REGULAR PAPERS
The Infrared And Raman Spectra Of Tetrachlorrphosphonium (V) Oxotetrachlorovanadate (V), PCL4VOCL4. An Experimental And Theoretical Study
Roldán, M. L.1 ; Lanús, H.1 ; Brandán, S. A.1 ; López, J. J.2 ; Varetti, E. L.3 * ; Ben Altabef, A.1.
1 Instituto de Química Física, Facultad de Bioquímica, Química y
Farmacia, Universidad Nacional de Tucumán. San Lorenzo 456, 4000 Tucumán,
Argentina.
2 Departamento de Química Física y Analítica, Facultad de Ciencias
Experimentales, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain.
3 CEQUINOR (Centro de Química Inorgánica), Departamento de Química,
Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962,
1900 La Plata, Argentina.
FAX:
+54 221 425 9485; e-mail: varetti@quimica.unlp.edu.ar
Received February
01, 2004. In Final Form March 15, 2004
Dedicated
to Professor Dr. P. J. Aymonino on occasion of his 75thAnniversary
Abstract
Infrared and Raman spectra were obtained
for tetrachlorophosphonium (V) oxotetrachlorovanadate (V), PCl4VOCl4,
and a complementary quantum chemistry theoretical study was made for both ions.
The molecular structures of the PCl4+ and VOCl4- ions were optimised by means of ab-initio and Density Functional Theory, using
different basis sets. Such structures present the expected Td
symmetry for PCl4+ and C4v symmetry for VOCl4-.
The wavenumbers corresponding to the normal modes of vibration were calculated
and used to confirm the spectral band assignments. The harmonic force constants
given by theory were scaled to reproduce adequately the experimental
wavenumbers.
Resumen
Se
obtuvieron los espectros de infrarrojo y Raman del oxotetraclorovanadato (V) de
tetraclorofosfonio (V), PCl4VOCl4, y se realizó un
estudio teórico complementario utilizando técnicas de la química cuántica. Las
estructuras moleculares de los iones PCl4+ y VOCl4- fueron optimizadas por medio de los métodos de la Teoría de los Funcionales de
Densidad, utilizando distintas bases. Esas estructuras presentan la simetría Td
esperada para el PCl4+ y la simetría C4v para
el VOCl4-. Se calcularon los números de onda
correspondientes a los modos normales de vibración, que fueron utilizados para
confirmar la asignación de bandas realizada. Las constantes de fuerza armónicas
suministradas por los cálculos teóricos fueron escaladas con el fin de
reproducir adecuadamente los datos experimentales.
Introduction
As part
of our studies about the structural and vibrational properties of vanadium
oxohalides, which comprised the VO2X2- anions
[1] and the VOX3 molecules [2], we considered the
oxotetrachlorovanadate (V) ion as the tetrachlorophosphonium (V) salt. As
neither structural data nor a vibrational study of this substance were found in
the literature, a study was undertaken which include measurement of its
infrared and Raman spectra and calculations of the molecular structure and
vibrational properties using quantum chemistry procedures. The VOCl4-
anion was first prepared in the form of salts of different organic cations by
Nicholls and Wilkinson [3], who reported only the corresponding
VO stretching wavenumber, besides the electronic spectra. Later,
Griffiths et al. prepared the PCl4VOCl4 salt and proposed
an assignment for the infrared spectra [4].
We present
now a more detailed study of the vibrational spectra for this compound and use
the revised assignment to define a force field for each ion.
Experimental
PCl4VOCl4 was prepared by reaction between a 1,2-dichloroethane solution of phosphorus
pentachloride and vanadium (V) oxide trichloride in a Schlenk apparatus under
dry nitrogen [4]. The red-brown precipitate was filtered off and dried in
vacuum, after washing with 1,2-dichloroethane. The product appeared as an
almost black solid with a green tinge.
The
infrared spectra of the pure substance between AgCl or polyethylene windows at
room temperature were run on a Bruker IFS 113v FTIR
spectrophotometer with a resolution of 2.0 cm-1 (LANAIS-EFO,
Universidad Nacional de La Plata, Argentina). The obtained infrared spectrum is
reproduced in Fig. 1, showing bands which are practically coincident with those
reported previously by Griffiths et al. [4]. The Raman spectrum were obtained
for the solid contained in a glass vessel by means of a Bruker FT Raman RF100S instrument,
which used 1.064 nm radiation from a Nd/YAG laser for excitation with a maximum
power of 1.5 W and 2.0 cm-1 of resolution (Servicios Técnicos de
Investigación, Universidad de Jaén, Spain). Excitation in the near infrared
resulted in an excellent Raman spectrum, reproduced in Fig. 2, which could not
be obtained by previous workers using visible light [4].
The
bands observed for PCl4VOCl4 in the infrared and Raman
spectra are collected in Table 1 and assigned to the normal modes of each ion,
in accordance with the previous proposal [4] and the present calculations.
Figure. 1.- The infrared spectrum of PCl4VOCl4.
Calculations
The
molecular geometries of the PCl4 + and VOCl4- ions were optimised by means of Hartree-Fock procedures and also with Density
Functional Theory (DFT) techniques, using in this last case the Becke’s
three-parameters exchange functional [5] in combination with the Lee-Yang-Parr
correlation functional [6], i.e. B3LYP, with several basis sets. The
wavenumbers corresponding to the normal modes of vibration were subsequently
calculated with the same methods and basis sets. All the calculations were
performed with the Gaussian 98 set of programs [7] considering the chemical
species in vacuo, that is, with no interaction with the surroundings.
The
harmonic force field in cartesian coordinates which resulted from the
calculations were transformed to symmetry coordinates through the
corresponding B matrix [8], obtained with a standard program. The resulting
force field was subsequently scaled using the scheme of Pulay et al. [9], in
which the diagonal force constants are multiplied by scaling factors fi
, fj and the corresponding interaction constants are multiplied
by ( fi . fj )1/2, adjusting these factors by
means of an iterative least squares procedure to reproduce as well as possible
the experimental wavenumbers. The potential energy distribution was
subsequently calculated with the resulting scaled quantum mechanics (SQM) force
field.
The
force field transformation, scaling and P.E.D. calculation were performed with
the program FCARTP [10]. The atomic displacements given by the Gaussian 98
programs for each vibrational mode were used to understand qualitatively the
nature of the molecular vibrations; for that purpose, the corresponding data
were represented graphically by means of the program GaussView [11].
Figure. 2.- The Raman spectrum of PCl4VOCl4.
The Wiberg bond indexes [12] in the
studied species were
calculated by means of the Natural Bond Orbitals (NBO) approach [13] as implemented
in the Gaussian 98 package, using the B3LYP/6-31+G combination.
Results of the geometry optimisations
and wavenumber calculations using the different theory levels, as well as the
SQM force constants matrices, are deposited as supplementary material
(available on the Internet at www.quimica.unlp.edu.ar/cequinor /varetti/PCl4VOCl4.pdf ).
Results
The
mid-infrared spectra of PCl4VOCl4 are dominated mainly by
the PCl4+ bands. In fact, with the exception of the 1017
cm-1 band, the VOCl4- bands appear below 370
cm-1. The assignment of the PCl4+ bands was
immediate, after comparison with the data of Griffiths et al. [4]. For the VOCl4- anion, however, the assignments rested heavily on the calculated wavenumbers.
The
results for both ions can be summarized as follows:
The tetrachlorophosphonium (V) ion, PCl4+
The calculations confirm the expected Td symmetry for this cation (Fig. 3). The molecular geometry optimized with the method mentioned below lead to a PCl distance of 1.98 Å, coincident with that measured by X-ray diffraction in PCl4ICl2 [14]. The nine normal modes of vibration should belong to the following symmetry species: A1 (R) + E (R) + 2 F2 (IR, R), where the infrared (IR) and Raman (R) activities are given in parentheses.
Figure. 3.- The calculated structures of the ions PCl4+ and VOCl4-.
Table 1.- Observed bands in the infrared and Raman spectra of PCl4VOCl4.
Experimental values (cm-1) |
Assignment |
|
Infrarrojo a |
Raman a |
|
1337.8 (vw) |
- |
2n3(PCl4+) |
1098.9 (m) |
- |
n1 + n3 (PCl4+) |
1015.6 (s) |
1017.0 (27) |
n1 (VOCl4-) |
827.9 (m) |
- |
n2 +n3 (PCl4+) |
763.9 (vw) |
- |
? |
699.8 (m) |
- |
n1 + n4 (PCl4+) |
657.0 (vs) |
656.0 (6) |
? |
640.0 (vs) |
641.0 (3) |
n3 (PCl4+) |
588.9 (s) |
- |
n5 + n6 (VCl5-) |
505.7 (m) |
- |
n3 + n4 (VCl5-) |
488.6 (m) |
456.0 (sh) |
? |
- |
454.0 (47) |
n1 (PCl4+) |
435.3 (m) |
- |
n5 (VCl5-) |
- |
359.0 (100) |
n2(VOCl4-) |
368.0 (vs) |
- |
n7 (VOCl4-) |
337.1 (s) |
- |
n2 (VOCl4-) |
256.0 (m) |
260.0 (30) |
n8(VOCl4-) |
246.0 (m) |
249.0 (28) |
n4 (PCl4+) |
- |
214.0 (12) |
n4 (VOCl4-) |
195.0 (w) |
196.0 (24) |
n6 (VOCl4-) |
- |
178.0 (15) |
n3 (PCl4+) |
- |
163.0 (11) |
n2 (PCl4+) |
161.0 (m) |
- |
n9 (VOCl4-) |
87.3 (w) |
86 (10) |
b |
68.8 (w) |
69 (13) |
b |
- |
56 (8) |
b |
a Relative intensity in parentheses.
b Probably lattice modes.
The
normal mode wavenumbers were calculated using several theory levels, as
mentioned before. Comparison of the measured and theoretical wavenumbers showed
that a very good reproduction of the experimental data is given by the
B3LYP/6-31G* combination, with a Root Mean Square Deviation (RMSD) equal to
12.6 cm-1. The calculations using different basis sets showed that
the inclusion of polarization functions caused a remarkable improvement in the
prediction of the vibrational wavenumbers for this ion. The calculated atomic
displacement vectors did agree with the previously proposed assignment [4].
The force field in cartesian coordinates given by the calculations
was transformed to a set of symmetry coordinates adequate for tetrahedral species [15]. The resulting force constants were
subsequently scaled using the approach of Pulay et al. [9] already mentioned in
the Calculations section, in order to reproduce as well as possible the
experimental wavenumbers.
The observed and calculated wavenumbers and their RMSD values,
theoretical infrared and Raman intensities and potential energy distribution
appear in Table 2. The SQM force field was used to calculate the following internal
force constants: f(PCl) = 3.62; f(PCl/PCl) = 0.22, in mdyn. Å-1 and
f(ClPCl) = 0.83; f(ClPCl/ClPCl) = -0.33, in mdyn.Å .rad-2.
Table 2.- Experimental and calculated wavenumbers (cm-1) and assignment for PCl4+.
Symmetry species |
Mode | Observed | Calculateda | Calc. SQMb | IR intensitiesc | Raman activitiesd | PED(³ 10%) | Assignment |
A1 | 1 | 454.0 | 435.0 | 453.1 | 0.0 | 25.9 | 100 S1 | ns (PCI4) |
E | 2 | 163.0 | 166.9 | 163.2 | 0.0 | 5.6 | 100 S2 | d(CIPCI) |
F2 | 3 | 640.0 | 639.2 | 640.6 | 158.0 | 3.4 | 102 S3 | na (PCI4 ) |
F2 | 4 | 246.0 | 245.7 | 245.5 | 6.5 | 7.2 | 88 S4 | d (CIPCI) |
RMSD (CM-1) | 12.6 | 0.56 |
Key to table 2. n, stretching; d, deformation.a B3LYP/ 6-31G* calculation. Observed and calculated values in cm-1 .b From scaled quantum mechanics force field. (see text).c Units are km.mol-1.d Units are Å4 (amu)-1.
The oxotetrachlorovanadate (V) ion, VOCl4-
The
optimized geometry obtained by the calculations for this anion using different
theory levels predict in all cases a C4v structure, as shown in Fig.
3. The twelve normal modes of vibration should be classified as 3A1
(IR, R) + 2B1 (R) + B2 (R) +3E (IR, R), where the
infrared (IR) and Raman (R) activities are given in parentheses.
Here
again, the wavenumbers corresponding to the normal modes of vibration were
calculated using several theory levels. The best agreement with the
experimental data was given by the B3LYP / 6-31+G calculation, with a RMSD
value of 18.4 cm-1. The corresponding optimized geometrical
parameters were: dVO = 1.584 Å; dVCl = 2.322 Å; αClVCl
= 86.9o; βOVCl = 103.5o. It should be
remarked that in this case, in contrast with the calculations made for PCl4+,
the introduction of polarization functions in the used basis sets degraded
notably the agreement between calculated and experimental wavenumbers, whereas
the introduction of diffuse functions gave better results. Such observation is
coincident with those made in the study of the VO2X2- [1] and VOX3 [2] species and point to a notably different behavior
of the theoretical calculations when the central atom is a metal or a non-metal
element.
As in
the case of the cation, the calculated force constants in cartesian coordinates
were transformed to the symmetry coordinates given in the literature for
chemical species having the same geometry [16] and subsequently adjusted with
the method proposed by Pulay et al. [9], generating a SQM force field which
reproduces the experimental data with a RMSD of 2.5 cm-1. The
observed and calculated wavenumbers, potential energy distribution and other
calculated data appear in Table 3. The SQM force field served to calculate the
internal force constants which follows: f(VO) = 7.26; f(VCl) = 1.50; f(VO/VCl)
= 0.14; f(VCl/VCl) = 0.33, in mdyn. Å-1 and
f(VOCl) = 0.52; f(ClVCl) = 1.00, in mdyn.Å .rad-2.
Table 3.- Experimental and calculated wavenumbers (cm-1) and assignment for VOCl4-.
Symmetry species |
Mode |
Observed |
Calculateda |
Calc. SQMb |
IR intensitiesc |
Raman activitiesd |
PED³ (10%) |
Assignment |
A1 |
1 |
1015.0 |
1067.9 |
1012.9 |
125.5 |
25.4 |
98 S1 |
n (VO) |
2 |
359.0 |
332.1 |
352.3 |
4.8 |
39.3 |
98 S2 |
ns (VCl4) |
|
3 |
178.0 |
188.1 |
175.8 |
1.4 |
7.4 |
98 S3 |
p (VOCl4) |
|
B1 |
4 |
214.0 |
217.4 |
213.6 |
0.0 |
6.3 |
98 S4 |
na (VCl4) |
5 |
- |
37.2 |
38.9 |
0.0 |
2.7 |
119 S5 + 22 S4 |
da (VCl4) |
|
B2 |
6 |
195.0 |
214.7 |
198.9 |
0.0 |
5.2 |
100 S6 |
ds (VCl4) |
E |
7 |
368.0 |
368.8 |
367.8 |
249.6 |
0.0 |
107 S7 |
nd (VCl4) |
|
8 |
256.0 |
256.2 |
255.6 |
1.3 |
3.3 |
76 S8 |
pd (VCl4) |
|
9 |
161.0 |
162.9 |
162.0 |
12.9 |
1.0 |
112 S9 |
dd (VCl4) |
RMSD (cm-1) |
|
|
18.4 |
2.5 |
|
|
|
|
n,
stretching; d, deformation; p,
deformation out of plane.
a B3LYP/6-31+G calculation. Observed and calculated
values in cm-1.
b From scaled quantum mechanics force field (see text).
c Units are km.mol-1.
d Units are Å4 (amu)-1.
Discussion
The internal force constants and other parameters obtained for VOCl4- and for the previously studied species VO2Cl2- and VOCl3 are compared in Table 4.
Table 4 - Comparison of internal force constants a and other parameters for VO2Cl2- , VOCl3 and VOCl4-.
|
VO2Cl2- (C2v) |
VOCl3 (C3v) |
VOCl4- (C4v) |
f(V-O) |
6.55 b |
7.73 c |
7.26 d |
f(V-Cl) |
1.87 |
2.67 |
1.50 |
f(VO/VCl) |
0.17 |
0.18 |
0.14 |
f(VCl/VCl) |
0.12 |
0.14 |
0.33 |
f(VOCl) |
0.50 |
0.58 |
0.52 |
f(ClVCl) |
0.46 |
0.41 |
1.00 |
d(VO) (Å) |
1.582 e |
1.570 f |
1.580 g |
WI (VO) h |
1.838 |
2.065 |
2.027 |
d(VCl) (Å) |
2.226 e |
2.142 f |
2.322 g |
WI (VCl) h |
0.698 |
1.091 |
0.841 |
α(ClVCl) |
110.8 e |
111.0 f |
86.9 g |
a Units are mdyn Å-1 for
stretching and stretching/stretching interaction and mdyn
Å rad -2 for angle deformation.
b Force constants from Ref. 1; c Force
constants from Ref 2; d Force constants from this work; e
Experimental value from Ref. 17; f Experimental value from Ref. 18; g
Calculated with B3LYP/6-31+G; h Wiberg index calculated with B3LYP/6-31+G.
It is
observed that the VO force constants and Wiberg indexes for the VO bond follow
the same trend, with a maximum in VOCl3. The VO distances, however,
are quite similar although the smaller value measured for VOCl3 seem
to correspond with the larger VO force constant. The comparison of the VCl
force constants show again the larger value for VOCl3, correlated
with a relatively large Wiberg index and smaller VCl bond distance.
The
VOCl4- anion, with a very different geometry in
comparison with the other two species, show notably high values for the ClVCl
deformation and VCl/VCl interaction force constants. Such observation as well
as the relatively large VCl bond distance could be explained by the small ClVCl
angle, which causes a strong interaction between the bulky chlorine atoms.
The
potential energy distributions obtained for both ions (Tables 2 and 3) shown
that there is only a small degree of mixing between the modes of vibration
defined by the used symmetry coordinates. This is shown by the participation
of the diagonal force constants in each mode, which are ³ 88 % for PCl4+ and ³ 76 % for VOCl4- .
Acknowledgements
The research grants provided by CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina) and UNLP (Universidad Nacional de La Plata) are gratefully acknowledged, as well as the financial help received from CIUNT (Consejo de Investigaciones de la Universidad Nacional de Tucumán).
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