast Hydrolysis of a Lewis Photoacid

Sodium-Benzophenone Eetyl as an Initiator of Anionic Polymerization of Methyl Methacrylate

Electron Transfer Luminescence in Solution

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