Low-Temperature Photolysis of Benzoyl Peroxide

Article abstract of DOI:10.1134/S0036024420010161

Abstract: The photolysis of dry benzoyl peroxide (BP) at 77 K in the 480–236 nm range of wavelengths and an ethanol solution is studied via EPR. It is determined that the main photochemical process in irradiating BP at λ = 480–365 nm is the direct photodi

Full text of DOI:10.1134/S0036024420010161

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 1, pp. 189–195. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Fizicheskoi Khimii, 2020, Vol. 94, No. 1, pp. 136–142.
PHOTOCHEMISTRY
AND MAGNETOCHEMISTRY
Low-Temperature Photolysis of Benzoyl Peroxide
a
a
a
a,
S. I. Kuzina , A. I. Bol’shakov , A. V. Kulikov , and A. I. Mikhailov *
a
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
*е-mail: alfaim@icp.ac.ru
Received March 4, 2019; revised May 15, 2019; accepted June 18, 2019
Abstract—The photolysis of dry benzoyl peroxide (BP) at 77 K in the 480–236 nm range of wavelengths and
an ethanol solution is studied via EPR. It is determined that the main photochemical process in irradiating
BP at λ = 480–365 nm is the direct photodissociation of the O–O peroxide bonds and C–H bonds of ben-
zene rings to form benzoyloxy radicals and atomic hydrogen, which likely forms via a two-quantum mecha-
−5
nism. The quantum yield of primary intermediates is ϕ ≈ 2.7 × 10 . Upon subsequent irradiation of a sample
using light with λ = 300–236 nm, benzoyloxy radicals release carbon dioxide and transform into phenyl rad-
−4
icals with a quantum yield of ϕ ≈ 3 × 10 . At 120–145 K, phenyl radicals react with double bonds of aromatic
rings of PB to form phenyl-substituted cyclohexadienyl radicals vanishing at 273 K. Quantum chemical cal-
culations of the radical structures of photolyzed peroxide are made with the density functional approach
(
B3LYP/6-311g), and good agreement between the experimental and theoretical parameters is noted. Calcu-
lations show that in the structure of benzoyloxy radicals, the spin density is distributed almost equally between
the two oxygen atoms of the radical, due to the conjugation of the unpaired electron to the lone p electron
pair on the oxygen atom of the C=O group. In the photolysis of BP solutions in ethanol at λ ≥ 365 nm, solvent
radicals form via sensitized photolysis through the reaction between primary benzoyloxy radicals and solvent
molecules.
Keywords: benzoyl peroxide, low-temperature (77 K) photolysis, radical intermediates
DOI: 10.1134/S0036024420010161
INTRODUCTION
there has so far been virtually no complete generaliza-
tion of the photolysis of this peroxide, due to both the
imperfection of investigations and the ambiguity of a
number of issues.
The aim of this work was to determine the nature of
the primary products of the photochemical destruc-
tion of benzoyl peroxide. The photolysis of dry BP and
its ethanol solution was therefore studied via 3-cm
EPR at 77 K using filtered light in the λ ≥ 500–236 nm
range of wavelengths. These data are of interest in
determining the role of peroxides as initiators of solid-
phase oxidation reactions and polymerization.
Benzoyl peroxide (BP) is a known initiator of radi-
cal reactions, curing agent for polyester resins, vulca-
nizer, oxidizer, bleaching agent, and deodorant. It is
widely used in polymer chemistry, organic synthesis,
medicine, and household activities. Intensive investi-
gations of BP began in the 1920s after it was discovered
that the BP molecule decomposes into free radicals in
solutions at temperatures of 60–80°C. The interpreta-
tion of reactions as radical processes was based on
analyses of the reaction products. A great many studies
of the thermal and photolytic decomposition of this
substance in various solvents were generalized by
Walling [1] and Nonhebel and Walton [2]. It follows
from the literature data that the thermal decomposi-
tion of BP in solution proceeds in two steps. The
homolytic cleavage of the O–O bond initially pro-
duces benzoyloxy radicals, and the decarboxylation of
EXPERIMENTAL
Benzoyl peroxide C H O was purified via triple
14
10
4
reprecipitation with methanol from cold chloroform
solution. The photolysis of the peroxide and 2% BP
solution in ethanol was conducted in quartz and glass
ampules in vacuum at 77 K. The UV light source was
a DRSh-1000 high-pressure mercury lamp (flux,
benzoyloxy radicals then yields CO and phenyl radi-
2
cals. In a solution, solvent radicals also form.
17
2
The photolysis of the peroxide has been studied to 3.8 × 10 quanta/(cm s) at λ = 236–500 nm). The
a lesser extent, although the photolytic decomposition reuired ranges of the lamp spectrum were cut using a
of BP is often used for the radical initiation of different set of glass light filters. The irradiated samples were
reactions by introducing BP to a reaction system as an heated (above 77 K) in a thermostat cooled with vapor
additive [2, 3]. Although there are many data on BP, of liquid nitrogen. Each irradiated sample was treated
189

1
90
KUZINA et al.
505 G
1
b 2.001
2
.065
2.065
*
100 G
1a
2.001
10 G
30 G
*
10 G
2.0027
2.0027
2.065
3
2
5
G
2.0018
4
Fig. 1. EPR spectra of UV-irradiated (at 77 K) dry benzoyl peroxide: (1) photolysis at λ = 480–365 nm for 1 h, and then (2) 5 min
at λ ≥ 236 nm; (3) photolysis of the initial peroxide at λ ≥ 236 nm; (4) heating of photoirradiated sample 3 to 145 K.
for several minutes at a given temperature, and the intensity of the signals grew along with the quantum
EPR spectra were recorded at 77 K using an EPR-21 energy (up to 365 nm) and exposure time. Figure 1
3
-cm radio spectrometer (ν = 9.6 GHz; microwave presents the spectrum recorded after irradiation at λ =
0
−3
4
80–365 nm. The spectrum simultaneously shows
field power, ∼10 W). Quantum chemical calcula-
tions of the radical structures were performed using
the GAUSSIAN 03 software package and the density
three signals with different g-factors: a broad two-
component signal with g = 2.065 (spectrum 1a); a
Н
functional theory (B3LYP/6-311g). The optical doublet of atomic hydrogen with line splitting αα =
absorption spectra of BP solutions in ethanol and ace- 505 G; and a weak structureless signal that, according
tone were recorded with a Specord M 40 spectropho- to the value of the g-factor (g = 2.001), should be
tometer using 1 cm cells. The gaseous products of the attributed to silyl radicals in the quartz ampule [4]
photolysis of BP were analyzed with a MI-1201B mass (before photolysis, the spectrum of the substance in
spectrometer.
the ampule did not contain the signal with g = 2.001).
The presented EPR data suggest that the photodis-
sociation of BP with formation of radical intermedi-
ates proceeds under the action of visible light with λ =
480–365 nm. However, the optical absorption spec-
trum of BP in ethanol solution (Fig. 2, range 365–
RESULTS AND DISCUSSION
Benzoyloxy Radicals and Atomic Hydrogen
To determine the boundary wavelength at which
photodissociation occurs, the BP sample was irradi-
ated in sequence using filtered light with λ ≥ 500,
480 nm) contains no bands.
The maxima of the absorption bands of two chro-
≥
480, ≥440, ≥400, and ≥365 nm (total irradiation mophore groups lie at ∼290 nm (the n–π* transition
time, 1 h), and the EPR spectrum was recorded after in the carbonyl group) and ∼235 nm (the π–π* transi-
each exposure. Signals in the EPR spectra emerged tion in the aromatic ring). The long-wave edge of the
under the action of light with λ ≥ 480 nm, and the optical spectrum of BP (in an acetone solution) ends
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LOW-TEMPERATURE PHOTOLYSIS OF BENZOYL PEROXIDE
191
A
(bond energy, 102 kcal/mol), and the excess energy of
the system after the absorption of the first quantum
1
0
0
0
0
.0
(
λ ≥ 480 nm) is only ∼35 kcal/mol. We must therefore
.8
.6
.4
.2
assume that radical pair (RP) instantly absorbs a sec-
1
ond quantum (owing to the participation of the n–π*-
and π–π*-excited states), and atomic hydrogen forms
in a second cell through a two-quantum mechanism (a
two-quantum mechanism of the formation of primary
radical intermediates in the photolysis of BP was also
noted in [5]):
BP + hν → [R …R ] + hν
2
1
1
1
2
00
250
300
350
400
500
λ, nm
(
RP)1
(2)
•
•
•
→
[R …Н ] → C H −C(O)−O + Н .
1
6
5
Fig. 2. Optical absorption spectrum of benzoyl peroxide in
an ethanol solution.
(RP)2
(R1)
The H atom easily leaves the second cell (RP) , as
2
at ∼360 nm. There is no absorption band from the
third chromophore of the peroxide group O–O
because the absorption spectra of peroxide com-
pounds are structureless even in the gas phase. It was
reported in [5] that after the absorption of a quantum
of light, BP instantly dissociates homolytically at the
O–O bond (by a mechanism of direct photodissocia-
tion) to form two oxy radicals. The activation by light
in the visible region is also due to the long-wave shift
of the band of the n–π* transition because of the dis-
tortion of the planar configuration of the BP molecule
is confirmed by the value (α = 505 G) of the hyperfine
splitting constant of the hydrogen doublet. In the rad-
ical pair, the hyperfine splitting constant of the dou-
•
•
blet of Н would be half that value [7]. After Н has left
the second pair (RP) , the remaining benzoyloxy rad-
2
ical experiences no cell effects and becomes single
benzoyloxy radical R .
1
In EPR spectra, oxy radicals are detected as sin-
glets without any hyperfine structure (the nuclear spin
of the oxygen atom is zero). However, spectrum 1a in
[
6]. The distortion of the molecule results in strong
interaction between the two electron pairs on the oxy-
gen atoms and their conjugation to the π electrons of
the double bonds of phenyl groups. Conjugation leads
to some mixing of n–π*- and π–π*-excited states. We
Fig. 1 showing the signal of radicals R is a superposi-
1
tioning of two signals. The first of these is a broad
intense singlet that dominates in the spectrum and
may therefore conclude that the primary photochem- overlaps the first line of the weak doublet. The second
ical process in the photolysis of BP using light with λ line of the doublet (indicated by an asterisk) is
=
480–365 nm is initiated by excited peroxide groups detected more clearly in the spectrum recorded with a
(
which decompose into two benzoyloxy radicals on narrow sweep (Fig. 1b, inset). The broad singlet in
O–O bonds) and the n–π*- and π–π*-excited states
of the carbonyl and phenyl groups (which cause the
cleavage of C–H bonds). The energy of a quantum of
light with λ = 480 nm is ∼65 kcal/mol, which is suffi-
cient for cleaving O–O bonds (∼30 kcal/mol). After
cleaving of O–O bonds, two benzoyloxy radicals lie
spectrum 1a (ΔH ≅ 100 G, g = 2.065) can be attributed
to the singlet of individual benzoyloxy radicals that
have left the cell, and the doublet (the line splitting α ≈
6
0 G, g ≈ 2.045) could be the signal of the small frac-
tion (∼8%) of radical pairs that have remained in the
cell. It is known that the signals of radical pairs with
g ≈ 2 (Δm = 1 transition) are recorded as broad dou-
blets [7], and their presence could be the reason for the
within the same structural cell as radical pair (RP) :
1
С Н –С(О)–О–О–С(О)–С Н + hν
6
5
6
5
broadening of the singlet. The quantum yield of R
∗
1
→
2С Н –С(О)–О .
(1)
6
5
and radical pairs (RP) in the range of λ = 480–
1
(R1)
−5
3
65 nm at 77 K was ϕ ≈ 2.7 × 10 .
According to the EPR data, the primary photo-
chemical process in the photolysis of BP also produces
atomic hydrogen. Its characteristic doublet with line
splitting α = 505 G is already noticeable under the
action of light with λ ≥ 480 nm. (Note that the gener-
ation of atomic hydrogen directly in the quartz ampule
requires the action of light with λ < 280 nm.) Hydro-
Benzoyloxy radical is a conjugate radical in which
the unpaired π electron on the oxygen atom is in the
π–р conjugation to the lone р electron pair on the oxy-
gen atom of the carbonyl group. Quantum chemical
calculations showed that due to conjugation and
exchange interactions, the spin density in radical R is
1
gen forms in BP upon the cleaving of C–H bonds distributed virtually equally (by ∼0.54) between two
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1
92
2
KUZINA et al.
000
500
000
Phenyl Radicals
1
8
44
Upon the irradiation of benzoyl peroxide contain-
ing benzoyloxy radicals using UV light with λ = 300–
236 nm, these radicals decompose on the C–C(O)O
1
1
28
bonds to form phenyl radicals R and carbon dioxide:
2
•
•
5
C H –C(O)–O + hν → C H + CO .
6
5
6
2
(4)
1
7
(R2)
5
00
During irradiation, the broad signal with g = 2.065,
which characterizes benzoyloxy radicals and radical
pairs, grows weaker. The signal of phenyl radicals
emerges at g = 2.0027. The considerable shift of the
g-factor (2.065 against 2.0027) allows us to simultane-
ously record the signals of R and R (as in Fig. 1,
1
12 16 19
32
0
0
10
20
30
40
50
60
m/z
1
2
Fig. 3. Mass spectrum of the gaseous products of the pho-
tolysis of BP at λ ≥ 236 nm at 77 K.
spectrum 2). In the photolysis of the initial BP using
harder light with λ ≥ 236 nm, the stage of the forma-
tion of benzoyloxy radicals is virtually indistinguish-
able, the spectrum of phenyl radicals is recorded from
the start of irradiation (Fig. 1, spectrum 3), and the
sample instantly becomes bright orange. The quantum
yield of the accumulation of phenyl radicals at 77 K at
oxygen atoms, and the structural formula of R can be
1
presented as
−4
λ ≥ 236 nm was ϕ ≈ 3.1 × 10 . In the photolysis of BP,
the wavelength of the light thus affects the nature of
the resulting radicals. Other radicals can also be
obtained, depending on the UV composition.
0
.54
.54
(3)
The EPR spectrum of radicals R is a binomial
2
triplet of triplets with nonuniform interaction between
the unpaired electrons and the protons of the ring.
0
Hyperfine splitting constant α = 23–24 G (from
1
Of interest is the great width of spectrum 1a (ΔH ≈ interaction with ortho-protons), α = 7.5 G (from
2
1
2
00 G) and the considerable shift of the g-factor (g = interaction with meta-protons), and g = 2.0027, which
.065) from the pure spin value. As noted above, the agrees with the known data [8]. The hyperfine splitting
main reason for the broadening of the spectrum could
be the presence of radical pairs with different distances
between the radicals in them [7]. It is also not impos-
sible that the singlet broadens because of the spin–
spin interaction between the two spin centers in the
radical. The considerable shift of the g-factor from the
pure spin value suggests a marked contribution from
the orbital magnetism to the total effective magnetic
moment of the radical.
constants for ortho- and meta-protons in the experi-
mental EPR spectra differ considerably (23–24 G
against 7.5 G), while the theoretical values of the con-
stants that were calculated by quantum chemical
means for radicals in the gas phase differ slightly: α =
1
8–15 G. The discrepancy between the experimental
and theoretical constants can be attributed to specific
features of the solid phase, and to the effect surround-
ing molecules have on the angles and length of the
C‒H bonds of the radicals.
In the literature, proofs of the formation of benzo-
yloxy radicals as the primary products of BP decom-
position were based on analyses of the reaction prod-
ucts that can be expected in the transformations of
benzoyloxy radicals [1, 2]. The EPR spectrum
recorded in the low-temperature photolysis of BP
Volatile Products of the Photolysis of BP
The cleaving of O–O, C–H, and C–C bonds in BP
molecules is accompanied by the formation of volatile
products of the photolytic decomposition of the per-
(
Fig. 1), which includes the singlet of individual ben-
zoyloxy radicals, the doublet of their radical pairs, and
the doublet of atomic hydrogen, is proof of the direct oxide. Figure 3 presents data on the main products of
photolysis (under the action of visible light) of the per- the photodestruction of BP. The mass spectrum shows
oxide O–O groups and the C–H bonds of the phenyl
groups to form benzoyloxy radicals and H atoms as water and carbon dioxide indicate the main products
primary radical intermediates.
•
traces of atomic hydrogen H , and the intense peaks of
•
of the low-temperature photolysis of BP.
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LOW-TEMPERATURE PHOTOLYSIS OF BENZOYL PEROXIDE
193
(a)
hydrogen atoms (H is contained in the structural for-
1
00
mula of R ) produces a doublet structure with splitting
3
8
6
4
2
0
α = 24 G, which agrees fully with the results from
quantum chemical calculations (α = 24 G). No addi-
tional splitting of the lines of the theoretical doublet on
the two protons of the ring (with hyperfine splitting
constant α = 3.4 G) is observed for the solid phase in
the experimental spectra.
0
0
0
0
2
924
In radical R , the dienyl fragment in the aromatic
4
000
3000
2000
1500
1000
400
3
ring in conjugation to the unpaired electron (π–π con-
jugation) and the lone electron pair at the oxygen atom
of the carbonyl group (π–р conjugation) constitute a
single conjugate system. Such a system with delocal-
ized electron density abruptly lowers the energy of the
radical. It thus increases its thermodynamic stability,
but simultaneously reduces the reactivity of the radi-
(b)
1
00
8
0
0
cal. In irradiated benzoyl peroxide, radical R lasts up
3
to 273 K.
6
Changes in Benzoyl Peroxide during Storage
4
000
2400
1400
400
ν, cm−
1
Upon heating irradiated (at λ = 480–236 nm and
7 K) benzoyl peroxide to room temperature, all but
7
Fig. 4. (a) Standard IR spectrum of dry benzoyl peroxide
and (b) the IR spectrum of the peroxide stored under lab-
oratory conditions for one year.
one signal from radical structures vanish; the only
remaining signal with g = 2.001 is that of the silyl rad-
icals in the quartz ampule. After re-irradiating this
sample with UV at 77 K, the doublet of atomic hydro-
gen disappears. Even greater changes are observed in
the EPR and IR spectra of peroxide that has been
stored for long periods under laboratory conditions.
The EPR spectra show no signals after this peroxide is
irradiated at λ = 480–365 nm (at 77 K), and the pow-
dered samples retained their original white color
Phenyl-Substituted Cyclohexadienyl Radicals
Upon heating the photolyzed (at 77 K) BP samples
containing phenyl radicals, these radicals remained up
to 120 K, and in the range 120–145 K, the spectrum of
R transforms into a doublet (Fig. 1, spectrum 4) with
2
H
β
hyperfine splitting constant α = 24 G and g = 2.0018. remains intact. In the IR spectrum in Fig. 4b (com-
According to [9], the doublet characterizes phenyl- pared to the standard IR spectrum of BP in Fig. 4a),
substituted cyclohexadienyl radicals R , which are
3
the most intense band of the stretching vibrations of
products of the reaction between phenyl radicals and
the double bonds of the aromatic rings of the BP mol-
ecule. In the interaction between phenyl radicals and
BP molecules, the double bonds in the rings are
cleaved, and the radicals are added to the peroxide
molecules:
−1
C–H bonds at ν ≈ 2924 cm is missing completely,
and the intensity of the bands in the range of 1600–
−1
1
450 cm , which represent the bonding vibrations of
CH bonds, also falls noticeably (the changes in the IR
2
spectra of BP require additional investigation). How-
ever, the irradiation of the peroxide using short-wave
light with λ ≥ 236 nm after long periods of storage is
accompanied by the formation of phenyl radicals
detected in EPR spectra. A bright-orange spot, the
color of phenyl radicals, emerges in the white BP sam-
ple. As they vanish, the orange spot disappears. Under
ordinary storage conditions, heat and light thus have a
harmful affect on BP, leading to its destruction. At the
same time, highly excitable and highly reactive pri-
mary benzoyloxy radicals cannot always be detected
via EPR. However, the ability to generate phenyl radi-
O
O
H
C O O C
(5)
(R )
3
After the formation of radicals R , a sample
3
becomes brown. Quantum chemical calculations show
that much of the spin density in radical R is localized
3
(
with respect to the substituent) on the carbon atoms
at the ortho- and para-positions of the ring (0.4, 0.4,
and 0.5, respectively). The interaction between
unpaired electrons (localized on the carbon atoms in cals persists, and BP can be used as an initiator of rad-
the ortho-position) and the nearest β protons of the ical reactions.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 94 No. 1 2020

1
94
KUZINA et al.
ting α = 7.5 G is caused by the interaction between the
2.0028
unpaired electron and the two protons of the ring:
H
H
*
O
O
C O O C
+ H
C(O)O (7)
~
*
R(CGD)
15 G
No radicals are detected after the photolysis of pure
alcohol (without BP) at λ ≥ 365 nm. The five-compo-
•
nent spectrum of radical CH C HOH lies at the center
3
of the spectrum in Fig. 5 between two extreme compo-
nents. The ratio between the intensities in the quintu-
plet (1 : 2 : 3 : 2 : 1) and different hyperfine splitting
constants (α = 18–11 G) suggest the nonequivalence
of the α and β protons, which is characteristic of radi-
cals of ethanol in the solid phase and agrees with the
published data [7]. In our spectrum of alcohol radicals
Fig. 5. EPR spectrum in the photolysis (at λ ≥ 365 nm and
7
7 K) of a 2% solution of benzoyl peroxide in ethanol.
(Ral), however, the hyperfine splitting constants are
sometimes underestimated, due probably to the super-
Photolysis of Benzoyl Peroxide in Solution
positioning with the signal of R . The contents of
CGD
R
and Ral in the mixture of radicals are 70 and
CGD
Several minutes after the start of the low-tempera-
3
0%, respectively.
ture (77 K) irradiation of 2% solution of BP in ethanol
at λ ≥ 365 nm, the EPR spectra show a well-resolved
In the low-temperature photolysis of solutions of
seven-component signal (Fig. 5). The concentration BP at λ ≥ 365 nm, solvent radicals thus form via sensi-
16
of paramagnetic centers reaches ∼7 × 10 spins per 1 g tized photolysis through the reaction between primary
of solution and falls upon further irradiation, due to benzoyloxy radicals and solvent molecules.
drop in the BP content.
Analysis of the shape of the spectrum showed that
radicals form from the aromatic and alcohol compo-
nents of the mixture, and the observed spectrum is a
superpositioning of two signals. Two extreme compo-
nents of the spectrum (marked with asterisks in the
spectrum and circles in the scheme of lines of the
hyperfine structure) at a distance of 90 G belong to a
triplet with αβ = 45 G of cyclohexadienyl (RCGD) rad-
icals formed by the reaction between atomic oxygen
and the aromatic rings of BP molecules.
CONCLUSIONS
The low-temperature (77 K) photolysis of benzoyl
peroxide (BP) was studied in the wavelength range
80–236 nm. The main focus was radical intermedi-
4
ates detected by EPR. It was shown that the wave-
length of the photolyzing light affects the nature of the
formed radicals. Light with λ = 480–365 nm causes
the direct photodissociation of O–O and C–HJ bonds
in the molecule to form two benzoyloxy radicals and
atomic hydrogen, which forms through a two-quan-
tum mechanism. The EPR spectrum of benzoyloxy
radicals, which is a broad singlet with ΔH ≅ 100 G and
g = 2.065, was recorded in superposition with the dou-
blet signal of radical pairs.
2H
The source of atomic hydrogen is alcohol mole-
cules. The detachment of H from the alkyl group of
•
alcohol produces alcohol radicals CH C HOH (Ral)
3
through the reaction of the sensitized photolysis of
benzoyloxy radicals and alcohol molecules:
The photolysis of BP at λ ≥ 236 nm generates phe-
nyl radicals formed by the decarboxylation of benzoy-
loxy radicals. At 120–145 K, the phenyl radicals in the
reaction with the double bonds of peroxide molecules
form stable phenyl-substituted cyclohexadienyl radi-
6) cals that last up to 273 K. Quantum chemical calcula-
tions of the radical structures of the photolyzed perox-
ide were performed using density functional theory,
and good agreement between the experimental and
calculated parameters was observed.
C(O)O + C H OH
2
5
(
COOH + CH CHOH.
3
(Ral)
Atomic hydrogen is effectively added to the double
bonds of the aromatic rings of BP molecules to form
In the photolysis of solutions of BP at λ ≥ 365 nm,
cyclohexadienyl radicals. It was reported in [7] that Н solvent radicals form via sensitized photolysis through
is added to the ortho- and para-positions of the ring the reaction between benzoyloxy radicals and solvent
(with respect to the substituent), and additional split- molecules.
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LOW-TEMPERATURE PHOTOLYSIS OF BENZOYL PEROXIDE
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FUNDING
5. O. N. Karpukhin and E. M. Slobodetskaya, Russ.
Chem. Rev. 42, 173 (1973).
The work was performed under the state task no. 0089-
019-0011.
6
. M. Ya. Mel’nikov and V. A. Smirnov, Photochemistry of
2
Organic Radicals (Mosk. Gos. Univ., Moscow, 1994)
[in Russian].
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2
8
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Translated by V. Glyanchenko
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