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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 75^thAnniversary

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 T_d symmetry for PCl₄⁺ and C_4v 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 T_d esperada para el PCl₄⁺ y la simetría C_4v 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 f_i , f_j and the corresponding interaction constants are multiplied by ( f_i . f_j )^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 T_d 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 C_4v 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: d_VO = 1.584 Å; d_VCl = 2.322 Å; α_ClVCl = 86.9^o; β_OVCl = 103.5^o. 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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