Quantum chemical study of the rearrangement of phenoxyl-hydroxyphenyl radicals

Article abstract of DOI:10.1134/S0036024417030220

The absorption spectra and decomposition kinetics of intermediates formed upon the photolysis of p-iodophenol are studied via flash photolysis. The extinction coefficient of the p-iodophenoxyl radical is calculated. It is found that p-iodophenol acts as a

Full text of DOI:10.1134/S0036024417030220

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 3, pp. 472–475. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.I. Porkhun, Yu.V. Aristova, E.V. Porkhun, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91, No. 3, pp. 455–458.
CHEMICAL KINETICS
AND CATALYSIS
Quantum Chemical Study of the Rearrangement
of Phenoxyl-Hydroxyphenyl Radicals
V. I. Porkhun, Yu. V. Aristova*, and E. V. Porkhun
Volgograd State Technical University, Volgograd, 400005 Russia
*e-mail: arisjulia@yandex.ru
Received February 24, 2016
Abstract—The absorption spectra and decomposition kinetics of intermediates formed upon the photolysis of
p-iodophenol are studied via flash photolysis. The extinction coefficient of the p-iodophenoxyl radical is cal-
culated. It is found that p-iodophenol acts as an inhibitor of light-independent liquid-phase oxidation reac-
tions.
Keywords: phenyl and phenoxyl radicals, iodophenol, absorption spectra, kinetics of radical decay
DOI: 10.1134/S0036024417030220
INTRODUCTION
The band at 350 nm in the absorption spectrum of
intermediate products (Fig. 1) is due to the formation
of p-hydroxyphenyliodohydroxycyclohexadienyl radi-
cals during the reaction between the solvated hydroxy-
phenyl radical and the initial phenol:
Direct Photolysis of p-Iodophenol
•
Phenyl radicals
are the simplest representatives
Ph
of aromatic radicals and, like phenol radicals, are pres-
ent as intermediates in many chemical, biological, and
photochemical systems. The study of reactivities of
phenyl and phenoxyl radicals is for this reason an
important problem in the physical kinetics of radical
reactions [1–7].
OHC6H•4 + JC6H4OH → R•
.
(3)
The absence of absorption at 350 nm in the spectrum
of intermediates upon the flash photolysis of solutions
containing p-iodophenol at low concentrations of
10‒5 M (Fig. 1) suggests reaction (3) occurs. Other
p-Iodophenol can serve as a convenient source of
phenoxyl and phenyl radicals upon photolysis: the for-
mation of p-iodophenoxyl and hydrophenoxyl radicals
upon its photolysis is possible in principle. The
absorption spectra and decay kinetics of intermediates
formed upon the photolysis of p-iodophenol at 20°C
have been studied via flash photolysis.
The flash photoexcitation (λ > 280 nm) of an oxy-
gen-free solution of p-iodophenol in benzene
(10‒2 М) results in the emergence of intermediate
products whose absorption spectrum is shown in
Fig. 1. According to experiments on flash photolysis of
an iodine solution in benzene, and to the data
obtained in this work (Fig. 1), the absorption band at
495 nm is due to complexation between atomic iodine
and benzene. The initial photolysis event is thus a
homolytic cleavage of the carbon–iodine bond:
evidence for the formation of the radical via reac-
R
tion (3) is experiments on the photolysis of aqueous
solutions of p-iodophenol. Figure 1d shows the
•
absorption spectrum (λmax = 370 nm) for the
radi-
R
cal formed by reaction (3) upon the flash photoexci-
tation (λ > 200 nm) of an aqueous solution of
p-iodophenol (10–2 M). The occurrence of such pro-
cesses in aqueous solutions has been shown via pulse
radiolysis, ESR, and the analysis of products. The
shift in the absorption maximum and the slight change
•
in the shape of the absorption band of the
radical
R
are due to differences in the solvating properties of
water and benzene.
The decay kinetics of the iodine–benzene com-
R•
plexes and the
radicals (measured at 495 and
350 nm, respectively) are governed by the second-
order law
•
•
.
(1)
HOC6H4J + hv → HOC6H4 + J
2C6H6J• → J2 + C6H6
2R → products,
,
(4)
(5)
The detached iodine atom participates in complex-
ation with the solvent:
•
•
k/ε350 = 1.3 ×105
cm s–1.
.
(2) and
472
J + C6H6 ↔ C6H6J

QUANTUM CHEMICAL STUDY OF THE REARRANGEMENT
ΔD
473
1
0.10
2
0.05
3
4
0
300
400
500
λ, nm
5
Fig. 1. Absorption spectra for intermediates at 5 × 10 s, obtained upon the flash photolysis of p-iodophenol in benzene:
–2
–2
–4
–5
(1) p-iodophenol in benzene (10 M), (2) p-iodophenol in benzene (2 × 10 M), (3) p-iodophenol in benzene (2 × 10 M),
and (4) p-iodophenol in water (10 M).
The quantum yield of reaction (1) upon steady- 320–360 nm) of an aqueous oxygen-free solution of
2,6-disulfoanthraquinone (5 × 10–5 M) in the pres-
ence of p-iodophenol (5 × 10–3 M) results in the for-
mation of p-iodophenoxyl and anthrasemiquinone
radicals (Fig. 2):
state irradiation using light with λ = 300 nm is 0.12.
At low concentrations of p-iodophenol (about
10‒5 M), the recombination of hydroxyphenyl radi-
cals,
2HOC6H•4 → products
*
*
;
(8)
A0 + hv → AS → A T
(6)
•
+
•
−
can compete with reaction (3), and the absorption
spectrum of intermediates displays only absorption at
495 nm. In the condensed phase, the hydroxyphenyl
radical absorbs in a farther UV region (λ < 300 nm). In
accordance with reaction (4), one of final products of
the photolysis of iodophenol was molecular iodine.
The amount of the liberated iodine was determined via
spectrophotometry using a calibration curve. This
allowed us to obtain the concentration of iodine atoms
•
*
.
(9)
A T + HOC6H4J → A + H + JC6H4O
Introducing oxygen into a solution results in the reac-
tion
•
•
A− + O2 → A0 + O2−,
becoming the main pathway for the decay of anthrase-
miquinone radicals. In accordance with reaction (10),
(10)
•
−
anthrasemiquinone radicals are replaced by
in a
O2
after a flash and thus the equal concentration of
R
period of 3 × 10–5 s in the presence of oxygen. The
absorption in the range under study is due only to the
phenoxyl radical (Fig. 2).
R• ε350
and the extinction coefficient of
= 1.2 ×
102 M–1 cm–1. The rate of reaction (5) was
=
K5
1.5 ×107
The extinction coefficient of the p-iodophenoxyl
M–1 cm–1.
radical at the absorption peak at 420 nm was calcu-
The equilibrium constant for process (2) was
ε = 2.8 ×103
lated to be
L M–1 cm–1. The decay of
obtained using the equation
p-iodophenoxyl radicals in air-saturated solutions is
[C6H4J•]
governed by the second-order law
= 3 ×10−3 M−1.
(7)
K2 =
[C6H4][J •]
2JC6H4O• → products,
K11 = 5.5 ×108 М–1s–1.
p-Iodophenol can liberate hydrogen atoms to form
phenoxyl radicals with the same spectral and kinetic
properties as the phenoxyl radicals of phenol and
cresols. This means that p-iodophenol should act as an
inhibitor of light-independent liquid-phase oxidation
(11)
The resulting equilibrium constant was considerably
lower than the one for complexation between atomic
iodine and mesitylene, due likely to the lower basicity
of the benzene ring.
Sensibilized Photooxidation of p-Iodophenol
The 2,6-disulfoanthraquinone-sensibilized photo- reactions. Under illumination p-iodophenol decom-
oxidation of phenyls is one way of preparing phenoxyl poses to form very reactive hydroxyphenyl radicals and
radicals. Photoexcitation through an UVS-6 filter (λ = should act as the catalyst of chain processes. It appears
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 3 2017

474
PORKHUN et al.
ΔD
0.2
1
0.1
0
2
400
500
600 λ, nm
5
Fig. 2. Absorption spectra for intermediates at 5 × 10 s, obtained upon the flash photolysis of aqueous solutions (pH 9) of
–3
–5
p-iodophenol (5 × 10 M) and 2,6-disulfoanthraquinone (5 × 10 M): (1) oxygen-free solution and (2) air-saturated solution.
that the study of oxidizing systems containing the hydroxyphenyl radical increases in the same order
p-iodophenol at certain concentrations will allow us to (calculations were performed using both the PM-3
control the oxidation chain reaction using light.
semiempirical method and the Hartree–Fock–
Roothan method in the 3-21d basis set, allowing for
electron correlation according to the MP-2 perturba-
tion theory). For structures I, II, and III, the dipole
moment of the phenoxyl radical is considerably higher
(by 150–400%) than that of the hydroxyphenyl radi-
cal.
The authors of [2] studied the oxidation of phenol
and proposed the prototropic rearrangement of the
phenoxyl-hydroxyl radical.
The PM-3 semiempirical method was used to cal-
culate the activation barriers of the intramolecular
rearrangement of phenoxyl radicals into hydroxyphe-
nyl radicals [8]. The radicals listed below were consid-
ered.
Our flash photolysis study did not confirm the pro-
totropic rearrangement. The decay of phenoxyl radi-
cals in oxygen-free solutions is strictly governed by a
second-order law. Upon the photolysis and radiolysis
of p-halophenols, one of primary products is the
hydroxyphenyl radical; however, no decay of the phe-
noxyl radicals was observed during our chromato-
graphic analysis of the reaction products.
Phenoxyl Radicals
O
H
O
O
H
Ph
Ph
Ph CH3
Ph CH3
CH3
CH3
We also studied whether rearrangement can pro-
ceed through the motion of the hydrogen atom in the
ring plane or at a certain angle to it; optimizing the
geometry of such systems produced a value of
H
kcal/mol. When studying radical reactions
ΔH* > 100
in the liquid phase, we must consider solvation. Since
the dipole moment of the transition state is higher
than that of the phenoxyl radical, we would expect
OH
OH
OH
Hydroxyl Radicals
The studied reactions are endothermic. The energy
profile of the reaction is shown in Fig. 3.
·
H
ΔH*
Reaction heats
for the structures under con-
ΔH
·
O
ΔH
sideration are almost identical (96.3–104.7 kJ/mol).
The saddle point lies in the output reaction channel.
The transition state corresponds to the position of the
hydrogen atom above the ring plane near the oxygen
atom. The dipole moment of the phenoxyl radical
diminishes in the order I–III–II (Fig. 3), while that of
(a)
(b)
(c)
Fig. 3. Potential energy surface cross section along the
reaction coordinate (angle ϕ): (a) phenoxyl radical,
(b) transition state, and (c) hydroxyphenyl radical.
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No. 3
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QUANTUM CHEMICAL STUDY OF THE REARRANGEMENT
475
some decrease in the activation energy when consider- The absorption spectrum was recorded, and the
ing rearrangement in polar solvents. The energy of sol- magnitude of the rate constant for the decay of the
vation of the phenoxyl radical and the transition state p-iodophenoxyl radical by the second-order law was
in water was estimated using the Kirkwood formula. obtained in experiments on the sensitized photooxida-
Assuming that equilibrium with the orientational and tion of p-iodophenol.
electronic components of the medium polarization is
established in the transition state, the drop in the acti-
vation energy is only 3 cal/mol. Rearrangement thus
has an extremely high activation energy, compared to
that of the recombination of phenoxyl radicals. At
room temperature, this rearrangement proceeds with
the π radical transforming into the σ radical; it is likely
to be symmetry-forbidden.
The experimentally observed absence of rearrange-
ment of phenoxyl radicals into hydroxyphenyl radicals
agrees with the high activation energy of this process
(>100 kcal/mol) calculated according to the PM-3
and Hartree–Fock–Roothan methods.
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(1976).
CONCLUSIONS
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(1975).
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Chem. A 86, 1915 (2012).
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were
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•
15 (2012).
and
radicals (5) is governed by a second-order law:
R x
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7
=
M–1 s–1 in benzene at 20°C.
K5 1.5 ×10
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The quantum yield of the photolysis of p-iodophe-
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Translated by K. Utegenov
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 3 2017