Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices
Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of malonaldehyde and acetylacetone are compared through theoretical and experimental data. Ground-state geometries, energies, and vibrational frequencies are calculated with the B3LYP/6–311++G(2d,2p) model ch...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1208842017-06-14T03:03:25Z Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices Trivella, A. Coussan, S. Chiavassa, T. Theule, P. Manca, C. Roubin, P. Molecular Solids Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of malonaldehyde and acetylacetone are compared through theoretical and experimental data. Ground-state geometries, energies, and vibrational frequencies are calculated with the B3LYP/6–311++G(2d,2p) model chemistry. The electronic delocalisation as well as the cis/trans rotamer properties are analysed. The hydrogen bond strength of the chelated forms can be estimated by the energy difference between chelated and non-chelated forms, and its enhancement due to methyl-induced electron release is estimated at 1.7 kcal·mol⁻¹. UV- and IR-induced reactivity of molecules isolated in nitrogen matrices is studied by means of FT–IR spectrometry. Interconversion between rotamers is the main process observed for both molecules, only some among the seven non-chelated forms being created. 2006 Article Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices / A. Trivella, S. Coussan, T. Chiavassa, P. Theule, C. Manca, P. Roubin // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1372–1381. — Бібліогр.: 59 назв. — англ. 0132-6414 PACS: 36.20.Ng, 31.15.Ar http://dspace.nbuv.gov.ua/handle/123456789/120884 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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Molecular Solids Molecular Solids |
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Molecular Solids Molecular Solids Trivella, A. Coussan, S. Chiavassa, T. Theule, P. Manca, C. Roubin, P. Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices Физика низких температур |
| description |
Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of
malonaldehyde and acetylacetone are compared through theoretical and experimental data.
Ground-state geometries, energies, and vibrational frequencies are calculated with the
B3LYP/6–311++G(2d,2p) model chemistry. The electronic delocalisation as well as the
cis/trans rotamer properties are analysed. The hydrogen bond strength of the chelated forms can
be estimated by the energy difference between chelated and non-chelated forms, and its enhancement
due to methyl-induced electron release is estimated at 1.7 kcal·mol⁻¹. UV- and
IR-induced reactivity of molecules isolated in nitrogen matrices is studied by means of FT–IR
spectrometry. Interconversion between rotamers is the main process observed for both molecules,
only some among the seven non-chelated forms being created. |
| format |
Article |
| author |
Trivella, A. Coussan, S. Chiavassa, T. Theule, P. Manca, C. Roubin, P. |
| author_facet |
Trivella, A. Coussan, S. Chiavassa, T. Theule, P. Manca, C. Roubin, P. |
| author_sort |
Trivella, A. |
| title |
Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| title_short |
Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| title_full |
Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| title_fullStr |
Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| title_full_unstemmed |
Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| title_sort |
comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2006 |
| topic_facet |
Molecular Solids |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120884 |
| citation_txt |
Comparative study of structure and photo-induced
reactivity of malonaldehyde and acetylacetone isolated
in nitrogen matrices / A. Trivella, S. Coussan, T. Chiavassa, P. Theule, C. Manca, P. Roubin // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1372–1381. — Бібліогр.: 59 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT trivellaa comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices AT coussans comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices AT chiavassat comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices AT theulep comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices AT mancac comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices AT roubinp comparativestudyofstructureandphotoinducedreactivityofmalonaldehydeandacetylacetoneisolatedinnitrogenmatrices |
| first_indexed |
2025-07-08T18:48:50Z |
| last_indexed |
2025-07-08T18:48:50Z |
| _version_ |
1837105712880156672 |
| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11, p. 1372–1381
Comparative study of structure and photo-induced
reactivity of malonaldehyde and acetylacetone isolated
in nitrogen matrices
A. Trivella1, S. Coussan1, T. Chiavassa1, P. Theul�1, C. Manca2,
and P. Roubin1
1Laboratoire PIIM, Universit� de Provence, Centre Saint-J�r�me, Marseille F-13 397, cedex 20, France
E-mail: pascale.roubin@up.univ-mrs.fr
2Laboratorium fur Physikalische Chemie, ETH Zurich, Zurich CH-8093, Switzerland
Received May 4, 2006, revised July 14, 2006
Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of
malonaldehyde and acetylacetone are compared through theoretical and experimental data.
Ground-state geometries, energies, and vibrational frequencies are calculated with the
B3LYP/6–311++G(2d,2p) model chemistry. The electronic delocalisation as well as the
cis/trans rotamer properties are analysed. The hydrogen bond strength of the chelated forms can
be estimated by the energy difference between chelated and non-chelated forms, and its en-
hancement due to methyl-induced electron release is estimated at 1.7 kcal·mol–1. UV- and
IR-induced reactivity of molecules isolated in nitrogen matrices is studied by means of FT–IR
spectrometry. Interconversion between rotamers is the main process observed for both molecules,
only some among the seven non-chelated forms being created.
PACS: 36.20.Ng, 31.15.Ar
Keywords: FT–IR spectrometry, cis/trans rotamer, properties, H-bond strength, electronic delo-
calization.
Introduction
Malonaldehyde (MA) and acetylacetone (AA) are
prototypical �-diketones, whose stable enolic forms
present a strong hydrogen bond (H-bond), with pro-
ton transfer between the two oxygen atoms. Figure 1
shows the eight enolic rotamers labeled XYZ: these
letters are either C or T, where C (T) stands for the cis
(trans) configuration relative to the C–C, C=C and
C–O bonds, respectively. The stable form is the
chelated CCC form, and numerous studies have at-
tempted to unravel fundamental questions as its C2v
(symmetrical H position between the two oxygen
atoms) or Cs (asymmetrical H position as shown in
Fig. 1) structure, the proton transfer barrier height,
and/or the estimation of the H-bond strength. Infor-
mation on this latter parameter could also be brought
through the dOO distance, the O–H stretching, height,
frequency, or the energy difference between non-
chelated and CCC forms. Microwave experiments
have determined a Cs structure for MA with a dOO
distance measured at 255 pm [1] whereas for AA,
photon electron spectroscopy [2,3] and x-ray diffrac-
tion [4,5] have predicted a C2v structure. Never-
theless, electron diffraction results are contradictory
[2,6,7], predicting a Cs or a C2v structure. A recent
ultra fast electron diffraction study of Zewail et al.
[8] has demonstrated that, at least at the short
timescale (� 100 ps), the AA structure is asymmetrical
with a dOO distance of 259 pm, 8 pm larger than the
previous estimation [6]. The proton transfer barrier
has been estimated at 6.6 kcal·mol–1 from the mea-
sured tunnelling splitting (21.6 cm–1) for MA [9–11],
and besides, many theoretical studies have focussed
on the description of this tunnelling process, trying
to reproduce the measured splitting and to estimate
the barrier height. Until the 90ies, this barrier
height has been found [11,14] ranging from 4.7 to
© A. Trivella, S. Coussan, T. Chiavassa, P. Theul�, C. Manca, and P. Roubin, 2006
10.0 kcal·mol–1, the lowest value coming from the
work of Frisch et al. [13] which emphasizes the role of
electron correlation. Consistently, more recent and
accurate calculations [15–27] agree with lower values
between 3.1 and 4.6 kcal·mol–1. Discrepancies
originate from the difficulty of describing the complex
multidimensional dynamics of this proton tunnelling.
For AA there is no experimental determination of the
tunnelling splitting and calculations of the proton
barrier are only a few [28–30], leading to similar
values as for MA, between 2.5 and 7 kcal·mol–1. Some
studies also calculate the energy release due to con-
version from the CCC form to the non-chelated forms,
generally CCT, which is the simplest way to quantify
the H-bond strength: the obtained values also decrease
with the calculation accuracy increase and range bet-
ween 12.4 and 15.8 kcal·mol–1 for MA [18,31,32] and
between 12.0 and 18.5 kcal·mol–1 for AA [28,29,
33,34]. For both molecules, infrared measurements
show that the O–H stretching mode is red-shifted and
broadened due to H-bonding: in gas phase, the �OH
band is clearly observed for AA [35,36] at � 2800 cm–1
(FWHM larger than 1000 cm–1) and has been ten-
tatively assigned at � 2950 cm–1 for MA [37,38]. In
the same way, the C=O stretching mode is red-shifted
and is measured in gas phase at 1620 cm–1 for AA and
at 1655 cm–1 for MA. These vibrational features
indicate that the mixing of vibrations due to electro-
nic delocalisation is larger for AA than for MA, and
therefore that the H-bond is stronger: this is due to
the electron release from the methyl groups. Vibra-
tional analysis seems therefore to bring clear informa-
tion on H-bond strength, whereas energy calculation
uncertainties are still too large to definitely conclude
about the relative H-bond strength of the two mo-
lecules. It should be noted that work allowing a direct
comparison of the two molecules, i.e. using same
experimental or theoretical methods, are a very few
[39–41].
Experimental data on the unstable non-chelated
forms are also rare and up to now there are obtained
only through photochemistry of molecules isolated in
cryogenic matrices (T < 10 K). This cold solid-state
environment prevents from both major fragmentation,
and relaxation to the stable CCC form. A � � �* UV
excitation leads to the formation of several non-
chelated rotamers [42–44] among the seven forms
shown in Fig. 1, and subsequent resonant UV or IR
irradiation of these created rotamers lead to other
rotamer conversions [45,46]. Combining different ex-
citations and a detailed vibrational analysis has allow-
ed us to provide new information, especially from
selective vibrational excitations, and to accurately
assign the observed new isomers in the case of AA
isolated in a nitrogen matrix [45,46].
The aim of this paper is to present new results
obtained with similar excitations in the case of MA
isolated in a nitrogen matrix, and to compare the main
features of MA and AA. The first part of the paper
is devoted to a comparative analysis of geometries,
energies and vibrational frequencies obtained with
density functional theory; vibrational analysis is in
this work the key ingredient for elucidating the pho-
to-induced processes. The second part of the paper is
devoted to the description of a selection of typical
UV- and IR-induced phenomena observed for MA and
AA isolated in nitrogen matrices and to a discussion of
the general features which are observed.
1. Structural, energetic, and vibrational analyses
All the geometry optimizations and vibrational
frequencies presented in this paper have been cal-
culated using the B3LYP/6–311++G(2d,2p) model
chemistry implemented in Gaussian 98 [47] (AA) and
in Gaussian 03 [48] (MA). Note that using Gaussi-
an 98 or Gaussian 03 leads to negligible differences for
the properties discussed here.
1.1. Structural analysis
Table 1 gathers the main geometric parameters for
the eight forms of the two molecules, and Fig. 2
summarizes geometrical parameters for the chelated
forms: the O–H distance is larger, and consistently
the O…H distance as well as the dOO value are
smaller, for AA than for MA. The C–O–H angle is
smaller while the O…H–O angle is larger, showing a
stronger linear character of the H-bond for AA, which
is consistent with a stronger H-bond. Note that the
methyl groups are arranged to ensure a staggered
configuration both with the O–H bond and with the
two sp2 electron pairs of the carbonyl O atom.
Concerning the non-chelated forms, XXC/XXT
rotamers are expected to be similar, being different by
Comparative study of malonaldehyde and acetylacetone
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1373
O
C
C
C
O
H
RR
H
CCC CCT
TCTTCC
CTC
O
C
C
C
O
H
R
R
H
CTT
O
C
C
C
O
R
R
H H
O
C
C
C
O
H
RR
H
TTTTTC
O
C
C
C
O
H
R
H
R
O
C
C
C
O
H
R
H
R
O
C
C
C
O
H
RR
H
O
C
C
C
O
RR
H H
Fig. 1. Diagram of the eight enolic isomers of malonalde-
hyde (R = H) and acetylacetone (R = CH3).
merely a rotation around the C–O bond which does
not involve main skeletal changes, excepted for the
CCC/CCT pair which corresponds to the chelat-
ed/non-chelated conversion. XXC/XXT pair cha-
racteristics are in fact evidenced in Table 1 by an
alternate large/small O–H and C=C bond lengths
and C=C–O angle, correlated to an alternate
small/large C–O bond length. These sensitive geo-
metrical parameters are those directly linked to the
C–O bond involved in the cis/trans conversion, and
their values systematically indicate a local larger
electronic delocalisation for the cis configuration. On
the contrary, all the COH angle values are similar, no
specific interaction differentiating the two partners of
a pair. The TCC value is the largest for the two
molecules, in agreement with steric hindrance expec-
ted to be the largest (Fig. 1). The C=C–C angle
values also reflect steric hindrance: for MA, its value
is larger for XCX than for XTX, the cis configuration
relative to C=C bond inducing more hindrance, while
for AA, its value increases in the series CTX, TCX and
TTX. This latter feature clearly shows the methyl
hindrance involved when the molecule is not in CXX
configuration and the maximum is obtained for
the methyl syn-1,3 configuration of TTX rotamers
(Fig. 1).
1.2. Energetic analysis
The relative energies of the non-chelated forms are
given for the two molecules in Table 2. They spread
1374 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
A. Trivella, S. Coussan, T. Chiavassa, P. Theul�, C. Manca, and P. Roubin
Table 1. Geometrical parameters of the eight enolic isomers of MA and AA, calculated with the B3LYP/6–311++G(2d,2p)
model chemistry. Bold characters emphasize some special features. Lengths are in � and angles in degrees.
d
O–H
d
C–O
d
C–C
d
C=C
d
C=O
C–C=C C=C–O C–O–H
MA
CCC 0.998 1.318 1.435 1.362 1.238 119.6 123.9 106.2
CCT 0.961 1.343 1.463 1.344 1.214 126.7 124.1 110.2
CTC 0.965 1.342 1.463 1.342 1.215 120.8 126.8 110.5
CTT 0.961 1.348 1.462 1.339 1.217 120.4 122.3 110.4
TCC 0.964 1.346 1.451 1.346 1.214 125.1 128.5 111.2
TCT 0.960 1.355 1.460 1.340 1.216 124.5 123.1 110.4
TTC 0.965 1.344 1.455 1.341 1.214 120.1 127.1 110.4
TTT 0.961 1.349 1.454 1.338 1.214 119.6 122.5 110.7
AA
CCC 1.005 1.324 1.441 1.368 1.246 120.8 121.7 105.9
CCT 0.963 1.350 1.472 1.349 1.218 126.8 121.4 109.9
CTC 0.964 1.358 1.469 1.352 1.222 126.9 121.3 110.0
CTT 0.962 1.362 1.468 1.348 1.224 126.5 116.9 109.9
TCC 0.963 1.355 1.466 1.352 1.221 130.5 126.6 111.1
TCT 0.961 1.364 1.474 1.345 1.222 129.8 121.6 109.9
TTC 0.964 1.363 1.469 1.349 1.223 132.4 120.3 109.8
TTT 0.962 1.368 1.468 1.346 1.223 131.9 115.8 110.3
MA AA
CC
O
H
C
H
O
H
H
1.680
1
.2
38
1.435 1.362
1
.318
0.998
1.078
1.084.1100
3
12
.9
122.4
.
10
6
2
119.6
12
0.
4
123.4
117.7
2.574
146.8
CC
O
C
C
H
O
H
C
HH
H
H
H
8
1.0
6 .1510
1.
24
6
.1441
1.079
. 6
13 8 1.492
1.324
1.005
109.9
118.7
121.6 12
1.7
120.8
01
5.
9
124.3
2.533
.1617
149.4
H
1.0
91 .
11
9
9
MA AA
CC
O
H
C
H
O
H
H
1.680
1
.2
38
1.435 1.362
1
.318
0.998
1.078
1.084.1100
3
12
.9
122.4
.
10
6
2
119.6
12
0.
4
123.4
117.7
2.574
146.8
CC
O
C
C
H
O
H
C
HH
H
H
H
8
1.0
6 .1510
1.
24
6
.1441
1.079
. 6
13 8 1.492
1.324
1.005
109.9
118.7
121.6 12
1.7
120.8
01
5.
9
124.3
2.533
.1617
149.4
H
1.0
91 .
11
9
9
AA
CC
O
H
C
H
O
H
H
1.680
1
.2
38
1.435 1.362
1
.318
0.998
1.078
1.084.1100
3
12
.9
122.4
.
10
6
2
119.6
12
0.
4
123.4
117.7
2.574
146.8
O
H
1.680
1.680.680
1
.2
38
1
. 2
38
1.435
1.435 1.362
1.362
1
.318
1
.318
0.998
1.078
1. 078
1.084
1.084.1100
.1100
1100
3
12
.93
12
.9
12
.9
122.4122.4
.
10
6
2.
10
6
2
10
6
2
119.6
12
0.
4
12
0.
4
123.4
123.4
117.7117.7
2.5742.574
146.8146.8
CC
O
C
C
H
O
H
C
HH
H
H
H
8
1.0
6 .1510
1.
24
6
.1441
1.079
. 6
13 8 1.492
1.324
1.005
109.9
118.7
121.6 12
1.7
120.8
01
5.
9
124.3
2.533
.1617
149.4
H
1.0
91 .
11
9
9
CC
O
C
C
H
O
H
C
HH
H
H
H
8
1.0
68
1.0
6
1.0
6 .1510.1510
1510
1.
24
6
1.
24
6
.1441
.1441
1441
1.079
. 6
13 8
. 6
13 86
13 8
13 8 1.492
1.324
1 .324
1.005
1.005
109.9
118.7118.7
121.6
121.6 12
1.7
12
1.7
120.8120.8
01
5.
9
01
5.
9
1
5.
9
124.3124.3
2.5332.533
.1617.1617
1617
149.4149.4
H
1.0
91 .
11
9
9.
11
9
9
11
9
9
Fig. 2. Malonaldehyde (MA) and acetylacetone (AA) CCC
geometries calculated by the B3LYP/6–311++G(2d,2p)
model chemistry. Distances are in �, angles in degrees.
Table 2. Relative energies (in kcal·mol–1) of the seven
non-chelated isomers of MA and AA calculated with the
B3LYP/6–311++G(2d,2p) model chemistry. The energy of
the CCC chelated isomer is –267.2482947 Hartree for MA
and 352.9242151 Hartree for AA
MA AA
CCC 0 0
CCT 12.8 15.6
CTC 8.6 11.1
CTT 9.1 11.8
TCC 9.6 12.7
TCT 10.0 13.6
TTC 6.7 14.3
TTT 7.9 16.6
among a similar range for both molecules
(5.5 kcal·mol–1 for AA and 6.1 kcal·mol–1 for MA)
but the AA rotamers are clearly at higher relative
energies than the MA’s. For both molecules, CCT is
among of the least stable rotamers, due to hindering
between the sp3 and sp2 electron pairs located on each
oxygen atoms, while for AA, TTX high energies
clearly originate from methyl syn-1,3 repulsive in-
teractions. Note that in a XXC/XXT pair, the «C»
partner is always the most stable, in agreement the
local larger delocalisation deduced from geometrical
parameters.
To compare the H-bond strengths of MA and AA,
we compare the relative energies of the CCT forms,
which are calculated at 12.8 kcal·mol–1 for MA and at
15.6 kcal·mol–1 for AA, i.e., a 2.8 kcal·mol–1 energy
difference. In a rough approximation, this energy
difference is due both to H-bond strength difference
and to H–H and H-methyl interaction difference. To
unravel the different origins, we have calculated with
the same method the energies of ethanoic acid (EA)
and formic acid (FA), for the cis and trans confi-
gurations. The cis configuration of both molecules is
the most stable due to a weak interaction between H
and O atoms which is similar for the two molecules,
whereas the trans configuration involves either a H–H
interaction for FA, or a H-methyl interaction for EA,
which can be compared with the interaction occurring
in the CCT forms of either MA or AA. The cis-trans
energy difference is found at 4.2 kcal·mol–1 for FA
and at 5.3 kcal·mol–1 for EA and the energy difference
due to H, H or H, methyl interaction is therefore
estimated at � 1.1 kcal·mol–1. Consequently, we esti-
mate the energy difference of the H-bond strength
between AA and MA at � 1.7 kcal·mol–1. This rough
estimation should be taken with care but is based on a
systematic comparison with the same level of cal-
culations which can give valuable trends, and this
methodology should be developed at a higher level of
model chemistry.
1.3. Vibrational analysis
Table 3 summarizes the calculated frequencies of a
selection of the most intense modes of MA and AA,
listed in decreasing frequency order, with their assign-
ment as defined using the main vibration contri-
butions obtained by the Potential Energy Distribution
(PED). Intensities are indicated without normali-
zation to allow comparison between the different
forms: this comparison must be done with care, taking
into account the lack of accuracy of intensity calcu-
lations. We discuss here these values to compare the
features of the two molecules and therefore we do not
introduce any scaling factor to better fit experimental
data. The main frequency features due to strong
H-bonding and electronic delocalisation are low �OH
and �C=O frequencies and high �OH frequency. This
also induces a strong vibration mixing which prevents
from identifying clearly the features related to the
other vibrations. The �OH frequency is lower for AA
(3023 cm–1) than for MA (3135 cm–1) and the same
feature is observed for the �C=O mode (1674 cm–1 for
AA and 1692 cm–1 for MA), whereas the �OH fre-
quency is higher for AA (1003 cm–1) than for MA
(937 cm–1). This is once again consistent with an
H-bond stronger for AA than for MA. It should be
noted that in the case of AA, the O–H stretching
vibration is pure whereas in the case of MA, it
is coupled with the central C–H stretching (15% in
the PED), whose calculated frequency is close
(3188 cm–1), in agreement with previous calculations
[49,50].
Figure 3,a–c shows an overview of the calculated
frequencies for the �OH, the �C=O, the �C=C modes
of the seven non-chelated rotamers. The �OH fre-
quencies are clearly split in two groups for MA,
a low-frequency (and low-intensity) group at
3790–3800 cm–1 corresponding to the XXC rotamers,
and a high-frequency (and high-intensity) group at
3848–3862 cm–1 corresponding to the XXT rotamers.
A similar classification, although frequency splitting
between the two groups is lower, is observed in
the case of AA. This difference in XXC/XXT �OH
splittings between AA and MA can be related to the
difference in XXC/XXT O–H bond length alternate
features: this latter feature is more noticeable for MA
than for AA, in agreement with a more noticeable
frequency splitting. The two most intense modes are in
the �C=O��C=C region, for which C=C and C=O
stretchings are strongly coupled. The PED analysis
shows that �C=C vibration generally corresponds to
the lowest frequency, except in the case of the TXT
forms of AA. The mean �C=O frequency is lower for
AA than for MA, indicating that, similarly to what is
observed for the chelated forms, the electronic de-
localisation is also larger for AA than for MA for the
non-chelated forms. This can be correlated to the
C=O bond length of the non-chelated forms which
is larger for AA (121.8–122.4 pm) than for MA
(121.4–121.7 pm). Figure 3,c also shows that the
�C=O frequencies are grouped, TXX frequencies being
lower than CXX frequencies, and CTT frequencies
being the highest. This region is especially interesting
for vibrational assignment [45,46] by considering the
difference �� = |�C=O – �C=C|, this value ranging
between 90 and 8 cm–1 in the case of AA and allowing
to clearly discriminate between rotamers. Figure 3,d
shows that the same ordering in �� values is obtained
Comparative study of malonaldehyde and acetylacetone
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1375
for the two molecules: the rotamers are grouped by
pair XTX/XCX and XTX values are always smaller
than XCX values, whereas all TXX values are smaller
than CXX values. The skeletal arrangement around
the C–C bond thus plays the most important role to
determine the �� value. The modes chosen in Table 3
are the most intense but are far from pure modes and
the OH/�CO/�CC region often shows a strong mix-
ing of the different vibrations. On the contrary, the
lowest frequency mode displayed in Table 3, �OH, is
always pure (more than 90% in the PED) and intense:
the XXC frequencies are always larger than the XXT
frequencies, indicating a stiffer skeleton in the cis
configuration, in agreement with geometrical para-
meters systematically showing a larger C=C–O angle.
This behaviour is directly related to that of �OH and is
1376 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
A. Trivella, S. Coussan, T. Chiavassa, P. Theul�, C. Manca, and P. Roubin
Table 3. Calculated frequencies (in cm–1) and intensities (in km·mol–1, italic) of the most intense vibrational modes of MA
and AA, calculated with the B3LYP/6–311++G(2d,2p) model chemistry. The contributions of the potential energy
distribution higher than 30% are indicated for each mode. As in Table 1, bold characters emphasize some special features.
�
OH
�
C=O
/ �
C=C
region �
OH
/ �
CO
/ �
CC
region �
OH
MA
CCC
3135 213
�
OH
1692 225
�
C=O
1627 259
�
OH
1391 109
�
OH
�
CO
1287 163
�
CO
1001 ��
�
CC
937 60
�
OH
CCT
3847 134
�
OH
1767 160
�
C=O
1666 333
�
C=C
1311 196
�
CO
1276 81
�
OH
920 77
�
CC
322 99
�
OH
CTC
3792 72
�
OH
1757 122
�
C=O
1640 644
�
C=C
1326 55
�
C=CH
1159 147
�
OH
�
CO
998 109
�
CC
551 84
�
OH
CTT
3848 138
�
OH
1762 116
�
C=O
1665 493
�
C=C
�
C=O
1376 162
�
O=CH
1247 228
�
CO
752 96
�
O=CC
436 102
�
OH
TCC
3800 41
�
OH
1743 339
�
C=O
1664 256
�
C=C
1242 166
�
OH
1153 113
�
CC
1012 102
�
CO
560 94
�
OH
TCT
3862 149
�
OH
1736 320
�
C=O
1700 159
�
C=C
1296 264
�
OH
1154 34
�
CC
1017 133
�
CO
285 115
�
OH
TTC
3796 75
�
OH
1747 326
�
C=O
1680 375
�
C=C
1240 59
�
CO
1137 377
�
OH
986 43
op
HCCO
op
OCCH
537 75
�
OH
TTT
3849 143
�
OH
1748 344
�
C=O
1710 264
�
C=C
1340 161
�
OH
1210 236
�
CO
1147 83
�
CC
418 94
�
OH
AA
CCC
3023 349
�
OH
1674 429
�
C=C
1643 205
�
C=O
�
OH
1463 149
–
1377 97
�
C=C
1270 ���
�
CC
1003 68
�
OH
CCT
3820 81
�
OH
1751 175
�
C=O
1669 412
�
C=C
1308 300
�
OH
1200 85
–
883 78
�
CO
388 82
�
OH
CTC
3797 44
�
OH
1734 173
�
C=O
1636 571
�
C=C
1466 104
�
aCH3
1182 210
–
949 71
�
CC
488 87
�
OH
CTT
3826 85
�
OH
1739 198
�
C=O
1657 387
�
C=C
�
C=O
1437 233
�
CCH
1237 147
�
OH
948 104
�
CC
387 94
�
OH
TCC
3809 90
�
OH
1707 223
�
C=O
1667 398
�
C=C
1383 73
–
1280 128
�
CC
1145 74
�
CC
481 71
�
OH
TCT
3839 92
�
OH
1712 29
�
C=C
1704 468
�
C=O
1381 66
�
CO
�
CCH
1307 183
�
OH
1280 143
�
CC
337 82
�
OH
TTC
3802 51
�
OH
1698 168
�
C=O
1671 446
�
C=C
1297 87
�
CC
1250 108
�
CO
1221 249
�
OH
488 93
�
OH
TTT
3836 91
�
OH
1703 25
�
C=C
1694 489
�
C=O
1398 128
�
OH
�
s CH3
1298 78
�
CC
1211 245
�
OH
�
CO
322 93
�
OH
consistent with the larger electronic delocalisation
induced by the cis configuration previously evidenced
through geometrical parameters.
2. Photoisomerizations
2.1. Experimental
Malonaldehyde was synthesised by protonation of
sodium malonaldehyde with an ether/HCl solution,
sodium malonaldehyde being synthesised by acid hyd-
rolysis of tetramethoxypropane followed by basifi-
cation [51,52], whereas acetylacetone was commer-
cially available (99% purity, Aldrich–Chemie).
Molecules were distilled under vacuum and mixed
with nitrogen (N60 grade, Air Liquid) in a vacuum
line using standard manometric techniques. Roughly
4 Pa·m3 of a 3:1000 mixture was sprayed onto a gold-
plated copper cube maintained at 17 K and the sample
was then maintained at 4 K, using a close-cycled
helium cryogenerator (Cryomech-PT405). Infrared
spectra (FT–IR) were recorded with an IFS 66/S
Bruker spectrometer in reflection-transmission mode,
at a 0.12 cm–1 resolution. Broad band UV irradiation
were performed using a 300 W Hg lamp. Selective IR
irradiation were performed using a Nd:YAG pumped
LiNbO3 cavity (pulse duration � 15 ns, pulse re-
petition: 10 Hz), leading to a tunable infrared beam
whose typical power was 5 mW and resolution
� 5 cm–1 in the 3560–3660 cm–1 region.
2.2. Malonaldehyde
Figure 4 shows the effects of UV and IR irradiation
for MA isolated in a nitrogen matrix, in the �OH and
�C=O/�C=C regions. Figure 4,a shows the spectrum
obtained after deposition: no band is present in this
�OH region, as expected due to the large red-shift
[37,38] of this mode. In addition, the broadening of
this vibration is large for the isolated molecule and is
expected to be even larger for the molecule in a solid-
state medium: this mode is therefore not observed for
the molecule isolated in matrices [53,54]. According
to our PED analysis, the two broad and structured
bands at 1652 cm–1 (FWHM: � 7 cm–1 for the main
peak) and 1593 (FWHM: � 15 cm–1) have a dominant
�C=O and OH contribution, respectively. Figure 4,b
shows the difference spectrum obtained after a 10 min
irradiation with a Hg lamp. Free �OH vibration bands
clearly appears, indicating the opening of the chelated
structure; consistently, in the �C=O/�C=C region, the
two broad bands totally disappear and a complex set
of new bands appear. The �OH region shows two
series of bands, at high frequency between 3650 and
3600 cm–1 and at low frequency between 3600 and
3550 cm–1. These two sets of bands are correlated to
the two sets of calculated frequencies (Fig. 3,a) and
this shows the presence of at least two different types
of rotamers, belonging to XXT and XXC groups,
respectively. The complexity of the �C=O/�C=C re-
gion indicates that probably more than one rotamer
per group is formed. In fact, Fig. 4,c shows the effect
Comparative study of malonaldehyde and acetylacetone
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1377
20 40 60 80 100
3800 3820 3840 3860
1700 1720 1740 1760
1640 1660 1680 1700
�C =O��
CXTTXT
TXC
TXX CTX
CCT
XXC XXT
Wavenumber, cm
–1
Wavenumber, cm
–1
CTC AA
AA
MA
MA
a b
cd
Fig. 3. Bar spectrum of MA and AA non-chelated isomer frequencies (in cm–1) calculated by the B3LYP/6–311++G(2d,2p)
model chemistry: �OH modes (a), �C=C modes (b), �C=O modes (c)� �� = ��C=O –�C=C� values (d).
of a long irradiation (difference spectrum between 105
min irradiation and 10 min irradiation, this latter
irradiation corresponding to that shown in Fig. 4,b):
decreases (at 3636 and 3587 cm–1) and increases (at
3628/3622 and 3584/3576 cm–1) are now observed
among each group, indicating the presence of at least
four different species. After the total conversion of the
chelated form into non-chelated forms, we observe
interconversion within the non-chelated forms. The
intensities of the increasing/decreasing bands at low
frequencies are clearly lower than the intensities of
the decreasing/increasing bands at high frequencies,
in agreement with what is expected by calculations.
Finally, Fig. 4,d shows the effect of an additional
selective IR irradiation centered at 3628 cm–1. This
�OH excitation is more selective than the previous UV
excitation and the main decrease of the excited band is
correlated to a main increase at 3584 cm–1. The reverse
effect is observed by exciting at 3584 cm–1. Irra-
diation have also been performed at 3622 and
3576 cm–1 and have shown similar interconversion.
The complete vibrational assignment of all the bands
is done by correlating along the whole spectrum the
increases and the decreases observed for different
irradiation times, for different types of irradiation, as
well as for temperature changes. In particular, tem-
perature effects allow us to differentiate intercon-
version between rotamers from trapping site effects.
The assignment is still puzzling and is currently under
study: the preliminary analysis indicates the presence
of the TTX and TCX pairs.
2.3. Acetylacetone
Figure 5 shows the effects of UV and IR irradiation
for AA isolated in a nitrogen matrix in the �OH and
�C=O/�C=C regions. Figure 5,a shows the spectrum
obtained after deposition: the features mentioned
for the �OH vibration are similar to those of MA
[35,55,56]. According to the PED analysis, the two
broad bands at 1638 cm–1 (FWHM: � 20 cm–1) and
1614 (FWHM: � 28 cm–1) have a dominant �C=C and
�C=O contribution, respectively. Figure 5,b shows the
difference spectrum obtained after a 120 min irra-
diation with a Hg lamp. The �OH region shows two
series of bands, at high frequency between 3630 and
3600 cm–1, and at low frequency between 3600 and
3560 cm–1. Thin bands appear in the �C=O/�C=C
region and, contrary to what is observed for MA, the
original bands do not completely disappear. Figure 5,c
shows the difference spectrum obtained after an irra-
diation at 3585 cm–1: the two sets of increasing and
decreasing bands in the �C=O/�C=C region are well-
separated (�� � 76 and 89 cm–1, respectively) and
should therefore unambiguously be assigned to two
rotamers among CCT, CTT and CTC (Fig. 3,d).
In addition, this infrared excitation does not bring
enough energy to induce a rotation around C=C and
we can deduce that these species are CTT and CTC.
Figure 5,d shows another example of selective infra-
red irradiation at 3622 cm–1. The main increasing �OH
band is in the same group of high-frequency �OH,
whereas the main increasing or decreasing bands in the
�C=O/�C=C region are separated by only �� � 33 and
16 cm–1. This latter feature shows that this irradiation
1378 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
A. Trivella, S. Coussan, T. Chiavassa, P. Theul�, C. Manca, and P. Roubin
Wavenumber , cm
–1
Wavenumber , cm
–1
356036003640
a
c
b
d
*
0.1
162016601700
0.5
a
c
b
d
*
3628 cm
–1
1580
Fig. 4. Infrared spectra of malonaldehyde isolated in a nitrogen matrix at T = 4 K, MA:N2 � 5:1000. After deposition (a),
difference spectrum after a 10 min Hg lamp irradiation minus a (b), difference spectrum after minus before an additional
95 min Hg lamp irradiation (c), difference spectrum after minus before a 35 min infrared irradiation at 3628 cm–1 (d).
The asterisk designates water impurity originating from malonaldehyde synthesis.
is a TXT
TXC interconversion (Fig. 3,d). The
information of the �OH region is less clear than for
MA, as expected by calculations and further in-
vestigation in the whole spectral domain shows that
only the CTT/CTC and the TCT/TCC pairs are
formed upon irradiation.
2.4. Discussion
The CCC forms of the two molecules clearly show
vibrational features characteristic to strong hydrogen
bonding like for example the shift and the broadening
of the �OH, �C=O, �C=C and �OH modes. No evidence
for proton tunnelling is found, similarly to what is
observed for molecules isolated in argon matrices
[42,53], probably due to a dynamical damping of the
proton movement through the matrix phonons. To our
knowledge, among similar molecules having an H
position in a symmetric double well-potential between
O atoms, this proton transfer in the ground electronic
state has been observed in the case of matrix isolation
only for tropolone [57] and only for the two modes the
most involved in this proton transfer, i.e., the stretch-
ings of the OH-bond and of the H-bond. In this case,
proton transfer occurs in a 5-atom quasi-ring which is
probably stiffer and less coupled to the environment
than the 6-atom quasi-ring of MA and AA. It should
be noted that temperature changes necessary to ob-
serve population changes and to unambiguously assign
a tunnelling effect are very easy to get in cryogenic
matrices for a tunnelling splitting measured at
� 21.6 cm–1 in the case of MA: cryogenic matrix
isolation would have been a nice way to evidence
proton tunnelling without this efficient damping. We
focus here on the observed �C=O and �C=C modes.
Both calculations and experiments show that they are
at lower frequency and that the �� value is smaller for
AA than for MA, in agreement with stronger H-bond-
ing for AA. The bandwidth is nevertheless very large
when compared to that of non-chelated forms (Figs. 4
and 5) and this indicates that H-bonding efficiently
acts as a doorway for relaxation through phonons. In
addition, broadening is larger for AA than for MA,
but this is not the case for the non-chelated forms, for
which the bandwidths are similar for AA and MA.
This suggests that the enhancement of H-bonding
plays a larger role than the presence of the methyl
groups in this broadening effect. Further analyses are
nevertheless necessary to definitely conclude: in parti-
cular it is necessary to test the homogeneity of the
bands observed for the non-chelated species.
The most striking feature is that irradiating the
� � �* transition of these molecules isolated in nitro-
gen matrices lead for both molecules to isomerisation
within the enolic rotamers, and not to another iso-
merisation as that to the keto form, whereas disso-
ciation is observed in the gas phase [58]; the solid-
state environment probably prevents from dissociation
through a cage effect. In the case of MA, broad band
irradiation with a Hg lamp lead to the complete
conversion to the non-chelated form while in the case
of AA, the conversion is limited to � 80%, indicating
the existence of a photostationary equilibrium and the
existence of non-chelated � chelated back-reactions.
For MA and AA isolated in a nitrogen matrix, the UV
absorptions of the chelated forms have been measured
at 265 and 270 nm, respectively, whereas the non-
Comparative study of malonaldehyde and acetylacetone
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1379
Wavenumber , cm
–1
Wavenumber , cm
–1
356036003640
a
c
b
d
0.1
3622 cm
–1
3588 cm
–1
1580162016601700
x 0.5
a
c
b
d
0.2
x 0.5
Fig. 5. Infrared spectra of acetylacetone isolated in a nitrogen matrix at T = 4 K, AA:N2 � 3:1000. After deposition (a),
difference spectrum after a 120 min Hg lamp irradiation minus a (b), difference spectrum after minus before a 45 min
infrared irradiation at 3585 cm–1 (c), difference spectrum after minus before a 11 min infrared irradiation at 3622 cm–1 (d).
chelated UV absorptions have been measured at
224 nm for MA (to be published) and at 230 and
249 nm for AA [46]. Chelated � non-chelated, as well
as non-chelated � chelated back-reactions are thus ex-
pected to occur under broad band irradiation. Laser
irradiation have allowed us to identify these back-
reactions in the case of AA [46] and further similar ex-
periments are necessary to also identify the back-
reactions in the case of MA, and to confirm that they
are less efficient. These experiments also give evidence
that infrared ��� excitations easily induce intercon-
version. Rotating around the single bonds necessitates
typically 5–8 kcal·mol–1, whereas rotating around the
double bonds necessitates typically 40–50 kcal·mol–1
and therefore, only rotation around single bonds are
expected to occur when exciting with infrared
irradiation. The only conversion observed here is
XXC
XXT and this shows that O–H stretching
couples efficiently with a neighbouring vibrator as the
rotation around the (C–O)-bond, but that far-neigh-
bour mode coupling is too weak to insure conversion.
Conversely, a similar study [59] of IR-induced inter-
conversions between a H-bonded form of malonic acid
(which also possess a quasi symmetric H-bond in a
HOCCCO quasi-ring), and non H-bonded forms, has
led to a more complex rotation around both C–O and
C–C single bonds: this is due to the different nature of
the rings, those of AA or MA being clearly reinforced
by a C=C double bond compared to that of malonic
acid. These vibrational excitations are in any case a
powerful tool to discriminate and identify the ro-
tamers. On the other hand, electronic excitations
suggest complex reactivity phenomena, the following
questions are assessed. Why only four among the
seven non-chelated rotamers are observed? Does iso-
merisation occur in the S2, S1 or S0 state? Is there
intersystem crossing, as suggested by a recent study
[34]? For this latter point, isolation in cryogenic
matrices can bring valuable information: the heavy-
atom effect, well-known to facilitate intersystem
crossing, increases in the series Ne, Ar, Kr, Xe, and
reactivity rates can be analysed by varying the host
material. This point is currently in progress.
Conclusion
The comparison of some calculated and measur-
ed properties of the eight enolic rotamers of ma-
lonaldehyde and acetylacetone has been investi-
gated through ground-state energy, geometry and
vibrational frequency calculations with the
B3LYP/6–311++G(2d,2p) model chemistry, and
through photoinduced isomerisation experiments for
the molecules isolated in nitrogen matrices. The re-
lative positions of vibrational bands as �OH, �CO, �OH
give a clear experimental evidence that H-bonding is
stronger for AA, as expected due to the electron
release induced by the methyl groups. The comparison
of geometrical parameters and vibrational frequencies
shows that calculations correctly take into account
this H-bond enhancement. The energy difference bet-
ween chelated CCC and the non-chelated CCT forms
gives an estimation of the hydrogen bond strength;
this estimation is nevertheless blurred by other contri-
butions like possible steric hindrance induced by the
rotation. We get here a rough estimation of
1.7 kcal·mol–1 for the hydrogen bond energy dif-
ference between AA and MA.
Alternate cis/trans properties are found only for
the cis/trans configuration around the C–O bond,
with evidence for a local larger electronic deloca-
lisation for the cis configuration: this effect is even
larger for AA. Although whole skeleton changes are
involved, the other cis/trans configurations around
the C–C or the C=C bonds do not lead to similar
alternate properties, probably because several types of
repulsive or attractive interactions are competing.
Reactivity has been analysed by means of FT–IR
spectrometry and vibrational analysis. We have
shown that, contrary to what is observed in gas phase,
in nitrogen matrices UV � � �* excitation produces
rotamers, only some among the seven rotamers being
observed. Selective O–H stretching excitation has
revealed a useful tool for identifying these rotamers.
The observed rotamers are not the same for AA and for
MA, suggesting complex reactional processes and
further experimental and theoretical studies are ne-
cessary to analyse the relaxation pathways involved in
this photochemistry.
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