2-Chloro-2,2-difluoracetamide (ClF2CC(O)NH2). Thermal decomposition, vapour infrared, mass spectrometry, low-temperature NMR, and theoretical studies. Solvent effects on conformational preferences

Article abstract of DOI:10.1071/CH10263

Gas-phase thermal decomposition of 2-chloro-2,2-difluoracetamide (CDFA) was studied at temperatures between 270 and 290°C. The rate constant for the decomposition follows the Arrhenius equation. k = (5.5 ± 0.3) · 10¹⁶s^-1 exp [-(104±4)kj mol^-1/RT] Mass spectrometry was used to analyze the decomposition pattern of the title compound. The FT-IR spectrum of the vapour phase and the infrared spectra of CDFA in protic and aprotic solvents were recorded. Potential energy surfaces were studied by theoretical calculations performed at the density functional theory level (PBEPBE and B3LYP methods) using the 6-31G*, 6-31+G*, 6-311+G**, aug-cc-pVDZ, and aug-cc-pVTZ basis sets. CSIRO 2011.

Full text of DOI:10.1071/CH10263

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2011, 64, 1366–1372
www.publish.csiro.au/journals/ajc
2-Chloro-2,2-difluoracetamide (ClF2CC(O)NH2). Thermal
Decomposition, Vapour Infrared, Mass Spectrometry,
Low-temperature NMR, and Theoretical Studies.
Solvent Effects on Conformational Preferences
A
B
A C
,
Ana G. Iriarte, Edgardo H. Cutin, and Gustavo A. Arguello
¨
A
Instituto de Investigaciones en F´ısico Qu´ımica de Co´rdoba (INFIQC) Departamento
]
de Fisicoqu´ımica, Facultad de Ciencias Qu´ımicas, Universidad Nacional de Co´rdoba,
Ciudad Universitaria, 5000 Co´rdoba, Argentina.
B
Instituto de Qu´ımica del Noroeste Argentino (INQUINOA), Instituto de Qu´ımica F´ısica,
San Lorenzo 456, T4000 CAN Tucuma´n, Argentina.
C
Corresponding author. Email: gaac@fcq.unc.edu.ar
Gas-phase thermal decomposition of 2-chloro-2,2-difluoracetamide (CDFA) was studied at temperatures between 270 and
2908C. The rate constant for the decomposition follows the Arrhenius equation.
ꢀ
ꢁ
ð104 ꢀ 4Þ kJ molꢂ1
k ¼ ð5:5 ꢀ 0:3Þ ꢁ1016 sꢂ1 exp ꢂ
RT
Mass spectrometry was used to analyze the decomposition pattern of the title compound. The FT-IR spectrum of the
vapour phase and the infrared spectra of CDFA in protic and aprotic solvents were recorded. Potential energy surfaces were
studied by theoretical calculations performed at the density functional theory level (PBEPBE and B3LYP methods) using
the 6-31G*, 6-31þG*, 6-311þG**, aug-cc-pVDZ, and aug-cc-pVTZ basis sets.
Manuscript received: 24 August 2010.
Manuscript accepted: 30 May 2011.
Published online: 23 August 2011.
Introduction
carbonylic band behaviour when increasing concentrations
of aprotic solvents were added to methanolic systems. They
concluded that, for some methanolic and aqueous systems,
there are two kinds of solvates. One type corresponds to
monosolvated C¼O species, whereas the other is related to the
di-hydrogen-bonded C¼O compounds. The occurrence of each
type depends on the relative concentration of the ‘free’ O–H
groups in the media, i.e. the availability of O–H groups to form
hydrogen bonds. In a recent theoretical investigation of the
N-methylacetamide/methanol system,[19] Kwac et al. concluded
that the non-Gaussian profile of the IR band was due to the
presence of two different solvated structures.
2-Chloro-2,2-difluoroacetamide (CDFA), which is one of the
simplest halogen-substituted amide compounds capable of pre-
senting more than one conformation around the sp3 carbon atom,
became of interest both because of its molecular conformation
in the gas phase and its vibrational spectrum in the crystal
form.[4,20] For this compound, the vibrational spectra have been
investigated by solid-phase IR and Raman techniques, and its
structure was solved by gas electron diffraction and quantum
chemical calculations.[14,20] Although this compound is
frequently used as a chemical precursor,[21–23] neither the gas-
phase infrared spectrum nor its thermal decomposition have
been reported, and neither experimental nor theoretical studies
have been carried out in solution.
Amide compounds are well known as peptide analogues, and as
such, they could help provide relevant information about the
conformation of proteins.[1–14] A large number of experiments
have been carried out in order to acquire experimental data
related to amide compounds and to reach a better understanding
of the different effects that help in the stabilization of these
species in different media.[9–16]
However, as far as we know, there is little information about
other properties of acetamide derivatives, such as, for instance,
the kinetics of thermal or photochemical decomposition. Only a
few studies of acetamide photolysis and pyrolysis mechanism
have been carried out and their products analyzed.[17,18] In
contrast, there are more data about the behaviour of acetamide
derivatives in solution. For example, the study of conformational
equilibria of N,N-dimethyl-2-haloacetamides,[9–12] N-methyl-
2-fluoroacetamide[10] and fluoro-, chloro-, bromo-, and cyano-
acetamide[11,12] deals with the presence of at least two stable
rotamers in some solvent systems. Also, the infrared spectra of
compounds such as formamide, N-methyl formamide, acet-
amide, and N-methylacetamide show marked shifts of the
C¼O stretching vibration bands with solvation in both protic
and aprotic solvents.[15] In order to study the latter fact in greater
depth, Eaton et al.[15] carried out the spectroscopic analysis of
some acetamide derivatives in solvents, and investigated the
Ó CSIRO 2011
10.1071/CH10263

Studies on 2-Chloro-2,2-difluoracetamide
1367
ϩ
H
°
ϩ
O
°
ϩ
F
C
°°
F
C
F
F
C
F C NH
F
C
ϩ
NH
ϪClЊ
ϪCFCl
H
ϪH2O
ϪOHЊ
C
F
N
N
C
N
N
Cl
Cl
m/z 46.05
(9 %)
Cl
F
F
F
F
ϩ
°°
ϪCFCl
ϩ
°
ϩ
°
O
O C NH
ϪClF2CH
ϩ
°
CFϩ
ϩ
F
H
F C N
m/z 43.05
F C N
N
H
(21 %)
Cl
m/z 45.05
(28 %)
F
ϪClF2CЊ
O
ϪC°
ϪOCNH
H
ϩ
O
N
F
H
ϩ
H
ϩ
°
H
N
H
m/z 44.05
(100 %)
F
F
F
Cl
m/z 94.05
(8 %)
ϪClЊ
H
F
ϩF
m/z 51.00
(10 %)
Scheme 1.
In this work, we provide new data to complete information
on the physicochemical properties of CDFA. In pursuing that
goal, we present the infrared spectrum of the gas phase, the
kinetics of its thermal decomposition, low-temperature NMR
spectra, the mass decomposition pattern to help understand the
decomposition mechanism, and IR spectra in different solvents,
such as CH2Cl2, CHCl3, CH3CN, CH3OH, CH3CH2OH,
CH3(CH2)2OH, (CH3)3COH, and (CH3)2CHCH2OH, together
with theoretical analyses. The latter made use of density
functional theory (DFT) methods combined with Tomasi’s
Polarized Continuum Model (PCM), in order to evaluate the
influence of the solvent on conformational stability.[24]
Infrared Gas-phase Spectra and Thermal Decomposition
We present in Fig. 1 a set of vibrational spectra of CDFA:
trace (a), the experimental IR spectrum recorded at 200 8C;
trace (b), the calculated IR spectrum and (e), the calculated
Raman spectrum, at the B3LYP/6–311þþg** level of approach.
Traces (c) and (d) correspond to the infrared and Raman spectra
respectively reported for the solid.[14] The corresponding
wavenumber values are collected in Table 1, in order to facilitate
comparison among the different phases, together with the root
mean square deviation (RMSD), used to figure out how much the
experimental values deviate from the theoretical ones.
The comparison between traces (a) and (b) shows remarkable
agreement not only in the position of the bands but also in their
intensities. Only a small difference is observed, due to an
inversion in the intensities of the bands located at 1171 and
Results and Discussion
1101 cmꢂ1
.
Analysis of Mass Spectra
Scheme 1 depicts the relevant fragment patterns responsible for
the peaks observed in the mass spectra, together with the pro-
posed mechanism to account for them.
A keto–enol equilibrium is postulated as there are H atoms
available bonded to the N atom vicinal to the C¼O bond. Thus,
peaks such as those at m/z 46.05 and 45.05 can be explained
through the decomposition of the enol form, whereas those
peaks at m/z 43.05, 44.05, 51.00, and 94.05 could come only
from the keto form.
A distinctive feature is the dominant peak of the spectrum,
which corresponds to m/z ¼ 44.05. Many halogenated acet-
amides reported in the literature show this peak, and it can be
directly assigned to the Csp3–Csp2 rupture coming from the
carbonylic form. As CDFA has very strong mesomeric delocali-
zation (denoted as ZN - p*C¼O),[16] which reinforces the
N–Csp2 bond, the occurrence of the Csp3–Csp2 rupture is quite
logical.
In the amide I and II region (1800–1600 cmꢂ1 approximately),
there are two distinctive features: the separation of the bands
corresponding to the amide I and II modes (from 1734 (solid
phase) to 1778 cmꢂ1 (gas phase) and from 1690 (solid phase) to
1591 cmꢂ1 (gas phase)), and the decrease in the intensity of the
NH2 deformation band, which reaches approximately one-third
of the intensity with respect to the original C¼O stretching band
absorption. This phenomenon is expected to occur, as it has been
observed for many primary amides in dilute solutions.[25]
In order to improve the vibrational assignments of the
solid,[14] we took advantage of the new information provided
by the gas-phase spectrum to verify or to correct the assignments
of those bands where the normal modes of vibrations seem to be
not well defined. For instance, the band assigned to CF2
antisymmetric stretching is reported at 1390 cmꢂ1 in the solid
phase, overlapping with the CN stretching vibration signal
(1400 cmꢂ1).[14] As our results fit very well with the calculated

1368
A. G. Iriarte, E. H. Cutin, and G. A. Argu¨ello
gas-phase spectra, it is logical to assume that the Raman spectra
also will. Besides, analysis of the vibrational spectra of mole-
cules containing fluorinated groups such as CF3 or CClF2 shows
a remarkable aspect: they have very intense bands in the
infrared, but very weak bands in the Raman spectra.[26] In this
context, it is possible to infer that the band located at 1368 cmꢂ1
(Raman calculated spectra) does not correspond to a vibration
involving the CF2 group, because that band has an appreciable
intensity. Thus, we assigned the CF2 antisymmetric stretching
vibration to the signal located at 1120 cmꢂ1 in the gas phase
spectra, and re-assigned this vibration mode in the solid to the
band at 1146 cmꢂ1 (IR).
Other substantial changes could be made using the consid-
erations outlined above. For example, the calculations predicts
the CF2 symmetric and NH2 deformation modes at 1101 and
1092 cmꢂ1 respectively. However, when considering the Raman
intensities and the infrared gas-phase experimental spectra, the
NH2 deformation modes could be attributed to the band at a
higher wavenumber (1101 cmꢂ1) whereas the CF2 symmetric
mode was attributed to those located at 1092 cmꢂ1 (see Table 1).
Continuing the study of CDFA, the thermal decomposition
of gaseous CDFA was investigated. The compound, at 2 KPa,
was investigated at four different temperatures between 270
and 290 8C. Dry air was allowed to enter the steel reactor at
atmospheric pressure, to ensure the rate constant for decompo-
sition was independent of pressure. In addition, in order to
examine the role that oxygen or air could be playing in the
decomposition, the experiments were performed under an N2
atmosphere, producing similar results.
(a)
(b)
(c)
(d)
(e)
3500
3000
2500
2000
1500
1000
500
Wavenumber [cmϪ1
]
Fig. 1. Infrared and Raman spectra of 2-chloro-2,2-difluoracetamide
(CDFA). (a) FT-IR gas phase, 200 8C; (b) and (e) calculated IR and Raman
spectra respectively (6–311þþg** basis set); (c) FT-IR solid phase;
(d) FT-Raman solid phase.
The disappearance of the reagent was followed using FT-IR
spectroscopy by considering the C¼O stretching vibration band
Table 1. Normal modes of vibration, approximate description and infrared and Raman spectra of 2-chloro-2,2-difluoracetamide (CDFA)
Mode
Approximate descriptionB
B3LYP/6–311þþG** [cmꢂ1 A
]
Gas IR [cmꢂ1 C
]
Solid IR [cmꢂ1 A
]
Solid Raman [cmꢂ1 A
]
n1
n2
n3
n4
n5
n6
n7
n8
NH2 asym. stretch.
NH2 sym. stretch.
C¼O stretch. (amide I)
NH2 def. (amide II)
C–N stretch.
3719
3590
1814
1622
1368
1171
1101
1092
949
759
660
605
556
512
421
372
344
332
259
189
44
3563
3444
1778
1591
1368
1179
1120
3355
3178
1734
1690
1400
1146
3331
3173
1703
1616
1435
1132
ꢂ
CF2 asym. stretch.
NH2 def.
ꢂ
CF2 sym. stretch.
C–Cl stretch.
C¼O def.
1075
969
781
729
668
610
527
436
395
343
279
210
160
–
n9
970
753
667
606
–
970
n10
n11
n12
n13
n14
n15
n16
n17
n18
n19
n20
n21
740
C–C stretch.
668
C¼O def.
609
NH2 def.
534
ClF2C def.
–
–
ClF2C def.
–
428
ClF2C def.
–
–
ClF2C def.
–
–
NH2 def.
–
–
CCN def.
–
–
Skeletal torsion
ClF2C torsion
Root mean square deviation
–
–
–
–
–
65 (20)D
158 (40)D
ARef. [14].
BStretch.: stretching; def.: deformation; sym.: symmetric; asym.: antisymmetric.
CPresent work.
DValues derived without the contribution of the N–H stretchings.

Studies on 2-Chloro-2,2-difluoracetamide
1369
0.2
0.0
Ϫ0.2
Ϫ0.4
Ϫ0.6
Ϫ0.8
Ϫ1.0
Ϫ1.2
Ϫ1.4
Ϫ1.6
(a)
(b)
Ϫ1.8
Ϫ2.0
Ϫ2.2
Ϫ2.4
Ϫ2.6
270
280
285
290
(c)
400
600
800
1000 1200
1400
Time [s]
(d)
Ϫ5.5
2400 2100 1800 1500 1200 900 600
3000 2700
Wavenumber [cmϪ1
]
Ϫ6.0
Ϫ6.5
Ϫ7.0
Ϫ7.5
Ϫ8.0
Fig. 3. Gas-phase decomposition product infrared spectra. (a) (Glass
þ Fe) system product. (b) Reference experimental spectrum of CHClF2.
(c) Stainless-steel cell system product. (d) Reference experimental spectrum
of CCl2F2.
The least-squares analysis of the data from Table 2 gave the
following expression for the activation energy:
kexptl½CDFAꢃ ¼ ð5:5 ꢀ 0:3Þ ꢁ 1016 sꢂ1
ꢀ
ꢁ
ð104 ꢀ 4Þ kJ molꢂ1
ꢄ exp ꢂ
0.00178
0.00180
0.00182
0.00184
RT
1/T [KϪ1
]
Our experimental results fit very well with a first-order kinetic
reaction equation, whereas those reported for acetamide and
some N-substituted acetamides[18,27] conform to a second-order
equation. A substantial difference between our results and those
reported could arise from the use of the iron reactor. In order to
better understand the experimentally observed behaviour, other
reactions were tested. In different glass ampoules, CDFA was
introduced with and without iron threads and finely divided flat
silicon wafers to simulate the whole steel reactor environment.
In order to investigate the thermal decomposition, the ampoules
were heated at 2808C for 20 min. The corresponding FT-IR
spectra of the products were recorded, except for the system
without iron threads or silicon wafers as there was no reaction
over this period of time. In Fig. 3 are shown the corresponding
infrared spectra obtained in both the steel and glass ampoule
reactors, together with the spectra of the possible products used
as reference.
Fig. 2. (Upper) First-order kinetic curves at selected temperatures for the
decomposition of 2-chloro-2,2-difluoracetamide (CDFA). (Lower) Arrhe-
nius plot of kinetic data obtained for CDFA.
Table 2. First-order rate constants from the decomposition of 2-chloro-
2,2-difluoracetamide (CDFA) at different temperatures
T [8C]
k [sꢂ1
]
270
280
285
290
5.16 ꢀ 0.12 ꢄ 10ꢂ4
1.30 ꢀ 0.04 ꢄ 10ꢂ3
1.70 ꢀ 0.06 ꢄ 10ꢂ3
2.69 ꢀ 0.14 ꢄ 10ꢂ3
The IR spectrum of the reaction carried out in the steel
reactor (Fig. 3c) shows different bands in comparison with that
registered from the glass ampoule/iron threads combination
(Fig. 3a). As far as we know, there have been no experimental
studies on the nature of the products obtained from the thermal
decomposition of the title compound; therefore, we could
postulate some products from the signals observed in the
infrared spectra. Apart from CO2, which is the major product
(see Fig. 3), it was also possible to observe CHClF2 as the most
important product in the glass system (compare with trace (b),
which corresponds to pure CHClF2[28]), and CCl2F2 (compare
with trace (d), which corresponds to pure CCl2F2[28]) in the steel
reactor.
intensity at each temperature. The data were analyzed according
to first-order kinetics:
d½ClF2CCðOÞNH2ꢃ
ꢂ
¼ kexptl½ClF2CCðOÞNH2ꢃ
dt
Fig. 2 shows the first-order decay at the four temperatures and its
associated Arrhenius plot.
Good straight lines are observed in all cases. At each
temperature studied, the first-order rate constant was calculated
from the plot by a least-squares method. Average rate constants
are given in Table 2.

1370
A. G. Iriarte, E. H. Cutin, and G. A. Argu¨ello
2.5
2.0
1.5
1.0
0.5
0.0
Chloroform
Gas
CH3Cl
CH2Cl2
CH3OH
Dichloromethane
Acetonitrile
0
20 40 60 80 100 120 140 160 180
Dihedral angle
Fig. 4. Potential energy curves calculated at 20 8C steps for different
phases, calculated at B3LYP/6–311þG** level or approach. The CH3Cl,
CH2Cl2, and CH3OH curves are shifted by 1, 2, and 3 kJ molꢂ1 from the gas-
phase curve respectively.
Methanol
Ethanol
Infrared and 1H NMR Analysis of CDFA in Solution
There is relevant information about structural conformations
and the solvation of amide and haloacetamide derivatives in
different solutions, as was mentioned above.[6,10,11,29] Taking
into account those results, we carried out experimental and
theoretical studies in solution to confirm whether CDFA exists
in more than one conformation. Potential energy curves, where
the dihedral angle capable of generating other conformers was
varied, were obtained using DFT methods with different basis
sets (see Computational Method section), using the Gaussian 03
program.[30] The PCM[24] was included to take into account the
solvent effect on rotational isomerism. In Fig. 4, curves corre-
sponding to the gas phase (bottom line) and results calculated in
different solvents (CH3Cl, CH2Cl2, and CH3OH) are presented.
One evident minimum is found corresponding to the gauche
form, with the angle between the ClꢂC bond and the C¼O
double bond at ,808. However, with the aim of examining all of
the possible configurations, geometry optimizations using stan-
dard convergence criteria without symmetry restrictions were
carried out. According to the calculations there is no possibility
that CDFA exists in other conformations but the gauche one,
either in the gas phase or in solution. The existence of imaginary
frequencies for the syn and anti conformations in both gas phase
and solution support this conclusion (see Fig. S1, Accessory
Publication). However, the potential curves obtained from
internal rotation around the C–C bond (in solution) show a
plateau at (ClꢂC–C¼O) E 1808, suggesting that some interac-
tions stabilizing this conformation could exist. TCA (trichloro-
acetamide) and TFA (trifluoroacetamide) in some solvent
systems present strong solute–solvent interactions, which have
been analyzed by FT-IR.[6] The authors observed that the
wavenumber of the C¼O bands did not change with concentra-
tion, concluding that all the molecules are associated with
solvent. In order to verify if this occurred in our system, a series
of IR spectra were recorded in CH3CN at different concentra-
tions of CDFA. The corresponding spectra are given in Fig. S2
of the Accessory Publication, where it is noticeable that the
C¼O band does not change with concentration either. Thus, it
can be presumed that CDFA presents this kind of interaction in
solutions.
2-methyl-1-propanol
Ter-buthanol
1770 1760 1750 1740 1730 1720 1710
Wavenumber [cmϪ1
]
Fig. 5. Spectra of 2-chloro-2,2-difluoracetamide (CDFA) in protic and
aprotic solvents, in the region of the carbonyl group band.
Nevertheless, in gas and solid phases, the CClF2 group
behaviour is determined by the intramolecular F?H hydrogen
bond and by O?F and O?Cl repulsions. In fact, the calculated
˚
Cl?H distance in the gas phase (2.51 A) is shorter than the same
˚
distance in solvents (2.57 A in dichloromethane and 2.58 A in
˚
methanol).
The experimental infrared spectra were recorded for CDFA
not only in aprotic solvents (chloroform, dichloromethane, and
acetonitrile; Fig. 5) but also in protic ones in order to further
investigate the experimental behaviour in solution.
In Fig. 5, the dashed-line traces correspond to CDFA in protic
solvents. There are two important points to note: the first one is
the shift of C¼O bands to lower frequencies as the nature of the
solvent changes on going from aprotic to protic, and the second
is the splitting of the C¼O bands in the CDFA/methanol and
CDFA/ethanol systems spectra. The H bridge between the
oxygen atom (of the C¼O group) and the OH of the protic

Studies on 2-Chloro-2,2-difluoracetamide
1371
decomposition follows first-order kinetics, and the temperature
for effective decomposition depends on the use of Fe as catalyst.
20
Experimental
223 K
225 K
CDFA was commercially available and its purity was checked
by NMR spectroscopy. The gas-phase infrared spectra (resolu-
tion 2 cmꢂ1) were recorded at 270, 280, 285, and 2908C with a
Bruker IFS-28 FT-IR spectrometer in the range 4000–600 cmꢂ1
.
15
10
5
For the solutions, the spectra were measured at room tempera-
ture in the range 4000–400 cmꢂ1, and at a concentration of
0.02 mol Lꢂ1 in solvents of different polarity, using an AgCl cell
with an optical path of 0.1 mm.
1H NMR spectra were recorded on a Bruker Avance II
400 MHz spectrometer using CDCl3. Chemical shifts are
reported in parts per million [ppm] downfield from TMS. Gas
chromatography–mass spectrometry (CG/MS) analyses were
performed in a Shimadzu CG-MS-QP 5050 spectrometer
equipped with a VF column, using helium as eluent at a flow
rate of 1.1 mL minꢂ1 and a heating ramp of 258C minꢂ1 from
1008 to 2808C. Mass spectra were obtained in electron-impact
mode (EI) with 70-eV ionization energy.
230 K
240 K
248 K
252 K
260 K
The gas-phase IR spectra were taken with a 10-cm-length
home-made stainless steel cell with Si windows at each end. The
steel reactor was surrounded with electrical tape and the tem-
perature was controlled with a thermocouple.
7
6
Computational Methods
[ppm]
Theoretical calculations were performed with GAUSSIAN 03,[30]
using DFT methods with 6-31G*, 6-31þG*, 6-311þG**,
aug-cc-pVDZ, and aug-cc-pVTZ basis sets.
Fig. 6. Low-temperature 1H NMR spectra at different temperatures and
with a small amount of water added.
Accessory Publication
solvent acts to weaken the C¼O stretching vibration, causing the
band to shift to lower frequencies, as has been clearly observed
for acetamide, N-methyl acetamide, dimethyl acetamide, etc.[15]
In the cases where the solvent is methanol or ethanol, two bands
are observed in the C¼O region. The band appearing as a
shoulder at higher frequencies could be associated with the
presence of the non-hydrated population, i.e. with the carbonyl
groups not forming hydrogen bonds with the solvent, as revealed
by the band position that coincides with the band in aprotic
solvents. However, as the existence of a doubly hydrated C¼O
population in some solvents such as methanol or ethanol
has been reported elsewhere,[15,19] we performed additional
experiments, recording 1H NMR spectra at seven different
temperatures (223, 225, 230, 240, 248, 252, and 260 K) in order
to disprove this assumption for CDFA (see Fig. 6).
Apart from the characteristic peaks corresponding to the
NH2 hydrogen atoms (with splitting due to the slower exchange
of the H atoms with the media), an extra signal appeared at
6.7 ppm as the temperature decreased. According to existing
data,[25] this signal could be assigned to the hydrogen atom
(from an OH group) bonded to the oxygen of the C¼O group,
forming an H bridge. The presence of only one signal rules out
the presence of a doubly hydrated C¼O population.
The following material is available online on the Journal’s
website: calculated frequencies for syn, anti, and gauche con-
formers (Fig. S1a, b, and c respectively) and infrared spectra
of the C¼O stretching band in acetonitrile solutions from
4.7 ꢄ 10ꢂ4 to 2.1 ꢄ 10ꢂ3 M (Fig. S2).
Acknowledgements
Financial support from CONICET, SECYT-UNC, and FONCYT is grate-
fully acknowledged. A.G.I. thanks Professor Clau´dio To´rmena.
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1
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