Two channels of electron transfer observed for the reaction of n-butyl chloride parent radical cations with naphthols and hydroxybiphenyls

Article abstract of DOI:10.1021/jp9808431

Pulse radiolysis of naphthols (NpOH) and hydroxybiphenyls (ByOH) in n-butyl chloride (BuCl) at room temperature exhibits electron transfer at a bimolecular rate constant of (1.0-2.8) × 10¹⁰ dm³ mol^-1 s^-1. The experiments reveal the direct formation of two types of transients: phenol type radical cations (NpOH^?+, ByOH^?+) and phenoxyl type radicals (NpO^?, ByO^?). This is explained by a mechanism involving two different electron-transfer channels. The solute radical cations exhibit two optical absorption bands in the 570-650 and 360-460 nm regions and undergo electron transfer with triethylamine and proton transfer with ethanol with bimolecular rate constants of (4-12) × 10⁹ and (3-6) × 10⁸ dm³ mol^-1 s^-1, respectively. NpO^? and ByO^? have relatively long lifetimes and show absorption bands in the 340-400 and 470-540 nm regions. By way of comparison, these phenoxyl type radicals are separately generated by pulse radiolysis in aqueous alkaline solution containing sodium azide, i.e., by oxidation of the solutes with N₃^? radicals. Under these conditions, the phenoxyl radicals decay by second-order kinetics with 2k = (1.2-4.5) × 10⁸ dm³ mol^-1 s^-1. The various modes of formation and decay of the phenolic radical cations are analyzed over a wide range of dose rate and solute concentrations. In comparison to radical cations of one-ring phenols, the increased stability of NpOH^?+ and ByOH^?+ is explained by the delocalization of the positive charge over the whole aromatic system, a postulate supported by open-shell quantum chemical calulations.

Full text of DOI:10.1021/jp9808431

5754
J. Phys. Chem. A 1998, 102, 5754-5762
Two Channels of Electron Transfer Observed for the Reaction of n-Butyl Chloride Parent
Radical Cations with Naphthols and Hydroxybiphenyls
†
‡
§
†
,‡
H. Mohan, R. Hermann, S. Naumov, J. P. Mittal, and O. Brede*
Chemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India, UniVersity of Leipzig,
Interdisciplinary Group Time-ResolVed Spectroscopy, Permoserstrasse 15, D-04303 Leipzig, Germany, and
Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany
ReceiVed: January 22, 1998; In Final Form: March 30, 1998
Pulse radiolysis of naphthols (NpOH) and hydroxybiphenyls (ByOH) in n-butyl chloride (BuCl) at room
1
0
3
-1 -1
temperature exhibits electron transfer at a bimolecular rate constant of (1.0-2.8) × 10 dm mol s . The
•+
experiments reveal the direct formation of two types of transients: phenol type radical cations (NpOH ,
•
+
•
•
ByOH ) and phenoxyl type radicals (NpO , ByO ). This is explained by a mechanism involving two different
electron-transfer channels. The solute radical cations exhibit two optical absorption bands in the 570-650
and 360-460 nm regions and undergo electron transfer with triethylamine and proton transfer with ethanol
9
8
3
-1 -1
•
with bimolecular rate constants of (4-12) × 10 and (3-6) × 10 dm mol s , respectively. NpO and
•
ByO have relatively long lifetimes and show absorption bands in the 340-400 and 470-540 nm regions.
By way of comparison, these phenoxyl type radicals are separately generated by pulse radiolysis in aqueous
•
alkaline solution containing sodium azide, i.e., by oxidation of the solutes with N
3
radicals. Under these
8
3
-1 -1
conditions, the phenoxyl radicals decay by second-order kinetics with 2k ) (1.2-4.5) × 10 dm mol s .
The various modes of formation and decay of the phenolic radical cations are analyzed over a wide range of
dose rate and solute concentrations. In comparison to radical cations of one-ring phenols, the increased
stability of NpOH and ByOH is explained by the delocalization of the positive charge over the whole
aromatic system, a postulate supported by open-shell quantum chemical calulations.
•
+
•+
10
1
. Introduction
cations were determined to be in the range (1-2) × 10 dm3
-
1
-1
mol s . It was found that the solvent radical cations react
with phenols (ArOH) by electron transfer (reaction 1a) as well
as deprotonation (reaction 1b), the latter process yielding
solvent-stabilized protons:
Increasing attention has been devoted to electron-transfer
processes involving phenols and their substituted derivatives,
since they play an important role in the stabilization of polymers,
food, and oils, etc. Their antioxidative properties are under-
1
,2
3
stood in terms of electron and H atom donor ability. Owing
BuCl•+ f ArOH•+ + BuCl
(1a)
(1b)
to their low ionization potential and the weak O-H bond,
phenols and substituted derivatives act as model systems for
understanding the antioxidative mechanism.4 Because of their
low pKa value and their instability even under nonprotic
conditions, phenol radical cations can hardly be detected and
phenoxyl radicals are usually the first observable oxidation
•
+
f ArO + H (BuCl)
,5
Since at room temperature in liquid phase the rapidly formed
phenoxyl radicals dominate, reaction 1b describes the main
channel of the reaction of parent radical cations with the
phenolic solute. Owing to delocalization of the positive charge,
in the case of p-methoxy-substituted phenols, the increase in
kinetic and thermodynamic stability resulted in the domination
of reaction channel 1a. Furthermore, the decay of ArOH• of
normal phenols proceeds by neutralization (reaction 2) as well
as deprotonation (reaction 3), the latter process mainly being
6
products of one-electron-transfer oxidation. Hence, in polar
solutions phenol radical cations have been observed in strongly
acidic media and characterized by EPR.7 In nonpolar solvents,
the radical cations of phenols and substituted derivatives were
characterized by optical absorption and EPR spectra taken in
,8
+
1
,9,10
glasses at 77 K
as well as by pulse radiolysis at room
temperature in nonpolar solutions.1
1
unimolecular:
Pulse radiolysis of phenols (ArOH) in n-butyl chloride (BuCl)
ArOH•+ + Cl f neutral products
-
(2)
(3)
provided evidence of the formation of solute radical cations
•
+
(
ArOH ) with very broad and unspecific optical absorption
•
+
1
bands. The time profiles gave a clear indication of the kinetics
f ArO + H
•+
of the formation and decay of ArOH . Hence, the bimolecular
rate constants for electron transfer from solute to solvent radical
To understand the mechanism of electron-transfer processes of
phenols and the effect of solute structure on various reaction
channels, pulse radiolysis studies of naphthols and hydroxy-
biphenyls in butyl chloride were initiated. The extended
aromatic moiety is expected to play an important role in the
†
Bhabha Atomic Research Centre.
University of Leipzig.
Institute of Surface Modification.
‡
§
S1089-5639(98)00843-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/26/1998

n-Butyl Chloride Radical Cations
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5755
stabilization of the solute radical cations as well as in the
conceivable formation of dimers of charge-transfer character,
analogous to other aromatic radical cations (ref 11a). In this
paper we report on pulse radiolysis of naphthols and hydroxy-
biphenyls under the aspect of radical cation kinetics.
2. Experimental Section
2
.1. Pulse Radiolysis. The liquid samples were irradiated
with high-energy electron pulses (1 MeV, 16 ns pulse duration)
generated by a pulse transformer type accelerator ELIT (Institute
of Nuclear Physics, Novosibirsk, Russia). The dose delivered
per pulse was measured with an electron dosimeter and was
usually around 50 Gy per pulse. The detection of the transient
species was carried out by the optical absorption technique,
which consists of a pulsed xenon lamp (XBO 900, Osram), a
Spectra Pro-500 monochromator (Acton Research Corporation),
an IP28 photomultiplier (Hamamatsu Photonics), and a 1 GHz
digitizing oscilloscope (TDS 640, Tektronix). Further details
are given elsewhere.1
Figure 1. Transient optical absorption spectra taken in the pulse
Pulse radiolysis experiments were performed in N2-saturated
radiolysis of pure n-butyl chloride purged with N : (O) immediately
2
•
n-butyl chloride solutions. Reaction with N3 radicals, generated
after the pulse; (b) after 100 ns. The insets show a typical time profile
and the Stern-Volmer plot to determine the rate of electron transfer
from 2-NpOH to the solvent radical cation.
via reactions 3a and 3b, was carried out in N2O-saturated
-
-2
-3
aqueous solution of N3 (2 × 10 mol dm , pH ) 10). The
solutions continuously flowed through the sample cell with a
path length of 1 cm.×
BuCl•+ (see Figure 1). These positively charged species decay
by fragmentation (reaction 5b) and the charge neutralization
process (reaction 7).
2
.2. Laser Photolysis. Laser photolysis experiments were
carried out using the fourth harmonic (266 nm) from a Quanta
Ray GCR-11 Nd:YAG laser (Spectra-Physics Lasers). The
pulse duration (fwhm) was measured to be 3 ns, and the pulse
energy was ∼10 mJ. The N2-saturated solution continuously
flowed through a quartz cell (0.3 cm × 0.3 cm). The probe
light of a pulsed 400 W xenon lamp (XBO-400, Osram) passed
through a SpectraPro-275 monochromator (Acton Research
Corporation) to a R955 photomultiplier tube (Hamamatsu
Photonics). The output signals were recorded on a 500 MHz
digitizing oscilloscope (DSA 602 A, Tektronix). Other details
are given elsewhere.1
•
+
•+
-
BuCl , Bu + Cl f products
(7)
BuCl was employed as a solvent for the generation and
investigation of solute radical cations due to (i) its high
ionization potential (Igas ) 10.7 eV), (ii) formation of very short-
•
+
lived (τ ) 90 ns) transient species (BuCl ) with a broad and
very small absorption in the 300-500 nm region, and (iii) the
relatively high yield of free ions (G ) 0.3). Figure 1
demonstrates this situation using optical absorption spectra and
a time profile taken in pure n-butyl chloride purged with
nitrogen.
2.3. Chemicals. Naphthols (1- and 2-NpOH) and hydroxy-
biphenyls (2-, 3-, and 4-ByOH) were commercially available.
n-Butyl chloride (BuCl, Merck) was additionally purified by
passing it through a column of an activated molecular sieve
3.1. Pulse Radiolysis of Naphthols. Pulse Radiolysis of
2-NpOH. In the presence of 2-NpOH (Igas ) 7.8 eV), the
positively charged species generated from the radiolysis of BuCl
react by an electron-transfer process (reaction 8), whereas radical
(
×13). The studies in aqueous solutions were performed in
purified water (Millipore, Milli-Q plus system).
•
species (Bu ) may react by H atom abstraction (reaction 9).
3
. Results and Discussion
BuCl•+ + 2-NpOH f 2-NpOH + BuCl
•+
(8)
(9)
The electrons produced during the radiolysis of BuCl are
•
•
Bu + 2-NpOH f 2-NpO + Bu
immediately scavenged by the solvent molecules by a dissocia-
tive electron capture process to form Cl anions. The highly
mobile positively charged holes (h ) move through the solvent
and are localized as BuCl or butene radical cations (Bu ).
-
•
+
The rate constants of these reactions, however, differ by some
orders in magnitude. Upon addition of the naphthols, the
•
+
•+ 11b
•+
transient absorption of BuCl at 500 nm (cf. time profile given
in the inset of Figure 1) showed accelerated decay and the
pseudo-first-order rate constant (kobs) increased linearly with
•
+
-
BuCl f h , e
(4)
solv
h•+ + BuCl f BuCl•+
-4
-3
solute concentration (1-10) × 10 mol dm . The bimolecular
(5a)
(5b)
(6)
rate constant calculated from the Stern-Volmer plot (inset of
f Bu•+ + HCl
10
3
Figure 1) of reaction 8 was found to be k8 ) 2 × 10 dm
-
1
-1
mol
(
s
(Table 1). The intercept of the Stern-Volmer plot
esolv- + BuCl f Bu• + Cl-
•+
Figure 1) represents the decay of BuCl in the absence of
any added solute and corresponds to a lifetime of 90 ns.
The transient optical absorption spectrum obtained during
pulse radiolysis of an N2-saturated solution of 2-NpOH (5 ×
In the absence of any added solute, the transient optical
absorption spectrum during pulse radiolysis of N2-saturated BuCl
exhibits broad absorption peaking near 500 nm caused by
-
3
-3
10 mol dm ) in BuCl 300 ns after the pulse, showed an

5756 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Mohan et al.
TABLE 1: Bimolecular Rate Constants for Electron
•
+
Transfer from Solutes to Solvent Radical Cation (BuCl )
(Reaction 1a)
solute
1-NpOH 2-NpOH 2-ByOH 3-ByOH 4-ByOH
1.0 2.0 2.8 1.9 1.8
electron transfer
rate constant ×
-
10
3
1
(
0
dm mol-1s-1
)
TABLE 2: Spectral Parameters for the Transient Species
Formed during Pulse Radiolysis of N
2
-Saturated BuCl
Containing Different Solutes (ArOH)
•
+
λ(ArO•)
(nm)
•+
2
λ(ArOH )
λ(ArOH)
Ar
(nm)
(nm)
1
2
2
-NpOH
380
340
390
530
350
380
475
360
400
420
410
440
4
5
20
80
-NpOH
-ByOH
360
4
5
60
70
360
3
6
80
00
3
4
-ByOH
-ByOH
390
50
390
50
340
530
340
540
420
430
6
6
absorption band at 350 nm with a shoulder at 380 nm, broad
absorption between 430 and 490 nm with a sharp peak at 475
nm, and broad absorption in the 530-650 nm region (Figure
2
a). During this time (0.3 µs after the pulse), the solvent
transient species ought to be decayed and the transient spectrum
observed is clearly due to the transient species generated from
2
-NpOH. Analysis of absorption time profiles (Figure 2b)
revealed that the band at 570 nm is relatively short-lived and
5
-1
decays by first-order kinetics at k ) 6.6 × 10 s . Absorption
at other wavelengths chosen was relatively long-lived and
displayed mixed kinetics. This suggests the presence of more
than one species, tentatively interpreted as a solute cation and
a solute radical.
Identification of Transient Species. To determine the nature
of these phenolic species, additional scavengers were used. In
the presence of low concentration (1 mM) of triethylamine
(
TEA), a positive charge scavenger, the absorption band at 570
nm was completely quenched (Figure 2a) and a considerable
decrease in absorption was observed at 360 nm. Absorption at
other wavelengths only showed a small decrease (Figure 2a).
Analogous to earlier studies of phenols, the short-lived transient
absorption, which was quenched by positive charge scavengers,
was attributed to the solute radical cation (2-NpOH ). The
relatively long-lived absorption (Figure 2a) showed very little
inpact on TEA and was classified as that of the naphthoxyl
Figure 2. (a) Transient optical absorption spectra obtained in the pulse
-
3
-3
radiolysis of N
BuCl in the absence (b) and in the presence of 1 × 10 mol dm
TEA (2). The difference of absorption spectra is shown as O. The
2
-saturated solution of 2-NpOH (5 × 10 mol dm ) in
-3 -3
1
-2
-3
transient spectrum of 2-NpOH (1 × 10 mol dm ) in BuCl is given
by 3. The inset shows the variation of absorbance as a function of
solute concentration (scavenger curve) at 350 nm (b) and 475 nm (2).
•
+
(
b) Absorption time profiles on pulse radiolysis of N
2
-saturated solution
of 2-NpOH (5 × 10 mol dm ) in BuCl at 360 (a), 470 (b), and 570
c) nm.
-
3
-3
(
•
radicals (2-NpO ). The difference between the spectra (in the
presence and absence of TEA) gives the absorption spectrum
reactions 10a and 10b.
•+
of 2-NpOH exhibiting bands at 360 and 570 nm. This enabled
•
+
the positions of the transient absorption bands of 2-NpOH
-
aq
•
-
N O + e f OH + OH + N
(10a)
(10b)
(10c)
•
2
2
and 2-NpO to be determined. The corresponding data are given
in Table 2.
•
-
3
•
3
-
•
OH + N
f N
+ OH
To verify the spectrum of naphthoxyl radicals (2-NpO ), pulse
radiolysis studies were performed in aqueous solutions. The
optical absorption spectrum of an aqueous solution of 2-NpOH
revealed that it exists in deprotonated form at pH g 10 (pKa )
2-NpO- + N f 2-NpO + N
•
•
-
3
3
-
•
•
8
.5). Therefore, one-electron oxidation of 2-NpO by N3
Figure 3 shows the transient optical absorption spectrum
obtained during pulse radiolysis of an N2O-saturated aqueous
solution of NaN3 (3 × 10 mol dm , pH ) 10) containing
12
radicals, a specific one-electron oxidant, ought to form 2-NpO
radicals via reaction 10c. The azide radical was generated via
-
2
-3

n-Butyl Chloride Radical Cations
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5757
Figure 4. Transient optical absorption spectra obtained in pulse
-
2
-3
Figure 3. Transient optical absorption spectrum obtained during pulse
radiolysis of an N
2
-saturated solution of 1-NpOH (1 × 10 mol dm
)
-
2
-4
-3
radiolysis of an N
2
-saturated aqueous solution of NaN
3
(3 × 10 mol
and chloranil (6 × 10 mol dm ) in benzene 0.1 µs (b) and 0.8 µs
(O) after the pulse. The inset shows time profiles at representative
spectral points.
-
3
-4
-3
dm , pH ) 10) and 2-NpOH (2 × 10 mol dm ). The inset shows
the absorption spectrum taken immediately after the pulse with 3-ByOH
-
4
-3
(
2 × 10 mol dm ) under the same conditions.
Triplet-Sensitized Electron Transfer. In view of the high
efficiency of triplet formation, the solute radical cations could
be generated by a triplet-sensitized electron-transfer process.
Chloranil (CA ) 2,3,5,6-tetrachloro-1,4-benzoquinone) has a
greater electron affinity than 2-NpOH and, therefore, should
be able to form anion radicals in accordance to reaction 12b.
TABLE 3: Kinetic and Spectral Parameters for ArO•
•
-
Radicals Formed during the Reaction of N
3
with ArO
(Reaction 10)
-
3
ꢀ
max × 10
decay (2k) ×
T1
λ
max
k × 10 (dm3
-
9
(dm mol
3
-1
10 (dm
-8
3
-
1
-1
-1
-1 -1
Ar
(nm)
mol
s
)
cm
)
mol
s )
T1
1
2
-NpOH
330
6.6
9.82
4.4
C H T1 + 2-NpOH f 2-NpOH + C H6 (12a)
6
6
6
4
5
00
30
1.76
0.72
5.84
1.50
1.46
2.96
1.07
0.93
0.17
1.78
0.43
T1
T1
-NpOH
350
6.2
4.5
2-NpOH + CA f [NpOH ‚‚‚CA] f
3
4
90
90
•+
•-
•
•
[
NpOH ‚‚‚CA ] f 2-NpO + HCA (12b)
2
3
4
-ByOH
-ByOH
-ByOH
350
00
350
40
350
40
1.7
0.6
4.6
3.4
1.2
2.1
This was performed using pulse radiolysis of benzene solutions
in which the naphthol triplets are formed by scavenging the
solvent triplets caused by reaction 12a. Figure 4 shows the
transient optical absorption spectra obtained during pulse
4
5
5
-2
radiolysis of an N2-saturated solution of 2-NpOH (1 × 10
-
3
mol dm ) in benzene containing a low concentration of
-4
-3
2
-NpOH (2 × 10 mol dm ), which exhibits absorption bands
-4
-3
chloranil (6 × 10 mol dm ). Under these conditions, the
with λmax at 350 and 480 nm and a shoulder in the 370-390
nm region (Table 3). The spectrum qualitatively matched that
obtained in BuCl (Figure 2a). The bimolecular rate constant
13b
primarily formed benzene triplet rapidly transfers its energy
to 2-NpOH, and the transient optical absorption spectrum taken
.1 µs after the pulse represents the triplet-triplet absorption
spectrum of 2-NpOH . During the course of reaction 12b the
transient optical absorption spectra 0.8 µs after the pulse showed
absorption bands peaking at 360, 440, and 480 nm and a long
tail to the red range. Regarding the spectra of 2-NpOH (Figure
a) and of 2-NpO (Figure 3), the bands at 360 and 480 nm
correspond well to those of the naphthoxyl radical (Table 2).
Because of the similiarity of the blue band of the radical cation
and the radical, at first glance the 10 nm shift of the radical
band in benzene compared with that in water seems to suggest
0
9
3
-1
(
k10c) was determined to be k10c ) (6.2 ( 0.3) × 10 dm mol
T1
-
1
•
s . The entire spectrum is due to 2-NpO , which decays by
8
3
-1 -1
second-order kinetics with 2k ) 4.5 × 10 dm mol
).
The radical cations of phenols (ArOH) were photolytically
s
(Table
3
•+
•
2
(
(
λ ) 266 nm) generated by a two-photon absorption process
reaction 11) in n-butyl chloride solution in which the electron
is immediately converted into Cl (cf. reaction 6).
-
1
ArOH + 2hν f ArOH + esolv-
•+
(11)
•+
the formation of 2-NpOH . However, the 480 nm band itself
and the relation between it and the blue band clearly argue for
•
However, in the laser photolysis of NpOH, we failed to observe
the 2-NpO . Following lengthy consideration of the absorption
•
+
2
-NpOH . Instead, we obtained the triplet-triplet absorption
behavior of the chloranil semiquinone radical by Gschwind and
T1
13a
13c
spectrum of 2-NpOH peaking at 430 nm.
Obviously, the
Haselbach, the 440 nm peak and the flat absorption between
•
singlet-triplet intersystem crossing efficiency was too high and
optical conditions for both singlet and triplet ionization were
unfavorable under our experimental conditions.
500 and 600 nm ought to be caused by this radical species CA .
Judging by the fast rate of process 12, we can conclude that
the triplet-sensitized electron transfer indeed took place but was

5758 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Mohan et al.
TABLE 4: Bimolecular Rate Constant for the Reaction of
Further Additives with Solute Radical Cations: TEA
of naphthoxyl radicals. It is concluded that the solute radical
cations are now converted to TEA (reaction 13) instead of
•+
(
Electron Transfer 13a), EtOH (Deprotonation 13b)
decaying by reaction 15. The TEA concentration-dependent
-
9
•
rate constant × 10
depletion of 2-NpO seems to be caused by the competition of
3
-1 -1)
solutes
additives
(dm mol
s
reactions 14 and 16.
1
-NpOH
-NpOH
-ByOH
-ByOH
-ByOH
TEA
EtOH
TEA
EtOH
TEA
EtOH
TEA
EtOH
TEA
7.0
0.5
4.0
0.6
11.6
0.3
BuCl•+ + TEA f TEA + BuCl
•+
(16)
2
2
3
4
Decay of Solute Radical Cations. When the fate of the solute
radical cations
•
+
(
2-NpOH ) is analyzed, the competition of at least three
reaction channels should be taken into account: (i) deprotonation
(reaction 15), (ii) neutralization (reaction 17)
9.6
12.0
EtOH
2
-NpOH•+ + Cl f products
-
(17)
followed by subsequent rapid proton transfer within the
intermediate contact ion pair, finally resulting in radicals instead
of the expected ions.
and (iii) self-deprotonation by naphthol (reaction 18),
Electron-Transfer Reactions InVolVing Naphthol Radical
Cations. To determine the kinetics of electron-transfer pro-
cesses, the decay of the 570 nm band of 2-NpOH•+ was analyzed
for increasing concentrations of TEA, which accelerated its
decay. The pseudo-first-order rate constant (kobs) was found to
-NpOH•+ + 2-NpOH f (2-NpOH ) + 2-NpO (18)
+
•
2
2
The initial part of the time profiles at 570 nm underwent slightly
faster decay, without the additionally delayed formation of
-
4
naphthoxyl radicals at increasing solute concentrations (5 × 10
-
4
be proportional to the TEA concentration (1-4) × 10 mol
-1
-3
to 1 × 10 mol dm ), which excludes reaction 18. The charge
neutralization process, however, is affected by the dose rate.
At the low dose rate of 20 Gy per pulse, the naphthol radical
cation decayed by first-order kinetics with a lifetime of 3.7 µs.
As the dose rate was increased to 50 Gy per pulse, its lifetime
decreased to 1.5 µs. At much higher doses per pulse (about
-
3
9
3
-1
dm from which k13a was calculated to be 4 × 10 dm mol
-1
s
(Table 4). Ethanol (EtOH), which is a relatively inefficient
positive charge scavenger, displayed another effect on the
•+
NpOH when analyzed at λ ) 570 nm. The overall absorption
decreased on the addition of EtOH, and the 570 nm band only
underwent slightly faster decay. The bimolecular rate constant
determined from the linear variation of kobs with the ethanol
2
00 Gy), the decay showed mixed kinetics, which (if at all)
could be explained by a superposition of inhomogeneous effects
with volume neutralization and reactions with hydrochloric acid
produced by reaction 5b. These studies show that the decay of
the naphthol radical cations is governed by neutralization and
that other channels such as reactions 15 and 18 only play a
minor role.
8
3
-1 -1
concentration was k13b ) 6 × 10 dm mol
s
(Table 4). As
is typical for radical cations,14 the transient band at 570 nm
remained unaffected in O2-saturated solutions. These findings
also confirm the cationic nature of the transient band at 570
nm.
The NpOH•+ are formed in the charge transfer from solvent
radical cations (reaction 8) and are subsequently scavenged in
the presence of TEA (reaction 13a). Ethanol competes with
the charge transfer (reaction 8) and, therefore, prevents the
The decay of naphthoxyl radicals in BuCl could hardly be
calculated exactly because of superpositions of radicals and
radical cations. However, in aqueous solutions they were
observed to decay by second-order kinetics with 2k ) 4.5 ×
•
+
efficient formation of NpOH (reaction 13c).
8
3
-1 -1
1
0 dm mol s , which should be corrected for BuCl by the
8
3
-NpOH•+ + TEA f TEA•+ + 2-NpOH
viscosity-determined factor of 2 to give about 8 × 10 dm
2
(13a)
(13b)
-1 -1
mol s .
•+
-NpOH•+ + C H OH f 2-NpO + C H OH
•
+
Effect of Solute Concentration. The 2-NpOH yield (moni-
2
2
5
2
5
2
tored at 570 nm) increased with solute concentration, reaching
-
2
-3
a saturation value at 10 mol dm 2-NpOH. In contrast to
BuCl•+ + C H OH f BuCl + C H OH
•
+
2
(13c)
2
5
2
5
•
this, the 2-NpO yield (determined at 350 and 475 nm) also
showed an appreciable increase at higher solute concentrations
An analysis of the time profiles at 360, 470, and 570 nm (Figure
b) shows that the 570 nm band of the solute radical cation
(
inset of Figure 2a). At naphthol concentrations higher than 2
10 mol dm , a slight change in the spectral shape appears
2
-2
-3
×
•
+
(
2-NpOH ) is formed immediately. Only slightly delayed
to be caused by increased absorption around 380-450 nm. At
•
formation may be supposed at 360 and 470 nm (2-NpO ). In
addition to the delayed formation of naphthoxyl radicals, a major
portion was formed immediately after the pulse, which is
explained by reaction 14.
-2
-3
5
× 10 mol dm 2-NpOH the formation of a new band could
clearly be seen. Figure 5 shows the transient optical absorption
spectra obtained during pulse radiolysis of an N2-saturated
-
2
-3
solution of 2-NpOH (5 × 10 mol dm ) in BuCl, 0.16 and
1
3
.44 µs after the pulse. Although the absorption in the 350-
BuCl•+ + 2-NpOH f 2-NpO + H (BuCl)
•
+
(14)
70 nm region (which is due to naphthoxyl radicals) showed
slight decay, increased absorption was observed in the 400-
50 nm region during the same time, i.e., a new transient
appeared.
Simultaneously with its delayed formation, the initial part of
The delayed formation ought to proceed by the decay of solute
radical cations by deprotonation (reaction 15), a process that is
markedly slower and less efficient than in the case of phenols.
4
1
•
+
•
+
•+
the 2-NpOH time profile taken at 570 nm (inset of Figure 5)
2
-NpOH , BuCl f 2-NpO + H (BuCl)
(15)
6
-1
showed slightly faster decay (k ) 1.1 × 10 s ), which matched
The absorption time profiles at 360 and 470 nm obtained in the
presence of TEA revealed the absence of the delayed formation
the formation of the new transient. Furthermore, it was found
-
3
-3
that adding a low concentration of TEA (1 × 10 mol dm )

n-Butyl Chloride Radical Cations
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5759
(a)
Figure 5. Transient optical absorption spectra obtained in pulse
-
2
-3
radiolysis of an N
in BuCl 0.16 (b) and 1.44 µs (O) after the pulse. The inset shows
absorption time profiles obtained during pulse radiolysis of an N
2
-saturated solution of 2-NpOH (5 × 10 mol dm
)
(b)
2
-
-
2
-3
saturated solution of 2-NpOH (5 × 10 mol dm ) in BuCl at 350
(a), 410 (b), and 570 nm (c).
quenched the formation of the new transient band around 400-
50 nm. Thus, we concluded that this band observed only at
4
high solute concentration is caused by a product of the ion
molecule reaction of naphthol radical cations with naphthol
(reaction 19) with a conceivable structure of a dimer stabilized
by charge-transfer interaction as is known for condensed
aromatics.1
1,15
2
-NpOH•+ + 2-NpOH a (2-NpOH)2•+
(19)
With k19 ) 3 × 10 dm mol s-1, the dimerization rate is
much smaller than those determined for pure aromatics. Such
an ion-molecule reaction was not observed in the case of phenol
and its sterically hindered derivatives.1 However, for the
naphthols it may be supposed that under favorable conditions
of orbital overlap, dimer radical cations could be observed at
high solute concentrations. Because of the planarity of the
molecule, such an orbital overlap may be achieved in the case
of naphthols.
7
3
-1
Figure 6. (a) Transient optical absorption spectra obtained in pulse
-3
-3
radiolysis of an N
2
-saturated solution of 3-ByOH (5 × 10 mol dm
)
-
3
-3
in BuCl in the absence (b) and in the presence of 1 × 10 mol dm
Analogous pulse radiolysis studies performed with 1-NpOH
in BuCl solution also revealed the formation of naphthoxyl
radicals and solute radical cations. The kinetic and spectral
parameters of the transient species formed from 1-NpOH are
shown in Tables 1-5.
TEA (1). The difference between the absorption spectra is shown as
O. The inset shows the variation of absorbance as a function of the
solute concentration at 340 (O) and 650 nm (b). (b) Absorption time
2
profiles obtained during pulse radiolysis of an N -saturated solution of
-
3
-3
3
-ByOH (5 × 10 mol dm ) in BuCl at 340 (a), 390 (b), and 650
nm (c).
3
.2. Pulse Radiolysis of Hydroxybiphenyls. Pulse Radi-
olysis of 3-ByOH. Figure 6a shows the transient optical
the 510-580 nm region. We state that in analogy to the findings
absorption spectrum obtained during pulse radiolysis of an N2-
of naphthol radiolysis, two kinds of transients are observed,
-
3
-3
•+
•
saturated solution of 3-ByOH (5 × 10 mol dm ) in BuCl
00 ns after the pulse. It exhibits absorption bands with λmax
390 and 650 nm with shoulders at 350, 370, 430 nm and a
3-ByOH and 3-ByO . As expected for cations, in the presence
-
3
-3
3
of 1 × 10 mol dm TEA, the transient absorptions at 650
and 390 nm were completly quenched (Figure 6a), whereas the
broad and long-lasting absorption bands at λmax ) 340 nm,
shoulder at 430, and low broad absorption in the 450-550 nm
region remained (Figure 6a), as is typical for aroxyl radicals.
)
small broad absorption in the 450-550 nm region. Kinetic
analysis of absorption time profiles indicates that the absorptions
at 650 nm (curve c in Figure 6b) and at 390 nm (curve b,
neglecting the slow background) showed similar kinetic behavior
•
In the presence of TEA, the delayed formation of 3-ByO at
5
-1
and decayed by first-order kinetics with k ) 5 × 10 s . But
the 340 nm absorption (curve a in Figure 6b) is long-lived and
showed a partly delayed formation estimated to proceed with k
340 nm also disappeared. Thus, we can conclude that (i)
•
+
3-ByOH is formed by the charge-transfer reaction with the
solvent parent ions (reaction 20), (ii) in competition with this,
6
-1
•
)
2.5 × 10 s . This delayed formation was also observed in
fast 3-ByO formation takes place by deprotonation as shown

5760 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Mohan et al.
-ByOH•+ + 3-ByOH a (3-ByOH)2•+
3
(24)
Analogous pulse radiolysis studies with 2-ByOH and 4-ByOH
in BuCl were carried out also. The kinetic and spectral
parameters for various transient species formed from these
compounds are presented in Tables 1-5.
3
.3. Effect of Solute Structure. Kinetic Considerations.
The charge transfer from radiolytically generated parent ions
to added solutes (e.g., reaction 25) is a satisfactory way of
producing radical cations of substances with an ionization
1
5,16
potential lower than the nonpolar solvent used.
BuCl•+ + S f S + BuCl
•+
(25)
-
3
In the case of scavenger (S) concentrations less than 2 × 10
-
3
mol dm , a free-ion reaction is concerned, comparable with
•
•-
those of the usual one-electron oxidants such as N3 , SO4
,
etc. in polar systems but exhibiting the advantage of the relative
stability of the product radical cation. Hence, alkanes, cyclo-
alkanes, and also alkyl chlorides are used for pulse radiolytic
studies of charge-transfer phenomena. When the rate constants
of solute cation formation are determined by analyzing the
influence of the scavenger on the time profiles of the parent
cations, the rate constants k25 are sometimes a little higher than
the diffusion-controlled limit (cf. Table 1). The reason for this
effect probably consists of the initially incomplete relaxation
Figure 7. Optical absorption spectra obtained during pulse radiolysis
-
3
of an N
2
-saturated solution of 3-ByOH (0.1 mol dm ) in BuCl 0.16
(b) and 1.4 µs (O) after the pulse. The difference between these two
spectra is shown as 1. The inset shows absorption time profiles obtained
during pulse radiolysis of 3-ByOH (0.1 mol dm- ) in BuCl at 420 (a)
and 650 nm (b).
3
14
of the solvent cations due to their “positive hole” character
in reaction 21, and (iii) a delayed deprotonation (reaction 22)
of the solute cation 3-ByOH•+ was observed.
17
and phasing-out of their inhomogeneous distribution. But the
•
+
solute cations S are in a completely free and relaxed state.
1
BuCl•+ + 3-ByOH f 3-ByOH + BuCl
•+
As already reported in a previous paper, in the case of
(20)
(21)
(22)
phenols (mainly sterically hindered), charge-transfer reaction
25 generates, within our time resolutions of about 10 ns, two
products in an apparently competing manner: phenol radical
cations and phenoxyl radicals, with the latter dominating by
far. This is described by reactions 1a and 1b). Furthermore, it
should be mentioned that the rapid formation of phenoxyls
(reaction 1b) does not proceed by the deprotonation of phenol
radical cations, which has been found to be a delayed process.1
The charge-transfer studies presented here with naphthols and
hydroxybiphenyls of different OH substitution patterns seem
to provide a deeper insight into the mechanism described above.
Figure 8 gives a general reaction scheme of the cation reactions
studied. In the reaction with parent cations for the naphthols
and hydroxybiphenyls, in all cases a much higher portion of
BuCl•+ + 3-ByOH f 3-ByO + H (BuCl)
•
+
•
+
•
+
3
-ByOH , BuCl f 3-ByO + H (BuCl)
This situation closely resembles those observed in the naphthol
system. Further evidence for the classification of absorption
•
bands of 3-ByO radicals came from the similarity of the
absorption spectrum obtained in BuCl to that spectrum obtained
•
-
in the reaction of N3 radicals with 3-ByO (inset of Figure 3).
-
•
3
•
-
3
3
-ByO + N f 3-ByO + N
(23)
The yield of 3-ByOH•+ increased with solute concentration,
-
3
-3
•+
reaching saturation at 5 × 10 mol dm (inset of Figure 6a).
radical cations ArOH was observed (cf. Table 5) than for
•
However, for 3-ByO a considerable yield increase was also
phenols. Furthermore, the lifetime of these species also lasts
much longer, being determined mainly by neutralization (reac-
tions 2a and 17) of the charge. At this point let us include in
the discussion the larger size of the aromatic system in
comparison to the OH substituent. Hence, to explain the
competition of the parallel cation (1a, 8, 20) and oxyl radical
(1b, 14, 21) formation, we propose a mechanism that takes into
account the geometry of the collision of the reacting species
observed at higher solute concentrations. The rate constant of
•
the delayed formation (reaction 22) of 3-ByO radicals remained
independent of solute concentration. However, for solute
-
2
-3
concentrations greater than 7 × 10 mol dm , another
absorption band (λmax ) 420 nm) appeared. Figure 7 shows
this situation using transient spectra taken of a 0.1 M solution
of 3-ByOH in BuCl. The delayed formation at 340 and 520
•
•+
nm of 3-ByO radicals is independent of the solute concentration
BuCl and ArOH, either at the aromatic part or at the polar
and appears at the same rate as at low concentrations. However,
hydroxyl group of the solute molecule. In the first case the
formation of an aromatic-dominated radical cation ought to
occur with charge delocalization throughout the whole molecule.
In the collision near the OH substituent just before charge
delocalization, an immediate deprotonation of the solute radical
cation in a complexed state should take place. This is indicated
in the reaction scheme as the charge-transfer situation given in
brackets. Hence, reaction channel 1b immediately yields ArO•
and protons that are stabilized afterward by solvatization by
the surroundings, in this case BuCl.
•
+
the 3-ByOH decay profiles at 380 and 650 nm were acceler-
ated in a pseudo-first-order manner, and simultaneously, the
increasing absorption at 410-430 nm was observed. Corre-
sponding to the 1-NpOH experiments described in the former
chapter, we attribute this delayed transient formation (inset of
Figure 7) to the ion molecule reaction 24 resulting in a product
of a conceivable dimer radical cation structure. The rate
constant of the forward reaction was determined to be k24 )
7
3
-1 -1
4
.5 × 10 dm mol s .

n-Butyl Chloride Radical Cations
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5761
The other reactions contained in the reaction scheme (Figure
) illustrate the reactivity of the solute radical cations against
8
added further scavengers (TEA, alcohol), which are commonly
used for radical cation identification. As experimentally
demonstrated in this paper, TEA acts in low concentration as a
•
+
-3
-3
charge scavenger for the ArOH , whereas at >10 mol dm
mainly the solvent parent cations are quenched. Ethanol rapidly
quenches the solvent parent ions by its deprotonation; the
•
+
deprotonation of ArOH seems to be slower and could not be
well characterized.
Charge Distribution and Geometry of the Transition State.
The lifetime of the radical cation formed under identical
conditions of dose and solute concentration was compared for
different phenols, naphthols, and hydroxybiphenyls (Table 5).
In the case of phenols, it was observed to increase in the
presence of the electron-releasing methoxy group, and alkyl
substituents seem to have a similar, albeit minor, effect. Hence,
lifetimes for phenol radical cations were in the range 0.5-0.7
µs. In the case of naphthols and hydroxybiphenyls, lifetimes
were in the ranges 1-2 and 2-3 µs, respectively. As already
postulated, the lifetime dependence on the molecular structure
could be due to the influence of the aromatic moiety. This also
seems to be supported by the remarkable difference in the
relative yields (Gꢀ) of the radical cations (cf. Table 5).
To learn more about the influence of the electronic structure
of the transient species, we undertook semiempirical open-shell
quantum chemical calculations. By use of PM31
8,19
wave
functions, the geometries and energies for the ground and the
excited states of phenols, naphthols, and hydroxybiphenyls were
calculated using HyperChem 5.01. The geometry optimization
of all compounds was performed in a vacuum with the Polac-
Ribiere conjugate gradient method in the unrestricted Hartree-
Fock (UHF) approximation. The direct inversion in the iterative
subspace (DIIS) method was used for better convergence of self-
consistent field (SCF) calculations of the electronic structure.
HyperChem uses atomic charges, which are commonly used in
molecular orbital theory. The calculated quantities are only
approximately related to experiment, but they provide a reason-
able representation of the 3D charge distribution within a
molecule.
Figure 8. Reaction scheme depicting the charge-transfer processes
involving different kinds of phenolic compounds such as one-ring
phenols, naphthols, and hydroxybiphenyls.
TABLE 5: Lifetime and Absorbance of the Solute Radical
Cations of Different Solutes ArOH Formed in BuCla
solute cation ArOH•+
λ
max (nm)
lifetime (µs)
Gꢀ × 10
-3
phenol
∼420
<0.2
0.3
0.5
2
4
2
,6-di-tert-butylphenol
-methoxy phenol
,6-di-tert-butyl-
440
400
460
3.5
1.1
As expected, in each case part of the positive charge is nearly
uniformly distributed among all H atoms of the aromatic rings.
The oxygen polarizes the molecule in such a way that the
phenolic proton and the neighboring C atom have the highest
charges. In the case of the unsubstituted phenol, the carbon in
the para position to the OH group represents an additional
positive charge localization center. In the case of the naphthol
as well as hydroxybiphenyl isomers, the second aromatic moiety
contributes to a further charge localization. There are no real
systematics for a marked charge localization difference in the
carbon atoms except that being substituted by OH. But the
charge delocalization in the H atoms shows more or less uniform
behavior, expressed in very similar positive values. Charge
delocalization thus takes place throughout the whole aromatic
molecule. Exceptions from this observation include 2-naphthol
and 3-hydroxybiphenyl, where in the carbon skeleton an
additional charge localization center has been found in the 4-
and 1-position, respectively. The typical charge distribution of
a phenolic radical cation is shown in Chart 1 using the example
of 2-naphthol and 3-hydroxybiphenyl.
0.7
4
-methoxyphenol
1
2
2
3
4
-NpOH
-NpOH
-ByOH
-ByOH
-ByOH
600
570
600
650
650
1.9
1.5
2.4
2.1
2.1
0.9
1.0
1.5
2.6
2.8
a
Concentration of the solute ) 5 × 10 mol dm-3. Dose ) 52 Gy
-3
per pulse.
This effect of competition between charge transfer 1a and
deprotonation 1b was only observed in the case of the reaction
of free ions with the phenolic scavenger. In the triplet-sensitized
electron transfer 12 checked for the donor-acceptor pair
T1
2
-NpOH and chloranil, only naphthol radicals are formed, i.e.,
•+
an immediate deprotonation of the NpOH occurred within the
ion pair. This can clearly be seen when comparing transient
spectra of triplet-sensitized reaction 12 given in Figure 4 with
those obtained in the ion-molecule reactions 1a and 1b
involving the free ions of the solvent given in Figure 5.
The time-resolved deprotonation of the stabilized and delo-
•+
calized radical cation ArOH (left side of the scheme) proceeds
in a delayed manner (reaction 15). Clear kinetic connections
were also found for this reaction channel by observing the decay
With respect to the geometry of the transition state of the
studied charge transfer reactions 1a and 1b, we analyzed possible
stable configurations for the electron jump using the same
approach. Generally speaking, there exists in each case a rapid
•
+
•
of ArOH and the formation of one part of ArO .

5762 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Mohan et al.
CHART 1: 2-Naphthol (Left) and 3 Hydroxybiphenyl (Right)
and geometry-unspecific charge transfer to the aromatic moiety,
as is also known to proceed over relatively long distances.
References and Notes
2
0
(
1) Brede, O.; Orthner, H.; Zubarev, V. E.; Hermann, R. J. Phys. Chem.
996, 100, 7097.
2) Schnabel, W. Polymer Degradation; Hanser: Munich, 1992.
(3) Scott, G. Atmospheric Oxidation and Antioxidants; Elsvier: Am-
sterdam, 1993; Vols. I, II.
4) (a) Becconsall, J. K.; Clough, S.; Scott, G. Proc. Chem. Soc. 1959,
08. (b) Bennett, J. E. Nature 1960, 186, 385.
5) (a) Brede, O.; Hermann, R.; Mehnert, R. Radiat. Phys. Chem. 1986,
However, the calculations also yielded a stable geometry for
the charge transfer from the BuCl•+ to the phenolic group, which
shows a clear distance between the Cl atom of the BuCl and
the phenolic proton of 1.8 Å. This gives rise to the postulated
mechanism involving synchroneous electron transfer and a
deprotonation of the phenolic group. In view of the proton
affinity of a halocarbon such as BuCl, there is a considerable
1
(
(
3
(
28, 507. (b) Brede, O. Radiat. Phys. Chem. 1987, 29, 369. (c) Brede, O.;
Hermann, R.; Mehnert, R. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2365.
-
1
proton affinity (PA) of about 150 kcal mol , i.e., quite
comparable with those of a corresponding alcohol (e.g., PACH3Cl
(
6) (a) Land, E. J.; Porter, G.; Strachan, E. Trans. Faraday Soc. 1961,
7, 1885. (b) Land, E. J.; Porter, G. Trans. Faraday Soc. 1963, 59, 2016.
c) Land, E. J.; Ebert, M. Trans. Faraday Soc. 1967, 63, 1181. (d) Bansal,
5
-
1
-1 21
)
163 kcal mol , PACH3OH ) 181 kcal mol ). Hence, in
(
the case of BuCl the stable electron-transfer geometry at the
phenolic group ought to favor electron transfer with immediate
deprotonation resulting in phenoxyl radicals and solvent-
stabilized protons. This interpretation supports the mechanism
established experimentally.
K. M.; Fessenden, R. W. Radiat. Res. 1976, 67, 1.
(7) (a) Dixon, W. T.; Murphy, D. J. Chem. Soc., Faraday Trans. 2
976, 72, 1221. (b) Dixon, W. T.; Kok, P. M.; Murphy, D. J. Chem. Soc.,
1
Faraday Trans. 2 1977, 73, 709. (c) Dixon, W. T.; Murphy, D. J. Chem.
Soc., Faraday Trans. 2 1978, 74, 432.
(
8) Holtman, D. M.; Murphy, D. J. Chem. Soc., Faraday Trans. 2 1979,
5, 1637.
(9) Kesper, K.; Diehl, F.; Simon, J. G. G.; Specht, H.; Schweig, A.
Chem. Phys. 1991, 153, 511.
10) Zubarev, V. E.; Brede, O. Acta Chem. Scand. 1997, 51, 224.
7
4
. Conclusions
Owing to their low ionization potential, the phenols are good
(
electron donors and can react with the solvent radical cations
(11) (a) Robinson, A. J.; Rodgers, M. A. J. J. Chem. Soc., Faraday
Trans. 1 1973, 69, 2036. (b) Mehnert, R.; Brede, O.; Naumann, W. Ber.
Bunsen-Ges. Phys. Chem. 1982, 86, 525.
•
+
(BuCl ) in two ways, charge transfer 1a and immediate
deprotonation 1b. The yield of radical cations of phenols (C6H5-
(
(
12) Alfassi, Z. B.; Schuler, R. H. J. Phys. Chem. 1985, 89, 3359.
13) (a) Carmichael, J.; Hug, G. J. Phys. Chem. Ref. Data 1986, 15,
1
OH) is low, and reaction 1b seems to dominate. In the case
of naphthols and hydroxybiphenyls, the solvent radical cations
react in comparable parts via charge transfer 1a and deproto-
nation 1b. By contrast, in the triplet-sensitized electron transfer
1
35. (b) Brede, O.; Mehnert, R.; Naumann, W.; Teply, J. Ber. Bunsen-Ges.
Phys. Chem. 1985, 89, 1036. (c) Gschwind, R.; Haselbach, E. HelV. Chim.
Acta 1979, 79, 941.
(
14) Brede, O.; B o¨ s, J.; Helmstreit, W.; Mehnert, R. Radiat. Phys. Chem.
982, 19, 1.
15) Busi, F. In The Study of Fast Processes and Transient Species by
(reaction 12) examined only for the pair naphthol triplet and
1
choranil, only naphthoxyl radicals seemed to be formed, which
is explained by electron transfer with subsequent proton transfer
within the ion pair.
(
Electron Pulse Radiolysis; Baxendale, J., Busi, F., Eds.; D. Reidel:
Dordrecht, 1982; p 417.
(16) Brede, O.; Mehnert, R.; Naumann, W. Chem. Phys. 1987, 115, 279.
As also supported by quantum chemical calculations, for
naphthols and hydroxybiphenyls the positive charge is stabilized
because of the extended aromatic moiety, which is also
experimentally demonstrated by comparison of the lifetimes of
(17) Warman, J. M. In The Study of Fast Processes and Transient
Species by Electron Pulse Radiolysis; Baxendale, J., Busi, F., Eds.; D.
Reidel: Dordrecht, 1982; p 433.
(18) Collins, J. B.; Schleyer, P. V.; Binkley, J. S.; Pople, J. A. J. Chem.
Phys. 1976, 64, 5142.
•
+
the radical cations ArOH . Owing to the increased stability
of the radical cations of naphthols and hydroxybiphenyls, they
decay to a considerable extent via the charge neutralization
process, whereas the radical cations of phenols decay exclusively
(19) Stewart, R. F. J. Chem. Phys. 1970, 52, 438.
(
20) Pilling, M. J.; Rice, S. A. J. Chem. Soc., Faraday Trans. 2 1975,
7
1, 1563.
21) Tabata, Y., Ito, Y., Tagawa, Eds. In Handbook of Radiation
Chemistry; CRC Press: Boca Raton, FL, 1991; p 204.
(
1
via the deprotonation mechanism.