Vibrational spectroscopy and aromaticity investigation of squarate salts: A theoretical and experimental approach

Article abstract of DOI:10.1016/j.molstruc.2006.01.035

Experimental and theoretical investigations of squarate salts [M₂(C₄O₄)] (M=Li, Na, K and Rb) were performed aiming to correlate the structures, vibrational analysis and aromaticity. Powder X-ray diffraction data show that these compounds are not isostructural, indicating that the metal-squarate and hydrogen bonds to water molecules interactions play a significant role on the the crystal packing. The infrared and Raman assigments suggest an equalization of the C-C bond lengths with the increasing of the counter-ion size. This result is interpreted as an enhancement in the electronic delocalization and consequently in the degree of aromaticity for salts with larger ions. Quantum mechanical calculations for structures, vibrational spectra and aromaticity index are in agreement with experimental finding, giving insights at molecular level for the role played by distinct complexation modes to the observed properties. Comparison between our results and literature, regarding molecular dynamics in different chemical environments, shows that aromaticity and hydrogen bonds are the most important forces driving the interactions in the solid structures of squarate ion.

Full text of DOI:10.1016/j.molstruc.2006.01.035

Journal of Molecular Structure 794 (2006) 63–70
www.elsevier.com/locate/molstruc
Vibrational spectroscopy and aromaticity investigation
of squarate salts: A theoretical and experimental approach
a
Stefanos L. Georgopoulos , Renata Diniz , Maria I. Yoshida , Nivaldo L. Speziali ,
a
b
c
´
d
d
Helio F. Dos Santos , Georgia Maria A. Junqueira , Luiz F.C. de Oliveira
a,
*
´
´
a
´ ´ ´
Nucleo de Espectroscopia e Estrutura Molecular, Departamento de Quımica, ICE, Universidade Federal de Juiz de Fora, Campus Universitario,
Martelos, 36036-900 Juiz de Fora, MG, Brazil
´
Departamento de Quımica, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil
b
c
´
Departamento de Fısica, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil
´
d
´ ´
Nucleo de Estudos em Quımica Computacional, Departamento de Quımica, ICE, Universidade Federal de Juiz de Fora, 36036-900 Juiz de Fora, MG, Brazil
Received 7 December 2005; accepted 24 January 2006
Available online 3 March 2006
Abstract
Experimental and theoretical investigations of squarate salts [M2(C4O4)] (MZLi, Na, K and Rb) were performed aiming to correlate the
structures, vibrational analysis and aromaticity. Powder X-ray diffraction data show that these compounds are not isostructural, indicating that the
metal-squarate and hydrogen bonds to water molecules interactions play a significant role on the the crystal packing. The infrared and Raman
assigments suggest an equalization of the C–C bond lengths with the increasing of the counter-ion size. This result is interpreted as an
enhancement in the electronic delocalization and consequently in the degree of aromaticity for salts with larger ions. Quantum mechanical
calculations for structures, vibrational spectra and aromaticity index are in agreement with experimental finding, giving insights at molecular level
for the role played by distinct complexation modes to the observed properties. Comparison between our results and literature, regarding molecular
dynamics in different chemical environments, shows that aromaticity and hydrogen bonds are the most important forces driving the interactions in
the solid structures of squarate ion.
q 2006 Elsevier B.V. All rights reserved.
Keywords: Oxocarbon; Squarate; Raman; Aromaticity; DFT and NICS calculations
1. Introduction
previously described [18]. NaC and KC squarate salts present
different numbers of water molecules: three molecules for
Na2C4O4, which present two different crystalline phases
[19,20], and only one in K2C4O4 salt [21]. Another major
difference is observed in the crystalline system of these salts,
one phase of NaC salt crystallizes in a triclinic system and
Oxocarbon dianions [(CnOn)2K, where nZ3, deltate; nZ4,
squarate; nZ5, croconate and nZ6, rhodizonate] (Fig. 1) were
recognized by West [1] in the 1960 as a new group of organic
compounds. These ions in aqueous solution present Dnh
symmetry and high degree of electronic delocalization,
which have been subject of several studies, mainly concerning
structural [2–6] and spectroscopic investigations [7–10], in
addition to some theoretical works [11–16]. Most studies of
oxocarbons involve squarate and croconate ions. The crystal
structures of croconate salts [17] show that LiC, NaC and KC
complexes present two water molecules bonded per M2C5O5
unit, although RbC and CsC salts are anhydrous. Unit cell and
crystal structure of NaC, RbC, KC and CsC squarate salts were
ꢀ
space group P1 [19] and another in a monoclinic space group
C2/c [20], and KC salt in monoclinic space group C2/c [21].
These differences in the crystalline system show that the
change of cation in squarate salts can be responsible for some
significant modifications in the crystal packing.
Ito and West [22] have investigated Raman and infrared
spectroscopy of such a family, initially identifying that
croconate ion [(C5O5)2K] in aqueous solution presents D5h
symmetry, which is stabilized by resonance. This outcome was
also reached from infrared spectra of potassium croconate,
where the band around 1700 cmK1 assigned to the stretching of
isolated CO groups [n(CO)] was not observed. Instead, a broad
band was just found at lower wavenumbers, which is an
indicative of resonance in the croconate ring. Similar
*
Corresponding author. Tel.: C55 32 3229 3310; fax: C55 32 3229 3314.
E-mail address: luiz.oliveira@ufjf.edu.br (L.F.C. de Oliveira).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.01.035

64
S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
dominant in the ring), whereas positive values indicate anti-
aromaticity (paramagnetic or paratropic current dominates in
the ring). This metodology has been used to calculated the
˚
NICS value of oxocarbons at 1 A above the ring plane [26] and
found values of K11 to deltate ion, K8.6 to squarate ion, K5.8
to croconate ion and K4.4 to rodizonate ion. For comparison
purposes, the NICS value calculated for benzene was K10, and
the authors have concluded that deltate ion is actually the most
aromatic of the oxocarbons ions, and the aromaticity degree
decreases with the ring size in the oxocarbon series. The
aromaticity of oxocarbons is still not completely defined so far
and several works have been done in this attempt [9,23,26,27].
The aromaticity concept can be defined in several ways.
Nowadays, it is known that the planarity of conjugated systems
is not enough for a molecule to be aromatic, and a complete
analysis must be done including geometric, energetic and
magnetic parameters [28]. The vibrational spectroscopic
studies of aromaticity is concentrated at the 800–1680 cmK1
region, where it is observed the CC bond stretching [n(CC)]
(800–1200 and 1640–1680 cmK1) and CC aromatic stretching
[n(CC)ar] (1200–1640 cmK1). In this work, the experimental
and theoretical spectroscopy and structural properties are
obtained and aromaticity investigations of squarate salts
[M2(C4O4)] (where MZLiC, NaC, KC and RbC) carried out
with all data correlated to the structural features, trying to bring
some light in the discussion of the aromatic properties of
oxocarbon ions and their relations with the vibrational
spectrum.
Fig. 1. The main cyclic oxocarbon ions: (a) deltate; (b) squarate; (c) croconate
and (d) rhodizonate.
observation was done for squarate ion [(C4O4)2K], which also
showed a symmetric planar structure stabilized by resonance.
From the normal coordinate analysis of these dianions, it was
found that the CC bond order increases and the CO bond
decreases for larger rings whereas the force constants of kCC
decreases and the kCO increases from (C4O4)2K to (C5O5)2K
.
The average force constant kCC for oxocarbons
(w4 mdynes AK1) is smaller than that for benzene
2. Methodology and experimental section
˚
(w6 mdynes AK1), but is larger than those observed for a
˚
single C–C bond (w2 mdynes AK1). This comparative
2.1. Calculation methods
˚
analysis was very important to West and co-workers establish
the croconate and squarate ions as aromatic species. There have
been some works [23,24] in the literature regarding to electron
delocalization in oxocarbons raising many discussions about
the aromaticity of these compounds. In 1981, Aihara [23] used
MO calculations of diamagnetic susceptibility of dianions and
neutral oxocarbons, concluding that only deltate ion presents
high degree of aromaticity, and the other oxocarbon dianions
were considered not aromatic. However, in 1983, Herndon [24]
used valence bond theory (VB) to analyze the stabilization
aspects, as for instance resonance energy, charge distribution
and bond order quantities. All of the dianions were considered
stabilized by resonance and were assigned as aromatic,
whereas the neutral oxocarbon species were also considered
to be stabilized by resonance, but not aromatics. Recently,
Schleyer and co-workers [25] have adopted, as an aromatic
criterion, a method based on the negative chemical shift of the
Geometries and Raman and infrared vibrational frequencies
and intensities were obtained at B3LYP/6-311CG(d) level of
theory to the squarate salts [M2(C4O4)] (were MZLiC, NaC
and KC) and at B3LYP/6-311CG(d)/LANL2DZ level for
RbC4O4 with the effective core potential (ECP) LANL2DZ
used for the Rb atom. Aromaticity indexes were obtained by
NICS method (nucleus-independent chemical shifts) at
Hartree-Fock level and 6-31CG(d) basis set. All calculations
were carried out using the GAUSSIAN 03 program [29].
2.2. Syntheses
The syntheses of the compounds were done by neutraliz-
ation reaction of squaric acid (H2C4O4) to lithium, sodium and
potassium hydroxide. Rubidium squarate was obtained by
reaction of lithium squarate and rubidium chlorate. All the
reactions were done in aqueous solutions, and crystals were
obtained by slow evaporation at room temperature. X-ray
crystal structure of sodium [19,20] and potassium [21] salts are
described in literature, but unfortunately, attempts to obtain
single crystals suitable to X-ray diffraction analysis of lithium
and rubidium squarates were unfruitful.
˚
center of the rings and at 1 A above, in a methodology known
as nucleus-independent chemical shifts (NICS). This method
gives results in good agreement with energetic (energy of
aromatic stabilization), geometric (bond order) and magnetic
(diamagnetic susceptibility) criteria. Negative values of NICS
indicate aromaticity (diamagnetic or diatropic current are

S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
65
2.2.1. Lithium squarate
2.5. Thermogravimetric measurements
An aqueous solution containing 8.3 mmol of LiOH were
added to aqueous solution of H2C4O4 (4.2 mmol), obtaining
3.9 mmol of the salt (yield: 93.8%). Thermogravimetric
analysis: mass loss at 100 8C of 20%, showing the presence
of two water molecules. Li2C4O4$2H2O (161.95 g molK1),
calculated C 29.7 and H 2.5%; found C 32.16 and H 2.39%.
The thermogravimetric data were obtained in a Mettler
STARe TG50 thermobalance, the samples were heated at
10 8C/min from ambient temperature to 800 8C in atmospheric
air flow. Thermogravimetric curves are deposited as Sup-
plementary materials.
3. Results and discussion
2.2.2. Sodium squarate
An aqueous solution containing 2.5 mmol of NaOH were
added to aqueous solution of H2C4O4 (5.0 mmol), obtaining
2.3 mmol of the salt (yield: 92.7%). Thermogravimetric
analysis: mass loss at 100 8C of 25%, showing the presence
of three water molecules. Na2C4O4$3H2O (212.07 g molK1),
calculated C 22.7 and H 2.8%; found C 22.90 and H 2.39%.
3.1. Structural analysis
From the thermogravimetric analysis of the squarate
compounds associated to elemental analysis and previous
crystal structure investigations, it can be concluded that
lithium, sodium, potassium and rubidium salts present water
molecules in the crystal packing. This water content is related
to two main features: the specific crystal packing of each
compound and the hydrogen bonds formed between these
water molecules and the ions present in the structure.
Thermogravimetric analysis of squarate salts M2C4O4 (MZ
LiC, NaC, KC and RbC) indicates the existence of a
correlation between cation size and quantities of water
molecules in the structures. In the LiC and RbC salts, there
are two water molecules for each M2C4O4 unit, three in NaC
salt and only one in KC. These results suggest that not only
cation size is important in crystal packing, since intermolecular
interactions also demand a strong influence in the supramole-
cular structure. Another important discussion can be done on
the thermal degradation profile of each squarate compound,
since after water loss the anhydrous salts (M2C4O4) of sodium
and rubidium are more thermal stable than lithium and
potassium salts. Qualitative analysis of TG curves suggests
that the squarate salts decomposition yields a carbonate and
then decomposes in oxide after water loss. The complete
decomposition was observed for potassium squarate and
partially for lithium squarate. Sodium carbonate is very stable
with decomposition occurring around 1000 8C, and sodium
oxide was not observed in this TG curve obtained up to 800 8C.
Similar result was obtained for rubidium squarate.
2.2.3. Potassium squarate
An aqueous solution containing 7.2 mmol of KOH were
added to aqueous solution of H2C4O4 (3.6 mmol), obtaining
3.2 mmol of the salt (yield: 88.9%). Thermogravimetric
analysis: mass loss at 100 8C of 10%, showing the presence
of one water molecules. K2C4O4$H2O (208.25 g molK1),
calculated C 23.1 and H 0.96%; found C 22.94 and H 0.72%.
2.2.4. Rubidium squarate
An aqueous solution containing 3.2 mmol of RbCl were
added to aqueous solution of Li2C4O4 (1.6 mmol), obtaining
1.6 mmol of the salt (yield: 37.4%). Thermogravimetric
analysis: mass loss at 100 8C of 10%, showing the presence
of two water molecules. Rb2C4O4$2H2O (319.01 g molK1),
calculated C 15.1 and H 1.25%; found C 11.14 and H 1.38%.
2.3. Vibrational spectroscopy
Infrared spectra were obtained in Bomem MB-102
spectrometer with CsI beam spplitter, using CsI disks, spectral
resolution of 4 cmK1 and 128 scans were accumulated to
improve the signal-to-noise ratio. Fourier-transform Raman
spectroscopy was carried out using a Bruker RFS 100
instrument and a Nd3C/YAG laser operating at 1064 nm in
the near infrared and CCD detector cooled with liquid N2. To
improve the signal-to-noise ratio, 500–1000 scans were
accumulated over a period of about 30 min, using 4 cmK1 as
spectral resolution. All spectra were obtained more than two
times to show reproducibility, and no changes in band positions
and intensities were observed.
In a recent investigation by Braga and co-workers [17], the
crystal structures of LiC, NaC, KC, RbC and CsC croconate
compounds show that most of salts crystallizes in monoclinic
system and C2/c space group, with exception made to
potassium salt which crystallizes in orthorrombic system and
Pbcn space group. These results indicate the crystal system is
not significantly affected by the cation size, whereas it has a
significant influence in the amount of water molecules in the
crystals. On the other hand, similar results were not observed
for squarate compounds. X-ray crystal structures of NaC and
KC [19–21] show that these salts do not present the same
number of water molecules; for instance, NaC salt present
three water molecules per M2C4O4 unit and only one in KC
salts.
2.4. X-ray diffaction
Powder X-ray diffraction were carried out using RIGAKU
GEIGERFLEX 2037 with graphite monocromator [0002] and
˚
2dZ6.708 A, NaI sinthilator detector and Co Ka radiation
The obtained crystals of lithium and rubidium squarate do
not present adequate quality to be analyzed by single crystal
X-ray diffraction. For this reason, the powder diffraction
˚
(lZ1.789 A), except to rubidium salt that was used Cu Ka
K1
˚
(lZ1.541 A) and scans of 0.1 8 min
.

66
S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
Fig. 2. X-ray diffraction pattern of (Li2C4O4), (Na2C4O4), (K2C4O4) and
(Rb2C4O4).
patterns of squarate salts were obtained, indicating that these
salts present different solid states structures, as can be observed
in Fig. 2. Experimental diffraction pattern of sodium squarate is
similar to the one simulated for triclinic phase [19], thus
suggesting that this phase was obtained. Diffraction pattern of
potassium salt is similar to the one simulated from the crystal
structure described by Macintyre and Werkema [21]. Exper-
imental and simulated powder diffraction patterns of sodium
and potassium squarate salts are deposited as Supplementary
materials.
Fig. 3. Structure of [M2(C4O4)] complexes in (a) cis and (b) trans geometries.
becoming smaller with the increasing of cation size, except
for RbC salt, which presents the largest dhkl value. Squarate
ions in RbC salt are probably arranged horizontally translated
in despite of vertical translation, justifying the interplanar
distance observed. Analyzing the croconate salts [17] it can be
observed that the cation size is not directly related to the
interplanar distances. This parameter presents the same value
in LiC, KC and RbC salts (3.30 A), with the smallest value
˚
The analysis of interplanar distances is very important to
define the p-stacking interaction between oxocarbon rings in
the solid state structure. Although the cations are monovalent
ions MC, the diffraction pattern shows that the cation size plays
a significant role on crystal packing. Table 1 shows the
C
observed in NaC salt (3.12 A) and the largest in Cs salt
˚
˚
(3.42 A). Although sodium croconate presents the smallest
interplanar distance, the rings are horizontally translated
˚
around 3.0 A, which is the biggest translation for these salts.
Similar translation was observed for RbC and CsC croconate
diffraction angles and the respective interplanar distances (dhkl)
for the most intense peaks observed in the powder diffraction.
˚
salts (1.92 and 2.11 A).
C
The interplanar distance ranged from 3.14 A (Li salt) to
˚
Structural and spectroscopic theoretical studies were
performed for [M2C4O4] compounds with MZLiC, NaC,
KC and RbC in cis e trans arrangements and for squaric acid
[H2C4O4]. As it can be seen in Fig. 3, the counter-ions can
interact with (C4O4)2K ring structure in two different ways. In
the first, named cis-[M2C4O4], the squarate anion acts as a bis-
bidentated ligand, with the counter-ions sharing a common
oxygen atom of the ring. In the other geometrical form, trans-
[M2C4O4], the squarate also act as a bis-bidentated ligand, but
with the counter-ions lying on opposite sides of the ring.
Analyzing the structural results obtained for the cis structure,
given in Table 2, it is observed a systematic increasing in the
M–O bond length with of the counter-ion size (LiC, NaC, KC
and RbC). The O–M–O bond angles show a remarkable
decreasing from 106.78 (LiC) to 70.58 (RbC). On the other
C
2.84 A (K salt), whereas RbC salt presents the largest value
˚
˚
of dhkl (3.30 A). Comparing these data to crystal structure of
croconate salts [17] it is possible to infer that the distance
between the squarate ions planes in crystal packing are
Table 1
Diffraction angles (2qhkl) and respective interplanar distances (dhkl) for the most
intense peaks (Irelative (%)) of Li2C4O4, Na2C4O4, K2C4O4 and Rb2C4O4
a
˚
dhkl (A)
Compound
Li2C4O4
Irelative (%)
2qhkl (8)
100
63
33.2
35.3
37.6
35.5
40.3
32.0
33.6
36.7
38.5
27.0
38.6
3.14
2.95
2.78
2.94
2.60
3.25
3.10
2.84
2.72
3.30
2.33
58
Na2C4O4
K2C4O4
100
77
90
78
hand, C(1)–O(7) and C(2)–O(5) bond length present a small
variation (w0.02 A) from Li to RbC. Analyzing the C–C
100
61
C
˚
˚
bond lengths, the C(2)–C(3) bond decreases by about 0.03 A
Rb2C4O4
100
32
and the C(1)–C(2) was found to be slightly longer with the
cation size. These results are in agreement with a recent
theoretical investigation [14] carried out at MP2/6-31CG(d)
a
˚
Values obtained using Co Ka (lZ1.791 A), except to Rb2C4O4 where was
˚
used Cu Ka (lZ1.542 A).

S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
67
Table 2
Table 4
Geometric parameters obtained for the cis and trans-[M2(C4O4)] complexes
NICS values for cis and trans-[M2C4O4] compounds and H2C4O4 obtained at
˚
0.6 A above ring center
LiCa
NaCa,b
KCa,c
RbCa,d
[M2C4O4] LiC
NaC
KC
RbCa
H2C4O4
cis-[M2(C4O4)]
M–O(7)
1.946
1.904
1.316
1.259
1.202
1.421
1.535
106.7
2.258(2.419) 2.586(2.860) 2.843
2.222(2.419) 2.508(2.860) 2.754
1.309(1.260) 1.304(1.260) 1.297
1.256(1.260) 1.255(1.260) 1.254
1.209(1.260) 1.214(1.260) 1.219
1.439(1.471) 1.449(1.457) 1.457
1.520(1.471) 1.511(1.457) 1.506
cis
K9.8
K11.0
K10.0
K11.0
K10.1
K11.0
K10.3
K11.0
K8.3
–
M–O(5)
trans
C(1)–O(7)
C(2)–O(5)
C(3)–O(10)
C(1)–C(2)
C(2)–C(3)
O–M–O
a
For Rb the ECP LANL2DZ was used.
cis forms are larger than those calculated for the trans isomers,
which might favor the cis forms in solution due to electrostatic
interactions.
90.1
78.1
70.5
trans-[M2(C4O4)]
M–O
Despite the presence of the same oxocarbon structure in all
compounds, the significant change observed for the interplanar
distances as function of the cation size can affect the
electrostatic interactions and lead to variations in electronic
delocalization that can be related to aromaticity of squarate
ions in the solid structures. In order to quantify the degree of
aromaticity, values of NICS were obtained by calculating the
1.897
1.258
1.469
1.467
102.5
2.234(2.419) 2.544(2.860) 2.795
1.256(1.260) 1.256(1.260) 1.254
1.478(1.471) 1.480(1.457) 1.484
1.469(1.471) 1.471(1.457) 1.473
C–O
C(3)–C(4)
C(2)–C(3)
O–M–O
87.7
76.4
69.5
Values in parenthesis are the average parameter obtained from X-ray
diffraction data.
a
˚
absolute shielding constant at 0.6 A above the ring center in
Geometry was optimized at B3LYP/6-311CG(d) level of theory.
Experimental data from Ref. [19].
b
order to reduce the local shielding of nearby s-bond [30] for
M2C4O4 (MZLiC, NaC, KC and RbC) in the cis and trans
forms (Fig. 3). The results are reported in Table 4. According to
the values of calculated NICS at HF/6-31CG(d)//B3LYP/6-
311CG(d) level for the cis compounds an increasing of the
aromaticity is obtained with the size of the counter-ion, with a
constant value equal to K11 found for the trans isomers.
Additionally, it is worth mentioning that all NICS values of the
salts are greater than those for the squaric acid (K8.3),
showing that the complexes must be more aromatic than the
respective acid form.
c
Experimental data from Ref. [21].
d
The ECP LANL2DZ was used for Rb atom.
level, where the structural and spectroscopic aspects of free
squarate ion [(C4O4)2K] and in its cis salts with LiC, NaC and
KC) were studied. Analyzing the results for the trans isomers
reported in Table 2, it can be seen an enlargement of M–O bond
˚
(w0.8 A) with the cation size, similar to the cis form and a
small effect in the C–C and C–O bonds. The O–M–O bond
angles are smaller for bigger cations. Experimental structural
parameters obtained from X-ray diffraction of NaC[19] and
KC[21] salts show that the M–O bond length increases from
NaC to KC (Table 2). The average M–O bond lengths are
2.419 and 2.860 A for Na and KC salts, respectively. The
3.2. Vibrational spectroscopy
C
˚
The spectroscopic analysis was done for squaric acid and
squarate complexes with LiC, NaC, KC and RbC counter-
ions. The experimental results are shown in Table 5, and the
vibrational spectra are depicted in Fig. 4 (infrared) and Fig. 5
(Raman). As in this investigation, the main goal is the
discussion of the aromaticity of squarate salts, the most
important vibrational modes related to the electronic deloca-
lization are the stretching modes of CC and CO bonds, which
can be observed in the 1700–1500 and 1200–1000 cmK1
region, respectively.
˚
greatest difference between C–C bonds is 0.024 A for both
salts, even though the average C–C bond is slightly smaller in
C
KC salt (1.457 A) than in Na salt (1.471 A). As can observed
˚
˚
in the cis compounds, the average C–O bond lengths present
˚
similar values for both salts (1.260 A), although the difference
between the largest and the shortest bonds for KC salt is 0.004
C
and 0.018 A for Na . The cis/trans relative energy and the
˚
dipole moment for the complexes are given in Table 3. For all
investigated species the trans form is found to be favorable in
gas phase with lower values found for heavier alkaline metals.
However, it is important to note that the dipole moments of the
Ito and West [22], in the pioneer investigation of the
vibrational spectra of squarate ion in aqueous solution have
assigned the D4h symmetry to this ion and then, seven bands
are expected in the Raman spectrum, and the normal
coordinate analysis pointed out two CO stretching (n1, A1g
and n9, B2g), one CC stretching (n5, B1g), one ring breathing
(n2, A1g), two CO bending (n11, Eg and n6, B1g) and one ring
bending mode (n10, B2g). Crystal structure of sodium [19] and
potassium salts [21] show that in solid state the symmetry is
reduced; the factor group analysis indicate Ci1 and C26h
symmetries for potassium and sodium salts, respectively. In
addition, vibrational spectra (Raman or infrared) obtained in
the solid state present more bands than the observed in
aqueous solution spectra [31].
Table 3
Gas phase relative energy values (DE and DEZPE in kcal/mol) and dipole
moments (m in Debye) for the cis and trans-[M2(C4O4)] isomers calculated at
B3LYP/6-311CG(d) level of theory
m
DEcis–trans
DEcis–transC
ZPE
cis
trans
Li
10.2
13.9
16.5
19.5
0.0
0.0
0.0
0.0
8.8
6.4
5.9
6.0
8.4
6.2
5.8
6.2
Na
K
Rba
a
The ECP LANL2DZ was used for Rb atom.

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S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
Table 5
Experimental vibrational bands of squaric acid and squarate salts with respective tentative assignment
H2C4O4
Li2C4O4
Na2C4O4
K2C4O4
Rb2C4O4
IR
Tentative assign-
ment
IR
R
IR
R
IR
R
IR
R
R
1813m
1646m
1508s
1317s
1823w
1617w
1515w
1810w
1640s
1810w
1650s
1592w
1780w
1649m
1592m
1798w
1686m
1596m
1711m
1802w
1611m
1524w
1749w
1653w
1524s
1802w
1615m
n(CaO)
n(CaC)
n(CaC)Cn(CaO)
n(C–C)
1551s
1528s
1462w
1271w
1128w
1068m
1295w
1172m
1048w
n(C–C)
1166w
1050w
928w
850w
721w
632m
383m
1161s
1128w
1088m
1165m
1113s
1100m
1088m
1127s
1126s
n(C–C)
1075m
1091w
1098m
n(C–C)
n(C–C)
864w
721w
607m
374w
821m
n(C–C)
726s
635s
380s
722s
649s
326m
718s
641s
304m
745m
683m
343m
726s
651s
319m
726s
651s
317m
Ring breathing
dring
594m
363m
685w
347w
d(CO)
Initially analyzing the IR spectra, an intense band is
observed between 1510 and 1550 cmK1 assigned to a
n(CC)Cn(CO) mode. From the analysis of the 800–
1680 cmK1 region, it can be observed seven vibrational
bands for the H2C4O4, seven for Li2C4O4, five for Na2C4O4,
three for K2C4O4 and Rb2C4O4. The vibrational bands related
to the CC bonds close to 1600 cmK1 were not observed for the
K2C4O4 and also a smaller number of bands assigned to n(CC)
single and n(CC) aromatic was found. Analyzing the Raman
spectra, the overall trend is the same as for the IR. An important
aspect in the Raman analysis is that a smaller number of bands
for the squarate salts is observed when the size of the counter-
ion increases, which can be related to the higher symmetry,
when comparing squarate structure in all compounds. In other
words, it seems that the squarate ion is less disturbed by
electrostatic forces when the counter-ion is very large in
volume, as the case of rubidium ion. An important region for
analysis in the Raman spectrum is located close to 1200–
1000 cmK1, involving the n(CC) modes. The squaric acid
presents a band of medium intensity at 1171 cmK1 and another
band of low intensity at 1047 cmK1, shifted from each other by
124 cmK1. The Li2C4O4 spectrum shows bands at 1161 and
1075 cmK1, separated by 86 cmK1. In the Na2C4O4 spectrum,
three bands were observed in this region, a band of low
intensity at 1165 cmK1, a band of medium intensity at
1113 cmK1 and a band of low intensity at 1063 cmK1, with a
difference of 50 cmK1 between two consecutive bands. For the
K2C4O4 complex two bands are observed at 1127 cmK1 of
medium intensity and at 1090 cmK1 of low intensity, with the
separation between them equal to 37 cmK1. The Rb2C4O4 salt
presents only one strong band at 1126 cmK1. The important
outcome from the previous analysis is that the difference
between these bands becomes smaller from LiC to KC, and for
RbC salt only one band was observed. These facts strongly
suggest equalization in the CC bonds lengths, and then an
increasing in the symmetry of the squarate ion in solid state
when bigger counter-ions are present. These results are in
agreement with X-ray crystal structure of NaC and KC salts,
which shows that potassium salt ðC26hÞ is more symmetric than
sodium salt ðCi1Þ in solid state.
The theoretical vibrational spectra were calculated for cis
and trans-[M2C4O4] complexes, (MZLiC, NaCKC and RbC),
Fig. 4. Infrared spectra of squaric acid (H2C4O4) and Li2C4O4, Na2C4O4,
K2C4O4 and Rb2C4O4 salts.
Fig. 5. Raman spectra of squaric acid (H2C4O4) and Li2C4O4, Na2C4O4,
K2C4O4 and Rb2C4O4 salts.

S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
69
Fig. 6. Correlations between experimental and theoretical vibrational frequencies of H2C4O4, Li2C4O4, Na2C4O4 and K2C4O4 molecules. Infrared (a) cis and (b)
trans and Raman (c) cis and (d) trans geometries.
and also for the squaric acid [H2C4O4] for comparison purposes
(the calculated frequencies and intensities are available as
Supplementary material). Theoretical results are in good
agreement with the experiment regarding to the values of the
vibrational frequencies, intensities and assignments of the
vibrational modes. In Fig. 6, the calculated vibrational
wavenumber are plotted against the experimental data; the
correlations are fairly good with rO0.99. These results indicate
a good agreement between experimental and theoretical
frequencies, and then no scale factor was used. For all the
investigated compounds it is calculated a vibrational frequency
around 1500 cmK1, very intense in IR, assigned as n(CC)C
n(CO). The important aspects observed experimentally in the
Raman spectrum are also verified theoretically for the bands in
the 1200–1000 cmK1 region, relative to the n(CC) mode. In
this region, the difference between the vibrational frequencies
with highest and lowest Raman activity decreases, in the
sequence: [Li2C4O4] (1267, 1089 cmK1, DZ178 cmK1);
[Na2C4O4] (1199, 1080 cmK1, DZ119 cmK1) and [K2C4O4]
(1162, 1077 cmK1, DZ85 cmK1). These results are in
agreement with the experimental data, and support the trend
of the C–C bond lengths equalization, suggesting an increasing
in the symmetry of the squarate ion throughout the series
analyzed.
Ribeiro and co-workers [32], investigating the molecular
dynamics of squarate anion in acetronitrile solution, have
shown that there is a blue shift of the Raman wavenumbers of
the carbonyl stretching and ring breathing modes, when
compared with the aqueous solution results [33]. In a very
recent work, Cavalcante and Ribeiro [34] have shown, by
vibrational dephazing analysis, that croconate ion when
submitted to very different chemical environments such as
water and acetonitrile solutions and also the ionic liquid
croconate tetrabuthylammonium, the carbonyl stretching and
ring breathing modes are shifted. The authors claim this
behavior is due to very strong hydrogen bond interactions
acting on the dephasing dynamics of C5O25K ion, and it can be
seeing a similar behavior in the present investigation with
squarate salts in the solid state, which could support the
conclusions obtained by those authors. If one compares the
Raman wavenumbers for both vibrational modes (carbonyl
stretching and ring breathing) in the series going from rubidium
to lithium salts, which can be seen in Table 5, almost the same
trend is observed with a blue shift from lithium to rubidium
salt, and in these structures there are also water molecules
involved in intermolecular hydrogen bonds. Summing up,
aromaticity is one of the most important interactions acting in
the crystalline structure of the squarate salts, and together with

70
S.L. Georgopoulos et al. / Journal of Molecular Structure 794 (2006) 63–70
[3] S. Matheus, G. Paul, K. Shivasankar, A. Choudhury, C.N.R. Rao, J. Mol.
Struct. 641 (2002) 263.
hydrogen bonds plays an important role in the analysis of the
vibrational spectra of oxocarbon ions.
[4] C. Brouca-Cabarrecq, A. Mohanu, P. Millet, J.C. Trombe, J. Solid State
Chem. 177 (2004) 2575.
4. Conclusions
¨
[5] A. Bulut, I. Uc¸ar, O.Z. Yesilel, H. Ic¸budak, H. Olmez, O. Bu¨yu¨kgu¨ngor,
¨
¨
O. Bu¨yu¨kgu¨ngor, Acta Crystallogr. C 60 (2004) m526.
[6] J.A. Cowan, J.A.K. Howard, Acta Crystallogr. E 60 (2004) m511.
[7] P.S. Santos, O. Sala, L.K. Noda, N.S. Gonc¸alves, Spectrochim. Acta A 56
(2000) 1553.
In the present work, the investigation of the experimental
and theoretical aspects of the structure and spectroscopic
properties of several squarate salts was carried out. From the
analysis of these parameters it could be concluded that the
molecular symmetry and consequently the electronic deloca-
lization are higher for the salts with bigger counter-ions.
Powder diffraction data showed that the salts analyzed present
distinct solid states structures. In the solid state, the cation size
affects the distance between parallel planes of squarate ions
and the cation–anion bond length, playing an important role in
the interatomic and intermolecular interactions in solid state.
The difference in these interactions can modify the electronic
delocalization of squarate ions, and the compounds can present
different degrees of aromaticity. Vibrational spectra of LiC,
NaC, KC and RbC salts showed a decreasing in the number of
bands and coincident bands at IR and Raman spectra with the
enlargement of the cation size. This result indicates a higher
symmetry of these compounds, and might be evidence that the
degree of aromaticity is also increasing throughout the series.
Considering only geometric parameters, the increasing of
aromaticity is observed from the equalization of C–C and C–O
bond lengths, which also can be used to explain the observed
decreasing of the number of vibrational bands and coincident
bands in the IR-Raman spectra. Theoretical results were in
good agreement to the experimental data regarding to structure,
vibrational spectrum and aromaticity trend. According to NICS
calculation in the ring center of the compounds cis: K9.8
[Li2C4O4], K10.0 [Na2C4O4], K10.1 [K2C4O4], and K10.3
[Rb2C4O4] it could be observed the increasing of aromaticity
degree with the cation size, supporting the experimental
proposal.
[8] J.G.S. Lopes, L.F.C. de Oliveira, P.S. Santos, Spectrochim. Acta A 57
(2001) 399.
[9] P.S. Santos, N.S. Gonc¸alves, J. Mol. Struct. 570 (2001) 75.
[10] T.M. Kolev, B.A. Stamboliyska, D.Y. Yancheva, V. Enchev, J. Mol.
Struct. 691 (2004) 241.
[11] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, Phys.
Chem. Chem. Phys. 3 (2001) 3499.
[12] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, Phys.
Chem. Chem. Phys. 4 (2002) 2517.
[13] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, Phys.
Chem. Chem. Phys. 4 (2002) 2919.
[14] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, Phys.
Chem. Chem. Phys. 5 (2003) 437.
[15] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, J. Mol.
Struct. (Theochem) 684 (2004) 141.
[16] G.M.A. Junqueira, W.R. Rocha, W.B. Almeida, H.F. Dos Santos, J. Mol.
Struct. (Theochem) 719 (2005) 31.
[17] D. Braga, L. Maini, F. Grepioni, Chem. Eur. J. 8 (2002) 1804.
[18] K. Peters, E.M. Peters, H.G. von Schnering, Acta Crystallogr. A 34 (1978)
S101.
[19] A. Ranganathan, G.U. Kulkarni, J. Phys. Chem. 106 (2002) 7813.
[20] V. Busetti, F. Marcuzzi, Zeitsch. Krist. 212 (1997) 302.
[21] W.M. Macintyre, M.S. Werkema, J. Chem. Phys. 42 (1964) 3563.
[22] M. Ito, R. West, J. Am. Chem. Soc. 85 (1963) 2580.
[23] J. Aihara, J. Am. Chem. Soc. 103 (1981) 1633.
[24] W.C. Herdon, J. Mol. Struct. (Theochem) 103 (1983) 219.
[25] P. Von, R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao,
N.J.R.E. Hommes, J. Am. Chem. Soc. 118 (1996) 6317.
[26] P. Von, R. Schleyer, K. Najafian, B. Kiran, H. Jiao, J. Org. Chem. 65
(2000) 426.
`
[27] D. Quin˜onero, A. Frontera, P. Ballester, P.M. Deya, Tetrahedron Lett. 41
(2000) 2001.
[28] T.M. Krygowski, M.K. Cyranski, Chem. Rev. 101 (2001) 1385.
[29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant,
J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross,
C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma,
G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S.
Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng,
A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen,
M.W. Wong, C. Gonzalez, J.A. Pople, GAUSSIAN 03, Revision B.05,
Gaussian, Inc., Pittsburgh PA, 2003.
Acknowledgements
The authors are grateful to CNPq, CAPES and FAPEMIG
for financial support and research scholarships and to
´
Laboratorio de Espectroscopia Molecular-USP-Sa˜o Paulo for
the Raman facilities.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2006.01.
035.
[30] D. Quin˜onero, A. Frontera, P. Ballester, Tetrahedron Lett. 41 (2000)
2001.
[31] P.S. Santos, J.H. Amaral, L.F.C. de Oliveira, J. Mol. Struct. 243 (1991)
223.
References
[32] A.O. Cavalcante, S.M. Urahata, M.C.C. Ribeiro, Phys. Chem. Chem.
Phys. 6 (2005) 2956.
[1] R. West, H.Y. Niu, D.L. Powell, M.V. Evans, J. Am. Chem. Soc. 82
[33] M.C.C. Ribeiro, L.F.C. de Oliveira, P.S. Santos, Chem. Phys. 217 (1997)
71.
(1960) 6204.
[2] D. Braga, G. Cojazzi, L. Maini, F. Grepioni, New J. Chem. 25 (2001)
1221.
[34] A.O. Cavalcante, M.C.C. Ribeiro, J. Raman Spectrosc. 36 (2005) 996.