Experimental and theoretical study of the photochemical and thermal decomposition of maleic and dichloromaleic anhydrides

Article abstract of DOI:10.1139/v02-038

The photochemical and thermal behavior of maleic anhydride 1a and dichloromaleic anhydride 1b, in cryogenic matrix were investigated by means of FT-IR spectroscopy. The ketenylcarbenes represent the key intermediate in the decomposition processes of the a

Full text of DOI:10.1139/v02-038

455
Experimental and theoretical study of the
photochemical and thermal decomposition
of maleic and dichloromaleic anhydrides
A. Ionescu, N. Piétri, M. Hillebrand, M. Monnier, and J.P. Aycard
Abstract: The photochemical and thermal behavior of maleic anhydride 1a and dichloromaleic anhydride 1b, in cryo-
genic matrix were investigated by means of FT-IR spectroscopy. The ketenylcarbenes represent the key intermediate in
the decomposition processes of the anhydrides, even if they were not observed experimentally. The yields of the differ-
ent products depend on the thermal or photochemical reaction conditions. The main photochemical products obtained
from 1a were cyclopropenone and acetylene, whereas the major products from 1b were dichlorocyclopropenone and
dichloroacetylene, along with small quantities of dichloropropadienone. The thermal reaction leads to dichloropropadienone,
CO, and CO2. MO calculations performed at the HF/6–31G*//HF/6–31G* level support the experimental mechanisms.
Key words: photolysis, thermolysis, ab initio calculations, cryogenic matrix, ketenylcarbenes.
Résumé : Le comportement photochimique et thermique de l’anhydride maléique 1a et dichloromaléique 1b, piégées
en matrices cryogéniques est suivi par spectroscopie IRTF. Les intermédiaires clés dans le processus de décomposition
des anhydrides sont des céténylcarbènes qui ne sont pas observés expérimentalement. La proportion des différents pro-
duits formés dépend des conditions de réaction. Les produits majoritaires obtenus photochimiquement à partir de
l’anhydride maléique 1a sont la cyclopropénone et l’acétylène tandis que l’anhydride dichloromaléique 1b produit éga-
lement une faible quantité de dichloropropadiénone. La décomposition thermique de l’anhydride dichloromaléique en-
traîne la formation de dichloropropadiénone, CO et CO2. Les états de transitions des différentes réactions ont été
modélisés en HF/6–31G*//HF/6–31G*.
Mots clés : photolyse, thermolyse, calculs ab initio, matrices cryogéniques, cétènes-carbenes.
Ionescu et al. 461
Introduction
mation rate constants — cyclopropenone is the major product.
Under continuous irradiation, both compounds decomposed to
dichloroacetylene (2232, 990, and 846 cm–1). The mecha-
nism of the reaction assumed the presence of the syn- and
anti-dichloroketenylcarbenes, but attempts to identify them
have failed.
In the case of difluoromaleic anhydride (R, R′ = F),
Brahms and Dailey (4) reported that pyrolysis leads to di-
fluoropropadienone, whereas photochemical decomposition
predominantly gives difluorocyclopropenone. In contrast
with this behaviour, the pyrolysis of the fluoromaleic anhy-
dride (R = H, R′ = F) yields fluoroacetylene (7). For the two
fluoromaleic anhydrides, irradiation initially produces cyclo-
propenones and, with time, produces the corresponding al-
kynes. In both cases, a ketenylcarbene was postulated as an
intermediate, but only in the matrix photolysis of the
fluoromaleic anhydride (7) was evidence found to support
the presence of the fluoro–ketenylcarbene. The carbene was
characterized by the IR bands at 2168, 1321, and 1102 cm–1,
the first value being the most intense.
For many years our laboratory has been interested in the
identification of highly reactive intermediate species pro-
duced by photochemical processes in cryogenic matrices (1).
Attention was paid to the chemistry and reactivity of some
conjugated carbenes, such as the ketenyl (2) and allenyl
carbenes (3). Both carbenes can undergo several reactions,
including ring closure, hydrogen or chlorine migration, and
(or) CO elimination (ketenylcarbenes), which are the precur-
sors of stable compounds. The presence of mono- or
disubstituted ketenylcarbenes was postulated in the thermal
and photochemical decomposition reactions of some cyclo-
butenediones and substituted maleic anhydride (4–9).
In our previous study (2) on the matrix photochemical de-
composition of 3,4-dichloro-1,2-cyclobutenedione, both
dichlorocyclopropenone (1896 cm–1) and dichloropropa-
dienone (2200 and 2260 cm–1) were identified by FT-IR
spectroscopy. A kinetic survey of the time evolution of the
reaction reveals a ratio of about 13:1 for the respective for-
Received 23 July 2001. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 17 April 2002.
A. Ionescu and M. Hillebrand. Department of Physical Chemistry, University of Bucharest, Bdul. Regina Elisabeta RO-7034,
Romania.
N. Piétri,1 M. Monnier, and J.P. Aycard. UMR CNRS 6633, Physique des Interactions Ioniques et Moléculaires, Equipe
Spectrométrie et Dynamique Moléculaire, Université de Provence, Case 252, Centre de St. Jérôme, 13397 Marseille cedex 20,
France.
1Corresponding author (e-mail: thalie@piimsdm8.univ-mrs.fr).
Can. J. Chem. 80: 455–461 (2002)
DOI: 10.1139/V02-038
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Can. J. Chem. Vol. 80, 2002
Fig. 1. Evolution of the FT-IR spectra of maleic anhydride 1a
isolated in an argon matrix at 10K during different irradiation
times. (a) Spectrum after deposition t = 0 min; (b) t = 152 min;
(c) t = 2800 min.
Fig. 2. Evolution of the FT-IR spectra of dichloromaleic anhy-
dride 1b isolated in an argon matrix at 10 K during different ir-
radiation times. (a) Spectrum after deposition, t = 0 min; (b) t =
35 min; (c) t = 568 min.
Taking into account the results of Dailey and co-workers
(4, 7), it seems that the photolysis and thermolysis of maleic
and dichloromaleic anhydrides could be a convenient method
to confirm the presence and the role of the ketenylcarbenes
in the reaction mechanism.
An extensive theoretical study on the formation of
ketenylcarbenes starting from bisketenes, and of their reac-
tivity in subsequent reactions of ring closure and further
decarbonylation, was done by Tidwell and co-workers (10).
In this study, they outlined the role of the substituents on the
relative stability of the ketenes and the cyclopropenones.
In the present study, we report experimental and theoreti-
cal results on the photochemical and thermal decomposition
of unsubstituted 1a and dichloro derivative 1b to provide
more information on the different possible reaction path-
ways. In addition, ab initio MO calculations were performed
to explain the chemical pathways.
Experimental
Dichloromaleic and maleic anhydride, supplied by
Aldrich, were used after purification by vacuum sublimation.
Matrix-isolation experiments
The matrices were obtained by spraying anhydride–argon
mixtures (1:750 and 1:360) onto a gold-copper plate cooled
to 20 K in a closed-cycle helium refrigerator. The IR spectra
were recorded at 10 K by reflectance with a Nicolet Series II
© 2002 NRC Canada

Ionescu et al.
457
Scheme 1.
Table 1. Experimental vibrational frequencies of products ob-
tained from photolysis of the dichloromaleic anhydride 1b.
Observed frequencies (cm–1)
Our work
Literature
2260a
Identification
2345.6
2281.1–2279.1
2142.7
1896.7–1892.9
1626.5
1036.4
CO2
4b
CO
3b
3b
3b
1896.3a
1629.9a
1029.9a
993.6a
993.1
5b
Note: The main peak frequencies are indicated in italic font.
aSee ref. 2.
750 FT-IR in the 4000–400 cm–1 spectral range with a reso-
lution of 0.12 cm–1. The UV spectra were recorded in hex-
ane and in the 200–600 nm range on an Unicam UV4
spectrometer. The spectrum of maleic anhydride shows an
absorption band at 224 nm. In the case of the dichloromaleic
anhydride, two absorption bands were observed at λ = 214
and 269 nm.
Photolysis of dichloromaleic anhydride
The time evolution of the IR spectrum of the matrix-
irradiated (λ > 230 nm) dichloromaleic anhydride (1b) is
presented in Fig. 2, and the vibrational frequencies used for
the identification of the products are listed in Table 1.
During irradiation, the bands of 1b (1803.7, 1624.9,
1247.2, 1205.0, 1006.8, 934.1, 885.9, 727.8, 701.5 cm–1)
gradually diminish in intensity and completely disappear af-
ter 568 min.
With time, three new band systems with different intensi-
ties were observed in the 2400–2100 cm–1 region. The two
most intense bands were assigned to CO (2142.7 cm–1) and
CO2 (2345.6 cm–1), while the band at 2281.1 cm–1, of lower
intensity, was assigned to dichloropropadienone (2).
The analysis of the whole range of frequencies revealed
that a group of bands at 1895.7 (the most intense), 1626.5,
and 1036.4 cm–1 have the same kinetic behavior: initially
they increased in intensity and then they decreased in inten-
sity. These bands were assigned to dichlorocyclopropenone
(2), and correspond to the νC=C, νC=O, νC-C + βC-C-C vibra-
tions, respectively. With time, these bands disappeared and a
band at 993.1 cm–1, assigned to the most intense vibration
mode of dichloroacetylene (2), gradually increased in inten-
sity. The time evolution of the integrated absorbancies of the
products is shown in Fig. 3. The experimental data are
reported in Scheme 2. The relative intensities of the bands
indicate that the major photochemical products are dichloro-
cyclopropenone and dichloroacetylene.
Irradiation techniques
Irradiation was carried out using an Osram 200 W high-
pressure mercury lamp equipped with a quartz envelope. The
broad band was filtered at λ > 230 nm. The light intensity
was the same in all experiments and the irradiation area was
wider than the surface used to record the spectrum.
MO calculations
Ab initio calculations at the HF/6–31G*//HF/6–31G*
level were performed using the program Gamess (11). The
critical points on the energy surface, minimum and saddle
points, were characterized by the Hess matrix.
Ab initio calculations (on the lowest excited electronic
states) using single-excitation CI (CI-single) at the 6–31G*
level were also performed.
Results and discussion
Photolysis of maleic anhydride
The behavior of maleic anhydride (1a) with UV irradia-
tion (λ > 230 nm) is shown in the IR spectra presented in
Fig. 1. Broad-band irradiation (λ > 230 nm) of the matrix-
isolated maleic anhydride causes a decrease in the absorp-
tion bands (the most intense located at ν = 1795.9 cm–1) and
the appearance of new bands in different areas of the spec-
trum.
Thermolysis
The thermolysis of dichloromaleic anhydride at 450°C
leads to the bands of the dichloropropadienone, CO, and
CO2 (Fig. 4). The reaction mechanism is also given in
Scheme 2. The presence of CO in the matrix cage suggests
formation of dichlorocyclopropenone. This compound, how-
ever, which is unstable in these temperature conditions, was
not observed.
On heating maleic anhydride at 450°C, no spectral
changes were observed. Apparently the thermal energy sup-
plied was not sufficient for decomposition of the molecule.
The bands at 2139.1–2143.5 and 2343.8 cm–1 were as-
signed to CO and CO2, respectively, while the intense bands
at 3285.2 and 3282.4 cm–1 (due to the different matrix sites)
and the less intense band at 742.3 cm–1 were assigned to
acetylene (12).
Analysis of the spectra shows weak absorption bands at
1896.0–1891.9, 1628.7–1626.7, and 1035.8 cm–1, which ini-
tially increase in intensity and then decrease after prolonged
irradiation (Fig. 1). These bands, assigned to cyclopro-
penone, demonstrate the intermediary presence of this com-
pound (13). The photochemical process is described in
Scheme 1.
Theoretical results
The experimental conditions used (irradiation of both
compounds in their absorption-band regions) indicate that
© 2002 NRC Canada

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Can. J. Chem. Vol. 80, 2002
Fig. 4. Spectrum of dichloromaleic anhydride 1b after
Fig. 3. Evolution of integrated absorbance during photolysis of
dichloromaleic anhydride 1b at λ > 230 nm. The band at
2281.1 cm–1, assigned to dichloropropadienone, is too small to
follow its evolution.
thermolysis at 450°C.
Fig. 5. Relevant optimized parameters (distances, Å; angles, deg)
for the transition states of the CO2 elimination from 1a and 1b
(values in parentheses) and activation energies (kcal mol–1) cal-
culated with HF/ 6–31 G* basis set.
Scheme 2.
Reactivity of ketenyl carbenes
The previously discussed experimental data show that
both ketenylcarbenes 2a and 2b undergo subsequent reac-
tions leading to the corresponding cyclopropenone, propa-
dienone, and alkyne.
the first-excited singlet was populated. The energies of the
Frank–Condon first-excited states depend on the number of
included configurations and (or) the number of the required
states. Their average value was approximately the energy of
the experimental radiation, attesting that the singlet state
was populated. We cannot tell if the energy furnished to the
system is also sufficient to overcome the transition barriers
in the excited state; we have considered that a radiationless
deactivation process to the ground state could be taking
place. Consequently, the reaction was based on the potential
energy surface of the ground state.
The ketenylcarbenes 2a and 2b were optimized at the
HF/6–31G*//HF/6–31G* level in the singlet and triplet
states for both the syn and anti conformations (Table 3). For
the unsubstituted ketenylcarbene, it was found that the
ground state is a triplet for both conformers, whereas for
dichloroketenylcarbenes, both conformers have singlet
ground states, with the syn conformer more stable.
The optimized geometries reflect the features generally
predicted for the singlet and triplet states of the syn-carbene
conformers, i.e., an X-C-Y angle (H6C4C3 (Scheme 1) and
Cl6C4C3 (Scheme 2)) of 109–115° for the singlet states and
130–131° for the triplet state. At the MP2 level, no mini-
mum could be located for syn-2a or syn-2b due to spontane-
ous ring closure to cyclopropenone. The same results were
observed by Tidwell and co-workers (10) for other syn-
ketenylcarbenes.
Formation of ketenyl carbenes
The first step in the decomposition (either photochemical
or thermal) of both anhydrides is CO2 elimination, which
yields ketenylcarbenes 2a and 2b. The localized transition
states (TS1, Fig. 5) for this process are planar and have acti-
vation energies of 68.1 and 62.9 kcal mol–1 for 1a and 1b,
respectively.
The differences between the lengths of the broken bonds
in the anhydrides and the corresponding transition states for
the CO2 elimination are listed in Table 2. It may be seen that
the bond breaking is asynchronous. In the transition state
TS1, the increase of the O-C-O angle supports CO2 forma-
tion.
Table 4 contains the compounds and transition-state ener-
gies modeled at the HF/6–31G* set level. The relevant geo-
metric parameters of the transition states and the activation
barriers for reactions of the syn- and anti-ketenylcarbenes
are presented in Fig. 6.
Ring closure and cyclopropenone stabilities
After this first step, it is likely that an equilibrium mixture
of syn- and anti-ketenylcarbene is present, as reported by
Dailey (7).
With regard to the experimental data, the most important
process (TS2) is the ring closure leading to the formation of
cyclopropenones (3), which starts from the syn-
© 2002 NRC Canada

Ionescu et al.
459
Table 2. Activation energies and relevant geometrical parameters (distances, angles) for the CO2-elimination reaction from 1a and 1b.
1a
TS1a
∆r
1b
TS1b
∆r
E (kcal mol–1)
0
68.1
0
62.9
rO-CO
rC-CO
O-C-O (°)
(Å)
(Å)
1.360
1.490
123.34
2.534
2.119
154.08
1.174
0.629
1.357
1.496
124.02
2.382
2.016
152.05
1.025
0.520
Table 3. Relative energies (kcal mol–1) for the syn and anti con-
Table 4. Total energies (Hartrees) of several compounds and
formers of ketenylcarbenes in the singlet and triplet states.
transition states.
1syn
3syn
1anti
3anti
Maleic anhydride
Structure
Dichloromaleic anhydride
2a
2b
5.14
0.00
0.04
7.90
8.35
6.86
0.00
7.03
Total energy
Structure
Total energy
1syn-2a
3syn-2a
1anti-2a
3anti-2a
TS2a
3a
TS3a
TS4a
–189.49584
–189.50397
–189.49072
–189.50404
–189.49196
–189.53397
–189.45918
–189.46254
1syn-2b
3syn-2b
1anti-2b
3anti-2b
TS2b
–1107.29974
–1107.28714
–1107.28881
–11077.28852
1107.29476
ketenylcarbene conformers. The calculated energy barriers
for this pathway are very low: 2.4 kcal mol–1 for syn-2a and
3.1 kcal mol–1 for the syn-2b.
3b
TS3b
TS4b
TS5b (S = 0)
TS5b (S = 1)
–1107.32690
–1107.24081
–1107.24757
–1107.18066
–1107.24574
The formation of acetylene in the decomposition reaction
of 1a and the accumulation of dichloroacetylene during pro-
longed irradiation of 1b raises the question of the stability of
the cyclopropenones. Two pathways to be considered are
(i) concerted decarbonylation of the cyclopropenones lead-
ing directly to the alkynes; (ii) ring opening of
cyclopropenones followed by decarbonylation of the syn-
ketenylcarbenes, which are in competition with ring closure.
The optimized transition states (TS3) for the first process
predicted high activation energies, 46.9 kcal mol–1 for 3a
and 53.9 kcal mol–1 for 3b. These values indicate that there
is a somewhat higher probability for the formation of the
alkyne for unsubstituted carbene 2a in comparison with sub-
stituted carbene 2b. This is in agreement with the experi-
mental findings.
For the second pathway, a larger activation energy was
calculated for the ring opening of 3a than for 3b, 26.3 vs.
20.1 kcal mol–1. (The transition state (TS2) is the same for
the ring-closing and ring-opening reactions.) The transition
states found for the CO elimination from the syn-
ketenylcarbenes were identical (structure and energy) with
those located in the previous path for the decarbonylation of
cyclopropenones (TS3). This means that energetically both
paths are equivalent.
TS5a (S = 0) –189.39443
TS5a (S = 1) –189.41132
1,2–Substituent migration
This reaction is specific to the anti-ketenylcarbenes and
may explain the formation of propadienones. The high prob-
ability of 1,2-halogen or 1,2-hydrogen migration in carbene
chemistry is due to the interaction of the electrons of the mi-
grating group with the vacant carbene orbital (14). This oc-
curs only in cases in which the electrons of the migrating
atom are located in an orbital quasi-parallel with the carbene
vacant orbital. In such cases the overlap of orbitals is en-
sured and the repulsive interaction avoided.
For the singlet state 2a and 2b, the double-occupied
carbene orbital is located in the molecular plane, whereas
the first vacant orbital is a π orbital with a high degree of lo-
calization on the carbene and the ketene C=O group. It is
therefore unlikely that such a migration, which might ex-
plain the formation of propadienone, can be favored, even
assuming a nonplanar transition state.
The calculated results reveal that the activation energies
are indeed very high; the larger value predicted for 2b is in
agreement with the assumption that the electronic repulsion
between the chlorine lone pair and the carbene in-plane elec-
trons prevents the migration. The structure of TS5 and the
activation energies are similar to those found by Dailey (7)
for the 1,2-fluorine migration in the fluoro-ketenylcarbene
(Ea = 50 kcal mol–1). It is apparent that the nature of the
substituent does not have a large influence on this process.
An enhanced possibility for a 1,2 migration reaction could
be expected starting from the triplet state, for which the
carbene in-plane orbital is only single occupied, implying a
decrease in electronic repulsions. The results in Fig. 6 indi-
cate that for triplet 3anti-2a the 1,2-migration remains a
high-energy process. For the triplet chlorine-disubstituted
ketenylcarbene (3anti-2b), however, the 1,2-migration in the
triplet state is a possible pathway and, therefore, the presence
Decarbonylation of the anti conformers
Assuming the anti conformers are present, they could be a
source of the alkynes. The decarbonylation reaction starting
from the anti-ketenylcarbenes and leading to the alkynes is
characterized by the transition state TS4. The geometries of
TS4 reflect the main features of the transition states for
decarbonylation reactions in the ketenyl group, i.e., rC-CO
values in the range 1.6–1.8 Å and C-C-O angles in the range
145–155°. There is a correlation between the activation en-
ergy and the length of the breaking C—CO bond in the
TS4—the value in carbene 2a (rC-CO = 1.680 Å (Ea
=
17.6 kcal mol–1)) is lower than that found for Cl,Cl-
disubstituted ketenylcarbene 2b (rC-CO = 1.71 Å (Ea
=
25.8 kcal mol–1)). The differences in the activation energies
for the decarbonylation process indicate that the CO elimina-
tion is highly favored for 2a.
© 2002 NRC Canada

460
Can. J. Chem. Vol. 80, 2002
Fig. 6. Relevant optimized parameters (distances, Å; angles, deg) for the transition states of the reactions of the syn- and anti-ketenyl
carbenes 2a and 2b, respectively, (values in parentheses) and activation energies (kcal mol–1) calculated with HF6–31G* basis set
(τ=H5C3C4H6).
of small quantities of dichloropropadienone can be ex-
plained.
this compound and the dichloropropenone, since the latter is
not observed.
Acknowledgment
Summary
Financial support from CNCSIS-Romania is kindly ac-
knowledged.
During the photolysis of maleic anhydride, only cyclo-
propenone, acetylene, CO, and CO2 are observed as prod-
ucts. The transition-state structure calculations are consistent
with these results. In fact, the cyclization process is sponta-
neous (2.4 kcal mol–1) from syn-ketenylcarbene. From the
anti-ketenylcarbene, decarbonylation is easier (17.6 kcal mol–1)
than the 1,2 migration process (63.5 kcal mol–1). These re-
sults explain why the propadienone was not experimentally
observed.
Photochemically, matrix-isolated dichloromaleic anhy-
dride 1b is converted to a mixture of anti- and syn-
ketenylcarbene. The syn conformer undergoes rapid ring clo-
sure to dichlorocyclopropenone, while for the anti isomer
there is competition between two possible pathways with
similar activation barriers (25–26 kcal mol–1) if we consider
the 1,2-migration in the triplet state. The presence of small
quantities of dichloropropadienone and dichloroacetylene
can, therefore, be explained. The formation of the latter
could also be produced from the syn-ketenylcarbene via
dichlorocyclopropenone, a pathway which is calculated to
have a lower barrier (37 kcal mol–1).
The reaction mechanism is the same for the thermolysis
and photolysis of dichloromaleic anhydride. The predicted
facile ring opening to syn-ketenylcarbene (20.1 kcal mol–1)
explains the failure to isolate dichlorocyclopropenone in the
matrix when 1b was pyrolyzed at 450°C. (Dichlorocyclo-
propenone is not stable under these conditions.) Although
the IR absorption bands seem intense, it is difficult to deter-
mine the quantity of dichloropropadienone produced be-
cause we cannot compare the relative intensities between
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