Matrix isolation and IR characterization of the benzoyl and benzoylperoxy radicals

Article abstract of DOI:10.1002/ejoc.201000153

The benzoyl radical 1 was synthesized in argon matrices by the thermal reaction of the phenyl radical 2 with CO. The IR spectrum, with the C=O str. vibration at: 1824.4 cm^-1 is in good agreement with DFT calculations. The formation of 1 is reve

Full text of DOI:10.1002/ejoc.201000153

FULL PAPER
DOI: 10.1002/ejoc.201000153
Matrix Isolation and IR Characterization of the Benzoyl and Benzoylperoxy
Radicals
Artur Mardyukov[a] and Wolfram Sander*[a]
Keywords: Radicals / Matrix isolation / IR spectroscopy
The benzoyl radical 1 was synthesized in argon matrices by
the thermal reaction of the phenyl radical 2 with CO. The IR
spectrum with the C=O str. vibration at 1824.4 cm–1 is in good
agreement with DFT calculations. The formation of 1 is re-
versible and UV irradiation results in the cleavage back to 2
and CO. The benzoyl radical 1 can react with molecular oxy-
gen in the matrix to produce the benzoylperoxy radical 3.
Radical 3 was also characterized by IR spectroscopy in com-
bination with DFT calculations.
Introduction
culations the activation barrier for the reaction of 1 with
CO is 3.33 kcal/mol while B3LYP predicts a very small acti-
vation barrier of only 0.73 kcal/mol.[10]
The benzoyl radical 1 is an important reactive intermedi-
ate that plays a role in the combustion of aromatic hydro-
carbons and as an initiator in polymerization reactions.[1,2]
Aryl ketones are frequently used as photoinitiators in free
radical polymerizations.[3,4] Upon irradiation these ketones
undergo Norrish type I cleavage to produce aromatic acyl
radicals with high efficiency.[5–7]
The benzoyl radical 1 is highly reactive, and to character-
ize this radical spectroscopically either low temperature
spectroscopy or time resolved spectroscopy (laser flash pho-
tolysis LFP) is necessary. Radical 1 was generated in the
solid state at 77 K and characterized by EPR spec-
troscopy.[11,12] The EPR spectrum could also be obtained
by time resolved EPR spectroscopy in solution.[5] The con-
clusion of the EPR studies is that the electronic structure
of 1 is described best as a σ-radical with an sp2 hybridized
radical center. A very weak and broad absorption maxi-
mum at 650 nm was assigned to the benzoyl radical in an
MTHF glass at 77 K.[13]
The benzoyl radical 1 has also been proposed to be an
intermediate in the combustion of aromatic hydrocarbons.
The combustion of aromatic hydrocarbons leads to the for-
mation of the phenyl radical 2 and CO which form an equi-
librium with 1. The thermochemistry and kinetics of the
unimolecular decomposition of 1 to give 2 and CO has been
studied by Solly and Benson.[8] At room temperature the
reaction enthalpy for the addition reaction was determined
to ∆H298 = –27.5 kcal/mol with an activation barrier of
2.3 kcal/mol.
The benzoyl radical 1 has also been generated as a tran-
sient species in solution at room temperature and investi-
gated by time resolved spectroscopy.[5,6,14–18] Since its ab-
sorptions in the UV/Vis region of the spectrum are only
weak, time resolved IR spectroscopy has proven to be par-
ticularly useful to detect 1.[5,6,16,18] The C=O str. vibration
of 1 was observed at 1828 cm–1 in n-hexane,[12,16,18,19] at
1824 cm–1 in CCl4,[5] and at 1818 cm–1 in acetonitrile.[6] The
lifetime of 1 in solution is in the order of miroseconds, de-
pending on the solvent and the presence of trapping rea-
gents. The reactivity of 1 towards various trapping reagents
is similar to that of simple alkyl radicals. If the solution in
which 1 is formed contains O2, the benzoylperoxy radical 3
is rapidly formed with a C=O str. vibration between 1820
and 1814 cm–1, depending on the solvent.[5,14]
Xia and Lin reported precise kinetic data and a reaction
enthalpy of –24.6Ϯ0.8 kcal/mol using cavity ring-down
spectroscopy for this reaction.[9,10] According to MP2 cal-
Here, we report the synthesis and IR spectroscopic char-
acterization of the benzoyl radical 1 in an argon matrix.
This allows to record the complete mid IR spectrum of 1,
which has so far been unknown. The thermal reaction of 1
with molecular oxygen and the photochemistry of the inter-
mediates is also reported.
[a] Lehrstuhl für Organische Chemie II der Ruhr-Universität
Bochum
44780 Bochum, Germany
Fax: +49-234-321-4353
E-mail: wolfram.sander@rub.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000153.
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Eur. J. Org. Chem. 2010, 2904–2909

Benzoyl and Benzoylperoxy Radicals
pyrolysis products in a large excess of argon at 15 K pro-
duced good yields of matrix-isolated 2.[23,24] In addition to
2 the matrix contains benzene (formed from 2 via hydrogen
abstraction), acetylene (from the thermal decomposition of
2) and unreacted starting material 4. If an argon matrix
containing 2 is doped with small amounts of oxygen (0.5–
2%) radical 2 is easily trapped to give the phenylperoxy
radical 5.[24] This bimolecular reaction is induced by warm-
ing the matrix from 10 K to approximately 30 K, where the
ridgid argon matrix is getting soft and diffusion of small
trapped molecules becomes rapid. We therefore used similar
conditions for the trapping experiments of 2 with CO.
Trapping of the products of the pyrolysis of 4 in argon
doped with 2% CO at 10 K resulted in the formation of a
new product with a strong IR band at 1824.4 cm–1 that was
not present in the absence of CO. If the matrix was an-
nealed at 30–35 K, all IR bands assigned to the new prod-
uct (Figure 1, Table 1) increased in intensity while that of
the phenyl radical 2 decreased. This indicates that the new
compound is formed by a bimolecular reaction between 2
and CO with a very low or absent activation barrier. It is
tempting to assign the new IR band at 1824.4 cm–1 to the
C=O str. vibration of the benzoyl radical 1. This is con-
firmed by comparison with the transient IR spectrum of
1 obtained by time resolved IR spectroscopy.[5] The C=O
stretching vibration of 1 in argon at 1824.4 cm–1 very nicely
matches the corresponding absorption in CCl4 at
1824 cm–1. The assignment of the IR spectrum of 1 was
further confirmed by comparison of the spectrum in solid
argon with calculations at the UB3LYP/cc-pVTZ level of
Results and Discussion
The Benzoyl Radical
The phenyl radical 2 (Scheme 1) can be easily generated
under the conditions of matrix isolation from a variety of
thermal and photochemical precursors.[20–24] To investigate
bimolecular reactions of matrix isolated 2 it proved to be
advantageous to generate it via flash vacuum pyrolysis
(FVP) followed by trapping in an inert matrix rather than
by photolysis of a matrix-isolated precursor. In the latter
case radical pairs are formed in the same matrix cage, and
annealing of the matrix results predominantly in recombi-
nation of these radical pairs rather than in reactions with
other small molecules trapped in the matrix. In our hands
the FVP of azobenzene 4 with subsequent trapping of the
Scheme 1. Reactions of the phenyl radical 2.
Table 1. IR spectroscopic data of two isotopomers of the benzoyl radical 1.
C6H5CO (1)
C6D5CO (d5-1)
Assignment
Mode[a]
Argon[b]
DFT[c]
Argon[b]
DFT[c]
Sym.
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
1824.4 (100)
1594.8 (2)
1581.0 (9)
–
1450.3 (4)
1307.8 (Ͻ1)
1288.0 (1)
–
1886.2 (100)
1635.0 (6)
1619.0 (4)
1518.8 (0)
1484.0 (5)
1354.6 (2)
1333.8 (2)
1203.4 (0)
1187.6 (0)
1163.5 (21)
1102.0 (2)
1048.6 (1)
1026.8 (0)
1022.2. (0)
1007.0 (0)
965.8 (1)
872.4 (0)
807.6 (5)
774.8 (18)
709.3 (10)
637.7 (4)
625.9 (3)
471.3 (1)
442.2 (0)
418.7 (0)
1815.8 (100)
1556.8 (6)
1550.0 (8)
1356.3 (Ͻ1)
1327.1 (7)
–
1297.0 (3)
866.6 (Ͻ1)
845.5 (2)
1100.1 (32)
817.2 (≈ 1)
831.5 (13)
–
959.6 (5)
–
765.1 (3)
–
749.3 (6)
545.5 (27)
624.9 (2)
603.7 (15)
587.0 (5)
–
1885.9 (100)
1597.3 (6)
1579.3 (5)
1386.4 (1)
1357.7 (3)
1061.5 (0)
1328.9 (3)
884.1 (0)
862.5 (1)
1124.3 (13)
834.4 (1)
850.8 (6)
861.6 (0)
978.8 (0)
820.8 (0)
791.4 (1)
678.0 (0)
762.2 (3)
557.7 (11)
643.5 (2)
618.9 (5)
602.0 (1)
418.1 (2)
431.2 (0)
363.5 (0)
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈЈ
AЈ
AЈЈ
AЈЈ
AЈЈ
AЈ
AЈЈ
AЈЈ
AЈ
AЈ
AЈЈ
AЈ
AЈЈ
C=O str.
C=C str.
C=C str.
C–H def.(in plane)
C–H def.(in plane)
C–H def.(in plane)
C–H def./C–C str (ring)
C–H def.(in plane)
C–H def.(in plane)
C–H def./C–C str.
C–H def.(in plane)
C–H def.(in plane)
C–H def.(out-of-plane)
ring str.
C–H def.(out-of-plane)
C–H def.(out-of-plane)
C–H def.(out-of-plane)
C–O str./ ring str.
C–H def.(out-of-plane)
C–H def.(out-of-plane)
CCO def./ring str.
CCO def./ring str.
C–H def.(out-of-plane)
CCC def.
–
1136.2 (21)
1070.4 (3)
–
–
–
–
935.8 (2)
–
789.7 (11)
755.9 (12)
687.8 (10)
624.7 (16)
610.4 (10)
–
8
7
6
5
–
–
–
–
4
C–H def.(out-of-plane)
[a] Mode numbers based on the C6H512C16O isotopomer. [b] Argon matrix at 10 K. Wavenumbers in cm–1 and relative intensities in
parenthesis. [c] Calculated at the UB3LYP/cc-pVTZ level of theory.
Eur. J. Org. Chem. 2010, 2904–2909
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Mardyukov, W. Sander
FULL PAPER
theory, which nicely reproduce the experimental spectrum
The formation of 1 from 2 and CO is photochemically
(Table 1). If d5-2 is used the corresponding d5-1 is formed reversible. Irradiation of matrix-isolated 1 at 10 K with UV
with the expected IR spectrum (Figure S1).
light (λ Ͼ 260 nm) leads back to 2 and CO (Figure 2). Sub-
sequent annealing at 35 K again results in the formation of
1.
The Benzoylperoxy Radical
In solution, the benzoyl radical 1 can be trapped by mo-
lecular oxygen to produce the benzoylperoxy radical 3.[5]
Under the conditions of matrix isolation the formation of
3 requires that the phenyl radical 2 first reacts with CO to
give 1, which in a second thermal step reacts with O2 to
give 3. Both thermal steps depend on the diffusion of CO
and O2, respectively, in the solid matrix. Since 2 is also
highly reactive towards oxygen, mixtures of 3 and the phen-
ylperoxy radical 5 are expected to be formed if mixtures of
CO and O2 are used as trapping reagents in argon. To in-
crease the yield of 3 we therefore used an excess of CO with
respect to O2 in the experiments.
If an argon matrix containing 2 doped with ca. 0.5% O2
and 2% CO is slowly warmed from 10 K to 35 K the forma-
tion of 1 as the primary product and 5 as a minor product
can be monitored by IR spectroscopy. However, 1 is also
trapped by O2 and during continuous warming of the ma-
trix all IR bands assigned to 1 decrease in intensity and a
new product with a prominent C=O str. vibration at
Figure 1. IR spectra showing the annealing of an argon matrix
doped with 1% CO containing phenyl radical 2. The band at 1824.4
assigned to 1 is growing in intensity during warming the matrix.
(a) Matrix at 10 K (b) The same matrix after annealing at 25 K for
10 min, (c) after annealing at 30 K for 10 min, and (d) after anneal-
ing at 35 K for 10 min.
According to DFT calculation the reaction 2 + CO Ǟ 1 1820.9 cm–1 is formed. If 18O2 is used in the experiment this
is exothermic by 24.58 kcal/mol with a barrier of 0.7 kcal/ band is almost not effected, indicating that the oxygen atom
mol at the B3LYP and 3.3 kcal/mol at the MP2 level of of the C=O bond comes from CO and not from O2
theory.[10] The reaction between 2 and CO in solid argon is (Figure 3, Table 2). Large 18O isotopic shifts are found
rapid at temperatures above 30 K where the diffusion of for a band at 1107.0 cm–1 (–68.1 cm–1) and 768.2 cm–1
small molecules in argon becomes possible. From that we (–21.6 cm–1) which are thus assigned to O–O and a C–O
conclude that the thermal barrier for this reaction must be stretching vibrations, respectively. For the transient benzo-
very small or absent, in agreement with the theoretical pre- ylperoxy radical 3 in CCl4 a C=O stretching vibration of
dictions.
1820 cm–1 was reported,[5] in excellent agreement with the
Figure 2. Difference IR spectra showing the photochemistry (λ Ͼ 350 nm) of 1, matrix-isolated in argon at 10 K. Bands pointing down-
wards are disappearing during irradiation and assigned to 1. Bands pointing upwards are appearing and assigned to 2. (a) IR spectrum
of 1 calculated at the UB3LYP/cc-pVTZ level of theory. (b) Difference IR spectrum after 30 min irradiation. (c) IR spectrum of 2
calculated at the UB3LYP/cc-pVTZ level of theory.
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 2904–2909

Benzoyl and Benzoylperoxy Radicals
band found in the CO/O2 trapping experiment. We there-
fore assign the new compound formed only in the presence
of both CO and O2 to radical 3 (Figure 4).
pVTZ level of theory (Figures S4 and S5, Table 2). These
experiments clearly show that the thermal reaction of 2 with
CO can compete with the reaction with O2 and that
the benzoyl radical 1 rapidly reacts with molecular oxygen.
DFT calculations do not indicate an activation barrier for
the oxygen trapping of 1 (Figure S2).
Two rotamers 3a and 3b are possibly formed in the ex-
periments. According to UB3LYP/cc-pVTZ calculations the
s-Z conformer 3a is by 3.47 kcal/mol (ZPE = 3.31 kcal/mol)
more stable than the s-E conformer 3b. The activation bar-
rier for the 3aǞ3b isomerization is calculated to 5.03 kcal/
mol (Figure S3). The calculated IR spectrum of 3a is in
good agreement with the experimental spectrum of 3a, and
there is no evidence for the formation of the less stable con-
former 3b.
UV irradiation (λ Ͼ 260 nm) of 3 at 10 K results in the
disappearance of all bands assigned to 3 and formation of
CO2 and several other new bands, in particular several
strong and broad absorptions in the range between 2140
and 2120 cm–1. If 18O2/16O2 mixtures are used in the experi-
Figure 3. IR spectra showing the annealing of an argon matrix
doped with 2% CO and 0.5% O2 containing phenyl radical 2. The ment new bands around 2089 cm–1 appear, which clearly
matrix was slowly (approximately 1 K/min) warmed from 10 K (t
indicates that one or several ketenes are formed. In ad-
= 0 min) to 40 K (t = 30 min). The band at 1824 cm–1 is assigned
dition, the three isotopomers C16O2, C18O16O, and C18O2
to benzoyl radical 1 and the band at 1820.9 cm–1 to benzoylperoxy
are found (Figure S6). We assume that the photolysis of 3
results in ring-opening and formation of ketenes. However,
due to the low intensity of other IR bands of the ketenes
radical 3.
The band positions, relative intensities, and isotopic
shifts of 3 (18O and d5 isotopomers) are in good agreement (presumably several isomers or conformers are formed) a
with predictions from DFT calculations at the UB3LYP/cc- definitive assignment was not possible.
Table 2. IR spectroscopic data of four isotopomers of the benzoylperoxy radical 3.
Mode[a] C6H5CO16O16O (3)
1820.9 (100) 1868.2 (100) 1820.6 (100) 1867.9 (100) 1821.1 (100) 1867.8 (100) 1821.3 (100) 1867.6 (100) C=O str.
1603.0 (10) 1643.1 (12) 1601.7 (13) 1643.1 (12) 1567.1 (22) 1605.1 (15) 1567.2 (48) 1605.1 (15) C=C str.
C6H5CO18O18O (3)
C6D5CO16O16O (3)
C6D5CO18O18O (3)
Assignment
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
–
–
1622.0 (1)
1529.1 (0)
–
–
1622.0 (1)
1529.1 (0)
–
1583.4 (1)
1404.8 (8)
1364.0 (5)
1066.3 (2)
1336.6 (2)
1195.6 (41) 1174.4 (54)
883.4 (5)
862.7 (1)
–
1583.4 (1)
1404.6 (8)
1363.9 (5)
1064.8 (4)
1336.5 (2)
1192.8 (41) C–C str./ C–H def.
883.3 (5)
862.6 (1)
C=C str.
C–H def.
C–H def.
C–H def.
1378.3 (10)
1333.4 (5)
1042.7 (3)
1300.2 (4)
1377.8 (20)
1333.3 (12)
1042.9 (4)
1299.8 (8)
1455.8 (Ͻ1) 1487.6 (6)
–
–
1228.4 (38) 1247.8 (53) 1225.8 (63) 1246.6 (57) 1174.7 (49)
1183.8 (8)
1455.2 (Ͻ1) 1487.6 (6)
1322.7 (Ͻ1) 1362.0 (2)
1309.8 (Ͻ1) 1340.9 (1)
1362.0 (2)
1341.2 (1)
C–H def./C–C str
1206.6 (13) 1183.1 (10) 1206.6 (14) 866.8 (9)
1189.9 (0) 1189.9 (0) 846.2 (1)
1155.2 (19) 1038.9 (8) 1090.6 (12) 1106.3 (12)
866.7 (22)
846.6 (6)
C–H def.
C–H def.
–
–
1107.0 (7)
–
–
–
1002.3 (3)
–
–
1154.0 (29) 1033.4 (18)
1092.2 (12) O–O str.
1112.6 (1)
1053.0 (0)
1028.5 (0)
1022.2 (1)
1008.9 (0)
973.0 (1)
948.7 (57)
870.9 (0)
807.5 (4)
772.7 (28)
710.5 (25)
694.7 (5)
687.2 (7)
631.7 (1)
462.2 (1)
443.2 (0)
–
–
–
1112.7 (1)
1052.9 (0)
1028.5 (0)
1022.2 (1)
1008.9 (0)
973.0 (1)
946.6 (58)
870.9 (0)
807.3 (4)
748.9 (28)
710.4 (24)
690.0 (3)
686.4 (7)
631.6 (1)
456.2 (1)
442.8 (0)
824.3 (4)
–
–
959.8 (7)
–
–
927.2 (60)
–
701.7 (3)
762.8 (9)
536.4 (14)
651.8 (10)
604.0 (2)
590.9 (Ͻ1)
–
842.3 (1)
839.2 (3)
867.6 (0)
978.6 (1)
822.7 (0)
815.3 (0)
941.2 (49)
677.2 (0)
719.6 (2)
768.0 (26)
548.0 (16)
666.8 (5)
620.6 (3)
605.5 (0)
452.0 (1)
398.8 (0)
824.3 (10)
813.7 (5)
–
959.7 (14)
–
842.2 (2)
838.7 (3)
867.6 (0)
978.5 (1)
822.7 (0)
815.2 (0)
939.3 (49)
677.2 (0)
719.0 (2)
742.7 (24)
547.9 (16)
663.9 (4)
620.2 (3)
605.5 (0)
446.1 (1)
398.5 (0)
C–H def.
C–H def.
C–H wag.
C–C str. (ring)
C–H wag.
C–H wag.
C–O str.
C–H wag.
C–H wag.
C–O str.
C–H wag.
ring skel.
ring out of plane
ring skel.
1002.3 (3)
–
950.3 (Ͻ1)
934.4 (67)
–
788.5 (2)
746.6 (10)
692.3 (20)
678.6 (5)
668.2 (7)
–
–
935.8 (65)
–
926.0 (98)
–
783.6 (5)
768.2 (10)
692.5 (27)
678.6 (11)
668.5 (8)
–
701.6 (4)
740.7 (12)
536.4 (32)
648.9 (10)
604.4 (6)
591.2 (2)
–
–
–
–
–
C–C–O def.
ring out of plane
8
–
–
[a] Mode numbers based on the C6H512C16O16O16O isotopomer.
Eur. J. Org. Chem. 2010, 2904–2909
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A. Mardyukov, W. Sander
FULL PAPER
Figure 4. Difference IR spectra showing the photochemistry (λ Ͼ 260 nm) of 3, matrix-isolated in argon at 10 K. Bands pointing down-
wards are disappearing during irradiation and assigned to 3. (a) IR spectrum of 3 calculated at the UB3LYP/cc-pVTZ level of theory.
(b) Difference IR spectrum after 60 min irradiation.
optics and dichroic mirrors in combination with cutoff filters (50%
transmission at the wavelength specified).
Conclusions
The phenyl radical is a highly reactive species that even
Computational Methods: Optimized geometries and vibrational fre-
quencies of all species were calculated at the UB3LYP[27–29] level
of theory using the 6-311+G(d,p) polarized valence-triple-ξ basis
set[30,31] and Dunning’s cc-pVTZ basis sets.[32–34] All DFT calcula-
tions were carried out with Gaussian 03.[35]
under the conditions of matrix isolation rapidly reacts with
CO and O2 as long as diffusion of these small molecules in
the solid matrix is possible. With CO the benzoyl radical
1 is formed which could be isolated and spectroscopically
characterized. Interestingly, conditions could be found were
mixtures of CO and O2 in the same matrix produce good
yields of the benzoylperoxy radical 3 by subsequent reac-
tions of the phenyl radical 2 with CO and O2. Obviously,
the reaction of 2 with CO can compete with the reaction
with O2. This is in line with DFT and ab initio calculations
which predict very shallow or absent activation barriers for
both reactions. The formation of the radicals 1 and 5 in
argon matrices depends on the statistical distribution and
rates of diffusion of CO and O2. An excess of CO compared
Acknowledgments
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG) and the Fond der Chemischen Industrie.
[1] N. S. Allen, M. C. Marin, M. Edge, D. W. Davies, J. Garrett,
F. Jones, S. Navaratnam, B. J. Parsons, J. Photochem. Pho-
tobiol. 1999, 126, 135.
[2] A. Ajayaghosh, S. Das, M. V. George, J. Polym. Sci., Part A:
Polym. Chem. 1993, 31, 653.
to O2 thus results in the preferential formation of 1 which [3] J. Eichler, C. P. Herz, I. Naito, W. Schnabel, J. Photochem.
subsequently reacts with O2 to 3.
1980, 12, 225.
[4] C. P. Herz, J. Eichler, Farbe Lack 1979, 85, 933.
[5] C. Kolano, G. Bucher, H. H. Wenk, M. Jaeger, O. Schade, W.
Sander, J. Phys. Org. Chem. 2004, 17, 207.
Experimental Section
[6] C. S. Colley, D. C. Grills, N. A. Besley, S. Jockusch, P. Matou-
sek, A. W. Parker, M. Towrie, N. J. Turro, P. M. W. Gill, M. W.
Azobenzene and [D10]Azobenzene: Azobenzene 4 (Acros Organics
George, J. Am. Chem. Soc. 2002, 124, 14952.
99.8%) was used without further purification. The perdeuterated
[7] G. W. Sluggett, P. F. McGarry, I. V. Koptyug, N. J. Turro, J.
Am. Chem. Soc. 1996, 118, 7367.
azobenzene d10-4 was synthesized according to a literature pro-
cedure.[25] 13C NMR (50.33 MHz, CDCI3, 25 °C, TMS): δC
=
[8] R. K. Solly, S. W. Benson, J. Am. Chem. Soc. 1971, 93, 2127.
[9] G.-J. Nam, W. Xia, J. Park, M. C. Lin, Chem. Phys. Processes
Combust. 1999, 304.
152.99, 78.06, 77.43, 76.79. MS (RI, 70 eV): m/z, % 192 [M+], 110,
82, 54, 45.
[10] G.-J. Nam, W. Xia, J. Park, M. C. Lin, J. Phys. Chem. A 2000,
Matrix Isolation: Matrix isolation experiments were performed by
standard techniques[26] using a closed cycle helium cryostat and a
CsI spectroscopic window cooled to 10 K. FTIR spectra were re-
corded with a standard resolution of 0.5 cm–1, using a N2(l)-cooled
MCT detector in the range 400–4000 cm–1. UV spectra were re-
corded between 800 and 200 nm with a standard resolution of
0.02 nm (Varian UV/Vis NIR spectrophotometer) from a sample
deposited on a sapphire window cooled to 10 K by a closed-cycle
cryostat. Flash vacuum pyrolysis was carried out by slowly sublim-
ing azobenzene 4 through a 7 cm quartz tube heated electrically
with a tantalum wire. Broadband irradiation was carried out with
mercury high-pressure arc lamps in housings equipped with quartz
104, 1233.
[11] P. J. Krusic, T. A. Rettig, J. Am. Chem. Soc. 1970, 92, 722.
[12] J. E. Bennett, B. Mile, Trans. Faraday Soc. 1971, 67, 1587.
[13] O. Ito, T. Sakaguchi, M. Matsuda, J. Chem. Soc. Faraday
Trans. 1978, 74, 1188.
[14] C. E. Brown, A. G. Neville, D. M. Rayner, K. U. Ingold, J.
Lusztyk, Aust. J. Chem. 1995, 48, 363.
[15] J. A. Martinho Simoes, D. Griller, Chem. Phys. Lett. 1989, 158,
175.
[16] A. G. Neville, C. E. Brown, D. M. Rayner, J. Lusztyk, K. U.
Ingold, J. Am. Chem. Soc. 1991, 113, 1869.
[17] M. Hoshino, R. Konishi, H. Seto, H. Seki, H. Sonoki, T.
Yokoyama, H. Shimamori, Res. Chem. Intermed. 2001, 27, 189.
2908
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 2904–2909

Benzoyl and Benzoylperoxy Radicals
[18] A. G. Merzlikine, S. V. Voskresensky, E. O. Danilov, D. C.
Neckers, A. V. Fedorov, Photochem. Photobiol. Sci. 2007, 6,
608.
[32]
[33]
D. E. Woon, T. H. Dunning Jr., J. Chem. Phys. 1993, 98, 1358.
R. A. Kendall, T. H. Dunning Jr., R. J. Harrison, J. Chem.
Phys. 1992, 96, 6796.
[19] L. Grossi, G. Placucci, J. Chem. Soc., Chem. Commun. 1985,
[34] T. H. Dunning Jr., J. Chem. Phys. 1989, 90, 1007.
[35] G. W. T. M. J. Krishnan, H. B. Schlegel, G. E. Scuseria,
J. R. C. M. A. Robb, J. A. Montgomery Jr., T. Vreven,
J. C. B. K. N. Kudin, J. M. Millam, S. S. Iyengar, J. Tomasi,
B. M. V. Barone, M. Cossi, G. Scalmani, N. Rega, H. N. G. A.
Petersson, M. Hada, M. Ehara, K. Toyota, J. H. R. Fukuda,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, M. K. H. Nakai,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, J. J. C. Adamo,
R. Gomperts, R. E. Stratmann, O. Yazyev, R. C. A. J. Austin,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, G. A. V. K. Morok-
uma, P. Salvador, J. J. Dannenberg, S. D. V. G. Zakrzewski,
A. D. Daniels, M. C. Strain, D. K. M. O. Farkas, A. D. Rab-
uck, K. Raghavachari, J. V. O. J. B. Foresman, Q. Cui, A. G.
Baboul, S. Clifford, B. B. S. J. Cioslowski, G. Liu, A. Liash-
enko, P. Piskorz, R. L. M. I. Komaromi, D. J. Fox, T. Keith,
M. A. Al-Laham, A. N. C. Y. Peng, M. Challacombe, P. M. W.
Gill, W. C. B. Johnson, M. W. Wong, C. Gonzalez, J. A. Pople,
Gaussian 03, rev. B.1, Pittsburgh, PA, 2003.
943.
[20] J. Pacansky, J. Bargon, J. Am. Chem. Soc. 1975, 97, 6896.
[21] J. G. Radziszewski, M. R. Nimlos, P. R. Winter, G. B. Ellison,
J. Am. Chem. Soc. 1996, 118, 7400.
[22] A. V. Friderichsen, J. G. Radziszewski, M. R. Nimlos, P. R.
Winter, D. C. Dayton, D. E. David, G. B. Ellison, J. Am. Chem.
Soc. 2001, 123, 1977.
[23] A. Mardyukov, E. Sanchez-Garcia, R. Crespo-Otero, W. San-
der, Angew. Chem. Int. Ed. 2009, 48, 4804.
[24] A. Mardyukov, W. Sander, Chem. Eur. J. 2009, 15, 1462.
[25] H. E. Bigelow, D. B. Robinson, Org. Synth. 1942, 22, 28.
[26] I. Dunkin, Matrix Isolation Techniques: A Practical Approach,
1998.
[27] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[28] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B: Condens. Matter
Mater. Phys. 1988, 37, 785.
[29] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett.
1989, 157, 200.
[30] A. D. McLean, G. S. Chandler, J. Chem. Phys. 1980, 72, 5639.
[31] R. Krishnan, J. S. Binkley, R. Seeger, J. A. Pople, J. Chem.
Phys. 1980, 72, 650.
Received: February 3, 2010
Published Online: April 14, 2010
Eur. J. Org. Chem. 2010, 2904–2909
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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