J. Phys. Chem. 1996, 100, 7097-7105
7097
Radical Cations of Sterically Hindered Phenols as Intermediates in Radiation-Induced Electron Transfer Processes Ortwin Brede,* Hans Orthner, Valentin Zubarev, and Ralf Hermann Max Planck Society, Research Unit Time-ResolVed Spectroscopy at the UniVersity of Leipzig, Permoserstrasse 15, D-04303 Leipzig, Germany ReceiVed: NoVember 29, 1995; In Final Form: February 5, 1996X
The radical cations of several preferably sterically hindered phenols have been observed through the radiationinduced electron transfer from the phenol (ArOH) to the parent cations of nonpolar solvents (n- and iso-butyl chloride, cyclohexane, freon-113). For p-methoxyphenol and its 2,6-di-tert-butyl-substituted derivative, ArOH•+ has been characterized after radiolysis at 77 K by EPR and optical absorption spectroscopies. Their broad optical absorption bands at λ ) 430 and 650 nm appeared (Figure 1b). The experiment suggests that the decay of 2•+ and 6•+ proceeds by deprotonation (4).
ArOH•+ f ArO• + H+
(4)
Figure 2a shows the EPR spectrum of γ-irradiated phenol 6 in a frozen freon-113 matrix. The slightly asymmetric singlet EPR signal of the cation 6•+ is changed upon annealing at 133 K. Instead, a narrow singlet EPR spectrum (shifted toward high field) with the peak-to-peak line width of 9 G of phenoxyl radicals 6• is observed (Figure 2b). The EPR spectrum obtained after annealing is not changed upon melting of the samples, and its room temperature solution EPR spectrum (cf. Figure 2c) provides evidence for the assignment to phenoxyl radicals 6• [1:3:3:1 quadruplet of 1:2:1 triplets with aH(OCH3) ) 0.172 and aH(2 meta-H) ) 0.114 mT, in agreement with data in ref 23]. The narrow singlet line EPR spectrum of 6• at 77 K could be due to the unresolved isotropic proton
coupling from the CH3 group, which is often expected to be free rotating.24 For the meta protons of the aromatic ring of 6• only weak anisotropic interaction could be expected at 77 K (ref 25), which contributes to the line width. Neglecting g anisotropy and fitting the experimental EPR spectrum results in a singlet Gauss line, which is indicative for the unresolved proton coupling.24 The spectrum of the cation radical 6•+ (Figure 2a) is broader than that of the phenoxyl radical 6• and is observed at a higher g value, as expected for phenolic cation radicals.11b The asymmetry of the spectrum is indicative for the g and A anisotropy. The latter arises from the magnetic dipole-dipole interaction of H atoms in the OH group with the unpaired electron on the 2p oxygen orbital.26 Isotropic proton coupling from the CH3 group in 6•+ is known to be about 50% higher11b in comparison with that in 6•, due to the increased spin density in the paraposition of the aromatic ring.27 Taking this into account, the EPR spectrum of 6•+ could be best simulated with AII(H,OH) ) 11 G, aH(OCH3) ) 2.5G, g - gII ) 0.0015, and a Gauss line width ) 4 G. The low-temperature optical absorption as well as EPR data provides direct evidence for the formation of stable phenol radical cations in frozen halogenated matrices. This is in line with earlier observations of ArOH•+ of phenol itself isolated in rare gas matrices9 as well as with the CIDEP detection of the 2,6-dimethyl-4-methoxyphenol radical cation in benzene at room temperature.13 The thermal deprotonation of the radical cations 2•+ and 6•+ upon warming of the matrices results in the formation of highly resonance stabilized phenoxyl radicals. Time-Resolved Measurements (Pulse Radiolysis). More detailed kinetic information resulted from pulse radiolysis measurements with the p-methoxy-substituted phenols 2 and 6 made in n-butyl chloride solutions at room temperature. Spectrum I in Figure 3a shows the optical absorption band due to the parent radical cations in n-butyl chloride. In the presence of 5 mmol of 6, the butyl chloride signal decays rapidly (reaction 2b), and spectrum II of the phenol species appears. This spectrum exhibits peaks with λmax ) 460 nm and about 310 nm, assigned in comparison to the low-temperature spectra to AroH•+, and further peaks with λmax ) 400 and 320 nm due to ArO•. By increasing the phenol concentration, the ArOH•+ yield
7100 J. Phys. Chem., Vol. 100, No. 17, 1996
Brede et al.
remains unchanged whereas the ArO• yield became higher (spectrum IV). Qualitatively the same situation has been observed for the charge transfer from phenol 2 to n-butyl chloride cations (reaction 2b). Taken under the same conditions as those for Figure 3a, Figure 3b shows the spectra of ArOH•+ and ArO• derived from the nonhindered p-methoxyphenol. In this case the transients seem to posses higher extinction coefficients. An analysis of the time behavior of the phenol cation and phenoxyl radical absorptions (see insets in Figure 3) shows that ArOH•+ is formed rapidly by charge transfer (2b) (k2b ) 1.5 × 1010 dm3 mol-1 s-1) and decays independent of concentration by the dissociation reaction 4 (k4 ) 2.2 × 106 s-1). As can be seen in the time profile for ArO•, a delayed formation according to reaction 4 is indicated, but there is also a considerable portion of rapidly, i.e., in the pulse, formed ArO•. Looking on the very slow reactions of solvent radicals in this system, this fast part of ArO• formation may be explained by a deprotonation reaction of the parent solvent cations by ArOH (5a) followed by the neutralization (5b). Here, reaction 5a competes with the charge transfer (2b) and represents a typical early event in radiation chemistry.
RX•+ + ArOH f RX• + ArOH2+
(5a)
ArOH2+ + esolv- or RCl•- f ArO• + H2 (+RCl) (5b) This could also be the reason for the observation that with increasing phenol concentration (>5 mmol) the ArOH•+ remains constant whereas that of ArO• becomes larger (see Figure 3 and comment given above). Electron Transfer Behavior of the Other Sterically Hindered Phenols 3-5. In a pulse radiolysis study about the reactions of hydrogen, alkyl, and alkylperoxy radicals with the model phenol 3 made in long-chain n-alkane solutions, for this reaction type unreasonable rapid ArO• formation was found.3c In subsequent work it became clear that the ionic reactions 2b and 4 are the reason for this phenomenon.6-8 For unsubstituted phenol 1 and the sterically hindered compounds 3-5 as well as 7 and 8, the kinetic and thermodynamic stability of radical cations is expected to be lower than those of the p-methoxy-substituted phenols 2 and 6. Although the low-temperature studies provide evidence for the phenol radical cations in halocarbon glasses in each case,10 in the pulse radiolysis of 1 and 3-5 in n-butyl chloride and in alkanes at room temperature the phenoxyl radicals seem to dominate. Hence, under these conditions at first sight in optical spectroscopy ArOH•+ could not be clearly distinguished from ArO•. Hence, at first the electron transfer (2b) has been characterized by analysis of the influence of ArOH on the time profiles of the solvent parent cation RX•+ in butyl chloride. Spectrum I of Figure 3a shows the optical absorption spectrum of n-BuCl•+. Based on the n-BuCl•+ time profiles and on Stern-Volmer plots, Figure 4 demonstrates for 1 and 3 the pseudo-first-order kinetics of reaction 2. The calculated rate constants are collected in Table 2. These k2b values are up to 50% higher than the diffusion-controlled limit in n-butyl chloride, which may be explained by a contribution of nonrelaxed and highly mobile positive holes.18 The decay of n-BuCl•+ in the pure solvent is characterized by the intercept on the ordinate in Figure 4a,b and amounts to about 1.1 × 107 s-1. The spectra of the products of the electron transfer (2) are demonstrated in Figures 5a,b for 1 and 3. Besides the wellknown phenoxyl radical with absorption maxima
