Spectroscopic and kinetic properties of the radical zwitterion and related intermediates in the one-electron oxidation of p-aminobenzoic acid

Article abstract of DOI:10.1021/ja953180z

The time-resolved resonance Raman spectra and acid-base properties of the transient prepared on the submicrosecond time scale by N₃^. oxidation of aqueous p-aminobenzoic acid (PABA) in near-neutral solutions identify it as the charge-neutral 4-^-O₂C-aniline^.+ radical. This zwitterion intermediate undergoes slow thermal dissociation (K ~ 2.4 x 10³ s^-1) by intramolecular electron transfer and CO₂ elimination. It reacts with common bases, such as OH^-, N₃^-, and HPO₂^2- ions, at the rate constants of 1.9 x 10¹⁰, 2.7 x 10⁶, and 2.2 x 10⁸ M^-1 s^-1, respectively, and converts into the nondissociative anilino radical form (pK(a) 6.7). In the ^.OH oxidation, formation of the zwitterion radical occurs via OH adducts (hydroxycyclohexadienyl radicals) of p-aminobenzoate anion, at a rate of 1.4 x 10⁵ s^-1 which is independent of the base concentration. In strongly acidic solutions the OH adduct of p-aminobenzoic acid reacts with H⁺, at a rate constant of 4.7 x 10⁸ M^-1 s^-1, to form the 4-HO₂C-aniline cation radical (pK(a) 1.1). The benzidine cation radical, observed as one of the transient secondary products in Raman experiments, results from the reactions of p-aminophenyl radical produced on dissociation of the 4-^-O₂C-aniline^.+ radical. The marked pH dependence of the nature and yields of the photoproducts of p-aminobenzoic acid, observed in several recent studies, is explained in terms of the dissociative properties of the zwitterion radical and its base-catalyzed conversion into the anilino radical. A two-state model has been developed for the explanation of the intramolecular electron transfer and bond dissociation in the radical intermediates with structurally stable ground electronic state.

Full text of DOI:10.1021/ja953180z

J. Am. Chem. Soc. 1996, 118, 2235-2244
2235
Spectroscopic and Kinetic Properties of the Radical Zwitterion
and Related Intermediates in the One-Electron Oxidation of
p-Aminobenzoic Acid†
G. N. R. Tripathi* and Yali Su‡
Contribution from the Radiation Laboratory and Department of Chemistry and Biochemistry,
UniVersity of Notre Dame, Notre Dame, Indiana 46556
ReceiVed September 18, 1995X
Abstract: The time-resolved resonance Raman spectra and acid-base properties of the transient prepared on the
•
submicrosecond time scale by N3 oxidation of aqueous p-aminobenzoic acid (PABA) in near-neutral solutions identify
it as the charge-neutral 4--O2C-aniline•+ radical. This zwitterion intermediate undergoes slow thermal dissociation
(k ∼ 2.4 × 103 s-1) by intramolecular electron transfer and CO2 elimination. It reacts with common bases, such as
2-
OH-, N3-, and HPO4 ions, at the rate constants of 1.9 × 1010, 2.7 × 106, and 2.2 × 108 M-1 s-1, respectively,
and converts into the nondissociative anilino radical form (pKa 6.7). In the •OH oxidation, formation of the zwitterion
radical occurs via OH adducts (hydroxycyclohexadienyl radicals) of p-aminobenzoate anion, at a rate of 1.4 × 105
s-1 which is independent of the base concentration. In strongly acidic solutions the OH adduct of p-aminobenzoic
acid reacts with H+, at a rate constant of 4.7 × 108 M-1 s-1, to form the 4-HO2C-aniline cation radical (pKa 1.1).
The benzidine cation radical, observed as one of the transient secondary products in Raman experiments, results
from the reactions of p-aminophenyl radical produced on dissociation of the 4--O2C-aniline•+ radical. The marked
pH dependence of the nature and yields of the photoproducts of p-aminobenzoic acid, observed in several recent
studies, is explained in terms of the dissociative properties of the zwitterion radical and its base-catalyzed conversion
into the anilino radical. A two-state model has been developed for the explanation of the intramolecular electron
transfer and bond dissociation in the radical intermediates with structurally stable ground electronic state.
Introduction
consumption by the reduced form of p- hydroxybenzoate
hydroxylase, p-aminobenzoic acid acts as a substrate that
undergoes a hydroxylation reaction in the enzyme,9 presumably
via a free radical path.10,11 Information on the spectroscopic
and kinetic properties of the redox states of unbound p-
aminobenzoic acid in solution is essential for modeling the
intermediate chemical steps in these various processes.
The redox reactions of p-aminobenzoic acid are of consider-
able interest even from a purely chemistry perspective. It is a
relatively simple chemical system in which strongly electron-
donating (-NH2) and electron-accepting (-CO2H) groups are
bound to a single phenyl ring. Therefore, it can be readily
oxidized or reduced. This prototype of the aromatic amino acids
has been the subject of numerous studies in recent years, mostly
focused on stable radiation products and photoproducts, with
and without nucleic acid bases in solution, and on transients
produced in the hydroxylation reactions in flavoenzymes.3-9,12-15
In spite of the chemical, biochemical, and photobiological
significance of the reaction properties of p-aminobenzoic acid,
there is very little direct information available on the absorption,
kinetic, and structural characteristics of its short-lived radical
intermediates in solution. Previously, ESR spectroscopy has
p-Aminobenzoic acid (PABA) is widely distributed in plant
and animal cells in free and bound forms, and is recognized for
its medicinal values.1,2 It plays an important physiological role
as metabolic inhibitor and has been accorded vitamin status
restricted to microorganisms where it is utilized in the synthesis
of folic acid compounds.2 In its commercial application as an
active ingredient of sunscreens this chemical prevents photo-
carcinogenesis, but it also increases photosensitivity toward
DNA damage in which a role for free radicals has been
implicated.3-8 In the mechanistic studies of molecular oxygen
† The research described herein was supported by the Office of Basic
Energy Sciences of the Department of Energy. This is Contribution No.
NDRL 3863 from the Notre Dame Radiation Laboratory.
‡ Present address: Pacific Northwest Laboratories, Richland, WA.
X Abstract published in AdVance ACS Abstracts, February 15, 1996.
(1) Wagner, A. F.; Folkers, K. Vitamins and Coenzymes; Interscience
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of Biochemistry; The MacMillan Company: New York, 1967; p 823.
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280, 1459. Willis, I.; Kligman, A. M. Arch. Dermatol. Res. 1970, 102, 405.
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1990, 282, 38.
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1990, 365, 1952.
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Proc. Indian Acad. Sci. 1986, 97, 573. Subramanian, P.; Rajaram, J.;
Ramakrishnan, V.; Kuriacose, J. C. Tetrahedron 1988, 44, 4631.
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1068. Rutherford, C. E.; Salter, L. F.; Thomas, R. C. Photochem. Photobiol.
A 1990, 52, 337. Gasparro, F. P.; Battista, J. Photochem. Photobiol. 1987,
45, 495. Sutherland, J. C.; Griffin, K. P. Photochem. Photobiol. 1984, 40,
391. Gasparro, F. Photodermatalogy 1985, 2, 151. Sutherland, B. M.
Photochem. Photobiol. 1982, 36, 95. Vasil’eva, S. V.; Zhizhina, G. P.;
Rapoport, I. A. Dokl. Akad. Nauk. SSSR 1980, 252, 755.
0002-7863/96/1518-2235$12.00/0 © 1996 American Chemical Society

2236 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Tripathi and Su
•
solution have been described in detail in several previous publications
from this Laboratory.27-29 Radiolysis by ∼8 MeV, ∼5 ns electron
pulses from a linear accelerator, which produced a radical concentration
of ∼3 × 10-6 M per pulse, was used for optical absorption measure-
ments. In the Raman experiments, 2 MeV, ∼100 ns electron pulses,
delivered by a Van de Graaff accelerator, at a dose rate sufficient to
product ∼3 × 10-4 M radical concentration per pulse, were applied.
In both experiments, a flow system was used to refresh solution between
consecutive electron pulses.
been used to examine radicals produced on OH oxidation of
p-aminobenzoic acid in basic aqueous solutions, and detection
of 4-carboxyanilino and 2-amino-5-carboxyphenoxyl anion
radicals has been reported.16 In spin-trapping studies of the
transient photoproducts in the presence of 2-methyl-2-nitroso-
propane, formation of 4-carboxyphenyl radical has also been
demonstrated.17 In the mechanistic formulation of the reaction
sequences leading to various products, it is generally assumed
that the oxidized radical states of p-aminobenzoic acid should
exhibit reaction properties analogous to other para-substituted
aniline radicals,18 although the nature of the products, such as
benzidine,12 does not support this assumption. It has been
suggested that the oxidation may involve electron transfer from
the CO2- group,12 but there has been no experimental evidence
to show that it actually happens. It has been recognized
previously that the reaction mechanism of the oxidized radical
state of p-aminobenzoic acid in mildly acidic solutions should
be drastically different from that in basic solutions.4,5,12 How-
ever, the molecular properties responsible for this behavior are
not understood.
In this paper, one-electron oxidation of p-aminobenzoic acid
in aqueous solution has been studied by optical absorption and
resonance Raman spectroscopy. The latter technique provides
structural identification of the transient absorbing species, and
is valuable for relating the structure with chemistry. We present
here vibrational spectroscopic evidence of a zwitterion radical
intermediate which undergoes thermal dissociation to produce
benzidine, via intermediary formation of p-aminophenyl radical.
This dissociative property of the zwitterion radical makes its
reactive behavior very different from that of the other structur-
ally similar aniline cation derivatives,18 with profound chemical
and biochemical implications.
Formation of Radical Zwitterion on Oxidation of PABA
•
It is generally accepted that the N3 radical reacts with organic
molecules by electron transfer. In several recent studies, this
radical has been used to prepare para-substituted aniline cation
and phenoxyl radicals, to measure their redox potentials.21,22 In
aqueous aniline, conclusive evidence of initial cation radical
(pKa ∼ 7) formation by N3• oxidation, even in basic solutions,
has come from Raman spectroscopy.29-31 In p-aminobenzoate
anion, electron transfer from two distinct sites, i.e., ring-NH2
and CO2-, has been suggested.12 In order to visualize the effect
of solvation on the oxidation reaction, it would be logical to
consider the resulting one-electron-deficient states as a combina-
tion of two electronic configurations, i.e., -O2C-aniline+ and
O2C-aniline. The hydration will favor and stabilize the dipolar
zwitterion configuration. However, it is difficult to predict the
relative contributions of the two configurations to the ground
electronic state, based on this qualitative argument. Since much
of the chemistry depends on the nature of the oxidation state, it
is important to ascertain it by a structure-sensitive method.
Transient Absorption Spectra. The transient absorption
observed 2 µs after electron pulse irradiation of an aqueous
solution containing 1 mM p-aminobenzoic acid (PABA) and
0.1 M NaN3 at pH 5.2 is shown in Figure 1a. The absorption
peaks are located at 445 (ꢀmax ∼ 2.6 × 103 M-1 cm-1) and 425
nm (shoulder peak, ꢀmax ∼ 2.3 × 103 M-1 cm-1). Temporal
evolution of this absorption at substrate concentrations of 0.5
to 10 mM was used to determine the reaction rate constant of
N3• radical with PABA, which was found to be 5 ((0.5) × 109
M-1 s-1. This rate constant shows little variation between pH
6 and 13. Because of strong overlapping absorption due to
Experimental Section
The one-electron oxidation of p-aminobenzoic acid (pKa 2.50 and
4.90)19 in aqueous solution was achieved by pulse radiolysis. Electron
-
irradiation of oxygen-free water produces eaq (2.6), hydroxyl radical
(2.7), and H atom (0.6) (numbers in parentheses are G values, i.e.,
yields of radicals per 100 eV of energy absorbed)20 on the ∼100-ns
-
scale. In moderately acidic and basic solutions, the eaq is converted
into •OH radical by saturating aqueous solutions by N2O.20 The
secondary oxidants N3• and Br2•- are prepared by reaction of •OH with
•
-
-O2CC6H4NH2 + N3 f (-O2CC6H4NH2)•+ + N3 (1)
-
an excess of NaN3 and KBr.21,22 In acidic solutions, the eaq reacts
with H+ (k ∼ 2.3 × 1010 M-1 s-1
)
to form H• atom. Therefore, the
20
products, an accurate determination of the transient decay
kinetics was difficult. A similar problem was encountered
previously in the case of aniline oxidation.30,31 The insert in
Figure 1 depicts a first-order plot for the decay of the transient
absorption in the 445-nm region with time, using background
absorption as reference. From this plot the decay rate constant
for the 445-nm transient at pH ∼ 6 was estimated as ∼2.4 ×
subsequent reaction is primarily by OH and H•. The OH radical is
scavenged by tert-butyl alcohol (t-BuOH), in order to identify the
transient absorption due to the H• reaction only. In basic N2-saturated
solutions containing t-BuOH, the eaq- is the primary reducing species,
and is frequently used to prepare electron and hydrogen adduct radicals
for transient absorption, ESR, and, more recently, Raman studies.23-26
The optical absorption and resonance Raman methods applied here
to study transient chemical species produced by pulse radiolysis in
•
•
103 s-1
.
The absorption spectrum in Figure 1a resembles the spectrum
(16) Neta, P.; Fessenden, R. W. J. Phys. Chem. 1974, 78, 523.
(17) Chignell, C. F.; Kalyaraman, B.; Mason, R. P.; Sik, R. H.
Photochem. Photobiol. 1980, 32, 563.
of the aniline cation radical,30-32 except that it is shifted to the
(18) Bacon, J.; Adam, R. N. J. Am. Chem. Soc. 1968, 90, 6596. Also
see: Solar, S.; Solar, W.; Getoff, N. Int. J. Radiat. Phys. Chem. 1986, 28,
229.
(19) Kortum, G.; Vogel, W.; Andrussow, K. IUPAC Dissociation
constants of orgnaic acids in aqueous solution; Butterworths: London, 1961;
p 381.
(20) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J.
Phys. Chem. Ref. Data 1988, 17, 513.
(21) Jonsson, M.; Lind J.; Eriksen, T. E.; Merenyi, G. J. Am. Chem.
Soc. 1994, 116, 1423.
(26) Tripathi, G. N. R.; Su, Y.; Bentley, J. J. Am. Chem. Soc. 1995,
117, 5540.
(27) Patterson, L. K.; Lilie, J. Int. J. Radiat. Phys. Chem. 1974, 6, 129.
Janata, E.; Schuler, R. H. J. Phys. Chem. 1982, 86, 2078.
(28) Tripathi, G. N. R. In Multichannel Image Detectors II; Talmi, Y.,
Ed.; ACS Symposium Series 236; American Chemical Society: Washington,
DC, 1983; p 171.
(29) Tripathi, G. N. R. In AdVances in Spectroscopy; Clark, R. J. H.,
Hester, R. E., Eds.; John Wiley & Sons: New York, 1989; Vol. 18, pp
157-218.
(30) Qin, L.; Tripathi, G. N. R.; Schuler, R. H. Z. Naturforsch. 1985,
40a, 1026.
(22) Lind J.; Shen, X.; Eriksen, T. E.; Merenyi, G. J. Am. Chem. Soc.
1990, 112, 479.
(23) Anbar, M.; Hart, E. J. J. Am. Chem. Soc. 1964, 86, 5633.
(24) See, for example: Jeevarajan, A. S.; Fessenden, R. W. J. Am. Chem.
Soc. 1992, 114, 10461.
(31) Tripathi, G. N. R.; Schuler, R. H.; Qin, L. Springer Proc. Phys.
1985, 4, 183.
(32) Tripathi, G. N. R.; Schuler, R. H. Chem. Phys. Lett. 1984, 110,
542. Tripathi, G. N. R.; Schuler, R. H. J. Chem. Phys. 1987, 86, 3795.
(25) Su, Y.; Tripathi, G. N. R. J. Am. Chem. Soc. 1994, 116, 4405.

One-Electron Oxidation of p-Aminobenzoic Acid
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2237
Figure 1. Transient absorption spectra obtained 2 µs after electron
pulse irradiation of N2O-saturated aqueous solution containing 1 mM
PABA, 2 mM phosphate buffer, and 0.1 M NaN3 at pH (a) 5.2 and (b)
10.5. The insert shows exponential decay of the 445-nm absorption
peak in Figure 1a with time.
Figure 2. Resonance Raman spectra obtained (A) 0.1 and (B) 10 µs
after electron pulse irradiation of N2O-saturated aqueous solution
containing 5 mM PABA, 0.1 M NaN3, and 2 mM phosphate buffer at
pH 5.4. Excitation at 440 nm.
Table 1. Rate Constants for PABA Reactions
reacting
species
a
state of PABA
k (M-1 s-1
)
Table 2. Comparison of the Resonance Raman Frequencies (cm-1
)
•
-O2CC6H4NH2
HO2CC6H4NH2
-O2CC6H4NH2
5 × 109
1.3 × 108
7 × 109
and Band Intensitiesa of the Oxidized Radical States of Anilineb and
PABAc
N3
•-
Br2
OH•
(8.2 × 109)b
(1.6 × 1010)c
-O2C-
C6H4NH2
-O2C-
•+
•+
C6H5NH2
C6H5NH• C6H4NH•
assignmentd
+
H•
HO2CC6H4NH3
6.7 × 108
1.7 × 109
2.8 × 1010
1 × 1010
O•-
-O2CC6H4NH2
HO2CC6H4NH2
-O2CC6H4NH2
1574 w
1494 vs
1380 m
1570 w
1504 vs
1384 m
1560 s
1505 m
1452 w
1324 m
1167 w
1557 m
1504 m
1463 w
1326 m
8a, ring stretch
7a, C-N stretch
19a, ring stretch
14, ring stretch
9a, CH bend
13, C-CO2 stretch
18a, CH bend
1, ring breathe
-
eaq
-
eaq
(2.1 × 109)d
•+
OH-
-O2CC6H4NH2
1.9 × 1010
2.7 × 106
2.2 × 108
(2 × 105 s-1
-
•+
-O2CC6H4NH2
1175 w
1185 m
1121 w
994 w
N3
HPO4
H2O
H2O
2-
•+
-O2CC6H4NH2
•+
e
-O2CC6H4NH2
)
1001 m
820 w
1003 m
817 w
-O2CC6H4NH2(OH)• (1.4 × 105 s-1
)
e
812 w
(OH adduct)
a Relative intensities in each spectrum: v ) very, s ) strong, m )
medium, w ) weak. b References 29 and 32. c This work. d Approxi-
mate mode description in terms of Wilson notations.
H+
HO2CC6H4NH2(OH)
(OH adduct)
4.7 × 108
a This work. b Reference 42. c Reference 43. d Reference 23. e First-
order rates.
The frequencies and relative intensities of the transient Raman
signals in Figure 2A are compared in Table 2 with those of the
aniline cation radical prepared by pulse radiolytic oxidation of
aqueous aniline in mildly acidic solutions.32 The two spectra
appear almost indistinguishable, except for the slight shifts in
some peak positions. For example, the most intense band at
1504 cm-1 in Figure 2A is at 10 cm-1 higher frequency than
the corresponding 1494-cm-1 band in the 432-nm Raman
spectrum of the aniline cation radical. A weak band at 1122
cm-1 is an additional feature in the spectrum of Figure 2A which
is not observed in the aniline cation radical (see Table 2).
Whether this band belongs to the same species as the strong
band at 1504 cm-1 or to some minor reaction product is a
question which relates to the transient identification. Therefore,
this aspect was carefully examined. First, the spectrum in the
combination and overtone region, 1800-3000 cm-1, was
recorded. Combination bands were observed at 2875 (1504 +
1384 ) 2888), 2689 (1504 + 1185 ) 2689), 2625 (1504 +
1122 ) 2626), and 2569 (1384 + 1185 ) 2569) cm-1. Since
the fundamental frequencies of two distinct species cannot
combine, the 1122-cm-1 frequency must be attributed to the
same transient as the other frequencies in the spectrum of Figure
2A. Secondly, the decay kinetics of the 1122-cm-1 band was
compared with that of the closeby 1384-, 1185-, and 994-cm-1
bands in the spectrum. The kinetic monitoring was difficult,
red by ∼15 nm. In basic solutions, the 445-nm transient decays
into another species, the spectrum of which is given in Figure
•-
1b. The Br2 radical produced by pulse radiolysis of N2O-
saturated aqueous solution containing 0.1 M KBr was also used
to oxidize PABA. This reaction was examined only in the pH
range 3.5 to 4.7,21,22 in which the Br2•- radical reacts with PABA
at a rate constant of 1.3 × 108 M-1 s-1. The transient absorption
observed 10 µs after the electron pulse was found to be identical
•
to the spectrum displayed in Figure 1a, showing that N3
•-
oxidation of p-aminobenzoate anion and Br2 oxidation of
p-aminobenzoic acid produce the same intermediate. Table 1
summarizes the various rate constants measured in this work.
Resonance Raman Spectra. For structural identification of
the reaction intermediates, the Raman spectra of transients
produced on pulse radiolytic oxidation of PABA were probed
at several excitation wavelengths, between 360 and 470 nm.
Raman spectra (600-1800 cm-1) obtained by 440-nm excita-
tion, 0.1 and 10 µs after electron pulse irradiation of a N2O-
saturated solution containing 5 mM PABA, 0.1 M NaN3, and 2
mM phosphate buffer at pH 5.4, are displayed in Figures 2A
and 2B, after proper subtraction of the background Raman
signals of the unirradiated solution. The two spectra are quite
different and it is clear that they represent two distinct species.

2238 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Tripathi and Su
as these bands are quite weak and appear in the vicinity of the
product bands which are relatively stronger (see Figure 2). The
1384-, 1185-, and 1122-cm-1 bands appear to decay at the same
time scale. Only the 994-cm-1 band grows slightly in intensity
with time, as a stronger product band also occurs at about this
frequency (996 cm-1).
In a less polar structure, one would expect the ring-NH2
stretching frequency to be significantly lower than in aniline
cation radical, and not slightly higher as observed. One would
also expect the C-CO2 stretching frequency to be much lower
than the observed frequency. Thus, the spectroscopic evidence
is fairly conclusive that the 445-nm transient is a radical
zwitterion.
The transient Raman signals in Figure 2A appear less than
100 ns after the electron pulse, and the spectrum attains its
maximum intensity when excitation approaches ∼440 nm,
relating it to the same transient as the absorption spectrum in
Figure 1a. The transient associated with the Raman spectrum
in Figure 2A, although structurally very similar to the aniline
cation radical, cannot be identified with the latter species because
of the minor differences noted above. The additional 1122-
cm-1 band suggests that the species observed is a substituted
aniline cation radical, with little perturbation of the vibrational
structure due to this substitution. In strongly acidic solutions
the 1122-cm-1 frequency shifts downward by 9 cm-1, suggest-
ing protonation of the substituent group. These facts, along
with the chemical conditions used in the observation, identify
the 445-nm transient with the CO2--substituted aniline cation
The zwitterion structures stabilized by hydration are an
important feature of the closed-shell amino acids and their free
radical states.2,36-39 In most closed-shell amino acids, the
-CO2H proton transfers to the -NH2 group on hydration,
forming the zwitterion structure which represents a lower energy
structure. The zwitterion radical state, -O2C-aniline+, estab-
lished above as the ground electronic state by Raman spectros-
copy, can be thought of as a state arising from transfer of an
electron from ring-NH2 to -CO2 group in the higher energy
state, O2C-aniline (hereafter referred to as the neutral state).
The solvent reaction field which stabilizes the zwitterion
structure may act as a barrier between the two states. Since
the neutral radical state is expected to undergo spontaneous
dissociation, direct observation of thermal equilibrium between
the zwitterion and neutral states is not possible. However,
because of this equilibrium, the zwitterion state will appear to
dissociate, at a relatively low first-order rate. Thus, we have a
very simple but logical two-state model for intramolecular
electron transfer leading to bond dissociation which is applicable
not only in the case of the oxidized PABA but also for a variety
of radicals, including those derived from aliphatic amino acids
(this model is discussed in detail in a later section). It should
be emphasized that the ground electronic state of these radicals
does not have to possess a dissociative bond. Since the solvent
reaction field depends on the dielectric constant (ꢀ) {2(ꢀ-1)/
(2ꢀ+1)},40 the model predicts a faster dissociation rate in the
solvents of low polarity, e.g., CCl4(ꢀ ) 2.3), compared to highly
polar solvents, e.g., CH3CN(ꢀ ) 36) and water (ꢀ ) 78). This
prediction is readily verified by the decarboxylation rates
measured by Ingold and co-workers for the photooxidized
benzoic and p-methoxybenzoic acids in CCl4 and CH3CN,41 and
the rate measured for the radiolytically oxidized p-methoxy-
benzoic acid in water in this work.
-
radical. Because of the CO2 group, the vibrational modes
containing ring-CO2- and symmetric CO2- stretching motions,
in addition to the modes enhanced in the aniline cation spectrum,
become likely candidates for resonance enhancement in the
Raman spectrum of the intermediate. The 1122-cm-1 band is
-
•
attributable to the ring-CO2 stretch. It is clear that the N3
oxidation of PABA in aqueous solution involves electron
transfer from the aniline moiety.
The resonance Raman spectra of the aniline cation and related
radicals,29-33 and isoelectronic phenoxyl radicals,34 have been
studied previously, and the spectrum in Figure 2A can be
interpreted in a straightforward way by comparison (Table 2).
The very strong band at 1504 cm-1 represents a vibrational
mode which primarily involves the C-NH2 stretching motion
mixed with ring vibrations. A 10 cm-1 higher frequency for
this mode than in aniline cation radical may suggest a slight
-
strengthening of the C-N bond due to the CO2 substitution,
although the effect must be small. A frequency in the region
of 1500 cm-1 for the C-N stretching mode in aniline cation
radicals (∼1280 cm-1 in aniline)35 is an indication of a C-N
bond which is close to a double bond (bond order >1.5). The
loss of an electron from the NH2 group would leave an unpaired
(p) electron on the nitrogen atom to compete for π-π bonding
with the adjacent ring carbon (p) electron, thus strengthening
the CN bond. Therefore, the substantial double bond character
of the C-N bond in the radical is an indication of the loss of
a large fraction of the electron from the NH2 group of the parent
molecule. The frequency of the 1122-cm-1 mode, which
Reaction of Zwitterion Radical with Base
The pH dependence of the nature and yields of the oxidation
products of PABA4,5 clearly indicates the critical role of the
bases in solution in controlling the chemistry. In aniline cation
radical, deprotonation by reaction with OH- has been investi-
gated,30 but the role of other bases and water is not well
understood. The reactions of the zwitterion radical of PABA
with three commonly used bases have been examined. The
decay rates of the 445-nm absorption were measured at several
concentrations of KOH in solution, and from the observed linear
relationship, the rate constant for the reaction of zwitterion
radical with OH- was determined as 1.9 × 1010 M-1 s-1. In
aniline cation radical, the corresponding rate constant has been
-
primarily represents the C-CO2 stretching motion, although
somewhat lower, is in the range where the stretching frequencies
of the C-C single bonds generally occur.36 This bond clearly
does not have any double bond character which would suggest,
although indirectly, that the CO2 plane is perpendicular to the
ring plane in the radical structure.
(37) Corey, R. B.; Donohue, J. J. Am. Chem. Soc. 1950, 72, 2899. Corey,
R. B.; Pauling, L. Proc. R. Soc. London 1953, B141, 10. Garfinkel, D.;
Edsall, J. T. J. Am. Chem. Soc. 1958, 80, 3807, 3818, 3823. Takeda, M.;
Iavazzo, R. E. S.; Garfinkel, D.; Scheinberg, I. H.; Edsall, J. T. J. Am.
Chem. Soc. 1958, 80, 2813. Ghazanfar, S. A. S.; Myers, D. V.; Edsall, J.
T. J. Am. Chem. Soc. 1964, 86, 3439 and references cited therein.
(38) Neta. P.; Fessenden, R. W. J. Phys. Chem. 1970, 74, 2263.
(39) Yu, D.; Rauk, A.; Armstrong, D. A. J. Am. Chem. Soc. 1995, 117,
1789. Armstrong, D. A.; Rauk, A.; Yu, D. J. Chem. Soc., Perkin Trans. 2
1995, 553.
(40) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486. Wong, M. W.; Frisch,
M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991, 113, 4782.
(41) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1987,
109, 897; 1988, 110, 2877, 2886.
It is evident from the resonance Raman spectrum of the 445-
nm transient that the molecular structure of the species is such
that the positive charge is mostly on the amine nitrogen and
-
the negative charge is largely confined on the CO2 oxygens.
(33) Poizot, O.; Guichard, V.; Buntinx, G. J. Chem. Phys. 1989, 90, 4697.
(34) Tripathi, G. N. R.; Schuler, R. H. Chem. Phys. Lett. 1983, 98, 594.
Tripathi, G. N. R.; Schuler, R. H. J Chem. Phys. 1984, 81, 113. Beck, S.
M.; Brus, L. E. J. Chem. Phys. 1982, 76, 4701. Johnson, C.; Ludwig, M.;
Asher, S. J. Am. Chem. Soc. 1986, 108, 905.
(35) Tripathi, G. N. R. J. Chem. Phys. 1980, 73, 5521.
(36) Dollish, F. R.; Fateley, W. F.; Bentley, F. F. Characteristic Raman
Frequencies of Organic Compounds; Wiley: New York, 1974.

One-Electron Oxidation of p-Aminobenzoic Acid
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2239
Figure 3. pH dependence of the 445-nm absorption, 20 µs after
electron pulse irradiation of N2O-saturated solution containing 1 mM
PABA and 0.1 M NaN3 (phosphate buffer). The sigmoidal curve
corresponds to a simple equilibrium with pKa ) 6.7.
estimated as 2.2 × 1010 M-1 s-1 by optical absorption, and ∼
5 × 1010 M-1 s-1 by Raman spectroscopy.30,31
The transient formed on reaction of the zwitterion radical
with base has a weak absorption at 425 nm (λmax), with ꢀmax
∼1 × 103 M-1 cm-1, which we attribute to 4--O2C-anilino
radical anion (Figure 1b). The absorption band shape and
intensity is similar to that of the anilino radical, but λmax is
shifted to the red by 15 to 20 nm. On Raman excitation at 423
nm, 100 ns after the electron pulse, a weakly enhanced Raman
Figure 4. (a) Absorption spectra obtained at different time intervals
after electron pulse irradiation of N2O-saturated solution containing 1
mM PABA and 1 mM phosphate at pH 7.3. (b) Time dependence of
the 400-nm absorption in (a) at different phosphate concentrations for
N2O-saturated solution with 10 mM PABA at pH 6.7.
spectrum, with bands at 1581, 1557, 1504, 1463, and 1326 cm-1
,
appears. The 1581-cm-1 band gains intensity in slow flow
experiments, showing its origin in some radiolytic product
accumulated in the Raman cell. Therefore, only the 1557-,
1504-, 1463-, and 1326-cm-1 bands can be associated with the
425-nm transient. These frequencies are similar to those
reported for the anilino radical32 (see Table 2). The carboxylate
group has a negligible effect on the vibrational structure of the
anilino radical, as noted previously for the aniline cation radical.
The ESR hyperfine constants of the anilino radical are also not
reported as 7.05.30 This shows that the presence of an electron-
withdrawing group, such as CO2-, at the para position has only
a small effect on the proton dissociation constant of the aniline
cation radical.
•
Reaction of OH Radical with PABA
In addition to the chemical interest in determining the rate
•
constants and understanding the mechanism of OH radical
reactions with PABA, the properties of the transient formed are
of relevance to the mechanistic studies of the reaction of oxygen
-
significantly affected by the CO2 substitution.16
The reactions of the -O2C-aniline•+ radical with other bases
were examined in near neutral solutions where the OH- reaction
is too slow. From a linear plot of decay rates of the 445-nm
absorption at different NaN3 concentrations, in the presence and
absence of phosphate buffer at pH 7.5 and 8.5, the rate constant
for the N3--catalyzed deprotonation of the zwitterion radical
was found to be 2.7 × 106 M-1 s-1. The rate of transient
formation in the reaction of molecular oxygen with p-hydroxy-
•
with certain enzymes.9-11 Since the mode of OH attack on
•
benzenoid systems is known to be different from that of N3 ,
comparison can be helpful in determining if electron transfer
from the CO2- group of PABA is of significance in the oxidation
•
reaction. The reaction of OH radical with p-aminobenzoate
anion initially produces a transient absorption with λmax 400
nm and ꢀmax ∼ 2.8 × 103 M-1 cm-1 (Figure 4). The absorption
spectrum of this species is different from the spectra of 4--O2C-
aniline•+ and anilino• radicals, depicted in Figure 1. From the
growth of the 400-nm absorption the rate constant for the
-
benzoate hydroxylase has been found to depend on the N3
concentration.9 It is likely that the role of azide anion in this
complex biochemical system is somewhat similar to its role in
aqueous solution, i.e., as a catalyst for proton transfer. This
also suggests that the pKa of the transferable proton is not too
high for the N3- anion to have perceptible effects on the kinetics.
Phosphate (HPO42-)-catalyzed deprotonation of the zwitterion
radical was examined at pH 8, which gave a rate constant of
2.2 × 108 M-1 s-1. There is also an additional decay component
(∼2 × 105 s-1) in the deprotonation reaction which is
independent of the substrate concentration. We attribute this
rate to the reaction of zwitterion radical with water acting as a
weak base.
•
reaction of PABA with OH was determined as (7.0 ( 1) ×
109 M-1 s-1. This rate constant is somewhat lower than 8.2 ×
109 and 1.6 × 1010 M-1 s-1, which were obtained previously
•
by competition studies.42,43 The OH radical generally reacts
with aromatic molecules by addition.44-46 We attribute the 400-
nm transient to the OH adduct of p-aminobenzoate anion.
The absorption of this species is shifted to the red by ∼35 nm
with respect to the OH adduct of p-hydroxybenzoate anion
(-O2CC6H4OH), and toward the blue by ∼20 nm with respect
to the OH adduct of p-oxybenzoate dianion (-O2CC6H4O-).46
These shifts are as expected, as the effect of the NH2 substitution
pKa of the Zwitterion Radical. As noted above, the 4--O2C-
aniline•+ radical undergoes deprotonation in neutral and basic
solutions. In the 450-nm region the absorption of the zwitterion
radical is very strong compared to that of its conjugate base.
Therefore, by monitoring the absorption in this region at
different pH, the pKa of the zwitterion radical can be estimated
with reasonable accuracy. Figure 3 shows a titration curve of
the 445-nm absorption Vs pH, which gives a pKa value of 6.7
((0.1). The pKa of the unsubstituted aniline cation radical is
(42) Anbar, N.; Meyerstein, D.; Neta, P. J. Phys. Chem. 1966, 70, 2660.
(43) Adams, G. E.; Boag, J. W.; Currant, J.; Michael, B. D. In Pulse
Radiolysis; Ebert, M., Keeme, J. P., Swallow, A. J., Eds.; 1965; p 131.
(44) Walling, C.; Camaioni, D. M.; Kim, S. S. J. Am. Chem. Soc. 1978,
100, 4814.
(45) Raghavan, N. V.; Steenken, S. J. Am. Chem. Soc. 1980, 102, 3495.
Steenken, S.; Raghavan, N. V. J. Phys. Chem. 1979, 83, 3101.
(46) Anderson, R. F.; Patel, K. B.; Stratford, M. R. L J. Chem. Soc.,
Faraday Trans. 1987, 83, 3177. Also, unpublished work from this
Laboratory.

2240 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Tripathi and Su
discussed later on, this absorption is identified with the 4-HO2C-
aniline cation radical by Raman spectroscopy. In acidic
solution, eaq- is partially converted to H atom, which gives an
adduct absorption at 337 nm (ꢀmax ∼1 × 104 M-1 cm-1). The
H atom reacts with PABA at pH 0.9, at a rate of 6.7 × 108
M-1 s-1
.
•
The 1-µs Raman spectrum obtained by OH oxidation of
PABA at pH ∼3 was identical to the spectrum in Figure 2A,
although the signal intensity was significantly lower. Since the
formation half-period of 4--O2C-aniline•+ via H2O elimination
from the OH adduct is ∼1.5 µs at this pH, the radical
concentration available for detection in Raman experiments is
very much reduced due to the loss of the OH adduct and
zwitterion radicals by second-order reactions on the microsecond
time scale. We do not observe any Raman signals attributable
to the adduct radicals, a problem we have encountered in almost
every aromatic system. We believe that the adduct radicals
generally undergo photodissociation induced by the Raman
probe pulse.
Protonation of the Zwitterion Radical. The acid constants,
readily measured for the -NH2 proton in aniline radicals by
optical absorption, are an important chemical parameter in
determining the redox potentials.21 The measurement of the
dissociation constant for the carboxylic proton in PABA radical
is not so straightforward. The absorption spectrum of the
4--O2C-aniline•+ radical in mildly acidic solutions is not readily
distinguishable from the spectrum in strongly acidic solutions.
Therefore, protonation of the zwitterion radical is difficult to
monitor by optical absorption. Unfortunately, the resonance
Raman spectrum also does not change significantly at low pH.
This is not unexpected, as carboxyl substitution has very little
effect on the aniline cation structure as discussed previously.
The only change which is readily noticeable is in the relative
intensities of the 1185- and 1122-cm-1 bands in the Raman
spectra. While the 1185-cm-1 band becomes very weak at low
pH, the 1122-cm-1 band shifts downward to 1113 cm-1. Thus,
it is clear that the species observed at very low pH, although
structurally similar, is distinct from the species produced in
mildly acidic solutions. We attribute the species observed at
low pH as 4-HO2C-aniline cation radical.
The pKa of the radical was determined by recording the
Raman spectra by oxidizing PABA with N3• radical at pH 6 (G
) 6) (Figure 6a), and by •OH radical at pH 0.5 (G ) 2.6) (Figure
6b), and combining them after multiplying by appropriate scaling
factors (Figure 6c), in order to simulate the spectrum for the
equal concentration of the two radical forms. The composite
spectrum was then compared with the spectra recorded at
different pH. It was found that the spectrum recorded at pH
1.1 (Figure 6d) matched the composite spectrum (Figure 6c).
Since the signals are very weak, this pKa value should be
regarded as approximate, and an error as large as (0.3 is not
unlikely.
Figure 5. Absorption spectra observed (a) 0.6 and (b) 7.5 µs after
pulse irradiating a solution containing 1 mM PABA at pH ∼3.5 (N2O
saturated). (c) Dependence of the decay rate of the absorption in (a)
on H+ concentration. The OH adduct of PABA reacts with H+ at a
rate constant of 4.7 × 108 M-1 s-1
.
on the physicochemical properties of the aromatic systems is
generally intermediate between those of the OH and O-
substitutions. The 400 nm transient decays with a rate constant
of 1.4 × 105 s-1 between pH 7 and 13.5. There is no evidence
that the decay is base-catalyzed or that the rate of decay depends
on buffer concentration (Figure 4b). Thus, the mechanism of
the adduct decay here is quite different from that of the OH
adducts of phenols. In basic solutions (pH >8) the product
formed is the 4--O2C-anilino• radical with absorption peak at
425 nm. At pH <8, however, the decay of the 400-nm
absorption results in absorption peaks at ∼425 and ∼450 nm,
i.e., both the anilino anion and zwitterion radicals are formed.
As can be seen from the 75-µs absorption spectrum in Figure
4a, taken at pH 7.3, the zwitterion absorption at 445 nm is
weaker than the absorption of the anilino anion radical at 425
nm, as expected from the acid-base properties of the radicals
discussed earlier.
•
In strongly basic solutions, the OH radical is converted to
O•- in <5 ns (pKa ) 11.9). At pH 13.1, where 94% of the
•OH radical converts into O•- in less than 1 ns, the reaction
rate constant 1.7 × 109 M-1 s-1 was found to be approximately
•
three times slower than in the OH reaction. The transient
product seen at 10 µs is very similar in absorption to the -O2C-
•
anilino• radical formed by N3 reaction at pH 11.4. At shorter
times, we observe an initial absorption in the 400-nm region
which decays at a rate of 1.5 × 105 s-1, i.e., the same as the
decay rate of OH adduct in mildly basic solutions, to produce
the 425-nm transient. It appears that the O•- reaction is similar
Raman Observation of Benzidine Cation Radical as a
Transient Product
•
in mechanism to the OH reaction in this system.
In acidic solutions, the OH adduct absorption shifts to higher
wavelengths with respect to the absorption observed at pH >7.
At pH 3.5, the λmax of the absorption is at 435 nm (ꢀmax ∼ 3 ×
103 M-1 cm-1) (Figure 5a). The 435-nm transient is attributed
to the OH adduct of the neutral PABA. This transient decays
at a rate which depends on H+ concentration (Figure 5c) from
which the H+-catalyzed decay rate constant was determined as
4.7 × 108 M-1 s-1. The transient formed at pH 0.9 by acid-
catalyzed loss of water from OH adduct has absorption λmax at
445 nm, and a shoulder at 425 nm, i.e., very similar to the
spectrum of the 4--O2C- aniline•+ radical in Figure 1a. As
Because of ∼100 times higher radiation dose in Raman
experiments than in transient absorption studies, complications
may arise due to product formation by second-order reactions
on the microsecond time scale. Figure 7 depicts the 440-nm
spectra, in the 1250-1700-cm-1 region, obtained on electron
pulse irradiation of a N2O-saturated solution containing 5 mM
PABA, 0.1 M NaN3, and 1 mM phosphate buffer at pH 5.5, at
different time intervals after the pulse. The spectra are displayed
after appropriate subtraction of background Raman signals of
unirradiated solution. The spectrum in Figure 7a was obtained

One-Electron Oxidation of p-Aminobenzoic Acid
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2241
recorded at later times, the intensity of the “A” signals decreases,
and signals due to another species labeled “B” start growing.
The Raman signals of the transient “B” attain their maximum
intensity ∼10 µs after the electron pulse (complete spectrum is
shown in Figure 2B). In the spectra, taken at longer times, the
signals “A” and “B” both decay, and at 100 µs after the electron
pulse, both signals become very weak. Experiments performed
at very fast flow of the solution through the Raman cell show
slight reduction in the intensity of the “B” signals, and complete
disappearance of signals marked “D”. A signal labeled “C” at
∼1500 cm-1 can also be seen in the 5-µs spectrum.
The transient “B” spectrum in Figure 7 (and Figure 2B) was
•
compared with the benzidine cation spectrum obtained by N3
oxidation of benzidine at pH 5.9 (Figure 7g), and also with the
published spectrum of the radical.47 Within experimental error,
the Raman spectra of the two species are identical. Therefore,
the transient “B” can be assigned to the benzidine cation radical
with certainty. It should be noted that the benzidine cation
radical has been detected because its absorption is in resonance
with the Raman excitation wavelength.47 It does not necessarily
imply that other transient secondary products, with absorption
far removed from the excitation wavelength, are not formed in
the reaction. In slow flow experiments, we have observed
Raman signals of unidentified species (e.g., signals marked “D”
in Figure 7) which result from radiolysis of the products formed
by previous electron pulses. Benzidine cation radical, observed
here, is mostly produced by the reactions initiated by a single
electron pulse. It can be seen that the decay of the “A” signals
is followed by the growth of “B” signals, establishing the 4--
O2C-aniline•+ radical as a precursor of the benzidine cation
radical. Signal “C” is attributed to the aniline cation radical.
•+
Figure 6. pKa estimation of the HO2CC6H4NH2 radical by Raman
spectroscopy. Excitation at 440 nm, 0.1 µs after pulse irradiation of 5
mM PABA solution: (a) 0.1 M NaN3, 2 mM phosphate added to
solution at pH ∼6.0, (N2O saturated); (b) pH ∼0.5, N2 saturated; (c)
) (a) + (b); (d) pH ∼1.1, N2-saturated solution. Spectra in (a) and (b)
are scaled to reflect equal concentrations of the species.
Mechanism of Benzidine Formation. Benzidine has not
been seen as a product of the photolytic oxidation of PABA by
every investigator.4,5,12,13 This is surprising, as most of the
proposed reaction schemes start with the 4--O2C-aniline•+ and
4--O2C-anilino• radicals as the initial transients. An under-
standing of the mechanism by which benzidine is produced may
provide a clue for not only these apparently contradictory
observations but also the overall chemistry. We want to look
for the reasons why a particular product should or should not
be observed under certain chemical conditions. This work
confirms that benzidine is indeed a product of PABA oxidation
in mildly acidic solutions. Here, we critically examine some
reaction mechanisms, proposed previously, for the formation
of benzidine in aniline related systems, and suggest improve-
ments based on the experimental data obtained in this work.
Mechanism I: This mechanism is based on a reaction scheme
proposed to explain benzidine formation in the photolytic
•
reaction of B(OH)3O•- radical with PABA.12 The N3 radical
(see reaction 1) may abstract an electron from the CO2- group
of p-aminobenzoate which would result in spontaneous dis-
sociation of CO2 and formation of p-aminophenyl radical. The
latter species, by bimolecular reaction, should produce benzidine.
Electron transfer from benzidine to 4--O2C-aniline•+, formed
by reaction 1, may convert it into the cation radical observed
in Raman experiments (reactions 2-4).
Figure 7. Raman spectra excited at 440 nm and observed (a) 100 ns,
(b) 500 ns, (c) 1 µs, (d) 5 µs, (e) 10 µs, and (f) 100 µs after pulse
irradiation of a N2O-saturated 5 mM PABA solution containing 0.1 M
NaN3 at pH ∼5.5. (g) Spectrum taken at 1.0 µs after pulse irradiation
of a N2O-saturated aqueous solution of benzidine (<1 mM) and 0.1 M
NaN3 at pH ∼5.9.
•
-O2CC6H4NH2 + N3 f •O2CC6H4NH2 f
•C6H4NH2 + CO2 (2)
•C6H4NH2 + •C6H4NH2 f (C6H4NH2)2
(3)
0.1 µs after the electron pulse (complete spectrum in Figure
2A). The Raman signals of the transient species formed
completely by this time are labeled “A”. In the spectrum
(47) Tripathi, G. N. R.; Schuler, R. H. Int. J. Radiat. Phys. Chem. 1988,
32, 251.

2242 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
-O2CC6H4NH2•+ + (C6H4NH2)2 f
Tripathi and Su
order reactions, which interfered with the first-order rate
determination in CH3CN by Ingold and co-workers, are of no
consequence. From the Onsager expression for the solvent
reaction field,40 which does not include specific solvent interac-
tions like hydrogen bonding, one would generally expect the
rate of decarboxylation in water and CH3CN to be comparable.
Hydrogen bonding is essentially a weak electrostatic interaction
which would slightly lower the effective negative charge on
the carboxylic group, thus reducing the zwitterionic character
of the radical structure and weakening the ring-CO2 bond.
Therefore, a slightly higher rate of decarboxylation in water
than in CH3CN is not unexpected. It is interesting to note that
the rate of decarboxylation drops by only a factor of ∼102, from
oxidized benzoic acid radical in CCl4 to p-methoxybenzoic acid
radical in water. The NH2 substitution is known to exert greater
stabilizing effect than OCH3 in radicals, but the difference is
far less in comparison to that between H and OCH3. Therefore,
the rate of decarboxylation of the radical zwitterion of p-
aminobenzoic acid (PABA) in water is expected to be lower
than 2 × 104 s-1, but greater than 2 × 102 s-1. Therefore, the
first-order decay of PABA zwitterion radical (rate ∼2.4 × 103
s-1), noted earlier, can be safely attributed to decarboxylation.
The benzidine, observed as a product in low dose photolysis12
or pulse radiolysis (this work), is a consequence of the
decarboxylation of the PABA radical. It can be shown that
benzidine formation does not occur by mechanism I or II.
(C6H4NH2)2•+ + -O2CC6H4NH2 (4)
The p-aminophenyl radical may also react with water to produce
aniline which is subsequently oxidized to form its cation radical.
Mechanism II: Bimolecular tail-tail addition of 4--O2C-
aniline•+ radicals may lead to coupled product which may
undergo intramolecular charge transfer to form benzidine
(reaction 5). This mechanism is frequently used to explain
benzidine formation in the oxidation of aqueous aniline.18,29-31,48
However, such a mechanism has been discounted in para-
substituted anilines,48 and in p-methoxybenzoic acid it is thought
to produce a peroxide (eq 5a), without the step in eq 5b.41
•+
-O2CC6H4NH2
+
-O2CC6H4NH2•+ f (-O2CC6H4NH2 )2
(5a)
(5b)
+
f (C6H4NH2)2 + 2CO2
Mechanism III: In this modified version of mechanism I,
the 4--O2C-aniline•+ radical is the precursor of the p-aminophe-
nyl radical (reaction 6) that produces benzidine by reaction 3.
-O2CC6H4NH2•+ f •O2CC6H4NH2 f •C6H4NH2 + CO2
(6)
•
In mechanism I the N3 radical undergoes two parallel
In a parallel reaction, this phenyl radical abstracts hydrogen from
water to produce aniline. Electron transfer from aniline to
4--O2C-aniline•+ forms aniline cation radical which is known
to produce benzidine.18,29-31,48 At high radical concentrations,
bimolecular head-tail electron exchange in the 4--O2C-aniline•+
radicals may also produce p-aminophenyl radical. The absorp-
tion spectrum of this extremely reactive species is not known,
but it is certainly not in resonance with the Raman excitation
wavelength to make it detectable. The benzidine and aniline
formation in this mechanism is essentially a manifestation of
the dissociative property of the zwitterion radical. The Raman
observation of the benzidine cation radical, depicted in Figure
7, represents a case of high radical concentration. It should
also be noted that the absorption of the Raman probe pulse by
the zwitterion radical state is likely to increase the yield of the
phenyl radical by photochemically populating the neutral radical
state.
reactions. Therefore, the radiation yield of 4--O2C-aniline•+,
•
prepared by reaction 1, should be much lower than that of N3 .
Comparison of absorbance at 445 nm in the spectra of 4--O2C-
aniline•+ radical, prepared by reactions of N3• with p-aminoben-
•
zoate anion (Figures 1a) and OH with p-aminobenzoic acid
(Figure 5b), shows that the yields are identical, within the
experimental uncertainties. Mechanism I is clearly not consis-
tent with these yields which do not change with the state of
ionization of the carboxylic group (electron loss from -COOH
group cannot occur with the same efficiency as that from the
COO- group). It is also inconsistent with the Raman observa-
tion of the 4--O2C-aniline•+ radical as a precursor of the
benzidine cation radical. Steric hindrance and Coulombic
repulsion of the CO2- groups should greatly reduce the reaction
probability at the C(4) site of 4--O2C-aniline•+, in comparison
to the other unobstructed reaction sites in the radical. Therefore,
benzidine formation in appreciable concentration by mechanism
II is improbable.18 Consistent with this argument, we did not
observe 4,4′-biphenoxide monoanion radical (isoelectronic with
benzidine cation radical) in the N3• oxidation of p-hydroxyben-
zoate anion in Raman experiments,46 although 4,4′-biphenol is
a major product in the bimolecular reaction of the unsubstituted
phenoxyl radical.29,48 Similarly, in the Raman studies of the
radical-radical reactions of anisole cation radical, 4,4′-
dimethoxybiphenyl cation radical appears as a dominant tran-
sient product, but not in the reactions of p-methylanisole cation
radical.50 In the electrochemical oxidation of para-substituted
anilines in aqueous solution, benzidine has not been seen as a
product,18 and in oxidized p-methoxybenzoic acid tail-tail
addition is thought to produce peroxide,41 which conforms with
these arguments.
As stated previously, from a consideration of the nature of
the electronic states in the oxidized PABA, that the radicals
with zwitterionic ground state are likely to undergo thermal
dissociation in solution. The oxidized radical state of p-
methoxybenzoic acid displays that behavior.41 The rate of
decarboxylation in this radical has been reported as 2 × 105
s-1 in CCl4 and e2 × 104 s-1 in CH3CN. The latter rate,
although approximate, appears to be qualitatively correct from
the trend seen in the first-order decay rates of comparative
radicals in the two solvents.41 We are currently examining the
structure and decarboxylation in the zwitterion radical state of
•
p-methoxybenzoic acid (H3COC6H4CO2 in Ingold’s designa-
tion) in water. The radical is prepared by pulse radiolytic
oxidation, as described in ref 49 and the Experimental Section
of this paper. Our preliminary results show the transient
absorption attributable to the zwitterion radical41,49 decaying by
Mechanism III explains benzidine formation without involv-
ing electron transfer from the CO2- site of p-aminobenzoate to
a pH-independent first-order rate of 1.5 ((0.5) × 104 s-1
.
•
N3 radical, as in mechanism I, or requiring tail-tail addition
Because of low radical concentration used in our work second-
of two 4--O2C-aniline•+ radicals, as in mechanism II, both of
which are inconsistent with the experimental observations. We
(48) Nonhebel, D. C.; Walton, J. C. Free Radical Chemistry: Structure
and Mechanism; Cambridge Unversity Press: Cambridge, 1974.
(49) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J. Phys. Chem. 1977,
81, 26, 31.
(50) Tripathi, G. N. R. Chem. Phys. Lett. 1992, 199, 409.

One-Electron Oxidation of p-Aminobenzoic Acid
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2243
discuss some interesting chemical consequences of this mech-
anism in the following section.
negligible in comparison to the initial second-order decay rate,
the product yields should reach saturation. Since the second-
order decay rate depends on radical concentration, and dimin-
ishes with time, a very slow rate of production of PABA radicals
can push the pH at which saturation in the product yields is
observed to a very high value. On the other hand, when fk is
greater than the second-order decay rate (i.e., high f values),
the PABA radical-radical adduct yields will be negligible. One
can readily estimate fk at any pH from the data on k (2.4 × 103
s-1) and pKa (6.7) of the zwitterion radical provided in this work.
The adducts formed by the reaction of p-aminophenyl radical
should generally show a pH dependence complementary to those
of PABA radical-radical adducts, i.e., the yields should
decrease as the pH of solution increases past the pKa of the
zwitterion radical (6.7). The yields of these products should
vanish as the zwitterion radical completely converts into anilino
radical at high pH. Benzidine is one such product which
displays this behavior. This is also true about the products
formed by the reaction of p-aminophenyl radical with other
chemicals in solution. The yields of the PABA-thymine and
PABA-thymidine adducts have been found to decrease with
pH, at pH >6.5.5 However, if the zwitterion radical is produced
in low concentration, the p-aminophenyl radical is more likely
to react directly with thymine and thymidine than with their
radicals. At high concentrations of the zwitterion radical, when
the second-order reactions are initially more dominant than the
first-order decay of the radical, the product yields will still follow
the same pattern as discussed above, although the pH depen-
dence of the yields will be relatively less pronounced. The
observation of benzidine by some investigators, but not by
others, as a photolytic product of PABA probably reflects the
differences in the experimental conditions in which the yields
were measured. It should be noted that benzidine is also
consumed, at least in part, by oxidation and subsequent
formation of more complex products.18 It can be seen that the
benzidine cation Raman signals in Figure 7 decay on the 100
µs time scale. In summary, the trends in the photoproduct yields
in various experiments can be readily understood if the
dissociative behavior of the zwitterion radical is taken into
consideration.
Trends in the pH Dependence of the Product Yields
Product formation is the central theme of the recent photo-
chemical and photobiological research on PABA. Several
interesting and unexpected results are reported in the literature.
Although this study is mainly concerned with the properties of
the primary radical intermediates, the mechanism discussed
above can be analyzed to make some generalized comments
on the pH dependence of the nature and yields of the products
observed in photolytic experiments. These photoproducts can
be broadly classified into two categories: (1) the products which
show negligible yields in near neutral solutions, but their yields
increase as the solution becomes more and more basic, and (2)
the products which exhibit the opposite trend, i.e., their yields
gradually decrease with the increase in the pH, and become
negligible at high pH. The products which have been seen by
some investigators, but not by others, essentially fall into one
of the above two categories. It should be noted that the steady-
state photolysis represents a complex chemical situation in which
reaction products of primary intermediates can also undergo
photolysis and enter into a variety of uncontrolled reactions.
We discuss here the behavior expected in the idealized condi-
tions where cross reactions do not occur.
The electronic structures of aniline cation and anilino radicals
are qualitatively similar,32 with C(4), C(2), and amine N sites
of high spin population and, therefore, high reactivity toward
-
covalent bonding. As discussed previously, the CO2H/CO2
substitution does not significantly alter the structural properties
of the two radicals, except for blocking the C(4) position so
that bonding distances are not approached on radical-radical
encounter. In the absence of thermally activated dissociation
of 4--O2C-aniline•+, one would expect addition reactions mainly
at the C(2) and N sites of these radicals. The spin population
on these atoms and number of equivalent reaction sites (2 for
C(2)) should mainly determine the relative yields of products.
One does not anticipate a drastic dependence of product yields
on pH, or formation of products with relatively high yields by
reaction at the C(4) site.18 These expectations are contrary to
the various observations in PABA.
Generalization of the Two-State Model for
Intramolecular Electron Transfer
Now let us examine the chemical consequences of mechanism
III (reaction 6 in particular) in the formation of products. If
the rate of first-order decay of the 4--O2C-aniline•+ radical (k)
is much faster than the rate of the second-order reactions (i.e.,
low concentration of radicals produced), PABA radical-radical
adducts at the C(2) and N sites will not be observed, as the
radical will dissociate prior to reactions at these sites. On the
other hand, in highly basic solutions where the zwitterion radical
completely converts into its nondissociative anilino anion form,
the product yields will reach their optimum values. In the
intermediate pH range the yields will be observed to increase
with pH. This predicted behavior has been seen recently in
the pH-dependence study of the 4-((2′-amino-5′-carboxyphenyl)-
amino)benzoic acid and 4-((4′-aminophenyl)amino)benzoic acid
(see ref 18 for the formation mechanism of this product) yields
in the photolysis of PABA.4 The yields of these products
become negligible at pH between 7 and 8. Considering
equilibrium between 4--O2C-aniline•+ and its deprotonated
anilino• form, attained on the microsecond or faster time scale
(see section on reaction rates with base), the decay rate by the
dissociative channel is given by fk, where f is the fraction of
the zwitterion radical in equilibrium. As f decreases with pH,
the dissociative decay rate becomes slower, increasing the yields
of the coupled products. At a fairly high pH, when fk becomes
Dissociation of the chemical bonds resulting from intramo-
lecular electron transfer has been observed in a number of
hydrated radical species produced on one-electron reduction.51,52
The dissociation rates and chemistry often depend on the pH
of the solution. The experimental and theoretical models used
for the description of the chemical events in these systems are
frequently based on charge reorganization in the ground
electronic state. Since the solvent response to the charge
distribution in a molecular system is extremely fast (<10-12
s), there is no apparent physical stimulus for the electron to
walk slowly (>10-6 s) from one molecular site to the other to
cause dissociation (model in ref 52), while the system remains
in the ground electronic state. Also, the theoretical treatment
of the electron as a classical particle in this context has
conceptual difficulties. A more satisfactory explanation for this
interesting chemical phenomenon in various radical systems can
be given by the analogy of the oxidized PABA, where two
(51) Neta, P.; Behar, D. J. Am. Chem. Soc. 1980, 102, 4798. Behar, D.;
Neta, P. J. Phys. Chem. 1981, 85, 690. Neta, P.; Behar, D. J. Am. Chem.
Soc. 1981, 103, 103. Bays, J. P.; Blumer, S. T.; Baral-Tosh, S.; Behar, D.;
Neta, P. J. Am. Chem. Soc. 1983, 105, 320. Norris, R. K.; Barker, S. D.;
Neta, P. J. Am. Chem. Soc. 1984, 106, 3140.
(52) Miller, K. E.; Kozak. J. J. J. Phys. Chem. 1985, 89, 401.

2244 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Tripathi and Su
closeby energy states, as discussed earlier, can be readily
visualized. To elaborate this point, let us consider kd as the
rate of dissociation of the upper electronic state (US) in a
hydrated radical. In the absence of other chemical reactions,
the ground state (GS) of the radical will decay by first-order
rate (ket) ) kd exp(-∆G°/RT), considering thermal equilibrium
between the two states. Here, ∆G° is the free energy change
in the standard state, and ket is the rate ascribed to intramolecular
electron transfer (a questionable terminology) in the litera-
ture.51,52 For the sake of numerical illustration, let us assume a
value of 1012 to 1014 s-1 for kd, which is quite reasonable for a
dissociative electronic state. For ∆G° ) 0.42 ((10%) to 0.6
((10%) eV, ket varies by three orders of magnitude, from ∼106
to ∼103 s-1. Most of the reported values of ket are in this range.
Thus, the observed slow rate of intramolecular electron transfer
in a radical can be readily understood in terms of the two-state
model. On changing the state of protonation of the radical, the
free energy difference changes to ∆G°′ ) ∆G° + (2.3RT)∆pKa,
∆pKa ) pKa(US) - pKa(GS), which can drastically affect the
magnitude of ket. If ∆pKa in oxidized PABA is approximated
by the pKa difference between the -N-H and -N+-H protons
in aliphatic amines (i.e., ∼25),53 one estimates ket in the
protonated radical to be ∼1025-times faster than in its depro-
tonated state. Without associating quantitative significance to
this factor, used only as an illustration, it can be concluded that
decarboxylation is possible only when the oxidized PABA is
in its zwitterion radical state, and not in the deprotonated anionic
form, contrary to a recent suggestion.4 On the other hand, in
the reduced radical states undergoing intramolecular electron
transfer and dissociation, a similar argument leads to pKa(GS)
. pKa(US), and ket several orders of magnitude higher in the
proton-ionized state than in the protonated state (if US remains
the dissociative state).
losing group in the oxidation of HX-ring-Y-. In the upper
dissociative state of the radicals, the acid property of the -BH
or -XH proton is expected to be close to that of the parent
molecules. However, in the ground electronic state, the negative
charge on -BH will increase the pKa by several units due to
the Coulombic attraction of the proton, and the positive charge
on -XH will greatly reduce the pKa due to the Coulombic
repulsion.53
Let us examine the effect of protonation of -A on ∆G°′ in
the above illustration. In that case, the free energy difference
changes to ∆G°′′ ) ∆G°′ - (2.3RT)∆pKa. If ∆G°′ <
(2.3RT)∆pKa, the dissociative state, HA- -R-B-, becomes the
ground electronic state (reversal of the state ordering) and
spontaneous dissociation of the AH fragment should occur. This
special situation arises in aliphatic amino acids where A ) NH2
and B ) CO2, and ∆pKa is close to ∼25. Thus, ∆G°′′ ) ∼∆G°′
- 1.5 eV. It should be noted that the HA- -R-B- state is the
radical ground state formed by electron addition to the amine
group in the H3N+-R-CO2- structure. Therefore, no intramo-
lecular electron transfer is involved in the radical dissociation.
The system reverts to the original state ordering (1.5 eV . ∆G°′
> 0) when electron is added to the anionic form of the amino
acid structures (H2N-R-CO2-), and deamination becomes a
slower process, consistent with the experimental observations.38
The ground electronic state is now H2N-R-CO22-, i.e., the
added electron is mostly on the carboxylic group, and deami-
-
nation occurs from the upper state, -H2N- -R-CO2 (see the
-
diagram). The dissociated fragment NH2 should react very
fast (∼10-11 s)26 with water to form NH3 (pKa ∼35).53 This
very simple but probably the only plausible explanation of the
intramolecular electron transfer leading to bond dissociation in
solvated radicals has been overlooked thus far, to our knowledge.
A temperature-dependence study of ket should provide informa-
tion on kinetic and thermodynamical parameters, such as kd and
∆G°. Such a study has been undertaken for the oxidized radical
states of p-XC6H4CO2- (X ) H, Cl, OCH3) in CCl4 by Ingold
and co-workers.41 Their observations fit extremely well with
the expectations of the model, with kd ∼ 1012.5. The model
explains the origin of this Arrhenius parameter and, more
importantly, it relates the measured activation energy with the
stabilization energy of the ring-CO2 bond in the ground
electronic state of the radical by the substituent (X) and the
solvent.
The pH dependence of the radical dissociation, in terms of
the two-state model, is illustrated in the following diagram. In
Acknowledgment. G.N.R.T. wishes to thank Professors D.
A. Armstrong, K.-D. Asmus, and R. W. Fessenden and Drs. J.
Bentley and J. Chateauneuf for stimulating discussions on this
work.
this diagram, -BH is an electron-accepting group in the
reduction of the molecule A-ring-BH, and -XH an electron
(53) Bell, R. P. The Proton in Chemistry; Cornell University Press:
Ithaca, NY, 1959.
JA953180Z