Synthesis and properties of trifluoromethoxyl fluoroformyl anhydride, CF3OC(O)OC(O)F

Article abstract of DOI:10.1016/j.jfluchem.2012.05.020

In the present work the synthesis of the trifluoromethoxyl fluoroformyl anhydride is presented for the first time. Two strategies were employed to obtain CF₃OC(O)OC(O)F: the thermal decomposition of CF ₃OC(O)OOOC(O)F in excess of CO, and the reaction between CF ₃OC(O)OOC(O)F, FC(O)OOC(O)F and CO. A mechanism was proposed taking into account reaction rates and thermal stability of the intermediates. DFT calculations at B3LYP/6-311+G* level were used to explore the conformational space of this molecule and the relative populations of conformers at room temperature, and to simulate the experimental vibrational spectrum. This molecule completes the family of the asymmetric oxygen bonded acyl compounds CF₃OC(O)OxC(O)F with x = 1-3.

Full text of DOI:10.1016/j.jfluchem.2012.05.020

Journal of Fluorine Chemistry 141 (2012) 16–20
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and properties of trifluoromethoxyl fluoroformyl anhydride,
CF3OC(O)OC(O)F
*
´
¨
Martın M. Manetti, Gustavo A. Arguello, Maxi A. Burgos Paci
Instituto de Investigaciones en Fı´sico Quı´mica de Co´rdoba (INFIQC) CONICET-UNC, Departamento, de Fı´sico Quı´mica, Facultad de Ciencias Quı´micas, Universidad Nacional de Co´rdoba,
Ciudad, Universitaria, X5000HUA Co´rdoba, Argentina
A R T I C L E I N F O
A B S T R A C T
Article history:
In the present work the synthesis of the trifluoromethoxyl fluoroformyl anhydride is presented for the
first time. Two strategies were employed to obtain CF3OC(O)OC(O)F: the thermal decomposition of
CF3OC(O)OOOC(O)F in excess of CO, and the reaction between CF3OC(O)OOC(O)F, FC(O)OOC(O)F and CO.
A mechanism was proposed taking into account reaction rates and thermal stability of the intermediates.
DFT calculations at B3LYP/6-311+G* level were used to explore the conformational space of this molecule
and the relative populations of conformers at room temperature, and to simulate the experimental
vibrational spectrum. This molecule completes the family of the asymmetric oxygen bonded acyl
compounds CF3OC(O)–Ox–C(O)F with x = 1–3.
Received 9 March 2012
Received in revised form 17 May 2012
Accepted 21 May 2012
Available online 12 June 2012
Keywords:
Fluorinated radicals
Gas phase reaction mechanism
Perfluorinated anhydride
DFT calculations
ß 2012 Elsevier B.V. All rights reserved.
1. Introduction
produce different species. In the case of the symmetric anhydride
CF3OC(O)OC(O)OCF3, it is obtained from decomposition of the
Research on fluorinated carbo-oxygenated compounds is highly
attractive for several reasons, among which are the applications of
fluorinated ethers, olefins, and fluoropolymers in the broad
spectrum of industries related with optics, petrochemistry,
aerospace, etc. [1,2]. Besides, study of these compounds is of
importance in atmospheric chemistry to get a deeper understand-
ing of the tropospheric and stratospheric fate of CFCs substitutes.
Following this line of research, synthesis and characterization of
several acyl anhydrides, peroxides, and trioxides have been
achieved in the last years. The families of compounds where
two RC(O)– groups are bounded by one, two, or three oxygen
atoms are known for the symmetric molecules (x = 1–3) FC(O)–Ox–
C(O)F [3–5], and CF3OC(O)–Ox–C(O)OCF3 [6–8]. The asymmetric
compounds CF3OC(O)–Ox–C(O)F with two and three oxygen atoms
are already known [9,10] but their isolation was rather difficult.
They were studied by gas electron diffraction, vibrational
spectroscopy, thermal properties, etc. [11–15].
parent trioxide [6]. The same strategy was followed to synthesize
the asymmetric molecule trifluoromethoxyl fluoroformyl anhy-
dride, CF3OC(O)OC(O)F. In this paper we present for the first time
the synthesis of this compound besides the analysis of its
vibrational spectrum and DFT calculations of the ground state
potential energy surface. The title molecule is the anhydride that
completes the family of the asymmetric oxygen bonded acyl
compounds CF3OC(O)–Ox–C(O)F with x = 1–3.
2. Materials and methods
The polyoxides CF3OC(O)OOOC(O)F and CF3OC(O)OC(O)F are
potentially explosive, especially in the presence of oxidizable
materials. All reactions should be carried out in millimolar
quantities only, and it is important to take safety precautions
especially when these compounds are handled in liquid and solid
state.
Perfluorinated acyloxy compounds are formed mainly by gas
phase photolysis of CF3 and/or FCO radical precursors in excess of
CO and O2 (an exception is the synthesis of FC(O)OOC(O)F obtained
from a F2/O2/CO mixture). The temperature at which the photolysis
is carried out is of central importance to stabilize the different
carboxy radicals CF3OCOx and FCOx, x = 1–3 that will recombine to
Commercially available samples of perfluoroacetic anhydride
CF3C(O)OC(O)CF3 (PCR), CO (PRAXAIR), and O2 (AGA) were used.
Dry oxygen was obtained from liquified oxygen at atmospheric
pressure in a trap immersed in liquid air and then transferred to
the reactor up to obtain the desired pressure. FC(O)C(O)F was
prepared by fluorination of oxalyl chloride [16].
Volatile materials were manipulated in a glass vacuum line
equipped with two capacitance pressure gauges (0–760 Torr,
MKS Baratron; 0–70 mbar, Bell and Howell), three U traps, and
valves with poly(tetrafluoroethylene) stems (Young, London).
* Corresponding author. Tel.: +54 351 4334180.
E-mail address: mburgos@fcq.unc.edu.ar (M.A.B. Paci).
0022-1139/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2012.05.020

M.M. Manetti et al. / Journal of Fluorine Chemistry 141 (2012) 16–20
17
The vacuum line was connected to the photoreactor (12 L round
flask equipped with a low pressure Hg Lamp – 254 nm) and to an
IR gas cell (double-walled quartz with Si windows, optical path
length 210 mm) placed in the sample compartment of a Fourier
transform infrared (FT-IR) spectrometer (Bruker IFS 66v). The
spectra were taken with a resolution of 2 cmꢀ1 and the addition
of 4 interferometer scans. This arrangement made it possible to
follow the course of the synthesis.
FCðOÞOꢁ ! Fꢁ þ CO2
(6)
(7)
CF3Oꢁ þ CO þ M ! CF3OCOꢁ þ M
Fꢁ þ CO ! FCO
(8)
CF3OCOꢁ þ FCðOÞOꢁ ! CF3OCðOÞOCðOÞF
CF3OCðOÞOꢁ þ FCOꢁ ! CF3OCðOÞOCðOÞF
(9)
3. Theoretical calculations
(10)
The conformational space of the ground electronic state of
CF3OC(O)OC(O)F was studied using the density functional theory
(DFT). The geometries of all conformers were optimized using the
hybrid density functional B3LYP with the 6-311+G* basis set.
Additionally, harmonic vibrational frequencies and zero-point
energies (ZPE) were calculated at the same level of theory to check
whether the obtained stationary points were either isomers or
first-order transition states; the calculated conformers possessed
no imaginary frequencies. The determination of the Hessian matrix
also enabled calculation of the thermochemical quantities for the
conformers at 298.15 K. The selected DFT method has been
extensively applied to the determination of geometric parameters
of related fluoro-carbon-oxygenated compounds, yielding accu-
rate results tested with gas electron diffraction experiments
[13,14,17]. Since we are interested in the minima of the potential
energy surface, and DFT methods take into account the electron
correlation energy to a partial extent [18], we think that the B3LYP/
6-311+G* method should be adequate to describe the relative
energies for the isomers. All symmetry restrictions were turned off
in the calculations. The GAUSSIAN03 program package [19] was
used in conjunction with Molden 4.1 [20].
Reaction (10) is rather speculative because it involves FCOꢁ
radicals whose formation is difficult at the temperatures of this
system, taking into account that FCO should come from the
decarboxylation of FC(O)O, with a rate constant of k6 = 0.22 sꢀ1
[21] followed by reaction (8). In comparison, the decarboxylation
of CF3OC(O)O, k5 = 35 sꢀ1 [22], is 160 times faster.
The expected products besides the anhydride are those shown
in reactions (9)–(14), which correspond to termination steps:
2FCðOÞO ! FCðOÞOOCðOÞF
(11)
(12)
(13)
(14)
2CF3OCðOÞOꢁ ! CF3OCðOÞOOCðOÞOCF3
CF3OCðOÞOꢁ þ FCðOÞOꢁ ! CF3OCðOÞOOCðOÞF
CF3OCOꢁ þ CF3OCðOÞOꢁ ! CF3OCðOÞOCðOÞOCF3
Once the reaction is terminated, the gas mixture is directed
through three U-traps kept at 77 K in dynamic vacuum. In this way,
the products are trapped but the remaining CO is pumped off. In
order to purify and identify the products, they were collected in a
glass trap connected before the U-traps. The temperatures of the U-
traps were set up 193, 153, and 77 K and the sample was pumped
through the traps. The most volatile products, retained in the 77 K
trap, were identified as CF2O, CO2, and a small amount of
FC(O)OOC(O)F and discarded. At 153 K a blend of the peroxides
FC(O)OOC(O)F, CF3OC(O)OOC(O)F, and CF3OC(O)OOC(O)OCF3 was
identified. CF3OC(O)OOC(O)F, CF3OC(O)OOC(O)OCF3, CF3OC(O)O-
C(O)OCF3, and CF3OC(O)OC(O)F were found in the last U trap at
193 K. Peroxides were separated from anhydrides by static
distillation at about 243 K. Both anhydrides CF3OC(O)OC(O)OCF3
and CF3OC(O)OC(O)F have similar vapor pressure and it is not
possible to separate them by distillation.
Another strategy for the synthesis of CF3OC(O)OC(O)F was the
reaction between CF3OC(O)OOC(O)F from our own repository,
FC(O)OOC(O)F, and CO. Under these conditions, formation of
CF3OC(O)OC(O)OCF3 is avoided. The experiment was conducted in
a 1L round glass flask filled with 30 mbar of CF3OC(O)OOC(O)F,
15 mbar of FC(O)OOC(O)F and CO to reach 500 mbar of total
pressure. The flask was immersed in a water bath at 333 K. At this
temperature the unimolecular decomposition of CF3OC(O)OO-
C(O)F and FC(O)OOC(O)F:
4. Results and discussion
4.1. Synthesis method
The title molecule was obtained from the recombination of
CF3OCO and FCO2 radicals produced in the reaction chamber from
thermal decomposition of CF3OC(O)OOOC(O)F in excess of CO. The
complete method of synthesis for the parent trioxide is described
elsewhere [10]. Briefly, it is obtained from the 266 nm photolysis of
trifluoroacetic anhydride (CF3C(O)OC(O)CF3) and oxalyl fluoride
(FC(O)C(O)F) in excess of CO and O2 at about 223 K. Once the
asymmetric trioxide is formed, the lamp is turned off and the
reactor bath is cooled down to about 193 K. At this temperature the
trioxide sticks on the reactor walls and the remaining products and
starting materials such as CF2O, CO2, CF3C(O)F, CF3OOCF3,
FC(O)OOC(O)F, CO, and O2 can be pumped out. After this, CO is
introduced in the reactor and the ethanol bath is allowed to slowly
warm up. In this way, the trioxide sublimates, and when
temperature reaches 253 K the dissociation of the molecule starts
according to reactions (1) and (2) producing CF3OC(O)Ox and FCOx
radicals that, in excess of CO, will give rise to the formation of
CF3OC(O)OC(O)F. The proposed mechanism is detailed from
reactions (1)–(10) as follows:
CF3OCðOÞOOCðOÞF ! CF3OCðOÞOꢁ þ FCðOÞOꢁ
FCðOÞOOCðOÞF ! 2FCðOÞOꢁ
(-13)
(-11)
CF3OCðOÞOOOCðOÞF ! CF3OCðOÞOOꢁ þ FCðOÞOꢁ
CF3OCðOÞOOOCðOÞF ! CF3OCðOÞOꢁ þ FCðOÞOOꢁ
2CF3OCðOÞOOꢁ ! 2CF3OCðOÞOꢁ þ O2
2FCðOÞOO ! 2FCðOÞO þ O2
(1)
(2)
(3)
(4)
(5)
has rate constants of
k
k
ꢀ13 = 2.9 ꢂ 10ꢀ4 sꢀ1 [23] and
ꢀ11 = 1.9 ꢂ 10ꢀ3 sꢀ1 [15] respectively. The former peroxide
decomposes into CF3OC(O)Oꢁ and FC(O)Oꢁ radicals according to
(-13), but CF3OC(O)Oꢁ decarboxylates very fast to give CF3Oꢁ and
CO2. CF3Oꢁ reacts efficiently with CO to give CF3OCOꢁ and its
combination with FC(O)Oꢁ yields the desired anhydride. It is worth
noting that the formation of CF3OC(O)OC(O)OCF3 is inhibited by
the decarboxylation rate of CF3OC(O)Oꢁ at 333 K and the relative
amount of FC(O)Oꢁ with respect to CF3OCOꢁ.
CF3OCðOÞOꢁ ! CF3Oꢁ þ CO2

18
M.M. Manetti et al. / Journal of Fluorine Chemistry 141 (2012) 16–20
Table 1
1.2
1.0
0.8
0.6
0.4
0.2
0.0
B3LYP/6-311+G* absolute and relative energies plus ZPE for the six conformers
found for CF3OC(O)OC(O)F.
Conformer
E+ZPE (H)
D
E+ZPE
Relative population
(%) at 298 K
(kcalmolꢀ1
)
s–s–s
s–a–s
s–s–a
a–s–s
s–a–a
a–s–a
ꢀ814.804749
ꢀ814.803011
ꢀ814.802679
ꢀ814.800631
ꢀ814.800112
ꢀ814.798763
0
77.38
12.28
8.64
0.99
1.09
1.3
2.58
2.91
3.76
0.57
0.14
Experiment
Simulation
4.2. Calculations
The conformational space of CF3OC(O)OC(O)F can be described
through the four main torsion angles of the skeleton F–C(F2)–O–
C(=O)–O–CF55O, which in turn can be conveniently reduced to
2000 1800 1600 1400 1200 1000
Wavenumber /cm-1
800
600
400
three because w0(F–C(F2)–O–C(O) always adopts the anti configu-
ration. These three main dihedrals can adopt either anti (ꢃ180.08)
Fig. 1. Experimental and calculated MIR spectrum of CF3OC(O)OC(O)F. Simulated
spectrum is the result of adding the three most stable conformers at the populations
shown in Table 1.
or syn (ꢃ0.08) configurations (i) the relative position of the CF3
group to the C55O bond,
w1(F3C–O–C55O); and (ii) the relative
position of each double bond to the anhydride bridge –O–,
2(O55C–O–COF) and 3(C–O–CF55O) respectively. Enantiomeric
w
w
The evolution of the reaction was followed by measuring the IR
forms are not taken into account since there are no energy
differences between them. The different conformers are designat-
ed according to the letter code ‘‘a’’ (anti) or ‘‘s’’ (syn) for w1 w2 w3 in
absorbance
of
CF3OC(O)OOC(O)F
(
s
1152 = 6.7 ꢂ 10ꢀ18 cm2
moleculeꢀ1). Once the reaction was terminated, the gas mixture
was condensed with liquid nitrogen in the U-traps. The low-
temperature distillation gave CF2O and SiF4 at 81 K, FC(O)OOC(O)F
at 193 K and pure CF3OC(O)OC(O)F (>99%) remained at 225 K.
Fig. 1 shows the IR spectrum of the molecule obtained with a
2 cmꢀ1 resolution.
the sequence w1–w2–w3. Six minima were found in the B3LYP/6-
311+G* PES being the s–s–s the most stable conformer. Eight
minima are possible according to the possibilities of the dihedrals;
however, in the a–a–a and a–a–s conformers the CF3 fragment
overlaps the COF being both configurations not at a minimum.
Fig. 2. Most stable conformer at the B3LYP/6-311*G+ level.

M.M. Manetti et al. / Journal of Fluorine Chemistry 141 (2012) 16–20
19
Table 2
electron diffraction (GED). Unfortunately, there is no GED study of
CF3OC(O)OC(O)OCF3 and the experimental values, ꢀ13.7 and
ꢀ21.98, result from the X-ray diffraction of the solid; however, the
calculated value for both angles is 25.18 for the conformation in the
Geometrical parameters for CF3OC(O)OC(O)F (B3LYP/6-311+G*) and the related
anhydrides FC(O)OC(O)F [6] and CF3OC(O)OC(O)OCF3 [14]. Numbering of atoms
follows from Fig. 1 and strictly applies only to CF3OC(O)OC(O)F. The values
informed for FC(O)OC(O)F and CF3OC(O)OC(O)OCF3 are associations for the same
type of bond or angle.
gas phase. In the present case, the dissimilarity between w2 and w3
shows the different interactions of CF3O and F with the carbonyl
groups and parallels the tendency observed in the symmetrical
anhydrides.
a
CF3OC(O)OC(O)F
FC(O)OC(O)F
1.31
CF3OC(O)OC(O)OCF3
˚
Distances (A)
C–F
1.33
1.39
1.36
1.18
1.37
1.36
1.17
1.31
1.37
1.35
1.17
1.36
1.36
1.17
C1–O2
O2–C3
C35O10
C3–O4
O4–C5
C55O11
4.3. IR analysis
1.17
1.38
1.38
1.17
CF3OC(O)OC(O)F has 27 vibrational modes, all of which are IR
active. All the conformers belong to the C1 symmetry group. Table
3 shows the vibrational modes experimentally accessible with the
instrument as well as the theoretical frequencies calculated for the
s–s–s rotamer without any correction. The IR transitions for the
symmetric anhydrides are also included for comparison. The
carbonyl stretching vibrations in the asymmetric molecule are
within the values for the symmetric ones, it can be seen that the
average value for these vibrations are 1873 and 1905 cmꢀ1 for
FC(O)OC(O)F and CF3OC(O)OC(O)OCF3 respectively and 1891 cmꢀ1
for CF3OC(O)OC(O)F. A similar effect is observed for the strongest
vibrations, which mainly corresponds to stretching of the C–O–C
fragment; for the symmetric molecules the bands are at 1168 and
1057 cmꢀ1 and the asymmetric is at 1116 cmꢀ1. This effect can be
thought of as a transition of properties going from one to the other
symmetric anhydride through the asymmetric one.
Fig. 1 shows the experimental IR spectrum of CF3OC(O)OC(O)F
after subtraction of the remaining impurities together with a
simulation. For the latter we used the B3LYP/6-311+G* frequencies
(without corrections) and intensities of the three most stable
rotamers at the populations in Table 1. Each vibrational transition
was fitted with a Lorentzian profile using 2 cmꢀ1 as FWHH, and the
intensities were normalized for suitable comparison with the
experimental spectrum. Taking into account the calculated
absorption cross sections and the relative populations of the
rotamers, the only transitions with quantitative differences are
those corresponding to the carbonyl stretching. The values of ns
and nas C55O for the s–a–s and s–s–a rotamers (1936–1916 and
1956–1900 cmꢀ1 respectively) are enclosed within the values
belonging to the s–s–s rotamer (1968–1886 cmꢀ1), so a theoretical
analysis of the spectra foresees a relatively wide and noisy
asymmetric C55O stretching when more than one rotamer is
present in the sample. Inspection of the figure in the carbonyl
region shows these features in the experimental and simulated
Angles (8)
F–C–F
109.2
106.0
118.4
128.3
103.9
119.1
129.2
125.1
105.6
F–C1–O2
105.0
128.0
106.4
C1–O2–C3
O2–C35O10
O2–C3–O4
C3–O4–C5
O4–C55O11
F6–C55O11
O4–C5–F6
127.9
103.5
118.5
127.9
118.2
128.0
105.0
Dihedrals (8)
w0
w1
w2
w3
179.3
ꢀ2.4
179.1
ꢀ24.0
ꢀ21.1
ꢀ13.7
ꢀ21.9
20.8
a
X-ray diffraction of a single crystal at ꢀ80 8C
Table 1 shows the absolute (Hartree units) and relative plus ZPE
(kcal molꢀ1) energies of the most stable conformer as well as the
relative population at 273 K.
Fig. 2 depicts the structure of the most stable calculated
conformer. Bond lengths and angles are presented in Table 2
together with the experimental data available for the two related
symmetrical anhydrides CF3OC(O)OC(O)OCF3 [6] and FC(O)OC(O)F
[14]. The geometrical parameters calculated agree with those
observed in similar compounds. The most interesting features are
those related with the O55C–O–C55O fragment. In the present
molecule the dihedrals O55C–O–COF and CF3OC–O–C55O are ꢀ24.0
and ꢀ21.1 degrees respectively, showing the syn-clinical confor-
mation of both C55O bonds found in others fluorinated anhydrides
like CF3C(O)OC(O)CF3, FC(O)OC(O)F and CF3OC(O)OC(O)OCF3. The
corresponding dihedral is 20.88 in FC(O)OC(O)F determined by gas
Table 3
Comparison of experimental and calculated (most stable conformer) mid-IR vibrational frequencies of CF3OC(O)OC(O)F, and its parent symmetric molecules. Relative
intensities higher than 10 are shown in brackets. Assignments are qualitative because of strong coupling.
CF3OC(O)OC(O)F
Calculateda
FC(O)OC(O)F
CF3OC(O)OC(O)OCF3
Approximate description of mode
1922 (13.6)
1860 (4)
1967.8 (29.9)
1885.7 (7.1)
1264.6 (7.71)
1244.9 (10.2)
1215.6 (18.82)
1208.2 (4.13)
1083.5 (100)
1061.2 (11.62)
944.1 (1.3)
875.0 (1.08)
812.5
1941 (58)
1870 (8)
1900 (15)
1846 (7)
n1
n2
n3
n4
n5
n6
n7
n8
ns C5O
nas C5O
n F2C–F
ns (O)C–O–C(O)
nas CF3
1297 (10.2)
1260 (22.3)
1293 (30)
1259 (48)
1198 (5)
1100 (67)
1057 (100)
964 (2)
1249 (25)
ns CF3/
nas C–O–C(O)/
ns C–O–C(O)/
n
asC–O–C
as(O)C–O–C(O)
as(O)C–O–C(O)
1116 (100)
1168 (100)
1030 (5)
938 (6)
n
n
988 (1.7)
943 (1)
n9
ds C–O–C/ds C–O–C
ns CF3/CF3–O
das C–O–C/das C–O–C
oop s OOC5O/OFC5O
oop as OOC5O/OFC5O
876 (1)
n10
n11
n12
n13
n14
n15
765 (1.4)
768.3 (2.68)
742
765 (6)
652 (8)
708.2
d
d
O–C–O
CF3/ O5C–F
609 (2.9)
610.4 (1.2)
d
a
The following are the rest of the calculated frequencies that were not observed experimentally:
n
16 604.4 (1.09),
n
17 559.9,
n
18 471.8,
n
19 428.1, 20 379, n21 288.3, n22
n
227.5, n23 140.9, n24 110, n25 88.2, n26 48.9, n27 38.2.

20
M.M. Manetti et al. / Journal of Fluorine Chemistry 141 (2012) 16–20
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5. Conclusions
The synthesis and characterization of the new asymmetric
anhydride CF3OC(O)OC(O)F have been accomplished through the
combination of CF3OCO and FCO2 radicals produced in the thermal
decomposition of CF3OC(O)OOOC(O)F in excess of CO. The key to
obtain the anhydride is the condensation of trioxide on the walls
of the reactor, removing all other products and adding CO. In this
way when the trioxide decomposes, the concentration of CF3OCO
and FCO2 radicals is appropriate to form CF3OC(O)OC(O)F
although with CF3OC(O)OC(O)OCF3 as impurity. Another method
for the synthesis was the reaction between CF3OC(O)OOC(O)F,
FC(O)OOC(O)F and CO, which prevents formation of CF3OC(O)O-
C(O)OCF3.
The conformational analysis performed using the B3LYP/6-
311+G* hybrid method indicates that the s–s–s conformer is the
most stable one, and that at room temperature the populations
of the s–s–s, s–a–s, and s–s–a rotamers could be appreciable.
The mid-IR experimental spectrum was simulated using the
frequencies obtained from the B3LYP calculations for the three
rotamers. Very good agreement can be found when comparing
both results, and the presence of the three conformers in the
experimental spectrum becomes evident when analyzing the
carbonyl region.
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Authors would like to thank CONICET, ANPCyT of Argentina, and
SeCyT-UNC for financial support. Language assistance by translator
R. Karina Plasencia is gratefully acknowledged.