International Journal of Organic Chemistry, 2011, 1, 37-40
doi:10.4236/ijoc.2011.12007 Published Online June 2011 (
Copyright © 2011 SciRes. IJOC
EPR Study of the Thermal Decomposition of Transannular
Peroxide of Anthracene
Lubomír Lapčík Jr.1, Barbora Lapčíková1, Andrej Staško2
1Centre of Polymer Systems, Faculty of Technology, Tomas Bata University, Zlín, Czech Republic
2Slovak Technical Un iversity Bratislava, Faculty of Chemical & Food Technology, Bratislava, Slovak Republic
Received April 7, 2011; revised May 15, 2011; accepted May 23, 2011
Thermal decomposition of transannular peroxide of anthracene (POA) (or 9,10-epidioxido anthracene) was
studied by means of electron paramagnetic resonance spectroscopy (EPR) in the solid as well as in the liquid
phases. Decomposition process proceeds via cleavage of the O-O bridge of the POA molecule, generating
thus an alcoxy intermediate radical. Its concentration increases to a certain equilibrium stage during the time
scale of the experiment. EPR spectra in the solid state were of the singlet type at the temperatures over 350 K,
a doublet like anisotropic spectra were measured at the room temperature, both having g-value 2.0033. EPR
spectrum from POA decomposed in benzene indicates four protons with higher (2aH = 0.305 mT, 2aH =
0.335 mT) and four protons with a lower (2aH = 0.075 mT, 2aH = 0.105 mT) splitting constants, correspond-
ing well the radical expected after cleavage of O-O bridge.
Keywords: Endoperoxide, Anthracene, Free Radicals, EPR, Decomposition, Kinetics
1. Introduction
UV light irradiated anthracene and its derivatives in de-
gassed solutions give anthracene dimers, while in the
presence of oxygen they form endoperoxides [1,2]. As
found by quantum chemical molecular orbital calcula-
tions based on Woodward-Hoffman orbital symmetry
conservation principle application [3], endoperoxides of
anthracene production is catalysed by transition metals
atoms. These thermally and photo chemically reactive
substances (depending on the reaction conditions) are
decomposed to the original hydrocarbon and simultane-
ously liberate singlet oxygen (1O2), or decompose by the
disruption of the O-O bonds, thus producing highly reac-
tive biradical, or finally by the transformation into the
other types of peroxides [4]. For instance, for anthra-
cene-9,10-endoperoxide (POA) it has been found, that its
photochemical excitation to Sn, with n 2 leads to
cycloreversion, producing anthracene and 1O2, whereas
excitation to the S1 state would initially cause homolytic
O-O cleavage, eventually resulting in a diepoxide rear-
rangement product [5,6].
Decomposition via disrupting of the O-O bridge can
be used for an effective crosslinking of some industrially
important polymers, producing thus highly crosslinked
matrices, e.g. for the optical information storage [7]. In
contrary to the latter mentioned polymer matrix
crosslinking reactions, depending on the reaction condi-
tions, a vigorous degradation of polysaccharide hyalu-
ronic acid by singlet oxygen liberated from POA was
reported earlier [8]. The possibility of endoperoxides to
release these reactive species by unimolecular decompo-
sition makes them interesting candidates for in vivo
site-specific oxidative targeting with singlet oxygen at
the present time [6,9]. The complex oxygen photosensi-
tization in the presence of water soluble anthracene-1-
sulphonate was studied by means of EPR spin trapping
method [10]. It was found, that the anthracene-1-sulpho-
nate in the presence of oxygen is converted to quinone
and oxygen is activated to superoxide radical intermedi-
ate. Consecutively, the latter superoxide radicals are
transformed into the highly reactive hydroxyl radicals.
In the paper presented thermal decomposition of tran-
sannular peroxide of anthracene (or anthracene 9,10-
endoperoxide, or 9,10-epidioxido-anthracene) is studied
by means of electron paramagnetic resonance spectros-
copy (EPR) both in the solid as well as in the liquid
2. Experimental
2.1. Materials
Transannular peroxide of anthracene was synthesized
using photosensitized reaction with methylene blue as a
photo sensitizer in chloroform by the method described
earlier by Foot et al. [2,11]. Final product was then puri-
fied using column chromatography (Florosil, Fluka,
Switzerland) and then several times re-crystallized in
vacuum. All chemicals used during synthesis were of
analytical grade purity. Final colourless crystals were
identified and characterized by IR spectroscopy [11]
characteristic absorption bands at 1170, 950, 880, 846,
and 745 cm–1 wavenumbers as shown in Figure 1, UV
VIS spectroscopy [12] and GC-MS [13].
2.2. Methods
Infra red spectra (IR) were recorded on computer con-
trolled Perkin Elmer 983 spectrometer (USA).
Electron paramagnetic resonance spectra were meas-
ured at room temperature on Bruker 200E SRC spec-
trometer controlled by Aspect 2000 computer (Bruker,
Germany). All EPR spectra were simulated using stan-
dard simulation software EPRSRC (Bruker, Germany).
3. Results and Discussion
In Figure 2 are shown EPR spectra of POA thermal de-
composition product as observed in the solid sample at
Figure 1. IR spectrum with the labelled characteristic ab-
sorption bands of POA measured in the form of thin film
prepared by evaporation of solvent (CDCl3) on KBr lens.
(a) (b)
Figure 2. EPR spectra of POA measured in the solid sample
at temperature: (a) 300 K; (b) 350 K (SW = 5.0 mT, g =
two different temperatures. Doublet anisotropic EPR
spectrum measured at 300 K was vanishing with the in-
creasing temperature, while at about 350 K the singlet
spectrum was observed. Probably, the hyperfine anisot-
ropic structure of the EPR spectrum at lower temperature
was possible to follow due to the polycrystalline charac-
ter of the studied sample. As the temperature was in-
creased to the temperature close to the melting point
(values given in the literature are ranging from 120˚C up
to 160˚C (Ref. [4]) the slightly anisotropic character dis-
appears and only purely isotropic singlet EPR spectrum
was observed. For both, the doublet as well as the singlet
spectrum the same g-factor was observed (g = 2.00330),
suggesting the same kind of the intermediate radical. The
time dependence of the intensity of the EPR line meas-
ured at 273 K is shown in Figure 3. Here, the intensity
of the signal is increasing with the time suggesting in-
creasing radical concentration in the system. After sev-
eral minutes, it is reaching its time independent plateau,
suggesting probably equilibrium radical concentration.
The same dependencies were observed at 393 K, 413 K
as well as at 433 K. At the temperature of 433 K, the
Figure 3. Time dependence of EPR spectra evolution meas-
ured at 373 K (*spectrum measured at room temperature).
Copyright © 2011 SciRes. IJOC
intensity of the EPR spectrum after reaching its maxi-
mum decreases. Most probably, the POA is starting to
decompose at this temperature. This phenomenon is
more evident from Figure 4, where the kinetic curves of
the time dependencies of the EPR line intensities meas-
ured at the different temperatures are shown. The initial
slope is decreasing at the lower temperature. This indi-
cates that the rate constant of the radical generation is
Formal first order kinetic rate constants of these proc-
esses are given in the Table 1. It is evident, that with the
increasing temperature the production of the radical in-
termediates is more effective.
Because of the fact, that the singlet EPR spectra can-
not be unambiguously assigned to a defined structure,
one can expect more exact structural information about
the radical surrounding from the hyperfine splitting of
the EPR spectra which can be observed in the solution.
For this reason, we have studied the thermal decomposi-
tion of POA also in the liquid phase.
Figure 4. Time dependencies of the radical concentrations
observed during thermal decomposition of POA at:
star—373 K; Empty circle—393 K; Empty diamond—413
K; Empty square—433 K.
Table 1. Obtained values of the first order kinetic mecha-
nism rate constants of POA thermal decomposition as cal-
culated for data shown in Figure 4.
Temperature (K) 104 × k1 (1/s) Corr. Coeff.
373 3.32 0.984
393 5.30 0.987
413 6.32 0.981
433 6.85 0.944
Figure 5 shows experimental and simulated spectra of
POA in benzene as observed at 300 K. It was simulated
with four higher (2aH = 0.305 mT, 2aH = 0.335 mT) and
four lower (2aH = 0.075 mT, 2aH = 0.105 mT) splitting
constants. This corresponds well to the two groups
(every one with approximately four equivalent protons in
xx and yy positions) own to POA structure.
To obtain a better fit between the experimental and
simulated EPR spectrum, also a contribution singlet
spectrum depicted in Figure 2(b) had to be considered
by the simulation. Similar spectra were observed also at
higher temperatures in different solvents (e.g. toluene,
xylene, chloroform).
4. Conclusions
The analysis of EPR spectra shown in Figure 5 moni-
tored during the thermal decomposition of POA suggests
formation of the following radicals:
Q+ R
where R is a radical capable to react with oxygen centred
radical from anthracene. R may originate from the sol-
vent molecules or from some another external sources.
The scheme is in accord with earlier published results by
Cowell and Pitts Jr. [14] assuming endoperoxide forma-
tion with consecutive radical reactions. Our theoretical
investigations [3] implied the role of transition metal by
the formation of antracene endoperoxide. Lazar et al. [7]
experimentally confirmed formation of alkoxy radicals
during thermal POA decomposition, and, furthermore,
analogous EPR spectra were found investigating similar
systems [15].
Experimental simulated
Figure 5. Experimental and simulated EPR spectra of POA
dissolved in benzene during thermal treatment at 300 K, g
= 2.0034, simulated as 2aH = 0.075 mT, 2aH = 0.105 mT, 2aH
= 0.305 mT, 2aH = 0.335 mT, pp = 0.045 mT with the
g-factor of 2.0034, SW = 2.5 mT. (aH and pp are splitting
constant and peak-to-peak width expressed in mT).
Copyright © 2011 SciRes. IJOC
Copyright © 2011 SciRes. IJOC
5. Acknowledgements
This article was created with support of Operational Pro-
gram Research and Development for Innovations
co-funded by the European Regional Development Fund
(ERDF) and national budget of Czech Republic within
the framework of the Centre of Polymer Systems project
(reg. number CZ. 1.05/2.1.00/03. 0111). This work was
also supported by the Scientific Grant Agency of the
Slovak Republic (Project VEGA/1/0018/09).
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