Experimental and theoretical vibrational study of the oxotetrachlorochromate(V) anion in [AsPh4][CrOCl4]

Article abstract of DOI:10.1002/zaac.200600219

Infrared and Raman spectra were obtained for tetraphenylarsonium oxotetrachlorochromate(V), [AsPh₄][CrOCl₄]. Equilibrium geometry and vibrational frequencies for the anion, CrOCl₄ ^-, were studied employing Density Functional Theory (DFT) methods. A comparative theoretical study was performed in order to determine the best level of theory and basis set to reproduce the experimental structure parameters and vibrational frequencies. Such frequencies served as a basis for the calculation of the scaled quantum mechanical (SQM) force field for the anion. The obtained results were compared with those obtained previously for the VOCl ₄^- anion.

Full text of DOI:10.1002/zaac.200600219

DOI: 10.1002/zaac.200600219
Experimental and Theoretical Vibrational Study of the Oxotetrachlorochromate(V)
Anion in [AsPh4][CrOCl4]
a
a
M. L. Roldan , S. A. Brandan , E. L. Varettib, 1), and A. Ben Altabefa, 1),
*
´
´
a
´
´
´
´
´
´
Tucuman / R. Argentina, Universidad Nacional de Tucuman, Instituto de Quımica Fısica, Facultad de Bioquımica, Quımica y
Farmacia
b
´
´
´
La Plata, R. Argentina, Universidad Nacional de La Plata, Centro de Quımica Inorganica, Departamento de Quımica, Facultad de
Ciencias Exactas
Received May 12th, 2006; revised July 10th, 2006.
Abstract. Infrared and Raman spectra were obtained for tetraphe-
nylarsonium oxotetrachlorochromate(V), [AsPh4][CrOCl4]. Equili-
brium geometry and vibrational frequencies for the anion,
CrOCl4Ϫ, were studied employing Density Functional Theory
(DFT) methods. A comparative theoretical study was performed in
order to determine the best level of theory and basis set to repro-
duce the experimental structure parameters and vibrational frequ-
encies. Such frequencies served as a basis for the calculation of the
scaled quantum mechanical (SQM) force field for the anion. The
obtained results were compared with those obtained previously for
Ϫ
the VOCl4 anion.
Keywords: Oxotetrachlorochromate (V); Vibrational spectra; DFT
calculations; Force field
Introduction
Experimental Section
[AsPh4][CrOCl4] was prepared by the method reported by Seddon
and Thomas [7]. In this procedure, a solution of CrO3 in glacial
acetic acid previously saturated with hydrogen chloride is mixed
with a similar solution of AsPh4Cl; the yellowish brown powder
formed is filtered off, washed with glacial acetic acid and dried in
vacuo. The solid is then recrystallized in CH2Cl2. All the synthesis
was carried out in a dry box under nitrogen atmosphere.
The [CrOCl4]Ϫ anion was studied in the form of its tetra-
phenylarsonium salt as part of a series of vibrational studies
on inorganic molecules or anions containing transition met-
als as central atoms. Such previous studies included the fol-
lowing chemical species: CrO2(NO3)2 [1], VO(NO3)3 [2],
Ϫ
VO2X2 (X ϭ halogen) [3], VOX3 (X ϭ halogen) [4],
Ϫ
VOCl4 (and PCl4ϩ) [5].
The infrared spectrum of the substance was obtained in a pellet of
KBr, prepared under dry nitrogen atmosphere, using a Perkin
Elmer Model 1600 FTIR spectrophotometer. The Raman spectrum
of the solid was registered in an FTϪRaman Bruker RFS 100 spec-
trophotometer, with light of a NdϪYAG (1064 nm) laser for exci-
tation and a Ge detector cooled to the liquid nitrogen temperature,
The crystalline structure of [AsPh4][CrOCl4] was deter-
mined by XϪray diffraction [6] but no detailed study is
known about its vibrational properties. It should be men-
tioned that the oxotetrahalochromates are very difficult to
synthesize and study because of their sensitivity to light,
moisture and heat. Besides, these substances are often im-
purified with CrIII and CrVI compounds, which difficult a
proper understanding of the infrared and Raman spectra.
with a resolution of 2 cmϪ1
.
The experimental Raman spectrum is shown in Figure 1. Only one
band due to the studied anion, located at 1021 cmϪ1, is observed
in the measured range (4000Ϫ400 cmϪ1) of the infrared spectrum,
which is not presented here. The frequencies, relative intensities and
assignment of the observed bands in the infrared and Raman spec-
trum are given as Supplementary material.
* Dr. A. Ben Altabef
Instituto de Quımica Fısica, Facultad de Bioquımica, Quımica y
Farmacia
´
´
´
´
´
Universidad Nacional de Tucuman
Calculations
San Lorenzo 456
´
4000 Tucuman / R. Argentina
All the calculations were performed with the Gaussian 03 set of
programs [8]. The equilibrium geometry was calculated using gradi-
ent standard techniques and the convergence criteria implemented
by defect in Gaussian. For geometry optimization the molecule was
modelled with the program GaussView [9] as a square pyramidal
structure to generate the input for an explorative series of calcu-
lations using different methods and basis sets. These calculations
served to determine the best theoretical approximation to predict
the structure and the vibrational frequencies. The employed DFT
functionals were mainly B3LYP [10, 11] and B3PW91 [10, 12], to-
Tel.: ϩ54 381 4311044
Fax: ϩ54 381 4248169
E-mail: altabef @fbqf.unt.edu.ar
1)
´
Members of the Carrera del Investigador Cientıfico, CONICET
(R. Argentina)
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
Z. Anorg. Allg. Chem. 2006, 632, 2495Ϫ2499
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. L. Roldan, S. A. Brandan, E. L. Varetti, A. Ben Altabef
Table 1 Geometric parameters calculated with the 6Ϫ31G* basis
set and different DFT functionals for [CrOCl4]Ϫ
a)
a)
b)
b)
Functionals
CrO
CrCl
ClCrCl
OCrCl
BLYP
BP86
PBEPBE
B3LYP
B3P86
PBE1PBE
B3PW91
1.558
1.549
1.547
1.529
1.520
1.515
1.521
2.298
2.274
2.272
2.274
2.252
2.251
2.258
86.3
86.3
86.3
86.4
86.4
86.4
86.4
104.7
104.7
104.7
104.6
104.6
104.5
104.5
Experimental values [6]
1.519
2.240
86.4
104.5
a)
Distances in A; b) Angles in degrees
˚
S2. In all cases the calculations predicted a structure with
the expected C4v symmetry (Fig. 2), in accordance with
experiment. The CrO and specially the CrCl bond distances
were systematically overestimated by the calculations. At
least part of the discrepancies may be attributed to inter-
molecular forces in the crystal since the calculations were
carried out in vacuo. It was found that the inclusion of pol-
arization functions in the basis sets significantly improved
the theoretical results and the lowest deviation with respect
to the experimental data was obtained for the 6Ϫ31G* and
6Ϫ311G* basis sets and the B3PW91 functional. The low-
est differences between experimental and calculated values
˚
˚
Figure 1 Calculated (upper) and experimental (lower) Raman
spectrum of [AsPh4][CrOCl4]. The anion bands are indicated with
arrows.
were 0.003 A for CrO and 0.018 A for CrCl bond distances,
and 0.1° for ClCrCl and 0.3° for OCrCl angles.
Additional calculations were made using different func-
tionals with the 6Ϫ31G* basis set and the results are shown
in Table 1. It can be seen that the BP86, PBEPBE and
BLYP functionals yield CrO and CrCl bond distances val-
ues which are too large. Bond angles, however, were well
predicted in all the calculations. The best results were ob-
tained with the B3P86/6Ϫ31G* combination. This level of
theory was used therefore to calculate the structural para-
meters of the [VOCl4]Ϫ anion, which has a predicted C4v
symmetry like [CrOCl4]Ϫ but which structure was not ex-
perimentally measured. The bond distances and angles pre-
gether with several basis sets as implemented in Gaussian. The ob-
tained results are added as Supplementary material (Table S2).
The harmonic force field in cartesian coordinates furnished by the
calculations was transformed to symmetry coordinates. The re-
sulting force field in these coordinates was subsequently scaled
using the scheme of Pulay et al. [13], in which the main force con-
stants are multiplied by scale factors fi, fj,... and the corresponding
interaction constants are multiplied by (fi fj )1/2 , adjusting the scale
factors to reproduce as well as possible the experimental wave-
numbers. The potential energy distribution was subsequently calcu-
lated with the resulting scaled quantum mechanics (SQM) force
field. That force field was also used to calculate the force constants
expressed in terms of simple valence internal coordinates.
˚
dicted by this calculation were d(VO) ϭ 1.541 A, d(VCl) ϭ
˚
2.275 A, α(ClVCl) ϭ 86.9°, β(OVCl) ϭ103.5°.
Vibrational frequencies
The conversion of the force constants in cartesian coordinates to
symmetry coordinates, the adjustment of the scale factors and the
calculation of the potential energy distribution was carried out with
the program FCARTP [14].
The 12 normal modes of vibration expected for the
[CrOCl4]Ϫ anion, of C4V symmetry, are classified as 3 A1 ϩ
2 B1 ϩ B2 ϩ 3 E, being the 3 A1 and 3 E modes active in
infrared and all modes active inRaman. Exploratory calcu-
lations using the B3LYP and B3PW91 functionals and dif-
ferent basis sets were also performed to determine the level
of theory which allowed a good agreement between theor-
etical and experimental frequencies, measured by the root
mean standard deviation (RMSD). In this case, the use of
diffuse functions improves clearly the quality of the calcu-
lated frequencies whereas the introduction of polarization
functions gives less satisfactory results, that is, a reverse be-
haviour of that observed in the calculation of geometrical
Results and Discussion
Structural parameters
The performance of the different theoretical approxi-
mations in the prediction of the structural parameters was
evaluated by comparison with the experimental data, ob-
tained with XϪray diffraction techniques [6]. Such com-
parison is included in the Supplementary material as Table
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Z. Anorg. Allg. Chem. 2006, 2495Ϫ2499

Oxotetrachlorochromate(V) Anion in [AsPh4][CrOCl4]
Table 2 Definition of symmetry coordinates for [CrOCl4]Ϫ (atom
Table 3 Scale factors for force field of [CrOCl4]Ϫ
numbers appear in Fig. 2)
Coordinate
ν (CrO)
Scale factor
0.936
1.128
1.150
1.150
1.015
1.150
0.860
1.150
A1:
νs (CrCl4)
π (CrOCl4)
νa (CrCl4)
δa (CrCl4)
δs (CrCl4)
νd (CrCl4)
πd (CrCl4)
δd (CrCl4)
S1 ϭ q (5Ϫ6)
ν1 (ν CrO)
S2 ϭ r(1Ϫ5) ϩ r (2Ϫ5) ϩ r (3Ϫ5) ϩ r (4Ϫ5)
ν2 (νs CrCl4)
S3 ϭ α (1Ϫ5Ϫ2) ϩ α (2Ϫ5Ϫ3) ϩ α (3Ϫ5Ϫ4) ϩ α (4Ϫ5Ϫ1) ν3 (π CrOCl4)
Ϫ β (1Ϫ5Ϫ6) Ϫ β (2Ϫ5Ϫ6) Ϫ β (3Ϫ5Ϫ6) Ϫ β (4Ϫ5Ϫ6)
B1:
1.150
S4 ϭ r (1Ϫ5) Ϫ r (2Ϫ5) ϩ r (3Ϫ5) Ϫ r(4Ϫ5)
ν4 (νa CrCl4)
S5 ϭ β (1Ϫ5Ϫ6) Ϫ β (2Ϫ5Ϫ6) ϩ β (3Ϫ5Ϫ6) Ϫ β (4Ϫ5Ϫ6) ν5 (δa CrCl4)
ν, stretching; δ, deformation; π, out of plane deformation
B2:
S6 ϭ α (1Ϫ5Ϫ2) Ϫ α (2Ϫ5Ϫ3) ϩ α (3Ϫ5Ϫ4) Ϫ α (4Ϫ5Ϫ1) ν6 (δs CrCl4)
E:
Assignment of the observed bands
S7 ϭ r (1Ϫ5) ϩ r (2Ϫ5) Ϫ r (3Ϫ5) Ϫ r(4Ϫ5)
S7Ј ϭ r (1Ϫ5) Ϫ r (2Ϫ5) Ϫ r (3Ϫ5) ϩ r(4Ϫ5)
ν7 (νd CrCl4)
The infrared spectrum of the [CrOCl4]Ϫ anion as a [PCl4]ϩ
salt was previously studied by Seddon and Thomas [15].
They assigned three of the six infrared active bands, which
were the CrO stretching mode at 1017 cmϪ1 and the sym-
metric and antysimmetric CrCl stretching modes at
401 cmϪ1 and 348 cmϪ1, respectively.
S8 ϭ β (1Ϫ5Ϫ6) ϩ β (2Ϫ5Ϫ6) Ϫ β (3Ϫ5Ϫ6) Ϫ β (4Ϫ5Ϫ6) ν8 (πd CrCl4)
S8Ј ϭ β (1Ϫ5Ϫ6) Ϫ β (2Ϫ5Ϫ6) Ϫ β (3Ϫ5Ϫ6) ϩ β (4Ϫ5Ϫ6)
S9 ϭ α (1Ϫ5Ϫ2) Ϫ α (3Ϫ5Ϫ4)
S9Ј ϭ α (4Ϫ5Ϫ1) Ϫ α (2Ϫ5Ϫ3)
ν9 (δd CrCl4)
ν, stretching; δ, angular deformation; π, out of plane deformation
The infrared spectrum of [AsPh4][CrOCl4] show the CrO
parameters. Such an influence of the additional diffuse
functions had already been observed for other compounds
containing transition metals [3, 4, 5]. The lowest RMSD
value (21.7 cmϪ1) was obtained with the combination
B3LYP/6Ϫ31ϩG (Table S3, Supplementary material) and
therefore such level of theory was used in subsequent calcu-
lations. Similar results were obtained in the study of the
[VOCl4]Ϫ anion [5].
Additional calculations were also carried out with other
functionals and the 6Ϫ31ϩG basis set. Calculated fre-
quencies with the BP86 and PBEPBE functionals were al-
most as good as those obtained with B3LYP. The frequenc-
ies predicted with the other functionals showed less agree-
ment with the experimental data (Table S4, Supplemen-
tary material).
stretching as a strong sharp band located at 1021 cmϪ1
,
with a strong counterpart Raman band at the same fre-
quency. The experimental Raman spectrum is shown in Fig-
ure 1 and is compared with the theoretical Raman spectrum
calculated at the B3LYP/6Ϫ31ϩG level of theory. The
anion bands are indicated by arrows, being other bands due
to the AsPh4ϩ cation. Out of the nine Raman active modes,
eight bands can be assigned to normal modes of vibration
of the studied anion. The measured frequencies compare
well with those observed for the [VOCl4]Ϫ anion [5] and
therefore the assignments were similar in both cases and are
detailed in Table 4. The calculated Raman intensities are
in general good agreement with relative intensities in the
experimental spectrum, although in some cases the anion
bands overlap partially with the bands of the cation.
Table 4 Experimental and calculated wavenumbers and assignment for [CrOCl4]Ϫ
a)
b)
c)
d)
Mode
Experimental
Calculated
SQM
IR Intensities
Raman Activities
PED (>10 %)
Assignment
A1
ν1
ν2
ν3
1021
346
208
1053
328
187
1021
346
200
92.8
4.0
0.8
45.0
37.0
8.0
98 % S1
95 % S2
95 % S3
ν (CrϭO)
νs (CrCl4)
π (CrOCl4)
B1
ν4
ν5
259
Ϫ
231
36
246
36
0.0
0.0
15.0
3.0
98 % S4 ϩ 45 % S5
145 % S5
νa (CrCl4)
δa (CrCl4)
B2
ν6
238
214
228
0.0
9.0
100 % S6
δs (CrCl4)
E
ν7
ν8
ν9
360
279
193
377
259
176
364
265
188
149.9
5.0
1.1
0.0
3.4
2.0
78 % S7 ϩ 40 % S8
48 % S8 ϩ 36 % S7
97 % S9
νd (CrCl4)
πd (CrCl4)
δd (CrCl4)
RMSD (cmϪ1
)
21.7
8.3
ν, stretching; δ, deformation; π, out of plane deformation; a) B3LYP/6Ϫ31ϩG calculation (observed and calculated values in cmϪ1); b) From scaled quantum
d)
4
mechanics force field (See text); c) Units in km.molϪ1
;
Units in A (amu)Ϫ1
˚
Z. Anorg. Allg. Chem. 2006, 2495Ϫ2499
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M. L. Roldan, S. A. Brandan, E. L. Varetti, A. Ben Altabef
Table 6 Comparison between [CrOCl4]Ϫ and [VOCl4]Ϫ
The force constants
The symmetry coordinates used for the calculations are de-
fined in Table 2 and follow the numbering of atoms shown
in Figure 2.
[CrOCl4]Ϫ a)
[VOCl4]Ϫ b)
Experimental Calculated
Experimental Calculated
ν (MO)
f (MO)
d (MO)
ν (MCl4)
f (MCl)
d (MCl)
1021.0
1021.0
7.39
1.520
364.0, 346.0
1.44
2.252
1017.0
1013.0
7.26
1.541
368.0, 352.0
1.50
2.275
c)
d)
1.519
Ϫ
360.0, 346.0
368.0, 359.0
c)
d)
2.240
Ϫ
a) Experimental frequencies, and calculated frequencies and force constants
using the SQM force field [this work]; b) Experimental frequencies, and calcu-
lated frequencies and force constants using the SQM force field [5]; c) Bond
distances calculated with B3P86/6Ϫ31G* (Table 1 and text); d) From Ref. [6]
A comparison between [CrOCl4]؊ and [VOCl4]؊
The stretching frequencies, calculated force constants and
bond distances of [CrOCl4]Ϫ are compared in Table 6 with
those obtained for [VOCl4]Ϫ. It can be seen that the CrO
bond is somewhat stronger than the VO bond, as evidenced
by the greater stretching frequency and force constant,
whereas an inverse behaviour is shown by the CrCl and VCl
bonds. The bond distances are in accordance with the
vibrational data in the case of the MO bonds (a smaller
bond distance correspond to a stronger bond) but not for
the MCl bonds. No clear explanation was found for such
discrepancy.
Figure 2 Structure and definition of internal coordinates for
[CrOCl4]Ϫ.
Supplementary material. Observed vibrational bands, calculated ge-
ometry and frequencies and SQM force field matrix are provided
as Supplementary material.
The force constants were computed at the B3LYP/
6Ϫ31ϩG level of theory and then scaled according to the
process previously described in the Calculations section,
leading to a final RMSD ϭ 8.3 cmϪ1. The computed scale
factors are collected in Table 3. The SQM force field ob-
tained in that way was used to calculate the potential energy
distribution (PED), which gives the relative contribution of
each symmetry coordinate to the normal modes of vi-
bration. These values appear also in Table 4 and show that
most of the modes are well described by single symmetry
coordinates of Table 2, with the exception of the ν4 mode,
showing a mixing of the S4 and S5 coordinates, and the ν7
and ν8 modes, which show a strong mixing of the S7 and
S8 coordinates.
´
Acknowledgements. S. A. Brandan thanks a grant of Banco Rio for
supporting this work. Grants from CIUNT (Consejo de Investiga-
´
ciones, Universidad Nacional de Tucuman) and CONICET
´
´
(Consejo Nacional de Investigaciones Cientıficas y Tecnicas, R.
Argentina) are also gratefully acknowledged.
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´
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´
´
´
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Oxotetrachlorochromate(V) Anion in [AsPh4][CrOCl4]
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