Charge transfer band of the benzyl radical-halide ion formed by dissociative electron attachment to benzyl halides in a rigid organic matrix

Abstract

In order to elucidate the anomalous features previously reported on electronic spectra of the benzyl radical generated from benzyl chloride, the spectra were studied by the fluorescence spectrophotometric method for benzyl radicals generated from benzyl fluoride, chloride, bromide, and iodide in a 3-methylhexane matrix by either uv or irradiation at 77 K. When benzyl radicals were generated by dissociative electron attachment to the benzyl halides in the 7-irradiated matrix, the halide ions close to the benzyl radicals were found to cause a red-shift of the spectra of the radicals, a change in the vibrational structure (especially an increase in the 0-0 band intensity of the fluorescence spectrum), and also the appearance of CT bands due to complexes between the halide ion and the benzyl radical at 265, 362,413 and 500 nm for F-, Cl", Br~, and I~, respectively. The dependence of the CT transition energy upon the halide ions agrees well with that expected from the ionization energy of the donors (the electron affinity of the halogen atoms) and the solvation energy of the halide ions in the matrix. For the benzyl radical from benzyl chloride, the effects of the chloride ion were found to be much less in 2-methyltetrahydrofuran matrix, and could not be observed at all in an ethanol matrix. Halide ions also caused a decrease in the fluorescence lifetime of the benzyl radical.

Effect of matrix polarity on the charge-transfer band of the benzyl radical-halide ion complex

“…Their spectral shape is that common to the low-resolution spectra of the "free" benzyl radicals such as those generated by the photodissociation of benzyl halides and toluene. The effect of the nearby I™ ion is absent because of its solvation by polar matrix molecules, as was found previously for CT. 2 When the benzyl radical is generated from Bzl in the nonpolar 3-MHx matrix, the observed spectra are significantly different as shown in Figure IB, which is essentially the same as reported previously.2 The excitation and fluorescence spectra of the benzyl radical are replaced with broad, unstructured bands with maxima at ca. 500 and 548 nm, which have been assigned to the CT excitation band and the CT fluorescence band of the Bz-T complex.…”

“…The solvation energy is approximately given by Born's formula Ea = (e2/2r)(ll/D) (2) where r denotes the effective radius of X" for Es(Bz-X") or that of Bz" for Es(Bz"---X). The static dielectric constant, Da, and the optical one, Dop, of the matrix should be taken as D for the initial and final states, respectively.…”

Radical ions of poly(silastyrene)

“…The absorption maximum and the band structure are very similar to those of the benzyl radical. 13 The spectra observed in MTHF and n-BuCl solutions containing polysilastyrene are quite different from the radical spectrum.…”

“…Although we assigned the absorption bands in the UV region observed in MTHF and n-BuCl to the radical anion and cation of polysilastyrene, respectively, a silyl radical bearing a phenyl group is also expected to have an absorption band in a similar region. 13 In order to eliminate the ambiguousness of the assignment, we also measured the absorption spectrum of dimethylphenylsilyl radical. The radical was produced by the dissociative electron attachment reaction of dimethylphenylchlorosilane.…”

Ab Initio MO and Density Functional Theory Study of Substituent Effects on Electron Attachment to Benzyl Chlorides

“…Thus, PUHF/6-31G(d) frequency calculations, performed with MP2 located extrema, are in a poor agreement with the experimental data which show that benzyl radicals may actually bind halide ions to form detectable species. 7,41 This discrepancy is most likely accounted for by a significant contamination of the UHF wave functions both in uncorrelated and in MP2 correlated calculations, which may have a significant impact on vibrational frequencies. 42 Indeed, PUMP2/6-31+G(d,p)//PUMP2/6-31G(d) calculated 〈S 2 〉 values for 6a-6h are as high as 1.25-1.27, and even after annihylation, 〈S 2 〉 is still not lower than 0.97-0.98.…”