Article
pubs.acs.org/JPCA
IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters:
Enol−Keto Tautomers and Ion→Solvent Proton Transfer
Published as part of The Journal of Physical Chemistry virtual special issue “Veronica Vaida Festschrift”.
Daniel T. Mauney, Jonathon A. Maner, and Michael A. Duncan*
J. Phys. Chem. A 2017.121:7059-7069.
Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/06/19. For personal use only.
Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
S Supporting Information
*
ABSTRACT: Protonated ions of acetylacetone, H+(Hacac),
and their argon-tagged analogues are produced via a pulsed
discharge and cooled in a supersonic expansion. These ions are
mass analyzed, selected in a time-of-flight spectrometer, and
studied with infrared laser photodissociation spectroscopy
using the method of rare-gas atom tagging. Computational
studies at the DFT/B3LYP level are employed to elucidate the
structures and spectra of these ions, which are expected to exist
as either enol- or keto-based tautomers. The protonated
acetylacetone ion is found to form a single enol-based isomer.
Adding one or two water molecules to this ion, for example, H+(Hacac)(H2O)1,2, produces primarily enol-based structures,
although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen
bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H+(Hacac)(H2O)3 ion
causes a significant change in the spectroscopy, attributed to proton transfer from the H+(Hacac) ion into the water solvent.
■
INTRODUCTION
Proton transfer is a key process in many areas of chemistry and
biology including acid−base reactions, electrochemistry, and
photosynthesis.1−8 It is also the basis for chemical ionization
mass spectrometry,9,10 plays a major role in hydrogen fuel
cells,11,12 and has been proposed in mechanisms for
atmospheric and interstellar chemistry.13−18 In 1805, Grotthuss
described the process of proton transfer in liquid water,19−21
and solvent assisted proton transfer has been implicated in a
number of mechanisms across chemistry and biology.5−7,22,23
Therefore, the study of protonated systems and their solvation
is an area of great interest. A number of recent studies have
provided insight into the structure and solvation in hydrogenbonded systems using infrared spectroscopy of size-selected
protonated water clusters in the gas phase, along with high-level
computational chemistry.24−51 These studies allow the careful
selection of specific ion−molecule complexes with known
composition. In the current work, we use these methods to
investigate protonated acetylacetone, hereafter denoted
H+(Hacac), and its mixed H+(Hacac)(H2O)n clusters (n =
1−3).
Proton-bound dimers represent intermediates in proton
transfer. These systems have been studied extensively using
mass spectrometry to determine binding energies and
reactivities.52−55 A number of proton-bound dimers containing
various molecular partners have been studied using infrared
photodissociation spectroscopy.56−69 Johnson and co-workers
showed that the frequency of the characteristic vibration arising
from the motion of the shared proton is related to the
© 2017 American Chemical Society
difference between the proton affinities of the neutral
monomers involved.61 An interesting variation on this idea is
intramolecular proton sharing. Intramolecular hydrogen bonds
in neutral molecules are well-known noncovalent interactions
contributing to the structures of many biological systems.70−76
Numerous examples of these systems have been documented in
crystallography and investigated computationally. Johnson and
co-workers investigated an ionized example of this kind of
bonding in the infrared spectroscopy of protonated 1,8
disubstituted naphthalenes.77 Another class of well-studied
compounds that exhibit OH···OC intramolecular hydrogen
bonds are the β-diketones.73−76
Acetylacetone (Hacac, also known as 2,4-pentanedione) is
the simplest β-diketone. It exists in the two tautomeric forms
shown in Scheme 1. The enol form features an intramolecular
Scheme 1
Received: July 20, 2017
Published: August 30, 2017
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The Journal of Physical Chemistry A
hydrogen bond and resonance stabilization through a
conjugated π-system, whereas the diketo form contains two
carbonyl groups with an ∼140° dihedral angle between the
oxygens. These tautomers are distinguished easily using 1H
NMR spectroscopy.78,79 Measurements using temperaturedependent photoelectron and UV spectroscopies have shown
that the enol tautomer is lower in energy by ∼4 kcal/mol.80−83
The equilibrium has also been shown to shift depending on the
environment, with the enol form dominating in gas phase
measurements, while the keto form dominates in polar
hydrogen-bonding solvents.84 The protonated form of Hacac
has tautomers similar to the neutral, which should also be
sensitive to solvation. H+(Hacac) may form an enol-like species
with two equivalent carbonyl protonation sites or a keto-like
species with a single proton bridging the carbonyl groups. The
water solvation in the H+(Hacac)(H2O)2,3 complexes has been
investigated previously by Chang and co-workers with infrared
spectroscopy in the 2800−3800 cm−1 region.85 Here, we
expand the investigation of this system, employing argon
tagging methods and additional cluster sizes, while extending
the spectra into the fingerprint region.
■
Figure 1. Infrared photodissociation spectra for the H+(Hacac)(H2O)n
complexes for n = 0−3. Each spectrum was measured by selecting one
of the various H+(Hacac)(H2O)nAr ion masses and recording the
fragment channel corresponding to the elimination of argon.
EXPERIMENTAL SECTION
but only in the 2800−3800 cm−1 region.85 Our spectra here
have somewhat sharper bands with additional structure, but
they are otherwise consistent with the spectra of Chang, which
were measured via the elimination of water molecules. The
water-free ion has three vibrational bands in the high-frequency
region, where O−H stretches are expected, and the larger
complexes have more bands in this region at slightly higher
frequencies, where the O−H stretches of water might be
expected. All of these ions also have signal in the low-frequency
region near 1600 cm−1, where carbonyl stretches are expected.
Each of the ions having attached water exhibits one or more
broad resonances in the 2700−3500 cm−1 region of the
spectrum, indicative of hydrogen-bonding vibrations. The n = 3
spectrum has less signal in the low-frequency range, possibly
from a combination of low parent ion intensity and low
photodissociation yield.
To investigate the structures giving rise to these spectra, we
performed computational studies on each of these ions with
and without attached argon atoms. Multiple isomeric structures
were found lying close in energy for each complex. These
included the expected enol−keto structures as well as those
with different attachment sites for water in the hydrated species.
The full details of these computations are provided in the
Supporting Information for this paper. We number these
isomers such that the lowest enol structure for the cluster with
one water is “EW1,” whereas the second-lowest keto isomer
with two waters is “K2W2.” The lowest few isomers for each
cluster size and their relative energies are shown as insets in
Figures 2−5, and the relative energies are presented in Table 1.
The argon atoms were found to bind weakly to each of these
ions and, with the exception of the n = 0 complex, had little
effect on the positions of vibrational bands or relative energies
of isomers (see binding energies in Table 1 and Supporting
Information). The infrared absorption spectra for each of these
ions are compared to the respective infrared photodissociation
spectra in Figures 2−5.
A. H+(Hacac). Figure 2 shows the experimental spectrum
obtained for H+(Hacac) (black) versus the spectra predicted by
theory for the two lowest enol structures (E1, blue and E2,
green) and the lowest keto structure (K1, red). The
H+(Hacac) and mixed H+(Hacac)(H2O)n ions are produced in
a pulsed discharge/supersonic expansion of 10% H2 in Ar
seeded with both Hacac (ReagentPlus, ≥99%, Sigma-Aldrich)
and water at room temperature and ambient vapor pressure.
The ions are mass selected using a reflectron time-of-flight
spectrometer and interrogated using infrared photodissociation
spectroscopy.41,42,62−69 The energy absorbed in a single photon
at these frequencies is not enough to break the strong bonds
present in the smaller clusters, so rare-gas tagging is
employed.32,37−51 For this experiment, H+(Hacac)(H2O)nAr
(n = 0−3) ions are produced and mass selected, and absorption
of an IR photon causes the elimination of the argon tag atom.
The spectrum is recorded as the fragment ion yield versus the
laser frequency. The IR laser system used is an optical
parametric oscillator/amplifier system (OPO/OPA; LaserVision) equipped with an external AgGaSe2 crystal, pumped
by a Nd:YAG laser (Spectra Physics Pro-230). The spectra
were recorded from 1000 to 4000 cm−1.
Computational studies were performed at the DFT/B3LYP/
6-311+G** level of theory using the Gaussian09 program
package.86 These computations investigated each of the
H+(Hacac)(H2O)n ions with and without attached argon. The
energetics presented are corrected for the zero-point energies.
Vibrational spectra were scaled based on a comparison between
the calculated and known vibrational frequencies for acetone.87
We derived a factor of 0.965 for frequencies above 3000 cm−1, a
factor of 0.973 for frequencies between 1750 and 3000 cm−1,
and frequencies below 1750 cm−1 were not scaled.
■
RESULTS AND DISCUSSION
Figure 1 shows a comparison of the infrared spectra obtained
for the different sized H+(Hacac)(H2O)nAr complexes (n = 0−
3), each measured in the mass channel corresponding to the
elimination of argon. The top trace in black is the spectrum for
H+(Hacac)Ar, whereas the spectra for the H+(Hacac)(H2O)1−3Ar ions are in order from top to bottom in blue,
red, and green, respectively. The spectra for the n = 0 and 1
complexes have not been reported previously. The spectra for
the n = 2 and 3 ions were reported by Chang and co-workers,
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Figure 2. Infrared photodissociation spectra for the H+(Hacac)Ar ion compared to the spectra predicted by theory for several low-energy isomeric
structures.
cm−1 region of the spectrum that is predicted to be an intense
feature for all keto species. Therefore, we are confident in
assigning the infrared spectrum for the H+(Hacac)Ar species to
the lowest-energy E1 isomer. This isomer has two OH groups,
with one forming an intramolecular COH···OC hydrogen
bond. The argon binds on the bridging hydrogen. This and the
hydrogen-bonding interaction shifts the O−H stretch to lower
frequency (the 3326 cm−1 band), whereas the free O−H stretch
has a higher frequency (3520 cm−1). A second enol-type isomer
has two OH groups facing away from each other, and the two
O−H stretches have similar intensities. Even though E1 and E2
lie close in energy, we apparently do not have appreciable
amounts of E2, as it would produce additional O−H stretching
bands. Note that the agreement between theory and experiment for this ion is not quantitative regarding either band
positions or their relative intensities. The discrepancy in band
positions likely reflects the limitations of theory for this system.
The relative band intensities predicted for linear absorption
spectra may not match the experiment, because we measure
photodissociation yields. Interestingly, the infrared spectrum of
the neutral acetylacetone spectrum was recently studied in
solution and modeled computationally.82 Its shared proton
vibration was much broader in that environment due to both
thermal and dynamical effects that are not expected for these
gas-phase systems.
The observation of the enol structure for this cation is not
too surprising. Neutral Hacac has the enol structure in the gas
phase, and this configuration is stabilized by an intramolecular
shared proton between the two oxygen atoms. This neutral
enol structure is therefore likely to be already present in the
vapor that mixes with our expansion gases. We add both
hydrogen and water vapor to the discharge mix, and therefore
protonation of Hacac most likely occurs by proton transfer
from either H3+ or H3O+ ions in the discharge. The most
probable protonation site is one of the lone pairs of an oxygen.
Protonation here could occur without disrupting the shared
proton. The second-lowest-energy enol isomer of the
protonated ion (E2) does not have a shared proton, and
Table 1. Isomers of H+(Hacac)(H2O)n=0−3Ar
relative energya
(kcal/mol)
isomer
bare
w/Ar
Ar binding energy (cm−1)
E1
E2
K1
E3
K2
K3
EW1
EW2
EW3
KW1
KW2
E2W1
E2W2
K2W1
K2W2
K2W3
E3W1
E3W2
K3W1
K3W2
0.0
2.8
5.6
12.2
15.6
17.1
0.0
7.1
9.8
11.5
13.0
0.0
4.6
7.3
8.9
11.3
0.0
1.7
5.9
6.5
0.0
2.8
5.6
12.2
15.6
17.1
0.0
6.5
9.3
11.1
12.6
0.0
4.4
7.2
8.7
11.4
0.0
1.6
5.8
6.6
63
206
4
254
390
390
14
177
168
159
140
42
100
81
120
30
22
10
39
23
a
Calculated at the B3LYP level of theory using Gaussian09 with the 6311+G** basis set. Relative energies are zero-point energy (ZPE)
corrected.
experimental spectrum has two sharp resonances in the O−H
stretching region at 3326 and 3520 cm−1, along with a peak at
2933 cm−1, where C−H stretches are typically found. The
fingerprint region has three resolved peaks at 1253, 1523, and
1625 cm−1 and weak unresolved features between 1300 and
1500 cm−1. The band positions and their relative intensities in
both the OH and fingerprint regions are in reasonably good
agreement with those predicted for the lowest-energy enol
isomer. There is no carbonyl stretch band in the 1900−2100
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Figure 3. Infrared photodissociation spectra for the H+(Hacac)(H2O)3Ar ion compared to the spectra predicted by theory for several low-energy
isomeric structures. The intensities in predicted spectra above 3550 cm−1 were multiplied by 10× to make the weaker bands visible.
molecule (3663 and 3750 cm−1), the O−H stretch of the
intramolecular hydrogen bond (3437 cm−1), and the much
more intense O−H stretch of the hydrogen bond to water
(2902 cm−1). The O−H stretches of water are both predicted
within ∼20 cm−1 of the highest-frequency bands observed, but
the hydrogen-bonded stretch is predicted much lower than the
next-lowest feature. This is not too surprising, as the
frequencies of shared-proton and hydrogen-bonding bands
are often difficult to reproduce with theory.30−39,42,69,89
However, the number of bands here accounts for the three
high-frequency bands and the broad structure in the hydrogenbonding region, although the measured intensities are mostly
higher than those predicted. It is also not unusual for our
photodissociation action spectra to have high-frequency bands
more intense than those predicted for absorption spectra, as
seen here, and hydrogen-bonding bands with water are often
broadened significantly.42,69 Additionally, multiple C−H stretch
vibrations are also predicted to fall in the same region as the
hydrogen-bonding vibration, and couplings here may account
for some of the width of the experimental feature. The bands
predicted for this isomer in the fingerprint region also match
reasonably well with the pattern detected between 1300 and
1700 cm−1. However, while the lowest-energy isomer accounts
for most of the bands detected, it does not reproduce the
features at 1928 or 3246 cm−1. These bands suggest that other
isomers might be present.
The second lowest-energy EW2 isomer also has the enol
configuration, but without the intramolecular hydrogen bond. It
is analogous to the second-most stable isomer of the solventfree ion in Figure 2 but now with water attached to one of the
OH groups. Its spectrum is shown in the third trace in Figure 3
(red). This isomer has bands predicted for the symmetric and
asymmetric stretches of the water (3648 and 3734 cm−1), the
O−H stretch attached to argon (3583 cm−1), and the O−H
stretch in the hydrogen bond to water (3215 cm−1). The band
predicted at 3583 cm−1 falls near some of the extra structure
seen at high frequency in the experimental spectrum, and the
3215 cm−1 band matches reasonably well with the experimental
therefore its formation from the stable neutral would involve
breaking this hydrogen bond. As an interesting note, the lowest
keto isomer (K1) also has an intramolecular shared proton.
However, the formation of this isomer by protonation of the
corresponding neutral keto form is less likely. The neutral has
the two carbonyl groups misaligned from each other by ∼140°
(Scheme 1), and therefore both internal rotation and
protonation would be required to form the keto ion from the
keto neutral. There is also much less of the neutral keto species
in the gas phase.
B. H+(Hacac)(H2O). Figure 3 shows the experimental
spectrum measured for the H+(Hacac)(H2 O) complex
compared to the spectra predicted for its different isomers.
Adding water to the H+(Hacac) ion produces several new
features. The experimental spectrum exhibits three sharp peaks
in the O−H stretching region at 3563, 3645, and 3729 cm−1
along with a small resonance at 3246 cm−1, a broad feature
spanning from 2500 to 3100 cm−1, four bands in the fingerprint
region at 1335, 1425, 1573, and 1652 cm−1 similar to those
seen for the H+(Hacac) ion, and a small new peak at 1928
cm−1. The broad signal centered near 2900 cm−1 is in the
region typical of hydrogen bonding, as seen in protonated water
clusters.25−51 New bands in the O−H stretching region are
likely from the attached water. The most interesting new
feature is the weak band at 1928 cm−1, which is in the region
typically attributed to CO stretches. This kind of vibration
should only be seen if a keto isomer is present.
The spectra predicted for the lowest-energy isomers
identified by theory are presented in the lower traces of Figure
3. The most stable structure (EW1) has the enol configuration
seen for the solvent-free H+(Hacac), with a water molecule
hydrogen bonding to one OH group, and the other OH
forming the intramolecular hydrogen bond. This structure with
the proton on Hacac makes sense, because its proton affinity
(894 kJ/mol) is much greater than that of water (697 kJ/
mol).88 The spectrum predicted for this isomer is shown in the
second trace of the figure in blue. In the high-frequency region
it includes the symmetric and asymmetric stretches of the water
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Figure 4. Infrared photodissociation spectra for the H+(Hacac)(H2O)2Ar ion compared to the spectra predicted by theory for several low-energy
isomeric structures. The intensities in predicted spectra above 3500 cm−1 were multiplied by 10× to make the weaker bands visible.
band at 3246 cm−1. Likewise, the structure predicted in the lowfrequency region is quite similar to that in the experimental
spectrum and also might account for the extra bands here.
Therefore, it seems likely that there is a small admixture of this
isomer present in the experiment in addition to the one
predicted to be most stable. However, neither of the lowest two
isomers has a band in the region of the experimental peak at
1928 cm−1. Likewise, no other higher-energy enol-based
isomers are predicted to have bands in this region (see
Supporting Information). The only ion structure with a band
here is the KW1 species, whose spectrum is shown in the lower
trace of Figure 3 (green). The strong band predicted in this
spectrum at 2143 cm−1 is the OH−OH2 stretch involved in the
hydrogen bond with water, but now it is for the protonated
carbonyl group. The position predicted for this vibration is
significantly higher than that of the 1928 cm−1 band in the
experiment, but again we note that shared-proton vibrations
like this are notoriously difficult to handle with harmonic
theory,30−39 and no bands for other isomers occur in this
region. We therefore conclude that there is also an admixture of
a keto-type isomer present in the experiment in addition to the
more abundant enol species. The experimental spectrum
therefore represents primarily the EW1 structure but with
minor concentrations of the less stable EW2 and KW1 isomers.
It is not too surprising that these minor isomers could be
present. The discharge conditions in the ion source are
energetic enough to sample different ion structures, but then
the cooling in the supersonic expansion is rapid enough to
freeze in these structures and inhibit rearrangements that might
equilibrate to the lowest-energy structures.
An interesting aspect of this spectrum is that it apparently
contains vibrations associated with two different types of
shared-proton moieties. The 2900 cm−1 structure is assigned to
the OH+−OH2 configuration of the enol species, whereas the
1928 cm−1 band is assigned to the same kind of vibration for
the keto species. Not too surprisingly, the local chemical
environment has a significant effect on the proton stretch
vibration. This idea has been explored in recent studies of the
stretch vibration for a number of A−H+−B proton-bound
dimers. Johnson and co-workers showed that the frequency of
the shared-proton stretch in these systems is correlated to the
difference in proton affinity (ΔPA) of the two species
involved.61 With the accepted proton affinity of Hacac in its
gas-phase enol structure (874 kJ/mol),88 and that of water (697
kJ/mol), the ΔPA value for the enol−H+−OH2 is ∼177 kJ/
mol. According to the empirical plot in ref 61, the proton
stretch vibration for this system should be ∼2800 cm−1, in good
agreement with our experimental finding. A lower proton
stretch vibrational frequency, such as the value of 1928 cm−1 for
the keto−H+−OH2 moiety, indicates a lower proton affinity for
the keto binding site. The proton affinity here has not been
measured, but if we apply the plot of ref 61 in reverse, the
frequency measured here would indicate a ΔPA of ∼95−100
kJ/mol. This suggests that the proton affinity of the keto
tautomer is ∼800 kJ/mol, which is 74 kJ/mol less than the
accepted gas-phase value for the enol species. The proton
affinity at this keto site should be more relevant for work done
in polar solvents, where the keto form is favored.
C. H+(Hacac)(H2O)2. The experimental spectrum for the
tagged H+(Hacac)(H2O)2 complex compared to the spectra
predicted by theory is presented in Figure 4. In contrast to the
spectrum for the single-water complex, there are now two
broad, intense features in the hydrogen-bonding region. The
single-water complex had a broad but much weaker feature
here. This could indicate that there are multiple hydrogenbonding environments present, each with differing vibrational
frequencies. This spectrum is also different in the O−H
stretching region, with a more tightly grouped multiplet of
three bands instead of the more widely spaced bands in the
single-water complex. The multiplet of bands in the 1300−1600
cm−1 region is more similar to that in the single-water
spectrum, as is the band at 1915 cm−1, which is shifted only 13
cm−1 from the position of the 1928 cm−1 band in the single7063
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the two OH−OH2 proton stretches. These two bands line up
with the two broad hydrogen-bonding features at 2988 and
3208 cm−1, but the intensity ratios do not match those
observed. This isomer is therefore difficult to rule out
completely. It has several weak features in the 1300−1650
cm−1 region, which might also contribute to the unresolved
structure in the experiment. Therefore, the E2W2 isomer may
be present in some concentration, but by itself it cannot explain
the most obvious structure. Its predicted spectrum also fails to
account for the peak at 1915 cm−1.
Because no enol-based isomers have any resonances near the
1915 cm−1 band, we investigated keto-based structures. Two
such isomers are shown in Figure 4. The green trace shows the
spectrum for the K2W1 isomer with hydronium bridging the
carbonyl groups and an external water. This spectrum has two
bands in the O−H stretching region, corresponding to the
symmetric and asymmetric stretches of the water at 3674 and
3765 cm−1. A band at 3007 cm−1 comes from the symmetric
stretch of the hydronium moiety, the one at 2835 cm−1 comes
from the asymmetric stretch of two hydronium OH groups
against the carbonyl groups, and the one at 2780 cm−1 is the
OH−OH2 hydronium−water hydrogen-bond vibration. The
1714 cm−1 band is the scissors bend of the hydronium against
the two carbonyl groups. The high-frequency O−H stretches
for this structure fall in the same region as those for the enol
isomers, and so these bands could also conceivably contribute
to the structure measured in this region. The more intense
bands at lower frequency do not match anything in the
experiment, but each of these vibrations has a component of
shared-proton motion, and their frequencies may not be
described well by theory. Therefore, it does not appear that this
isomer makes a contribution to the spectrum, but it is also
difficult to rule it out completely. The purple trace corresponds
to the K2W2 isomer with a bridging Zundel-type water dimer
spanning across the two carbonyl groups. The three highfrequency bands at 3566, 3671, and 3729 cm−1 are all O−H
stretches on the water. The 2528 cm−1 band is a hydroniumlike hydrogen-bonding stretch, whereas the 2139 cm−1 band is a
Zundel-like shared-proton stretch. Both of these lowerfrequency vibrations involve shared-proton motions that are
difficult to handle with harmonic theory. The shared-proton
stretch predicted at 2139 cm−1 may well be lower in frequency,
perhaps explaining the 1915 cm−1 experimental band. This is
probably the best explanation for this band; the carbonyl
stretch is also predicted in this same frequency region, but its
IR intensity is much lower than that of the shared-proton
motion.
The infrared spectrum of this particular ion was studied
previously by Chang and co-workers.85 Their experiment did
not employ tagging but rather used the elimination of water
molecules to detect photodissociation, and their infrared laser
only covered the 2800−3800 cm−1 region. In the limited range
of their experiment, our spectrum is consistent with theirs,
although they did not detect the hydrogen-bonding feature we
see at 2988 cm−1. Their O−H stretch bands are also broader
than ours, presumably because of higher ion temperatures.
With the limited structure available in their spectrum, Chang
assigned it to a keto isomer with an open-chain water dimer
attached to one carbonyl group, even though this isomer was
computed to be less stable than others. However, as shown in
Figure S37 in the Supporting Information, we find that the full
IR spectrum of this isomer does not agree with the
experimental spectrum.
water complex, although there is additional weaker structure in
this region.
Theory suggests that there are several isomeric enol and keto
structures lying close in energy. The most stable E2W1
structure is again an enol species but with two water molecules
attached in a sequential arrangement at an OH protonation site,
with a separate OH−O intramolecular hydrogen bond. A
second-most stable enol structure (E2W2) has no intramolecular hydrogen bond, with a single water attached at
each of the two roughly equivalent OH sites. Two low-lying
keto isomers each have protonated water structures bridging
their carbonyl groups. In K2W1, a hydronium moiety forms
this bridge, with a water attached to it on the back side via a
hydrogen bond. In K2W2, a Zundel-like protonated water
dimer forms the bridge. It is shown in the figure in its
equilibrium hydronium-water configuration, but symmetry
suggests that vibrational averaging of the proton position may
occur between equivalent hydroniums as it does in the Zundel
ion.
As with the single-water complex, comparison with theory
indicates that this spectrum is not completely consistent with
any single isomer. However, the lowest-energy structure
(E2W1, blue trace) accounts for many of the experimental
bands. It is analogous to the lowest-energy structure for the
single-water complex (EW1, Figure 3, blue), with the second
water connected to the first via a hydrogen bond. In the O−H
stretching region, the predicted spectrum has a triplet of peaks
that match the pattern in the experiment reasonably well,
although each member of the triplet is ∼20 cm−1 higher than
the frequencies in the experiment. This same kind of offset was
noted above for the spectrum of the single-water complex. The
intensities of the predicted bands in this region are multiplied
by a factor of 10 to allow them to be seen in the figure. The
predicted spectrum also has two hydrogen-bond stretches at
3284 and 3407 cm−1 representing the shared proton between
the two water molecules and the intramolecular OH−O
vibration in the Hacac structure, respectively. These overlap
with the broad structure in the higher-energy part of the
hydrogen-bonding region. In the fingerprint region, the
spectrum for this isomer has bands at 1375 and 1637 cm−1
that match reasonably well with the more intense bands here.
However, the experiment again has a higher-energy band at
1915 cm−1 that is not present in the theory. The most
prominent feature in this region in the predicted spectrum is
the H-bond stretch at 2432 cm−1 coming from the shared
proton between water and Hacac. This band is well above the
experimental bands in the fingerprint region but well below
those in the hydrogen stretching region. Although it does not
match the experimental spectrum, harmonic theory can have
difficulty predicting these shared proton stretches, as noted
earlier. If we assume that this band is either very broad or
higher than predicted, it could explain the broad, weak
hydrogen-bonding structure below 2900 cm−1 or even perhaps
the more intense structure in the 2988 cm−1 region.
The next-lowest E2W2 structure (Figure 4, red) corresponds
to the water molecules each attaching to a separate OH from an
enol-based structure. Bands predicted at 3669 and 3756 cm−1
correspond to the symmetric and asymmetric stretches of the
water molecules. The doublet predicted here does not match
the experiment, but some contribution from these bands might
explain the unresolved doublet structure of the 3651 and 3747
cm−1 bands. The spectrum predicted also contains peaks at
3030 and 3309 cm−1 in the H-bonding region corresponding to
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Figure 5. Infrared photodissociation spectra for the H+(Hacac)(H2O)3Ar ion compared to the spectra predicted by theory for several low-energy
isomeric structures. The intensities in predicted spectra above 3550 cm−1 were multiplied by 10× to make the weaker bands visible.
Overall, as shown from this discussion, we find that the most
stable isomer (E2W1, Figure 4, blue trace) reproduces most of
the measured vibrational bands. Small admixtures of the other
isomers, particularly the K2W2 species, are necessary to
account for other features detected. It is clear that the exact
hydrogen-bonding frequencies for these various isomers are
quite important, but unfortunately these are not described well
enough by theory to give us more confidence in these
assignments.
D. H+(Hacac)(H2O)3. The experimental spectrum obtained
for the H+(Hacac)(H2O)3 ion is shown in Figure 5, where it is
compared to the spectra predicted by theory for different
isomeric structures of this ion. It is immediately clear that this
spectrum is significantly different from those of the smaller
complexes, with only a single main sharp band at high
frequency (3750 cm−1), a single feature at low frequency (1617
cm−1), and two broad bands in the hydrogen-bonding region
(3162 and 3363 cm−1). There are three much weaker peaks in
the high-frequency region. The hydrogen-bonding bands are
somewhat sharper than those for the other complexes, and
shifted to higher frequencies.
Just as for the smaller complexes, theory finds a number of
both enol and keto isomers lying close together in energy. The
structural patterns are similar to those for the H+(Hacac)(H2O)2 ion but with the additional water bound in different
hydrogen-bonding positions. As seen for the other complexes,
the enol-type isomers lie lower in energy than the keto forms.
In the higher-frequency free O−H stretching region, none of
the predicted spectra reproduce the experiment perfectly, but
the doublet for isomer E3W1 has the correct spacing between
two of the measured features and has a strong asymmetric
stretch band that matches the 3750 cm−1 reasonably well. In
the lower-frequency region, all of the enol or keto forms of this
complex have spectra dominated by the intense bands
associated with hydrogen-bonding vibrations on the water
molecules. Isomer E3W1 has a water attached to the enol OH
group, with two flanking neutral waters. It has a 1779 cm−1
band predicted for the OH−OH2 scissors vibration and a
doublet near 3354 cm−1 predicted for the symmetric and
asymmetric stretches of the water molecule involved in this
proton sharing. Isomer E3W2 has its three waters in a chain,
with intense OH−OH2 (2118 cm−1) and OH2−OH2 (3030
cm−1) hydrogen-bonding vibrations along the length of this
chain. Isomer K3W1 has a water dimer structure bridging the
carbonyl groups much like isomer K2W2, with strong
hydrogen-bonding vibrations (2429 and 2833 cm−1) associated
with these bridging waters. Isomer K3W2 has a hydronium-like
structure bridging the carbonyl groups, with a strong
hydronium stretch vibration at 2204 cm−1. Unfortunately, as
shown in Figure 5, none of the predicted spectra in the mid-IR
frequency range match the experiment. The same is true for the
low-frequency region, where all isomers have predicted spectra
with multiple bands, but the experiment has only a single peak
at 1617 cm−1. Chang and co-workers assigned their spectrum of
this ion in only the higher-frequency region to be that of a keto
isomer,85 but our data in a wider range shows that the spectrum
does not match those predicted for either enol or keto forms of
H+(Hacac)(H2O)3. We mentioned earlier how harmonic
theory has severe difficulties in the treatment of hydrogen
bonding and shared-proton vibrations. This ion is apparently
another example where such problems arise and apparently
become even more significant. We examined the present
H+(Hacac)(H2O)3 ion with dispersion-corrected density functional theory (DFT) and with MP2 calculations using
comparable basis sets and found no significant improvement
in the results.
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dynamics of each system. Each cluster also has free−OH
stretches at higher frequencies. The top trace shows the
spectrum for the H+(H2O)4 ion, which has three water
molecules surrounding a central hydronium, and which is
known as the “Eigen” ion.42,44,49 The second trace shows the
spectrum when this Eigen ion binds to a neutral benzene
molecule via an OH−π hydrogen bond (its vibration is the
3473 cm−1 band).69 The inductive forces from binding to
benzene result in stiffening of the hydrogen-bonding network,
and the vibrations shift to higher frequencies while maintaining
the same basic intensity pattern. A similar effect occurs when
hydronium is surrounded by three heavier nitrogen molecules
rather than water molecules (third trace).68 Here, the
hydrogen-bonding frequencies are even higher, while maintaining a similar intensity ratio between the two hydronium bands.
As shown, the spectrum of the H+(Hacac)(H2O)3 ion fits nicely
into this same sequence, with hydrogen-bonding vibrations that
are even higher in frequency than the others in the series.
We therefore suggest that the best assignment for the
spectrum of the H+(Hacac)(H2O)3 ion is to a structure like the
lowest-energy E3W1 species, but one in which the proton
involved in the OH−OH2 bond has transferred over to produce
a CO−OH3+(H2O)2 (solvated hydronium) configuration in the
water. This makes sense for several reasons. The spectrum for
the H+(Hacac)(H2O)3 ion changes significantly from those of
the smaller clusters, indicating the presence of a new infrared
chromophore. The E3W1 structure is otherwise computed to
be stable and differs from the proposed structure only in the
position of the bridging proton. The hydrogen-bonding pattern
fits the trend shown in Figure 6 for a solvated hydronium
species. It makes sense that a hydronium tethered to a carbonyl
group would be in a strong binding interaction, pushing the
hydrogen-bonding modes to higher frequencies. The E3W1
structure has a higher symmetry arrangement for the flanking
water molecules, which is consistent with the simple pattern in
the O−H stretching region. If the charge is on water, then it
makes sense that the water-based vibrations would be more
intense than those of the organic framework. This works for the
hydrogen-bonding vibrations but also for the hydronium bend
at 1617 cm−1. This scenario also makes sense energetically. The
proton affinity of the enol form of Hacac (874 kJ/mol) is
greater than that for a single water molecule (697 kJ/mol), and
consistent with this the smaller complexes here that have the
proton on the Hacac moiety. However, the proton affinity of
water clusters increases with their size, and that for the n = 3
cluster has been estimated to be 893 kJ/mol, favoring the
proton transfer into the water.92 Although the E3W1 isomer
works for this assignment, it is clear that we have lost spectral
information about the organic moiety and cannot distinguish
clearly between enol versus keto forms there.
Proton transfer processes are an appealing aspect of
molecular cluster science, and several examples of these
processes have been suggested to occur previously. For
example, in the case of ROH-H+−water mixtures, changes in
collisional fragmentation channels or infrared spectral features
were noted by several groups.28,93−95 However, the present
system provides more highly resolved spectroscopic features,
with clear evidence for changes in the infrared spectrum at a
specific cluster size. The present system also demonstrates the
complexity of vibrational patterns for intermediate-sized ions
undergoing solvation. Harmonic theory at its present level
cannot be expected to provide a clear picture of the vibrational
patterns in such systems, and full-dimensional anharmonic
The problems with harmonic theory for hydrogen-bonding
and shared-proton vibrations are now well-established for many
examples of protonated water clusters or their mixtures with
other hydrogen-bonding partners. Even for small protonated
water clusters such as the H+(H2O)n (n = 3,4) ions, strong
hydrogen-bonding vibrational bands are predicted by theory in
regions where no signal is observed.26−51 These small
protonated water clusters have now been examined with
anharmonic theory in different forms to investigate these
issues.90,91 The anharmonic studies demonstrate convincingly
how unreliable the harmonic calculations can be for such
systems. In larger systems like the ions described here,
anharmonic theory is simply not feasible. We encountered a
similar problem in the clusters of protonated water−benzene
mixtures.69 In that system, computations were insufficient to
explain the measured vibrational patterns, but we used
comparisons between the spectra for protonated water−
benzene mixtures and those for pure protonated water clusters
to analyze the patterns. A similar approach could be useful here,
because the infrared pattern for H+(Hacac)(H2O)3 looks very
much like those of other protonated water clusters we studied
previously.42,44,49,68,69 In particular, the free OH stretch, two
kinds of hydrogen-bonding vibrations, and the single band
where water bending is expected are all consistent with the
vibrations being localized on a protonated water center. To
illustrate this further, we compare the vibrational spectra of
other related protonated water clusters to that measured here
for the H+(Hacac)(H2O)3 ion in Figure 6. In each complex, the
Figure 6. Infrared photodissociation spectra for selected ions
containing hydronium in different environments compared to the
spectrum of the H+(Hacac)(H2O)3 ion.
cluster is interpreted to contain hydronium in slightly different
hydrogen-bonding environments, and its vibrations dominate
the spectrum. The hydrogen-bonding region has two broad
bands, with the lower-frequency asymmetric stretch of
hydronium more intense than its higher-frequency symmetric
stretch, although line widths vary because of the individual
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■
CONCLUSION
Complexes of protonated acetylacetone and water were
produced with a pulsed electrical discharge, and their infrared
spectra were obtained between 1000 and 4000 cm−1 using
mass-selected infrared photodissociation spectroscopy. The
experimental spectrum for H+(Hacac) shows that a single enolbased isomer can account for most of the spectrum, although
small concentrations of other isomers must also be present,
including keto-based structures. As this H+(Hacac) ion is
solvated with water molecules, the spectrum becomes more
complex, including different forms of hydrogen-bonding
vibrations that are not well-described by theory. Although
many spectral features are consistent with those predicted for
low-lying enol isomers, multiple isomers, including contributions from keto forms, are identified for the single- and doublewater complexes. The spectrum for the H+(Hacac)(H2O)3
complex changes significantly from those of the smaller
clusters, with virtually no active vibrations from the organic
framework of the molecule. Instead, new features are
characteristic of a protonated water cluster attached to the
organic scaffold. This pattern makes sense if the protonated
H+(Hacac) ion has transferred its proton into the solvating
water.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.7b07180.
Full details of the DFT computations done in support of
the spectroscopy presented here, including the structures,
energetics, vibrational frequencies, and predicted spectra
for each of the complexes considered. The full citation
for ref 86 is also given (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: maduncan@uga.edu.
ORCID
Michael A. Duncan: 0000-0003-4836-106X
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors would like to gratefully acknowledge funding for
this research by the National Science Foundation (Grant No.
CHE-1464708).
■
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