A short hydrogen bond investigation by polarized Raman spectra of Co2+ and Zn2+ salts of pyromellitic acid

Article abstract of DOI:10.1016/j.saa.2006.07.025

Cobalt and zinc salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C₆H₂(COO)₄H₄], have been synthesized and investigate by polarized Raman spectroscopy. These compounds present short intramolecular hydrogen bonds (SHB) between adjacent carboxyl groups. Raman spectra indicate the presence of this interaction in both salts. Three specific vibrational of SHB modes have been investigated: O-H-O symmetric [ν_sym(OHO)] and asymmetric [ν_asym(OHO)] stretching modes and O-H stretching mode [ν(O-H)], which they were observed around 300, 850 and 2500 cm^-1, respectively. In crystallographic point of view, the cobalt salt presents a symmetric SHB while the zinc salt presents an asymmetric SHB. In cobalt salt all three vibrational modes of O-H-O groups in polarized Raman spectra occur in A_g orientation although in zinc salts two of them are observed in A_g orientation and one in B_g. Spectra analysis indicate that ν_sym(OHO) mode is observed as A_g to cobalt salt and B_g to zinc salt. This mode occurs in a crowed spectral region and its identification was made by deconvolution techniques. Comparing spectra of the two salts, it is observed a small difference in relative intensity and wavenumber shift of ν_sym(OHO) (deviance of 43 cm^-1) and ν(OH) (deviance of 21 cm^-1) modes due probably to differences in O...O distance between salts and in orientation of pyromellitate anion in unit cell. The ν_asym(OHO) mode does not present significant wavenumber shift due difference in SHB. The ??(OH) band presents a great potential for hydrogen bond studies due to the fact that in its vibrational region (around 2500 cm^-1) it is not observed other vibrational modes of these compounds.

Full text of DOI:10.1016/j.saa.2006.07.025

Spectrochimica Acta Part A 67 (2007) 372–377
A short hydrogen bond investigation by polarized Raman spectra of
Co2+ and Zn2+ salts of pyromellitic acid
Renata Diniza,1, Maria S. Dantasb, Nelson G. Fernandesa, Maria T.C. Sansivieroa,∗
a Departamento de Qu´ımica, ICEx, Universidade Federal de Minas Gerais, Av. Antoˆnio Carlos, 6627, 31270-901 Belo Horizonte-MG, Brazil
b Departamento de F´ısica, ICEx, Universidade Federal de Minas Gerais, Av. Antoˆnio Carlos, 6627, 31270-901 Belo Horizonte-MG, Brazil
Received 14 March 2006; received in revised form 14 July 2006; accepted 19 July 2006
Abstract
Cobalt and zinc salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C6H2(COO)4H4], have been synthesized and investigate by
polarized Raman spectroscopy. These compounds present short intramolecular hydrogen bonds (SHB) between adjacent carboxyl groups. Raman
spectra indicate the presence of this interaction in both salts. Three specific vibrational of SHB modes have been investigated: O–H–O symmetric
[νsym(OHO)] and asymmetric [νasym(OHO)] stretching modes and O–H stretching mode [ν(O–H)], which they were observed around 300, 850 and
2500 cm−1, respectively. In crystallographic point of view, the cobalt salt presents a symmetric SHB while the zinc salt presents an asymmetric
SHB. In cobalt salt all three vibrational modes of O–H–O groups in polarized Raman spectra occur in Ag orientation although in zinc salts two
of them are observed in Ag orientation and one in Bg. Spectra analysis indicate that νsym(OHO) mode is observed as Ag to cobalt salt and Bg to
zinc salt. This mode occurs in a crowed spectral region and its identification was made by deconvolution techniques. Comparing spectra of the
two salts, it is observed a small difference in relative intensity and wavenumber shift of νsym(OHO) (deviance of 43 cm−1) and ν(OH) (deviance of
21 cm−1) modes due probably to differences in O· · ·O distance between salts and in orientation of pyromellitate anion in unit cell. The νasym(OHO)
mode does not present significant wavenumber shift due difference in SHB. The ν(OH) band presents a great potential for hydrogen bond studies
due to the fact that in its vibrational region (around 2500 cm−1) it is not observed other vibrational modes of these compounds.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Short hydrogen bond; Raman spectroscopy; Pyromellitate salts
1. Introduction
related, where H atom is located in a special position (two-fold
axe, inversion center or mirror plane). In asymmetric HB the
atoms are symmetrically independent. Crystal structure of acid
salts of pyromellitic acid shows the predominance of asymmet-
ric intramolecular SHB interaction, therefore, these compounds
become very interesting systems for charge density studies [17].
Vibrational studies of HB are concentrated in infrared spec-
troscopy (IR), mainly in O–H stretching mode [ν(OH)]. How-
ever, at ν(OH) region, very broad bands appear in IR spectra
of hydrate compounds, complicating the assignment of other
HB modes. As vibrational modes of water molecules are, in
general, very weak in Raman spectra, they do not perturb the
analysis of HB bonds in hydrate polycarboxylic compounds
[18]. Reid [19] interpreted the changes in the infrared spectra by
making a semiempirical treatment of the hydrogen bond based
on the Lippincott and Schroeder potential [20]. This treatment
used a harmonic oscillator approximation to calculate the O· · ·O
stretching frequency with respect to the O· · ·O equilibrium dis-
Polycarboxylic acids are important systems to hydrogen
bonding studies [1–10]. Anions of these acids can form
intramolecular hydrogen bonds (HB), as can be seen in Fig. 1. In
particular, the divalent anion of 1,2,4,5-benzenetetracarboxylic
acid (pyromellitic acid) has adjacent carboxyl groups that favour
intramolecular short hydrogen bond (SHB) formation. Several
X-ray structural studies of its derivative compounds show SHB
interaction, mainly in divalent anion [11–16].
In crystallographic point of view, hydrogen bonds can be
classified as symmetric and asymmetric one. Symmetric HB is
the interaction observed between atoms that are symmetrically
∗
Corresponding author. Tel.: +55 31 3499 5723; fax: +55 31 3499 5700.
E-mail address: mtcaruso@netuno.lcc.ufmg.br (M.T.C. Sansiviero).
Present address: Departamento de Qu´ımica, ICE, Universidade Federal de
1
Juiz de Fora, Campus Universita´rio, Bairro Martelos, 36036-900 Juiz de Fora-
MG, Brazil.
˚
tance. For a short O· · ·O distance (<2.5 A), a frequency around
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.07.025

R. Diniz et al. / Spectrochimica Acta Part A 67 (2007) 372–377
373
Fig. 1. Representation of short intramolecular hydrogen bond in policarboxylic
acids.
Fig. 2. Raman spectra of Ag and Bg symmetry orientations of CoH2Bt. The
symbol * refers to νsym(OHO), ᭹ to νasym(OHO), and # to ν(OH).
850 cm−1 was found. Miller at al. [21] studied the O· · ·H· · ·O
hydrogenbondinCsH(CF3CO2)2 andKH(CF3CO2)2, thatshow
very short O· · ·O distances. Using Raman spectroscopy, they
assigned the O· · ·H· · ·O mode at 798 and 792 cm−1 for Cs and
K salts, respectively. Hadzi et al. [2] have also discussed the
Raman and infrared spectra of several MH(CX3COO)2 species
and assigned values of 720 and 320 cm−1 for the νasym(OHO)
and νsym(OHO) bands, respectively. In recent years, some stud-
ies of polarized vibrational spectra of compounds presenting
HB have been reported [22–25]. In these works the focus of
HB investigation was on the ABC band structures [26] and
they have investigated the transition dipole moment orientation
of HB modes, mainly for ν(OH) and OH deformations. The
ABC band structures are related to Fermi resonance between the
fundamentals ν(OH) modes of HB and overtones of the bend-
ing modes [27]. Spectra of a hydrogen-bonding system show
broader, more intense and shifted bands than non-hydrogen-
bonding systems [28]. The shift of frequency of ν(OH) is corre-
lated to O· · ·O bond distances [29]. For SHB, this mode occurs
in the region of 2000–2500 cm−1. However, for medium and
weak HB this mode is found in the region of 3500–3000 cm−1
[29].
hexaaquazinc(II) dihydrogen 1,2,4,5-benzenetetracarboxylate
[ZnH2Bt]. Powder Raman spectra of these compounds and their
deuterated analogues were analyzed by our group [18,30]. The
three main short hydrogen bond modes were observed around
300, 850 and 2500 cm−1 assigned to the following SHB stretch-
ing modes, νsym(OHO), νasym(OHO) and ν(OH) respectively.
The wavenumbers of SHB bands obtained from Raman powder
data are similar in both salts, even though the different sym-
metries of SHB observed in crystalline structures. Polarized
Raman spectra allow a better assignment of the SHB vibrational
bands and can be an interesting tool in its spectral differen-
tiation.
2. Experimental
CoH2Bt and ZnH2Bt single crystals were obtained by
slowly evaporation at room temperature from aqueous solu-
tion of cobalt/zinc(II) nitrate hexahydrate (Aldrich 23,037-5,
99%/22,873-7, 98%) and pyromellitic acid (Aldrich B400-
7, 96%). Single crystals were oriented by X-ray diffraction
method. Raman spectra were recorded on microRaman DILOR
X-Y in backscattering configuration and triple monochroma-
tor equipped with a CCD detector at room temperature. The
514.5 nmlineofanargonionlaserwasusedasexcitingradiation.
The measurements have been performed at room temperature in
In the present work, the stretching modes of O H O groups
of the SHB interaction have been investigated by polarized
Raman spectroscopy using single crystals of hexaaquacobalt(II)
dihydrogen 1,2,4,5-benzenetetracarboxylate [CoH2Bt] and
Table 1
Correlation diagram and analysis of the fundamental modes of CoH2Bt and ZnH2Bt
C2h
Internal modes
Salt
Raman selection rules
H2BT2−
CoH2Bt
SHB
CoH2Bt
ZnH2Bt
ZnH2Bt
CoH2Bt
ZnH2Bt
Ag
Bg
Au
Bu
28
26
26
31
28
28
27
28
15
15
14
15
15
15
15
15
3
0
2
1
2
1
2
1
(xx), (yy), (zz), (xz)
(xy), (zy)
Inactive
Inactive
SHB: short hydrogen bond modes.

374
R. Diniz et al. / Spectrochimica Acta Part A 67 (2007) 372–377
Fig. 3. Raman spectra of Ag and Bg symmetry orientations of ZnH2Bt. The
symbols are defined as Fig. 2.
the region 150–950 and 2300–2900 cm−1 region for scattering
geometry z(xx)z, z(yy)z, y(zz)y, y(xz)y, z(xy)z and x(yz)x that cor-
respond to Ag (the first four) and Bg (the last two) symmetries.
Thex, yandzorthogonalcoordinatesystemwaschoseninaman-
ner that the y-axis were parallel to b crystallographic axis. The
dissensions of single crystals are about 2 mm × 5 mm × 2 mm
for CoH2BT (prism form) and 3 mm × 2 mm × 5 mm (hexago-
nal distorted form) for ZnH2BT in the a, b, and c axes, respec-
tively. Discussions of vibrational results were made using the
factor group analysis of the crystals [31]. The tentative assign-
ments of bands were done by deconvolution techniques and
previous ab initio calculations [18,30].
3. Results and discussion
Polarized Raman spectra for z(xx)z, z(yy)z, y(zz)y, y(xz)y,
and z(xy)z and x(yz)x scattering geometry of CoH2BT and
ZnH2BT are shown in Figs. 2 and 3, respectively. Single crys-
tal X-ray diffraction data show that CoH2Bt [11] and ZnH2Bt
[16] are ionic, meaning that in both cases the metal is coordi-
nated to water molecules only. They crystallize in monoclinic
system, P2/m and C2/c space groups, respectively. These two
space groups are correlated to C21h and C26h symmetry group,
respectively. Factor group analyses of CoH2Bt and ZnH2Bt
crystals show 1 1 1 normal modes of vibration (excluding the
acoustical modes), where 51 from cation ([M(H2O)6]2+) and
60 from anion (H2Bt2−). In CoH2Bt these modes are dis-
tributed in Γ = 28Ag+26Bg+26Au+31Bu, and for ZnH2Bt in
Γ = 28Ag+28Bg+27Au+28Bu. In this group there are four sym-
metry types: Ag and Bg (Raman active) and Au and Bu (IR
active). The Ag symmetry is observed in z(xx)z, z(yy)z, y(zz)y and
y(xz)y orientations, while Bg symmetry is observed at z(xy)z and
x(yz)x orientations. Table 1 displays the classification and selec-
tion rules for fundamental modes of the salts. Selected Raman
frequencies of cobalt and zinc salts in powder and in crystal with
different orientations are listed in Table 2. In Table 3 are listed
the SHB geometrical parameters of CoH2Bt and ZnH2Bt.

R. Diniz et al. / Spectrochimica Acta Part A 67 (2007) 372–377
375
Table 3
Geometrical parameters of short hydrogen bond in CoH2Bt and ZnH2Bt
O–H–O (◦)
˚
˚
Salt
O· · ·O (A)
O–H/H–O (A)
CoH2Bt [11]
ZnH2Bt [16]
2.381(2)
2.413(2)
1.211(7)
1.06(4)/1.36(4)
159(2)
171(4)
In CoH2Bt, a mirror plane is perpendicular to intramolecu-
˚
lar SHB interaction and the O· · ·O distance is 2.381(2) A. The
H atoms of SHB are located in mirror plane symmetry and
this interaction is oriented parallel to b crystallographic axis.
In ZnH2Bt the SHB is asymmetric which O· · ·O distance of
˚
2.413(2) A. These SHB are oriented almost along to a crystal-
lographic axis.
3.1. Short hydrogen bond vibrations
Fig. 4. Raman spectra around νsym(OHO) band of CoH2Bt. The symbol * refers
to νsym(OHO).
To both salts, three SHB fundamental modes present Raman
activity. As can be observed in Table 1, in CoH2Bt it is
expected all of them in Ag spectra, although in ZnH2Bt only
two are expected. Semi-empirical calculations indicate that
stretching modes of O–H–O groups are related to O· · ·O dis-
tances [19]. In systems that show SHB (O· · ·O distance between
parallel to b crystallographic axis although in zinc salt is almost
perpendicular to the same axis.
The vibrational region of νsym(OHO) (∼300 cm−1) is very
complicated to be analyzed due the presence of deformation
modes of aromatic ring and water molecules. Due this fact,
deconvolution methods were used in single crystal polarized
Raman spectra to help the tentative assignment of this mode.
These analyses are shown in Figs. 4 and 5 for CoH2Bt and
ZnH2Bt, respectively. InCoH2Btspectrawasidentifiedabandin
this region in both symmetry orientation (Ag and Bg). Deconvo-
lution treatment (Fig. 4) shows that in Ag symmetry orientation
˚
2.4 and 2.5 A) the symmetric [νsym(OHO)] and asymmetric
[νasym(OHO)] stretching modes are expected at about 300 and
850 cm−1, respectively, and the OH stretching [ν(OH)] at around
2500 cm−1. In powder Raman spectra of cobalt [18] and zinc
[30] salts, weak and broad bands were observed in these regions
(Table 3).
The best region of hydrogen bond investigation is that related
to O–H stretching mode, which is very sensitive to formation of
this kind of interaction and it is less influenced by other modes
in these compounds. This mode occurs only in Ag orientation
at 2572 and 2593 cm−1 to CoH2Bt and ZnH2Bt, respectively.
In cobalt salt, this mode has a smaller wavenumber than zinc
salt (21 cm−1). These results probably are more related to dif-
ference in O· · ·O distance of SHB than to difference in their
crystallographic symmetry. This distance in Co salts (2.381(2)
two bands are present, one at 277 cm−1 and another at 300 cm−1
.
In Bg orientation just one band at 304 cm−1 is observed. Crystal
data show that in this salt the SHB is symmetric and factor group
analysis (Table 1) shows that in cobalt salt all SHB modes are
Ag. The band shape of Ag orientation is more similar to that
observed in powder spectrum, and due these facts, the weak and
broad band observed in Ag orientation at 277 cm−1 was identi-
˚
˚
A) is smaller than that observed in ZnH2Bt (2.413(2) A). This
band in CoH2Bt does not present a significant intensification in
the different Ag orientations, however in ZnH2Bt is observed
intensification in z(yy)z orientation. This effect could be related
to difference of anions orientation in crystal cell. In powder
Raman spectra of this compounds, the ν(OH) of medium HB
is observed at 3450 and 3271 cm−1 in CoH2Bt and ZnH2Bt,
respectively [18,30].
The νasym(OHO) was also observed in Ag orientation at
858 and 861 cm−1 for CoH2Bt and ZnH2Bt, respectively. Sim-
ilar band was not observed in Bg orientation. No significant
wavenumber shift is observed to νasym(OHO) due to crystal-
lographic difference in SHB. In both compounds this mode
presents intensification in some orientations. In CoH2Bt this
bandismoreintensein z(xx)zandz(yy)z, althoughinZnH2Btthis
intensification was just observed in z(yy)z. Similar to ν(OH), this
effect can be associated to the different orientation of pyromel-
litate ion in crystal cells. In cobalt salt the O–H–O groups are
Fig. 5. Raman spectra of ZnH2Bt in νsym(OHO) band region. The symbol *
refers to νsym(OHO).

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R. Diniz et al. / Spectrochimica Acta Part A 67 (2007) 372–377
fied as νsym(OHO) mode. The band observed at 304 cm−1 in Bg
orientation was tentative assigned as aromatic ring deformation
mode [δ(φ)]. As can be seen in Table 1, one of the SHB modes
in ZnH2Bt is expected in Bg orientation. Two SHB modes were
identified in Ag spectra [νasym(OHO) at 856 cm−1 and ν(OH)
at 2593 cm−1] and it will be suppose that in this compound the
νsym(OHO) would be observed at Bg spectra. In ZnH2Bt a weak
broad band, similar to cobalt salt, is observed at 320 cm−1 in
Bg spectra (Fig. 5). Ag spectra [y(zz)y and x(yz)x] present bands
in the same region but the shape and intensity are different to a
typical O–H band.
Theνsym(OHO)modeofcobaltandzincsaltspresentnotonly
a shift in wavenumber values (around 43 cm−1), like observed in
νasym(OHO), but in its symmetry too. In CoH2Bt this stretching
is Ag and the SHB is symmetric, although in ZnH2Bt this mode
is observed at Bg spectra and the SHB is asymmetric. Similar
to that observed in ν(OH), CoH2Bt spectra present νsym(OHO)
mode in lower wavenumber than in ZnH2Bt spectra, due mainly
to the difference in O· · ·O distance of SHB.
Ag, however the νsym(OHO) is Ag in CoH2Bt and Bg in ZnH2
Bt.
Small differences are observed in relative intensity and
wavenumber shift of νsym(OHO) and ν(OH) modes. Compar-
ing ZnH2Bt spectra it can be seen a great intensification of
SHB bands. The νsym(OHO) band is more intense in y(zz)y and
y(xz)y orientations although νasym(OHO) and ν(OH) bands are
intensified in z(yy)z orientation. On the other hand, in CoH2Bt
spectra only ν(OH) band showed a small intensification in z(xx)z
orientation. This difference in intensity could be related to differ-
ent orientations of pyromellitate anions in crystal cell. Another
difference in Raman spectra of these compounds is the shift
in wavenumber in νsym(OHO) and ν(OH) bands. In CoH2Bt
these modes occur in smaller wavenumber (277 and 2572 cm−1
)
than in ZnH2Bt (317 and 2593 cm−1). The wavenumber shifts
observed to these modes (40 and 21 cm−1, respectively) are sig-
nificant and suggest that occurs due the difference in O· · ·O
˚
˚
distance (2.381(2) A to CoH2Bt and 2.413(2) A to ZnH2Bt)
than to difference of SHB symmetry.
Acknowledgments
3.2. Other internal vibrations
˜
We are grateful to Dr. Marcos Assunc¸ao Pimenta to per-
Some vibrational frequencies and tentative assignments
are listed in Table 2. In the vibrational region analyzed
(200–1000 cm−1)ofcobaltandzincsaltsisobservedmorebands
inAg spectrathaninBg orientations(Figs. 2and3). Theseresults
indicate that the most deformation modes of aromatic ring [δ(φ)]
and carboxyl groups [δ(COO)] reveal Ag symmetry. The bands
observed in Bg spectra of cobalt salts are tentative assigned to
benzene ring deformation at 304 cm−1, in plane carboxyl group
deformation at 796 cm−1 and in plane benzene ring deformation
at 936 cm−1 [32,33]. In Bg spectra of zinc salt were observed
bands at 340, 494, 612 and 816 cm−1 assigned to in plane car-
boxyl deformation, benzene ring deformation (two bands) and
in plane C–H deformation, respectively.
In CoH2Bt Ag spectra were observed bands assigned to ben-
zene ring deformation (300, 495 and 615 cm−1), C–H in plane
deformation (692 cm-1), carboxyl group deformation (355 and
787 cm−1) and M–O stretching mode (451 cm−1). Ag spectra
of ZnH2Bt present bands at 262, 492 and 610 cm−1 assigned to
benzene ring deformation, at 696 and 816 cm−1 to C–H in plane
deformation, at 787 cm−1 assigned to carboxyl group deforma-
tion and at 492 cm−1 to M–O stretching mode.
mit the use of Microraman spectrometer of Departamento de
Fısica of Universidade Federal de Minas Gerais. This work has
been supported by Fundac¸ao de Amparo a Pesquisa do Estado
de Minas Gerais, FAPEMIG (Grant CEX 1123/90). R.D. is
grateful to Conselho Nacional de Desenvolvimento Cientıfico
e Tecnologico, CNPq for providing the graduate fellowship and
´
˜
`
´
´
FAPEMIG to for Postdoctoral fellowships.
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