Spectrochimica Acta Part A 79 (2011) 1145–1148
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
j ourna l ho me page: www.elsevier.com/locate/saa
Vibrational spectra of double oxalates of the type M
I
2
Cu(C
2
O
4
)
2
·2H
2
O
(M
I
= Na
+
, K
+
, NH
4
+
)
Daniel Palacios
a
, Alejandra Wladimirsky
a
, María C. D’Antonio
a,b
, Ana C. González-Baró
c
,
Enrique J. Baran
c,∗
a
Departamento de Ciencias Básicas, UTN-Facultad Regional Santa Cruz, 9400 Río Gallegos, Argentina
b
Departamento de Ciencias Exactas y Naturales, Universidad Nacional de la Patagonia Austral, 9400 Río Gallegos, Argentina
c
Centro de Química Inorgánica (CEQUINOR/CONICET, UNLP), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900 La Plata, Argentina
a r t i c l e i n f o
Article history:
Received 13 October 2010
Received in revised form 13 April 2011
Accepted 15 April 2011
Keywords:
Double oxalates
Infrared spectra
Raman spectra
a b s t r a c t
The infrared and Raman spectra of Na
2
Cu(C
2
O
4
)
2
·2H
2
O, K
2
Cu(C
2
O
4
)
2
·2H
2
O and (NH
4
)
2
Cu(C
2
O
4
)
2
·2H
2
O
were recorded and briefly discussed on the basis of their structural peculiarities and by comparison with
the vibrational spectra of other metallic oxalates.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
In recent papers we have investigated the vibrational spectro-
scopic behavior of an important number of oxalate complexes [1–6]
related to naturally occurring oxalates found in rocks and soils as
well as among a variety of living organisms [7,8]. As an exten-
sion of these studies we have now investigated the infrared and
Raman spectra of some double oxalates containing copper(II) and
a monovalent metal cation.
The compound Na
2
Cu(C
2
O
4
)
2
·2H
2
O occurs naturally as the min-
eral wheatleyite, and is one of the few known and well characterized
secondary oxalate minerals [8,9]. The structure and magnetic
properties of the analogous synthetic compound have also been
investigated [10] whereas the structures of M
I
2
Cu(C
2
O
4
)
2
·2H
2
O
with M
I
= K
+
[11,12] and NH
4
+
[13] are also known. These two last
compounds are isomorphous, and present some structural differ-
ences with Na
2
Cu(C
2
O
4
)
2
·2H
2
O, which on its part shows a close
structural relationship to CuC
2
O
4
·nH
2
O [10].
2. Experimental
The starting material for the synthesis of the investigated com-
pounds was CuC
2
O
4
·nH
2
O, prepared as previously described [1].
Copper(II) oxalate was slowly dissolved, under continuous stir-
∗
Corresponding author. Tel.: +54 221 4259485; fax: +54 221 4259485.
E-mail address: baran@quimica.unlp.edu.ar (E.J. Baran).
ring, in warm (ca. 50
◦
C) aqueous solutions of the corresponding
monovalent cation oxalates up to a final molar ratio (soluble
oxalate/CuC
2
O
4
·nH
2
O) equal to 2. After slow evaporation of the
resulting solutions in air, well formed crystals of the three com-
pounds were obtained. They were characterized by X-ray powder
diffractometry and chemical analysis, confirming the purity of all
of the obtained samples.
The infrared spectra in the spectral range between 4000 and
400 cm
-1
were recorded as KBr pellets with a Bruker IFS 66 FT-IR
spectrophotometer. A total of 60 scans were accumulated. Raman
spectra, in the range 4000–100 cm
-1
were obtained with the FRA
106 Raman accessory of the same Bruker instrument. A total of 60
scans were accumulated, using the 1064-nm line of a solid state
Nd:YAG laser for excitation. Spectral resolution was ±4 cm
-1
in
both measurements.
3. Results and discussion
3.1. Structural characteristics of the compounds
The Na
2
Cu(C
2
O
4
)
2
·2H
2
O salt crystallizes in the triclinic space
group P
¯
1 with Z = 1 [9,10]. The structure consists of stacked, quasi-
planar [Cu(C
2
O
4
)
2
]
2-
units and of linear chains of Na
+
cations and
of H
2
O molecules, running parallel to the c axis of the unit cell.
The Cu(II) ion lies on a center of symmetry and is coordinated to
two oxalate anions, each acting as a bidentate ligand and generat-
ing a practically square CuO
4
moiety. The environment of copper
is octahedrally completed with two centrosymmetrically related
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.04.034