Metal isotope and density functional study of the tetracarboxylatodicopper(II) core vibrations

Article abstract of DOI:10.1016/j.saa.2005.02.038

Vibrational spectra of tetrakis(acetato)diaquadicopper(II) complex have been deeply examined in order to provide a detailed description of dynamics of [Cu₂O₈C₄] core being a typical structural unit of most copper(II) carboxylates. Low frequency bands related to significant motions of metal atoms were detected by metal isotope substitution. Observed spectra and isotope shifts were reproduced in DFT calculations. For clear presentation of computed normal vibrations, a D_4h symmetry approximation was successfully applied. Basing on observed isotope shifts and calculation results, all skeletal vibrations have been analyzed including normal mode with the largest Cu...Cu stretching amplitude assigned to Raman band at 178 cm^-1.

Full text of DOI:10.1016/j.saa.2005.02.038

Spectrochimica Acta Part A 62 (2005) 703–710
Metal isotope and density functional study of the
tetracarboxylatodicopper(II) core vibrations
∗
Piotr Dro z˙ d z˙ ewski , Anna Bro z˙ yna
Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Faculty of Chemistry, Wrocław University of Technology,
Wybrze z˙ e St. Wyspia n´ skiego 27, 50-370 Wrocław, Poland
Received 14 December 2004; received in revised form 22 February 2005; accepted 22 February 2005
Abstract
Vibrational spectra of tetrakis(acetato)diaquadicopper(II) complex have been deeply examined in order to provide a detailed description
of dynamics of [Cu ] core being a typical structural unit of most copper(II) carboxylates. Low frequency bands related to significant
2
8 4
O C
motions of metal atoms were detected by metal isotope substitution. Observed spectra and isotope shifts were reproduced in DFT calculations.
For clear presentation of computed normal vibrations, a D4h symmetry approximation was successfully applied. Basing on observed isotope
shifts and calculation results, all skeletal vibrations have been analyzed including normal mode with the largest Cu· · ·Cu stretching amplitude
−1
assigned to Raman band at 178 cm
.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Copper(II) carboxylates; IR and Raman spectra; Normal vibrations; DFT calculations
1
. Introduction
vibrations, but without more precise characterization. Only
two Cu O stretching vibrations have been proposed by Fani-
ran and Patel [3] for copper complexes with acetic acid and
its chloro-derivatives. These bands were found at about 330
A variety of polynuclear metal carboxylates have been
prepared and investigated so far with special attention placed
on possible metal–metal interactions. Large group of these
compounds consists of copper(II) complexes with the for-
mula [Cu2(OOCR)4(L)2] (R: organic group, L: Lewis base)
containing the characteristic [Cu2O8C4] core. In most cases,
these compounds were investigated by X-ray diffraction
method accompanied by magnetic and selected spectroscopic
measurements [1]. However, the majority of vibrational at-
tempts were limited to discussion of the carboxylate group
vibrations. Much less work was done on [Cu2O8C4] core
vibrations absorbing in the far-IR region. Lever and Ra-
maswamy [2] have investigated this problem for several cop-
per(II) carboxylates, employing the deuteration, 65Cu iso-
tope substitution and low temperature techniques. Based on
those experiments, several bands located between 200 and
−
1
−1
and 370 cm . At similar frequency (334 cm ), the anti-
symmetric Cu O stretching vibration was postulated for
copper(II)-N-acetyl-␤-alaninato complex [4]. The vibrations
of water molecules, frequently present in the metal acetates,
were studied by Baraldi and Fabbri [5]. However, only a gen-
−1
eralstatementwasmadethatbandsobservedbelow450 cm
refer to metal–oxygen vibrations, without distinguishing the
metal–water modes. Metal–metal interactions were studied
by Raman spectroscopy for Re, Mo, Rh, Ru, Fe and Cu com-
plexes [6]. For Rh and Cu acetates, the transitions at about
−
1
170 cm were postulated as ν(M–M) vibrations, but this
assignment was not unambiguous. In another work [7], the
−
1
excitation profiles of 700 and 320 cm Raman bands of cop-
per(II) acetate were measured in order to characterize the
electronic transitions of this compound.
−1
4
50 cm were pointed out as being generated by Cu O
As results from the above review, the vibrations of
∗
2 8 4
[Cu O C ] core are not completely localized and charac-
Corresponding author. Tel.: +48 71 3203909; fax: +48 71 3284330.
E-mail address: piotr.drozdzewski@pwr.wroc.pl (P. Dro z˙ d z˙ ewski).
terized. Among eight expected Cu O stretching vibrations
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.02.038

7
04
P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
Table 1
(some may be degenerate) only two, anti-symmetric were
Selection of optimized and measured structural parameters ( A˚ , ) of the
tetrakis(acetato)diaquadicopper(II) skeleton
◦
assigned so far. The discussed [Cu2O8C4] group must also
show several O Cu O bending modes coupled to a vary-
ing extent with Cu O C and O C O vibrations as well
as with some Cu O stretching ones. It is difficult to ana-
lyze such a complicated system only in an empirical way.
Thus, the theoretical predictions will be very helpful in this
matter.
Parameter
Optimized
Observed
[17]
[18]
2.614(2)
Cu1–Cu5
Cu1–O2
Cu5–O4
Cu1–O9
Cu5–O11
Cu1–O31
C3–O2
C3–O4
C10–O9
C10–O11
C3–C15
C10–C17
O2–Cu1–Cu5
O4–Cu5–Cu1
O9–Cu1–Cu5
O11–Cu5–Cu1
O2–Cu1–O9
O4–Cu5–O11
Cu1–O2–C3
Cu5–O4–C3
Cu1–O9–C10
Cu5–O11–C10
O2–C3–O4
O9–C10–O11
O2–C3–C15
O9–C10–C17
O31–Cu1–O2
O31–Cu1–O9
2.610
2.001
2.032
2.030
2.001
2.305
1.266
1.271
1.271
1.266
1.527
1.527
85.6
2.616(1)
1.950(3)
1.945(4)
1.994(3)
1.986(3)
2.156(4)
1.248(6)
1.274(7)
1.268(6)
1.251(6)
1.506(7)
1.495(5)
–
1.952(2)
1.941(2)
1.992(1)
1.990(1)
2.161(2)
1.261(2)
1.259(2)
1.261(2)
1.256(2)
1.502(2)
1.500(2)
82.8(1)
In the present work, the far-IR region vibrations of
the [Cu2O8C4] system were elaborated with help of the
6
3
65
Cu– Cu isotope replacement and quantum calculations on
the density functional theory (DFT) level. The experiments
and calculations were performed for copper(II) acetate mono-
hydrate, a relatively simple molecule containing the typical
[Cu2O8C4] dinuclear unit.
84.7
84.5
85.8
89.7
–
–
–
86.1(1)
82.7(1)
86.1(1)
89.2(1)
2
. Experimental
89.1(1)
91.1(1)
124.9(4)
120.7(3)
124.9(3)
121.9(3)
125.4(5)
124.2(4)
118.8(5)
117.0(4)
93.3(2)
97.7(2)
2
.1. Preparation of the complexes
89.6
87.4(1)
121.6
121.1
121.3
121.4
127.0
127.0
116.9
116.2
106.0
83.7
124.5(1)
121.2(1)
125.2(1)
121.4(1)
125.3(1)
124.4(1)
117.5(1)
117.4(1)
93.5(1)
The copper(II) acetate monohydrate was prepared by
treating a light excess of freshly precipitated copper hy-
droxide with aqueous solution of the acetic acid. The iso-
topically labeled complexes were obtained on the same
way, but on the 0.1 mmole scale. The deuterated com-
pound was prepared by three-fold re-crystallization from
D2O.
93.1(1)
2
.2. Physical measurements
The far-IR spectra were recorded with 0.5 cm 1 reso-
3
. Results and discussion
.1. Far-IR region
For the theoretical study of the [Cu2O8C4] vibrations,
−
lution on a Fourier-Transform PE 2000 spectrometer with
samples as Nujol mulls between polyethylene plates. For
the middle-IR region, a FTIR 1600 instrument and KBr disc
technique were used. Raman spectra were collected with the
right angle optics configuration on a Jobin-Yvon T64000
spectrophotometer using the 514.5 nm Ar–Kr laser excita-
tion.
3
the complete molecule of [Cu2(OOCCH3)4(H2O)2] has been
chosen. The simplest model, like copper formate was re-
jected because the nature of the atom (hydrogen or carbon) di-
rectly bonded to carboxylate carbon significantly influences
the metal-carboxylate group vibrations, as has been demon-
strated before [16]. Thus, the results obtained for copper ac-
etate are much better compared with respective data of similar
complexes containing the variously substituted carboxylates
as bridging ligands.
2
.3. Computations
Quantum calculations on DFT level were performed using
GAUSSIAN98 package [8]. The Becke’s three-parameter hy-
brid functional B3LYP [9] and LanL2DZ basis [10,11] aug-
mented with d-polarization functions on heavy atoms were
applied. The vibrational frequencies and intensities were cal-
culated using the energy-minimized structure and harmonic
approximation. The potential energy distribution was calcu-
lated by Balga programs written by Lapinski and Nowak [12].
Molecule drawing and theoretical spectra were prepared with
help of Molden [13] and Animol [14] applications. Before
plotting the theoretical Raman spectrum, the computed inten-
TheinitialstructuralparametersweretakenfromtheX-ray
[17] and neutron [18] diffraction analysis of copper acetate
monohydrate. The geometry optimization was performed un-
der relaxation of all internal degrees of freedom. A selec-
tion of optimized, skeletal parameters is listed in Table 1 and
compared with the respective data observed. The computed
molecular structure together with atom numbering system
is presented in Fig. 1. Despite the very symmetric appear-
ance of the [Cu2(OOCCH3)4(H2O)2] molecule, its real sym-
4
sities were corrected for ν and Boltzman distribution factors
metry is C because acetate bridges are not structurally the
i
[15].
same. The largest differences are observed for the Cu O bond

P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
705
are observed for two IR bands at 376 and 331 cm 1. Simi-
−
lengths, which are related to intermolecular hydrogen bond-
ing in which only one pair of opposite carboxylate groups is
involved [18]. These differences are approximately twice as
small as in the optimized structure. The Cu O distances are
slightly overestimated, but their relative lengths differ less
than 1.5%. Among the skeletal angles, the largest differences
concern the location of water molecules, which are more dis-
placed from the Cu· · ·Cu line in the optimized structure than
in the real molecules.
lar shifts were computed for closely positioned bands at 344
and 341 cm . Two lower frequency bands showing the 1.0
and 0.5 cm isotope shifts were correlated with two theo-
retical transitions exhibiting the 1.0 and 0.6 cm shifts and
well corresponding intensities. In the Raman spectrum, the
.0 cm shift was detected only for one band at 178 cm .
This effect indicates that the band calculated at 168 cm
1.2 cm shift) is the best candidate for the corresponding
mode. A smaller, 0.5 cm displacement was observed for
the 253 cm band correlated with Raman active transitions
computed at similar frequencies. The discrepancies in the ob-
served and calculated shifts are partially due to the shape of
several Raman bands being worse than observed in the IR
spectrum. In all spectra, the remaining bands were correlated
on the basis of their similar frequencies and intensities.
In order to get a better insight into vibrational character
of presented transitions, the potential energy distribution
PED) terms were calculated for all normal modes of
Cu2(OOCCH3)4(H2O)2] molecule. For this purpose, a
complete set of 102 symmetrized internal coordinates was
constructed. Since for metal carboxylates, like this presented
−
1
−
1
−1
−
1
−1
2
−
1
−
1
(
−
1
Basing on the optimized structure, the harmonic vibra-
tional frequencies, IR intensities and Raman activities were
subsequently calculated using the numerical differentiation
of analytical gradients. All frequencies were positive, con-
firming that the optimized structure corresponds to the po-
tential energy minimum. The theoretical spectra produced
from the calculated data are presented in Fig. 2, together with
the observed ones. For the detection of the metal–ligand vi-
brations, the isotope effects due to 63Cu– Cu replacements
were measured and computed. The resulting wavenumber
downshifts, listed in Fig. 2 by band maxima, are very useful
in proper attribution of computed normal vibrations to ob-
served IR and Raman transitions. The largest isotope shifts
−
1
65
(
[
Fig. 1. Computational model of the [Cu2(OOCCH3)4(H2O)2] complex.

7
06
P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
Fig. 2. Observed and DFT calculated spectra in far-IR region.
here, one of the most interesting problems is the metal–metal
interaction, the respective Cu–Cu stretching coordinate
has been implemented. As a result, the five-membered
Cu O C O Cu rings appear and their bending coordinates
could be defined as recommended by Pulay et al. [19]. The
most significant atom displacements have been presented for
these coordinates in Fig. 3. Pulay’s recommendations were
alsousedformethylgroupcoordinates. Severalattemptswere
made for the symmetry of [Cu2(OOCC)4] skeletal coordi-
show also small Raman activity. The reverse phenomenon is
noticed for g modes. Also the A2g, A1u, B1u and B2u modes,
which are not active at all for ideal D4h symmetry, here have
appeared as very weak transitions. The coordinates related to
the water molecules were treated under the C symmetry. The
i
results of the above procedure have been presented in Table 2.
nates starting from simple C up to D symmetry. Calculated
i
4h
PED characterizations of the respective normal vibrations
were the simplest and the most clear for the D4h symmetry.
For lower symmetry, most of the skeletal vibrations were
described as the sums of several PED terms of comparable
contributions. Therefore, taking into account that the real
and calculated structure of the [Cu2(OOCC)4] skeleton only
slightly deviate from D4h symmetry, it was decided to use
the D4h symmetry as a good approximation for the clear pre-
sentation of calculated [Cu2(OOCC)4] skeletal vibrations. A
small disadvantage of this approximation is that the mutual
exclusion rule is not exactly valid. Some of the calculated u
type vibrations, besides the expected significant IR intensity,
Fig. 3. Atom displacements in deformations of five-membered ring. Plus
and minus represent the atom displacements out of the ring plane.

P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
707
Table 2
Selected normal vibrations of tetrakis(acetato)diaquadicopper(II) complex, representing the dynamics of the [Cu2(OOCC)4] core (D4h) and some H2O related
modes (Ci)
−
−1
Assignmenta
PED(%)
Species
D4h
Observed (cm 1)
IR
Calculated (cm
IR
)
Ci
R
R
Eg
1535
1584
ν(C O)-82
B2u
A2u
A1g
Eu
B1g
A1g
Eu
B1g
A1g
Eu
B1g
B2g
Eu
1620sh
1605
1551
1530
ν(C O)-84
ν(C O)-84
1444
1422
ν(C O)-64 + ν(C C)-18
ν(C O)-65 + ν(C C)-14
ν(C O)-57 + ν(C C)-18
ν(C C)-47 + ν(C O)-26 + δ1(R)-24
ν(C C)-41 + ν(C O)-20 + δ1(R)-17
ν(C C)-50 + ν(C O)-25 + δ1(R)-22
δ1(R)-62 + ν(C C)-28
δ1(R)-58 + ν(C C)-22
δ1(R)-64 + ν(C C)-25
τ2(R)-59
1421
944
692
629
1414
950
678
633
1426
951
1404
958
951
692
947
689
706
635
671
637
τ2(R)-63
A2g
Eg
A2u
B2u
Eu
n.o.
530
623
535 (0.5)
τ2(R)-71
δ(O
δ(O
δ(O
C
C
C
C)-43 + ν(Cu O)-34
C)-57 + ν(Cu O)-28
C)-71 + ν(Cu O)-16
n.o.b
n.o.
376 (2.5)c
520
482
344 (2.5)
341 (2.5)
ν(Cu O)-85
331 (2.5)
A1g
Eg
323
253 (0.5)
253 (0.5)
279
ν(Cu O)-85
ν(Cu OH2)-46 + ν(Cu O)-26 + δ(O
ν(Cu O)-51 + δ(O
ν(Cu O)-31 + ν(Cu OH2)-28 + δ(O
Ag
256 (2.0)
247 (1.6)
233 (1.8)
C
C
C)-18
C)-17
C C)-32
232
A2u
B2g
B1u
Eu
272 (1.0)
241 (1.0)
δ2(R)-58 + δ(O
δ(R–R )-65 + τ(Cu
C
C)-18 + ν(Cu O)-12
ꢀ
(232)
228
O
C
C)-36
n.o.
253
223 (0.1)
221
ν(Cu OH2)-41 + τ1(R)-39
ꢀ
τ(R–R )-43 + τ(Cu
O
C
C)-27
2
232
34
220
216 (0.1)
ꢀ
Au
ν(Cu OH2)-42 + τ(R–R )-13
ν(Cu O)-78
τ1(R)-41 + τ(H2O)-32
τ1(R)-81 + δ2(R)-13
B1g
Eg
224sh
210sh
215 (0.2)
207
197 (0.3)
Au
Ag
175sh
190
τ(H2O)-80
n.o.
178 (2.0)
182 (0.3)
168 (1.2)
τ(H2O)-57 + τ1(R)-29
ν(Cu Cu)-75 + ν(Cu OH2)-22
δ2(R)-45 + ν(Cu O)-40 + δ(O
A1g
B2u
A2g
A2u
Eg
140
153
C
C
C)-15
C)-16
n.o.
132
τ(Cu O C C)-81 + τ2(R)-12
ν(Cu O)-58 + δ2(R)-28 + δ(O
δ2(R)-59 + δ1(O Cu OH2)-25
114 (0.5)
113 (0.6)
117
103 (0.1)
99 (0.1)
105
δ2(R)-62 + δ2(O Cu OH2)-18 + τ1(R)-14
δ2(O Cu OH2)-47 + δ2(R)-27 + ν(Cu O)-19
B2u
n.o.
74
78
Ag
Au
64
64
46
73 (0.1)
69
δ1(O Cu OH2)-47
ꢀ
ꢀ
ꢀ
Eu
B2g
Eu
71 (0.1)
67
τ(Cu
τ(Cu
τ(Cu
O
O
O
C
C
C
C)-17 + τ(R–R )-16
C)-22 + δ(R–R )-15
C)-35 + τ(R–R )-39 + τ2(R)-13
74
Eg
65 (0.1)
δ2(O Cu OH2)-78 + δ2(R)-19
δ1(O Cu OH2)-70
56
40
n.o.
61
44
10
B2u
A1u
a
ν(Cu O)-19 + δ2(R)-18 + δ2(O Cu OH2)-41
τ1(R)-99
ν, Stretching; δ, in-plane bending; τ, out-of-plane bending; R, Cu
n.o., Not observed
O C O Cu ring.
b
c
Downshifts upon 63Cu–65Cu substitution.
tude of calculated and observed 63Cu– Cu displacements al-
65
Among eight ν(Cu O) vibrations expected, these approx-
imated as the Eu species, show the largest isotope shift. Since
normal frequencies were calculated for the molecule slightly
−1
low for unambiguous attribution of 376 and 331 cm bands
−1
to recently mentioned normal vibrations. The 45 cm dif-
ference corresponds well to structural data. As results from
the X-ray measurements, in copper(II) acetate monohydrate,
deviated from D4 symmetry, the discussed doubly degener-
h
atedEu modehastwo, 344and341wavenumbers. Themagni-

7
08
P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
a pair of opposite carboxylate groups form the Cu O bonds
about 0.04 A longer than the remaining ones. These carboxy-
To complete the vibrations in which the copper atoms
are involved, the inter-ring bending and torsion modes have
to be considered. In Table 2, these vibrations were labeled
˚
late oxygen atoms are also involved in hydrogen bonding
with water molecules. Both effects may cause the mentioned
ꢀ
ꢀ
as δ(R–R ) and τ(R–R ), respectively. The Raman active
−1
ꢀ
−1
4
5 cm splitting of the ν(Cu O) vibrations approximated
δ(R–R ) mode (B2g) contributes significantly in 228 cm
−1
here by Eu species. The second IR active ν(Cu O) mode
vibration and has been attributed to 232 cm band together
−
1
−1
(
A2u) was predicted at 113 cm with 0.6 cm isotope shift.
with previously discussed ν(Cu O) of Eg symmetry. The de-
−
1
ꢀ
A 114 cm band of comparable intensity and isotope sensi-
tivity has been proposed for that vibration.
generate τ(R–R ) mode (Eu) is dispersed over four normal vi-
brations and coupled with τ(Cu O C C) deformation. The
ꢀ
ThemostintenseRamanbandoflowfrequencyregionwas
highest PED term of τ(R–R ) coordinate has been found in
−
1
−1
calculated at 279 cm as due to totally symmetric A1g vibra-
221, 220 cm degenerate vibration, assigned to more split
absorption maxima at 253 and 234 cm
−
1
−1
−1
tion. Its observed position (323 cm ) is 44 cm higher than
that calculated, which again is due to mentioned differences
in real Cu O bonds. Other Raman active ν(Cu O) modes
described by Eg species were computed at 247 and 233 cm
and correlated with similar energy and intensity bands at 253
.
Since calculations were carried out for whole
[Cu2(OOCCH3)4(H2O)2] molecule, it was possible to
evaluate the vibrations related to water groups. In order
to provide an additional proof of computation results, the
IR spectrum of the complex substituted with D2O was
measured and computed. In calculations, the stretching
Cu OH2 modes are predominant in three normal vibrations.
The highest frequency mode with significant contribution of
−
1
−
1
and 232 cm . The last ν(Cu O) transition of B1g symme-
−
−
1
try has been predicted as medium intensity band at 215 cm
,
strongly overlapping with ν(Cu OH2) vibration at 216 cm 1.
−1
Both vibrations were attributed to 232 cm Raman band and
−
1
−1
its shoulder at 224 cm . The only inactive ν(Cu O) vibra-
tion (B2u) is strongly coupled with other vibrations of the
same symmetry and with O Cu OH2 bending modes.
Special attention is usually focused on the Cu· · ·Cu in-
teraction. In the present calculations, the corresponding
stretching ν(Cu Cu) coordinate has been defined in order
to get some recognition about the frequency of the motion
where Cu· · ·Cu distance is the most changed. It should be
pointed out that this coordinate, corresponding frequency
and force constant do not have to be related to chemical
Cu Cu bond. It is also possible to describe the discussed
vibration as totally symmetric O Cu O bending mode, but
this in turn, causes much more complicated description of
eight-membered Cu O C O Cu O C O ring deforma-
tions. In present work, the ν(Cu Cu) stretching coordinate
ν(Cu OH2) is predicted at 256 cm as a very weak Raman
−1
transition with an H-D shift below 1 cm . In theoretical
Raman spectrum this mode is overlapped by about six times
−
1
more intense vibration at 247 cm , thus it will be difficult
to observe the former mode. Two remaining IR active
−1
vibrations computed at 223 and 216 cm are also weak
and positioned close to much more intense 221 cm band.
Upon deuteration, the first vibration remains unchanged,
−1
−
1
whereas the second vibration shifts to 212 cm . Since
−
1
the IR bands observed between 400 and 180 cm are not
sensitive to H2O D2O replacement, the recently mentioned
Cu OH2 vibrations should be considered as unobservable.
The only observed deuteration effect in the far-IR spectrum
−
1
is the disappearance of the 175 cm shoulder, which may
be correlated with twisting mode of H2O groups calculated
−
1
−1
−1
contributes mainly in 168 cm normal A1g vibration ex-
hibiting 1.2 cm metal isotope shift. Based on the calculated
frequency and strongly on the isotope shift, the discussed
mode has been attributed to 178 cm Raman band showing
.0 cm displacement upon Cu– Cu substitution.
The Cu Cu O bending vibrations are mainly represented
at 190 cm . The main band at 182 cm is probably due
to lattice vibrations which may also generate some other
−
1
−
1
weak absorptions below 150 cm , not attributed so far to
molecular transitions. The contribution of lattice modes is
also possible for relatively intense Raman bands located in
the same frequency region.
−
1
−1
63
65
2
by δ2(R) Cu O C O Cu ring deformation. As expected,
these vibrations are more coupled with other modes. The
highest PED percentages of δ2(R) were calculated for 103,
3.2. Middle-IR region
−
1
9
9 and 241 cm normal vibrations of Eg and A2u symmetry,
The bands observed in the middle-IR region between 1800
−
1
respectively. Third, B2u mode is dispersed over three vibra-
and 600 cm may be classified into three groups: the skele-
tal modes of CH3 COO groups, the C H deformations and
bending vibrations of water molecules. The last absorption
−
1
tions with the highest contribution (40%) in the 153 cm
transition.
−1
The torsional deformation of the O–Cu· · ·Cu–O subunit is
implemented in the τ1(R) coordinate. The calculations show,
that all corresponding vibrations should produce very weak
bands – more than twenty times weaker than in, e.g., the
has been easily located as the 1650 cm shoulder, which
disappears upon H2O D2O replacement with simultaneous
appearance of new 1213 cm band due to δ(D2O) vibration.
As has been presented in Fig. 4, very similar effect results
from calculations.
The expected eight ν(C O) vibrations (with two dou-
bly degenerated) are well separated into symmetric and
anti-symmetric modes with respect to local COO group
−
1
− −1
1
2
79 cm Raman band and 344, 341 cm IR absorptions.
−
1
Thus, a small shoulder at 210 cm in Raman spectrum has
been proposed for Eg type vibrations predicted at 207 and
−1
1
97 cm
.

P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
709
Fig. 4. Observed and DFT calculated spectra in middle-IR region.
between 1470 and 1500 cm 1, but their predicted low inten-
sity suggests that discussed bands are mainly due to ν(C O)
vibrations. Similar phenomenon is observed in the IR spec-
trum, but according to calculations, the δ(CH3) vibrations
−
symmetry or σh plane of D4h symmetry used here. The
anti-symmetric vibrations are observed in the IR spectrum
as intense band with maximum at 1605 cm and shoul-
−
1
−
1
der at about 1620 cm . The corresponding normal vibra-
−
1
−1
tions were computed at 1530 (A2u) and 1551 cm (B2u) as
very intense and weak transitions, respectively. Third anti-
symmetric mode (Eg) was computed at 1584 cm , but the
only candidate for this mode is the weak Raman band at
have the intensities similar to that of ν(C O) at 1414 cm
Eu). Thus, the absorption maximum at 1421 cm has been
proposed for ν(C O) mode, whereas the 1445 cm peak
assigned to δ(CH3) vibrations.
The recently presented ν(C O) vibrations are coupled
with the ν(C C) modes, calculated mainly at close posi-
tions around 950 cm . Two of these vibrations at 958 and
47 cm are Raman active and may be correlated with the
intense band, observed at 951 cm . The IR active (Eu) mode
is predicted as 950 cm band of very low intensity, compa-
−1
(
−
1
−1
−1
1
535 cm (higher frequency band is probably due to δ(H2O)
vibrations). The symmetric ν(C O) vibrations are less sepa-
−
1
−
1
rated and has been calculated at 1422 (A1g) and 1404 cm
B1g). These two strong Raman transitions have been cor-
−
1
(
9
−
1
−
1
related with two bands at 1444 and 1426 cm , being the
most intense in the discussed area. However, these two bands
are probably also generated by δ(CH3) vibrations calculated
−
1
−1
rable with very weak absorption observed at 944 cm
.

7
10
P. Dro z˙ d z˙ ewski, A. Bro z˙ yna / Spectrochimica Acta Part A 62 (2005) 703–710
The 700–600 cm 1 part of the middle-IR area is occupied
−
The most intense copper–oxygen stretching vibrations
−
1
were attributed to 376, 331, 114 cm IR absorptions and
by “in plane” δ1(R) and “out-of-plane” τ2(R) deformations
of bridging rings in which the motions of carboxylate car-
bon atoms are predominant. As results from the respective
IR spectra (Fig. 4), for both Eu deformations the bands com-
puted at 678 and 633 cm correspond very well to those
observed at 692 and 629 cm . Raman transitions are a little
more complicated, because four calculated transitions have
different intensities: B1g mode at 671 cm is medium inten-
sity, A1g (689 cm ) and B2g (637 cm ) are weak, whereas
the A2g (623 cm ) is very weak. Following these relative
intensities, the most intense band at 706 cm has been pro-
−
1
3
23, 224 cm Raman transitions. One of the most interest-
ing mode related to elongation of Cu· · ·Cu distance was cal-
−
1
−1
culated at 168 cm and assigned to 178 cm Raman band
−
1
−
1
exhibiting the 2.0 cm isotope shift.
−
1
−1
Acknowledgements
−
1
−1
−
1
Thanks are due to Pozna n´ Supercomputing Centre for
computation facilities and to Tomasz Misiaszek for help in
Raman measurements.
−
1
−
1
posed for B1g mode and band at 692 cm for A1g species.
Among two Raman active τ2(R) transitions, only the B2g was
−
1
recorded at 635 cm
.
In the frequency range between 600 and 400 cm 1,
two kinds of vibrations were predicted and observed: the
δ(O C C) bending and water motions. Only one of the for-
mer vibrations was observed in the Raman spectrum. In the
IR spectrum, this region is mostly covered by broad band with
−
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−
1
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−
1
and rocking (547 cm ) vibrations of water molecules is
−
1
shifted to 405 cm by H-D replacement.
The remaining bands of discussed middle-IR region are
related to CH3 group vibrations. Vibrations of these groups
also cover the highest frequency area together with intense
and well separated H2O stretching vibrations at 3475 and
−
1
−1
3
375 cm , shifted to 2590 and 2490 cm upon deuteration.
4
. Conclusions
Vibrational spectra of [Cu2(OOCCH3)4(H2O)2] complex
have been measured and interpreted with help of deuteration
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tions (DFT) reproducing the spectra and mentioned isotope
labeling effects.
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[
[
[12] L. Lapinski, M.J. Nowak, Balga—computer program for PED cal-
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Cu2O8C4] core only slightly deviate from D4h point group,
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simple and clear description of most important normal vi-
brations related to Cu Cu, Cu O, C O and C C stretching
coordinates and some deformation modes.
[
14] M.M. Szczesniak, B. Maslanka, AniMol—computer program In-
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[
[
[
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Side effects resulting from applied D4h approximation
(frequency splitting of some degenerated vibrations, very
[
[
18] G.M. Brown, R. Chidambaram, Acta Cryst. 29B (1973) 2393.
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(1979) 2550.
weak activity of selected forbidden transitions) appeared to
benotdisturbingtheinterpretationofobservedfundamentals.