Femtosecond electron ejection in liquid acetonitrile: Evidence for cavity electrons and solvent anions

Abstract

Articles you may be interested inPhotodetachment and electron reactivity in 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide J. Chem. Phys. 137, 034512 (2012); 10.1063/1.4736569Collisional electron transfer to photoexcited acceptor radical anions Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopyExcess electrons were studied in liquid acetonitrile at room temperature by femtosecond pumpprobe spectroscopy. Using Ϸ200 fs, 265 nm laser pulses, electrons were ejected into the liquid by photodetachment from iodide ions and, in separate experiments, by photoionization of indole. A strong and broad absorption band with a maximum near 1400 nm was observed in both systems. A second absorption band was observed at wavelengths below 620 nm for iodide solutions, but was not seen in photoexcited indole due to overlapping excited state absorption. The bands are in good agreement with ones seen previously in nanosecond pulse radiolysis experiments ͓I. P. Bell, M. A. J. Rodgers, and H. D. Burrows, J. Chem. Soc., Faraday Trans. 1 73, 315 ͑1977͔͒. Bell, Rodgers, and Burrows assigned the visible and IR bands to absorption by acetonitrile dimer and monomer anions, respectively. Our results strongly question this interpretation. Instead, we assign the short-wavelength absorption band to a solvent-bound valence anion consisting of one or two acetonitrile molecules and the IR band to a solvated or cavity electron. Low-level quantum chemical calculations indicate that valence anion formation is strongly correlated with CCN bending, but do not provide a clear indication of whether a monomer or dimer valence anion is favored. The highly mobile cavity electron is scavenged by added chloroform at a bimolecular reaction rate of (1.02 Ϯ0.03)ϫ10 11 M Ϫ1 s Ϫ1 . The appearance of both absorption bands within our time resolution suggests that the two forms of the excess electron are produced by prompt reaction with the iodide charge-transfer-to-solvent ͑CTTS͒ excited state. In support of this mechanism, strong static scavenging by chloroform was observed at both visible and IR wavelengths. For iodide in acetonitrile, the signal in the IR decays biexponentially due to competition between geminate recombination of the cavity electron with the parent iodine atom and its reaction with the solvent. Geminate recombination between the solvated electron and the parent iodine atom occurs with a characteristic time constant of Ϸ30 ps, while additional solvent anions are formed in a slow reaction with a time constant of Ϸ260 ps. Approximately 30% of the solvated electrons photodetached from iodide undergo geminate recombination. There is no evidence for geminate reaction between the promptly formed solvent anion and iodine, suggesting that these species are formed at larger initial separation than the IR-absorbing cavity electron/iodine atom pair. In indole, geminate recombination occurs on a slower time scale of Ϸ135 ps.

Toward Electron Encapsulation: Polynitrile Approach

“…16,26 These two pathways of electron localization are also found in large acetonitrile clusters. 27 The electron is solvated by methyl groups of acetonitrile, as 10.…”

“…26,33 Reaction (3) shifts equilibrium (2) to the right side which makes it necessary to take it into account in order to determine the equilibrium parameters of reaction (2). For glutaro-and succino-nitrile ( Figures 2 and 3, respectively), the rate constants for dimerization reaction (3) are close to those for diffusion-controlled reactions in n-hexane; for adiponitrile this reaction is ca.…”

“…32a Shkrob et al 33 and others 26,31 found that acetonitrile molecules in the dimer and the monomer anions have similar geometry, with the C-C-N angle of 130 o . In the centrosymmetrical dimer anion, there is a 3-electron-2-center bond between the two cyanide carbons with the C-C distance of 1.65 Å.…”

“…Even in neat liquid acetonitrile 16,26 and (CH 3 CN) n -clusters, 27,28 the electron is solvated by methyl groups rather than cyano groups of the acetonitrile which point away from the cavity. A similar situation occurs for electrons in amides, where polar C=O groups also point away from the cavity.…”

Formation of Large Molecular Cluster Anions and Elucidation of Their Electronic Structures

“…Shkrob and Sauer 104 have conducted a definitive study on excess electron localization in liquid CH 3 CN using pulse radiolysis-transient absorption and timeresolved photoconductivity methods. A second group, led by Xia et al, 105 has also studied this system using femtosecond pump-probe spectroscopy. Although different techniques have is much lower than that of isomer II and if no transformation between two isomers takes place on the timescale of the measurement, the lower EBE ''isomer I'' can be selectively photodetached by using the HB laser (h 1 ).…”