Time resolved resonance Raman observation of the extreme protonation forms of a radical zwitterion in water

Article abstract of DOI:10.1063/1.1860311

The reactions of the aqueous proton with the zwitterionic p-aminophenoxyl radical in strongly basic to extremely acidic aqueous solutions have been investigated using time-resolved resonance Raman spectroscopy. The dynamic stability of the different protonation forms of the radical, observed on the microsecond time scale in this work, has been achieved by controlling the proton exchange rate in water. In strongly acidic solutions we observe a rare ring-H⁺ bonded dication species, a key intermediate in the amine hydrolysis. The neutral p-aminophenoxyl radical undergoes NH₂- deprotonation in strongly basic aqueous solutions, which has no analogues in closed-shell amines.

Full text of DOI:10.1063/1.1860311

Time resolved resonance Raman observation of the extreme protonation forms of a
radical zwitterion in water
G. N. R. Tripathi
Citation: The Journal of Chemical Physics 122, 071102 (2005); doi: 10.1063/1.1860311
View online: http://dx.doi.org/10.1063/1.1860311
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THE JOURNAL OF CHEMICAL PHYSICS 122, 071102 ͑2005͒
Time resolved resonance Raman observation of the extreme protonation
forms of a radical zwitterion in water
G. N. R. Tripathi
Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
͑
Received 29 November 2004; accepted 29 December 2004; published online 4 February 2005͒
The reactions of the aqueous proton with the zwitterionic p-aminophenoxyl radical in strongly basic
to extremely acidic aqueous solutions have been investigated using time-resolved resonance Raman
spectroscopy. The dynamic stability of the different protonation forms of the radical, observed on
the microsecond time scale in this work, has been achieved by controlling the proton exchange rate
+
in water. In strongly acidic solutions we observe a rare ring-H bonded dication species, a key
intermediate in the amine hydrolysis. The neutral p-aminophenoxyl radical undergoes
NH -deprotonation in strongly basic aqueous solutions, which has no analogues in closed-shell
2
amines. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1860311͔
INTRODUCTION
EXPERIMENTAL METHODS
One-electron oxidation of p-aminophenol in oxygen-free
Proton interactions are fundamental chemical interac-
tions in aqueous systems that can drastically alter the physi-
cochemical properties of a chemical species. They provide a
•
•
•
water was accomplished by reactions of OH, HSO , and N
4
3
10–12
radicals produced by pulse radiolysis.
The proton con-
1
centration in solution was varied with H SO and KOH. On
2
4
natural probe for the charge distribution in solvated mol-
•
−
aq
electron pulse irradiation of oxygen-free water, OH and e
1
–5
ecules and radicals. We have examined in this work the
reactions of protons with the p-aminophenoxyl radical, a re-
action intermediate of considerable chemical and biochemi-
are the main reactive species present in solution on the
−
1
00 ns time scale. In mildly acidic to basic solutions, e was
aq
•
converted into OH by its reaction with N O-saturated water.
Secondary oxidants, such as N , were prepared in basic so-
lutions by the reaction of OH ͑pK 11.9͒ or O with an
excess of N . In H SO solution, the HSO radical is pro-
duced by the OH oxidation of the acid and also by direct
action of the radiation. The pulse radiolysis–time resolved
resonance Raman techniques used in this work have been
previously described. The specific experiments use 2 MeV,
2
6
–8
•
3
cal importance,
which acquires a strongly zwitterionic
8
,9
•
−•
electronic structure in water.
a
•
−
3
•
4
The p-aminophenoxyl radical ͑H N␾O ͒ is isoelectronic
2
2 4
•
with the biologically important p-benzosemiquinone radical
−
2
•
anion ͑␾O ͒ and p-phenylenediamine radical cation
+
2
•
1
͓
␾͑NH ͒ ͔, the semiquinone models. The p-aminophenoxyl
2
radical can mimic many of the functions of the latter radicals
7
,8
1
00 ns electron pulses, delivered by a Van de Graaff accel-
in an aqueous environment. It is a prototype of the active
site free radical intermediates in enzymes, such as cerulo-
plasmin and copper amine oxidases, and in metabolic utili-
−
4
−5
erator at dose rates that produced about 10 –10 M radical
concentration. The Raman scattering was probed by an
excimer-pumped dye laser pulse tuned in resonance with the
optical absorption of the radicals. The spectra were recorded
using an optical multichannel analyzer, accompanied by an
intensified gated diode array detector, with the gate pulse
synchronized with the Raman signal pulse. Extensive signal
averaging was performed to improve the signal-to-noise ratio
in the Raman spectra, with the accelerator and laser operated
at a repetition rate of 7.5 Hz. In both experiments, a flow
system was used to refresh the solution between consecutive
electron pulses. Raman band positions were measured with
reference to the known Raman bands of common solvents,
such as ethanol and carbon tetrachloride. They are accurate
7
zation of human health related chemicals, such as serotonin.
The semiquinone properties of the p-aminophenoxyl radical
•
͑
H N␾O ͒ arise due to the effects of hydrogen bonding and
2
the solvent reaction field in water that induce a positive
charge on the amine-N and a negative charge on the O site of
8
the radical. The zwitterionic character of the radical is fur-
ther enhanced on incorporation of ions into the hydration
9
shell. Because of its unusual charge distribution this radical
displays interesting protonation behavior in aqueous solution
that is not seen in closed shell amines.
The reactions of the aqueous proton with
p-aminophenoxyl radical were examined in extremely acidic
to very strong basic solutions by time-resolved resonance
Raman spectroscopy. Here we present a rare observation of
proton addition to the cation form of the radical in extreme
acid solutions. In strongly basic solutions, the radical exists
in its amine-deprotonated anionic form. The spectral changes
that occur on successive proton additions establish the most
−1
−1
to within ±2 cm for sharp bands and ±5 cm for broad
and shoulder bands.
RESULTS AND DISCUSSION
The absorption spectra of the p-aminophenoxyl radicals
show only a small variation in strongly acidic to moderately
strong basic aqueous solutions, with absorption maxima in
+
acidic form of the radical as a ␲-H bonded dication.
0
021-9606/2005/122͑7͒/071102/4/$22.50
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122, 071102-1
© 2005 American Institute of Physics
1

071102-2
G. N. R. Tripathi
J. Chem. Phys. 122, 071102 ͑2005͒
−
1
shoulder band on its higher frequency side ͑ϳ1666 cm ͒.
The NH scissors is not enhanced in other protonation forms
2
of the radical, apparently because the frequency is not in
proximity of the 8a mode or the ring and the amine moieties
8
,9
are non-coplanar. A high Raman enhancement of the 8a
mode occurs due to elongation of the ring along this mode in
*
2
2
the excited state of the resonant ␲ � ␲ ͑ B � B ͒ elec-
2
2
−
1
tronic transition. The moderately enhanced 1432 cm mode
represents in-phase C-NH and C-O stretching motions ͑Wil-
2
son mode 7a͒ with a small ring stretching component similar
to the Wilson mode 19a. On the other hand, the very weakly
−
1
enhanced 1516 cm mode represents out-of-phase C-NH2
and C-O stretching motions coupled with the 19a ring vibra-
tion. The CO and CN bonds in the aqueous radical are typi-
cal of the three electron bonds, with bond lengths estimated
as 1.284 and 1.34 Å, respectively. This recent study on the
8
neutral p-aminophenoxyl radical provides valuable guide-
lines for the spectra-structure correlations presented in this
work for the different protonation states of the radical.
In the semiquinone state, exchange between the unpaired
electron and electron pair on two equivalent atomic sites of a
conjugated system leads to formation of two three-electron
FIG. 1. Transient Raman spectra observed 1 ␮s after electron pulse irradia-
tion of oxygen-free aqueous solutions containing 2 mM p-aminophenol and:
͑
A͒ 0.1 M NaN , 8 M KOH; ͑B͒ 0.1 M NaN , pH 11; ͑C͒ pH 1.8; and ͑D͒
3
3
spectrum with 16 M H SO in solution–spectrum in ͑C͒ ͑see text͒. Excita-
2
4
tion was at 470 nm for the spectrum in A and 441 nm for B, C, and D.
1
bonds. Therefore, the bond properties in the semiquinone
radicals are intermediate between completely oxidized
͑quinone-like͒ and reduced ͑hydroquinone-like͒ states. A loss
in the unpaired ␲-electron density from the ring sites of the
radical increases its quinoid character and, as a consequence,
8
,13
the 435–445 nm region.
However, with 8 M KOH in so-
3
lution, the absorption shifts to the red by ϳ30 nm. The Ra-
man spectra were recorded 1 ␮s after the electron pulse from
which solvent background was subtracted. At very high acid
concentration, this subtraction procedure leads to consider-
able noise in the difference Raman spectra, specifically in the
frequency region of strong acid bands. In this frequency re-
gion very weak signals cannot be unambiguously attributed
to the transients formed. Fortunately, the structure sensitive
bands in p-aminophenoxyl radicals appear in the
2
the 8aCC frequency. In the centrosymmetric ͑D2h͒
p-benzosemiquinone anion and p-phenylenediamine cation
radicals, the 19a ring vibration is symmetry forbidden in Ra-
man. It couples with the asymmetric stretching motions of
the substituent groups and produces a Raman inactive vibra-
−
1
tion in the ϳ1505 cm region. On the other hand, the sym-
metric stretching motion of the substituent groups ͑Wilson
−
1
Ͼ1200 cm region and are fairly prominent, so they can be
readily distinguished from the solvent bands and can be posi-
tively identified with the species.
mode
7a͒
in
p-benzosemiquinone
anion
and
p-phenylenediamine cation radicals produces a vibration in
−
1
the ϳ1409 cm region which is quite prominent in the reso-
nance Raman spectra of these radicals. A slight non-
equivalence of the substituent groups ͑CO/CN͒ in the
p-aminophenoxyl radical is the reason why the 1516 cm−1
mode is seen with a very low intensity in the spectrum ͓Fig.
1͑b͔͒, i.e., it is not strictly forbidden. In essence, the intensity
Figure 1 depicts transient Raman spectra in the
1
1
300–1800 cm− region, excited in resonance with radical
absorption maxima. Four protonation forms of the radical are
evident. All four forms show bands at 1622±30 cm ,
505±25 cm , and 1409±25 cm ͑we will use these aver-
age frequencies for general discussion͒ with relative intensi-
ties changing with the state of protonation. The nature of the
vibrational modes responsible for these bands, their relative
enhancement in the spectra and frequency shifts, provide in-
sight into the protonation sites and their structural effects.
The neutral p-aminophenoxyl radical ͑H N␾O ͒, seen in
moderately acidic to basic solutions in H O and D O, has
−
1
−
1
−1
1
−
1
of the ϳ1505 cm mode is an indicator of the non-
equivalence of the ring-substituent bonds. It represents the
8
most intense band in the 400–430 nm resonance Raman
•
spectra of aqueous phenoxyl ͑␾O ͒ and aniline cation
+
2
•
͑␾NH ͒ radicals, attributed to predominantly the CO and
•
CN stretching motions ͑ϩsmall 19a component͒,
2
11
respectively. These CO and CN bonds are close to a double
bond ͑␲-bond order Ͼ0.5͒. The strengthening of the ring-
substituent bonds weakens the adjacent ring CC bonds in-
2
2
been a subject of numerous experimental and theoretical in-
vestigations, with and without incorporation of solvent inter-
3
,8,13,14
−1
actions in calculations.
Convergence has occurred re-
volved in the 19a mode. Therefore, the ϳ1409 cm mode in
•
+•
2
cently in the spectroscopic and theoretical interpretations of
phenoxyl ͑␾O ͒ and aniline cation ͑␾NH ͒ radicals is pre-
8
the structure of the radical in water. In brief, the most in-
dominantly a 19a mode which contains a small CO or CN
stretching component, and is very weakly enhanced in the
resonance Raman spectra. As the CO or CN bonds become
−
1
tense band at 1632 cm in the spectrum ͓Fig. 1͑b͔͒ is as-
signed to the 8a mode ͑Wilson notation͒ which primarily
involves the stretching motion of the central ring CC
•
weaker in para-substituted radicals ͑e.g., X␾O ͒, the relative
1
5
bonds. The NH scissors is mechanically coupled to the 8a
mode, derives intensity from this coupling, and appears as a
contribution of the CO or CN stretch decreases in the
2
−
1
−1
ϳ1505 cm mode, and increases in the ϳ1409 cm mode.
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1

071102-3
A radical zwitterion in water
J. Chem. Phys. 122, 071102 ͑2005͒
On the other hand, the stretching motion of the CX bond
which is strengthened and acquires a partial double bond
character ͑␲-character͒ at the cost of the CO or CN bonds
At very high acid concentration in solution, further pro-
tonation occurs. In 16 M acid solution, the singly protonated
cation radical still persists, but additional signals at slightly
different frequencies also appear. On subtraction of the
mono-cation radical signals from the spectrum, the differ-
ence spectrum that is obtained is shown in Fig. 1͑d͒. This
spectrum is not of the hydroquinone cation radical or its ␲
−
1
also contributes towards the ϳ1409 cm mode, particularly
when X=O or N. The resonant electronic transitions in these
*
radicals are ␲ � ␲ transitions. It is the ␲-component of the
bonds that can possibly change on electronic excitation.
Therefore, the ␲-bond order of the CO or CN bonds quali-
tatively correlates with the relative intensities of the ϳ1505
and ϳ1409 cm− modes. To summarize, the simple qualita-
2
proton-adduct, indicating that hydrolysis of the amine group
has not occurred. The 8aCC frequency shifts upwards by
1
−1
−1
21 cm to 1652 cm , a general trend that we observe on
−
1
2,11
tive description of the complex ϳ1505 and ϳ1409 cm
formation of radical cations. However, the relative Raman
−
1
−1
modes presented here, which was established on the basis of
the systematic experimental studies of closely related sys-
tems, explains why the strength of the ring-substituent bonds
has only a small effect on the vibrational frequencies, but a
drastic effect on their relative enhancement in the resonance
intensities of the 1519 cm and the 1396 cm modes are
not significantly affected and, in that respect, the spectrum
remains similar to that of the mono-cation form of the radi-
cal. It should be pointed out that the proton adduct of the
mono-cation form of the p-aminophenoxyl radical is ex-
11
+
Raman spectra. In p-aminophenoxyl radical, the contribu-
tremely short-lived which implies that the H -bonding is ex-
−
1
tions of the CO and CN stretches to the 1432 cm mode are
comparable. In the following discussion we will use the neu-
tral p-aminophenoxyl radical as reference for interpreting the
observed spectral trends that occur on addition or loss of a
proton from the radical.
tremely weak in the dication radical. However, in 16 M
H SO solution, the dication radical is formed at a rate faster
2
4
than its decay rate which gives it dynamic stability and al-
lows observation on the microsecond time scale.
The amine-N, hydroxyl-O, and the ring are the sites
where proton addition to the mono-cation form of the
In very basic solutions ͑e.g., 8 M KOH͒, the
+
2
•
NH -deprotonated anion form of the radical is observed ͓Fig.
p-aminophenoxyl radical ͑HO-␾=NH ͒ may occur. How-
2
1
͑a͔͒. The chemical species that are seen in this strongly
ever, protonation of amine-N, where a large fraction of the
positive charge on the radical resides, or hydroxyl-O, is not
consistent with the observed spectrum. If N or O-protonation
−
9
basic solution are very short-lived ͑Ͻ10 s͒ in pure water
from which they abstract a proton and disappear by protona-
tion. However, in the presence of very high base concentra-
tion ͑Ͼ1 M͒ in solution, their formation rate from the neutral
radical can exceed their rate of decay, facilitating observation
of the equilibrium concentration on a much longer time
scale, as in the present case. NH and O largely share the
negative charge on the radical, forming three-electron
␲
+
occurred, one would essentially observe an H N -substituted
3
+
+•
+
phenol cation ͑H N -␾=OH ͒ or an H O -substituted
3
+
2
+•
2
aniline cation ͑H O -␾=NH ͒ radical. The formation of the
ϾC=OH or ϾC=NH double bonds in such species would
transfer the unpaired ␲-electron density from OH /NH to
2
+
+
2
1
6
+
+
2
the ring. The ␲-electron density, however, cannot be effec-
1
7
+
-bonds through the ring. Because of the higher electrone-
tively shared by the substituent groups, such as -NH or
3
+
2
gativity of O, a slightly more electronic charge resides on it
than on NH, which may be the reason why a slightly lower
-OH , which lack vacant p␲ orbitals to accommodate the
electron. The sharing requires the capability to form a ␲
bond. Therefore, the spectrum would be closer to that of the
aniline cation ͑␾NH ͒ or phenol cation radical ͑␾OH ͒,
with a significant drop in the 8aCC frequency. The ob-
1
9
−
1
1
frequency ͑1385 cm ͒ of the CN/CO stretching mode than
in the neutral radical is observed. The 1385 cm−1 band is
+•
2
+•
−
1
20
about three times more intense than the 1482 cm band,
which suggests almost comparable CO and CN ␲-bond or-
served spectrum does not display these characteristics.
−
1
ders. The 8aCC frequency ͑1593 cm ͒ in the anion radical is
It can be readily visualized that loss of ␲-electron from
1
5
+•
closer to that of a hydroquinone structure. To our knowl-
edge, this is a rare example of deprotonation of a neutral
the HO-␾vNH2 radical ͑oxidation͒ would produce a
quinone-like structure and increase the 8aCC frequency. The
sharing of the unpaired ␲-electron on the ring by the proton
amounts to a partial loss of electron from the ring, without a
change in the oxidation state. Therefore, an increase in the
8aCC frequency should occur. Since the frequency of the
1
8
amine observed in water.
The neutral radical protonates on its O ͓Fig. 1͑c͔͒ in
1
3
mildly acidic solutions ͑pHϽ2.2͒. The 8aCC frequency
−
1
͑
-
1631 cm ͒ remains virtually unchanged. However, the C
−
1
NH bond becomes significantly stronger than in the neutral
species. As a result, the 1519 cm mode containing the CN
stretch becomes more prominent in the spectrum, as com-
1396 cm mode containing the CO stretch is lower than in
the mono-cation form of the radical, the proton must add to
2
−
1
the ring ␲-system on the side of the -OH group ͑HO-͑H͒␾
−
1
+•
2
pared to the 1406 cm mode that contains the C-OH stretch.
The cationic form of the p-aminophenoxyl radical is more
appropriately described as the p-hydroxy-aniline cation radi-
cal, with a major fraction of the positive charge on the amine
group. Thus, p-aminophenoxyl, a nominal oxy radical, be-
comes an amine radical on proton addition. The
O-protonation of the p-aminophenoxyl radical in moderately
=NH ͒, thus reducing the ␲-component of the CO bond and
the frequency. A ␴-bonding must be ruled out, as the CO
bond would become comparable to or weaker than a single
bond and a far greater reduction in the CO frequency would
be expected. In the cation form of the p-benzosemiquinone
+
•
radical anion ͓͑HO=␾=OH͒ ͔, proton addition occurs to
the ring ␲-system at the center. It is not surprising that in a
+
•
acidic solution and -NH deprotonation in very basic solu-
relatively less symmetric HO-␾=NH2 radical, the proton
addition to the ring ␲-system occurs closer to the OH side, as
2
tions are chemical consequences of a zwitterionic structure.
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1

071102-4
G. N. R. Tripathi
J. Chem. Phys. 122, 071102 ͑2005͒
Mason, in Free Radicals in Biology, edited by W. A. Pryor ͑Academic,
New York, 1982͒, Vol. 5; P. D. Josephy, T. E. Eling, and R. P. Mason, Mol.
Pharmacol. 23, 461 ͑1983͒, and references cited in these papers.
G. N. R. Tripathi, J. Chem. Phys. 118, 1378 ͑2003͒.
it experiences less electrostatic repulsion than from the posi-
tive charge on -NH . The most acid form of the
2
+
p-aminophenoxyl radical is essentially a rare ␲-H bonded
carbonium radical species.
8
9
0
2
1
G. N. R. Tripathi, J. Phys. Chem. 108, 5139 ͑2004͒.
1
The role of proton-induced double bond character of the
C. von Sonntag, The Chemical Basis of Radiation Biology ͑Taylor and
Francis, New York, 1987͒; B. Lesigne, C. Ferradini, and J. Pucheautt, J.
Phys. Chem. 77, 2156 ͑1973͒.
+
ϾC=X bonds in the hydrolysis of closed-shell molecules is
well recognized, although transitory existence of such spe-
11
G. N. R. Tripathi, in Time Resolved Spectroscopy, Vol. 18, edited by R. J.
J. H. Clark and R. E. Hester ͑Wiley, New York, 1989͒, pp. 157–218; G. N.
R. Tripathi, in Multichannel Image Detectors II, edited by Y. Talmi, ACS
Symposium Series 236 ͑Am. Chem. Soc., Washington, D.C., 1983͒, p.
171;
1
8
cies has precluded direct observation. One and two proton
adducts of the p-aminophenoxyl radical ͑X=NH ͒ are essen-
2
tially the radical analogues of such close-shell intermediates.
+
In hydrolysis mechanisms, the ϾC=X bond facilitates H O
2
12
1
B. Lesigne, C. Ferradini, and J. Pucheautt, J. Phys. Chem. 77, 2156
+
addition at C, by transferring positive charge on X to O of
3͑
1973͒.
H O, which is followed by XH elimination and replacement
G. N. R. Tripathi and R. H. Schuler, J. Chem. Phys. 76, 4289 ͑1982͒; J.
Phys. Chem. 88, 1706 ͑1984͒; J. Chem. Soc., Faraday Trans. 89, 4177
2
1
8
of X by OH. We did not observe conversion of the proto-
nated p-aminophenoxyl radicals into the hydroquinone cat-
ion radical presumably because, under experimental condi-
tions used, radical–radical reactions dominate slow
hydrolysis. However, in steady-state oxidation of acidic
͑
146
1993͒; Q. Sun, G. N. R. Tripathi, and R. H. Schuler, J. Phys. Chem. 94,
273 ͑1990͒.
R. Liu and X. Zhou, J. Phys. Chem. 97, 9613 ͑1993͒; 97, 9618 ͑1993͒; K.
S. Raymond and R. A. Wheeler, J. Chem. Soc., Faraday Trans. 89, 665
͑1993͒; D. M. Chipman, J. Phys. Chem. A 103, 11181 ͑1999͒.
Wilson notations used for convenience. See F. R. Dollish, W. G. Fateley,
and F. F. Bentley, Characteristic Raman Frequencies of Organic Com-
pounds ͑Wiley, New York, 1974͒.
1
5
−
8
2
p-aminophenol by mild oxidants, such as S O , hydrolysis
2
was observed to be complete in a few hours.
In summary, we have presented vibrational spectroscopic
evidence of unusual protonation forms of a zwitterionic ␲
radical in water that mimics redox-active sites in a variety of
biological systems.
16
Assuming a diffusion-controlled rate ͑ϳ10 s−1͒ for the OH− reaction in
10
−
−
MH+OH ↔M +H O, a pK of Ͼ14 for MH would imply a life-time of
2
a
Ͻ10− s for M− to decay by the reaction with water. However, a steady-
10
state concentration of this species can be observed at a much longer time-
1
0 −1
scale if it is produced at a faster rate ͑Ͼ10
concentration of OH .
s ͒ using an appropriate
−
ACKNOWLEDGMENT
17
−
−
The loss of p␲-electron from the hydroquinone state, HN-␾-O , is
largely confined to NH and O. The three-electron bonds formed on ex-
change of ͑p␲͒2 and p␲ electrons on HN and O in ͑HN-␾-O͒ have a
minor effect on the central ring ͑hydroquinone-like͒ bonds.
The research described herein was supported by the Of-
fice of Basic Energy Sciences of the Department of Energy.
This is Contribution No. NDRL 4200 from the Notre Dame
Radiation Laboratory.
−•
1
1
8
9
C. H. Rochester, Acidity Functions ͑Academic, New York, 1970͒. See, for
example, W. M. Shubert and R. H. Quacchia, J. Am. Chem. Soc. 85, 1278
͑
1963͒; 85, 1284 ͑1963͒. Olah and co-workers pioneered the method of
1
detection of such intermediates. See G. A. Olah, A. Molnar, and K. B.
L. Pauling, The Nature of the Chemical Bond ͑Cornell University Press,
Loker, Hydrocarbon Chemistry ͑Wiley-Interscience, New York, 1995͒.
Ithaca, New York, 1960͒.
+
+
2
The transfer of the electronic charge to -OH and -NH is possible only at
G. N. R. Tripathi, J. Am. Chem. Soc. 120, 5134 ͑1998͒.
2 3
3
the cost of weakened bonding with the ring and likely dissociation of the
radical. Since the cation and dication forms of the radical are interconvert-
ible by changing the acid concentration, the possibility of the radical dis-
sociation was ruled out.
G. N. R. Tripathi, J. Phys. Chem. 102, 2388 ͑1998͒.
4
G. N. R. Tripathi, Y. Su, J. Bentley, R. W. Fessenden, and P.-Y. Jiang, J.
Am. Chem. Soc. 118, 2245 ͑1996͒; G. N. R. Tripathi and Y. Su, ibid. 118,
2
235 ͑1996͒; G. N. R. Tripathi, Y. Su, and J. Bentley, ibid. 117, 5540
2
2
0
1
−1
The 8aCC frequency is lower than 1600 cm in aniline cation and anisole
5͑
R. P. Bell, The Proton in Chemistry ͑Cornell University Press, Ithaca, NY,
959͒.
1995͒; Y. Su and G. N. R. Tripathi, ibid. 116, 4405 ͑1994͒.
and p-anisole cation radicals ͑Ref. 11͒. Also see G. N. R. Tripathi, Chem.
Phys. Lett. 199, 409 ͑1992͒; G. N. R. Tripathi and R. H. Schuler, J. Chem.
Phys. 86, 3795 ͑1987͒.
1
6
S. G. Anchell, The Darkroom Developer ͑Butterworth-Heinemann, Lon-
don, 2000͒.
M. Inal and G. Kanbak, Med. Sci. Res. 25, 323 ͑1997͒; R. H. Bisby, S. A.
Johnson, and A. W. Parker, J. Phys. Chem. B 104, 5832 ͑2000͒; R. P.
Structural calculations on the radicals in question lack predictive value and
7
a
satisfactory calculation of the vibrational frequencies of
2
+•
͑HO͑H͒␾NH2͒ could not be performed.
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