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Keywords:

  • X-ray diffraction;
  • Matrix;
  • Phosphoryltrifluoride;
  • Vibrational spectroscopy;
  • Quantumchemical calculation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Phosphoryl fluoride was characterized by Raman spectroscopy and X-ray diffraction analysis. The X-ray structure was obtained by in-situ crystallization. Phosphoryl fluoride crystallizes in the trigonal space group Pequation imagem1 with two formula units in the unit cell. In the crystal structure zigzag chains are observed which are formed by intermolecular P–O contacts. The Raman spectra of neat and matrix isolated POF3 display an extra line, which indicates intermolecular interaction in the solid state. Therefore quantum chemically calculation of a POF3 oligomer was performed. The theoretical calculation indicates that the extra Raman line is caused by side splitting of the P–O valence vibration.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Experimental spectroscopic and structural data of phosphoryl fluoride (POF3) in the solid state have only been described rarely in literature. IR spectroscopy in inert gas matrices and calorimetric measurements were performed to characterize condensed POF3.13 Structural data are only known for the gas phase.4 Smitskamp, Olie, and Gerding reported an additional line in the Raman spectra of the analogous phosphoryl halogenides (POX3, X = Br, Cl).5 This line is suggested to be an intermolecular interaction of the molecules in the solid state. In order to investigate possible intermolecular interactions of phosphoryl trifluoride in the solid state, Raman spectroscopic investigations and the X-ray structure of the compound were studied.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Raman Spectroscopy

Raman spectra of phosphoryl fluoride were recorded in all three aggregation states (Figure 1). The lines of gaseous and liquid POF3 are in agreement with the literature values.4,5.

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Figure 1. Raman spectra of solid, liquid, and gaseous POF3. Experimental parameters: 150 μm slith width, resolution 0.25 cm–1, laser power 250 mW.

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The Raman spectrum of solid POF3 displays an additional vibration mode in the area of the P–O stretching vibration, similarly to phosphoryl halogenides (POX3, X = Br, Cl) described in the literature.6 Due to this mode at 1368 cm–1, intermolecular interactions are suggested. No additional vibration modes were found in the Raman spectrum of gaseous POF3.7 Intermolecular interactions seem to be only present in the solid state. Additionally Raman matrix spectra of POF3 in argon and xenon were recorded. Matrix isolated POF3 in argon display side-splitting of mode ν2 (Table 1). Raman matrix spectra of POF3 in xenon at a dilution of 1:200, 1:500, and 1:1000 are shown in Figure 2.

Table 1. Experimental and calculated frequencies of POF3.
SolidLiquidGasTetramer b)Assign.
  1. a

    a) Matrix spectra at a dilution of 1:1000. b) Major Raman frequencies are scaled with an empirical factor of 1.008. c) Side-splitting of ν2 in argon matrix at a dilution ratio of 1:1000.

equation image /cm–1equation image /cm–1equation image /cm–1equation image /cm–1 
   XenonArgon  
138213941415140314061382ν(P–O)
1368  139113941329 
     997 
989  985988973νas(PF3)
886876873871876 c)869νs(PF3)
    870 c)  
493  479482473δas(PF3)
479478 467470437δs(PF3)
344339 336336311δ(PF3)
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Figure 2. Raman spectra of matrix isolated POF3 at different dilution ratios. Experimental parameter: 150 μm slith width, resolution 0.25 cm–1, laser power 250 mW.

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Matrix isolated phosphoryl trifluoride at a low dilution ratio of 1:200 is represented by a liquid-like situation. In contrast to the 1:200 xenon / POF3 matrix, POF3 molecules at high dilution ratio of 1:1000 can be described by a gas-like situation. Intermolecular interaction between the molecules decreases with higher dilution. Surprisingly, the line at 1368 cm–1 is present in all Raman matrix spectra. This suggests relatively strong interactions between the molecules, which may be present as oligomers, even at a dilution ratio of 1:1000.

Theoretical Calculations

The additional line x1 in the Raman spectrum of solid POF3 is in the range of the P–O stretching vibration. The extra line cannot be explained by a combination mode or overtone. A possible explanation for the line at 1368 cm–1 could be side splitting due to the crystal packing. As example for packed molecules with intermolecular interactions a POF3 tetramer was calculated. The input geometry of the tetramer was taken from the crystal structure. POF3 molecules were optimized at the M06-2X/ aug-cc-pVDZ level of theory with the Gaussian program package.8 The M06–2X9 functional was used because of the implemented dispersion correction. The structure of the POF3 tetramer is shown in Figure 3.

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Figure 3. Optimized structure of POF3 tetramer calculated at the M06-2X/ aug-cc-pVDZ level of theory.

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The calculated Raman spectrum now displays an extra line in the area of the P–O stretching vibration. The additional line is the P–O valence mode of the central POF3 molecule in the tetramer structure (Figure 4). Furthermore a new strong P–F vibration in the Raman spectrum of the calculated tetramer is present at 998 cm–1. The Raman matrix spectrum of solid POF3 in xenon at a dilution of 1:200 displays also an additional line in this area. Hence the intermolecular interactions of the Raman matrix spectra may be best characterized by the calculated tetramer. The side splitting of the P–O stretching vibration can therefore be illustrated by the calculated tetramer structure of POF3. Experimental and calculated vibrational modes of POF3 are summarized in Table 1.

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Figure 4. Calculated Raman spectrum of POF3 tetramer at the M06-2X/cc-aug-pVDZ level of theory.

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X-ray Diffraction Analysis

For the structure determination, single crystals were grown in situ in a glass capillary, which is mounted on a diffractometer and cooled by a nitrogen stream at a controlled temperature. The focus of the gas stream can be modified by moving the capillary in the vertical direction. Crystals were obtained by sublimation at –50 °C (b.p. –39.7 °C and m.p. –39.1 °C10). A single crystal of dimension 0.2 × 0.2 × 0.1 mm was obtained (Figure 5; right) after repeating of the procedure for several times. The structure was determined at 173(2) K.

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Figure 5. Molecular unit of POF3 (50 % probability displacement ellipsoids). Symmetry codes: I = 1–y, xy, z; ii = 1–x+y, 1–x, z. (left); a single crystal in the capillary (right).

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Phosphoryl trifluoride POF3 (Figure 5; left) crystallizes in the trigonal space group Pequation imagem1 with two formula units in the unit cell. The phosphorus atom is coordinated distorted tetrahedral with bond angles of 116.37(6)° (O–P–F) and 101.77(7)° (F–P–F). Compared with the gas electron diffraction studies of Moritani et al.11 the coordination of the phosphorus atom is almost the same (F–P–F: 103.3°). In comparison with phosphoryl trichloride POCl3 described by Olie12 the bond angles Cl–P–Cl (104.9° average) are smaller. The P–F bond length is with 1.506(1) Å about 0.01 Å shorter than a formal P–F double bond (1.52 Å). Also compared with the gas phase structure the P–F bond is shortened (1.524 Å). The P–O bond length is with 1.428(2) Å about 0.09 Å shorter than a formal P–O double bond (1.52 Å). The P–O bond length of the gas phase structure described by Moritani is with 1.436 Å a little bit longer. Figure 6 shows the super cell of POF3 along the b axis. The shortest F···O contact is 3.126(2) Å and the F···F contact is 3.055(1) Å, which are slightly longer than the sum of the van der Waals radii (2.99 Å and 2.94 Å).13 However, the intermolecular P···O contacts within the unit cell are with 3.248(1) Å shorter than the sum of the van der Waals radii (3.32 Å)13 (Figure 6). This result confirms the assumption that the additional line in the Raman spectrum at 1368 cm–1 is caused by an intermolecular interaction.

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Figure 6. View of the cell along the b axis (50 % probability displacement ellipsoids). P–O contacts are drawn as dashed lines.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

The condensed phase of POF3 was characterized by Raman spectroscopy and X-ray diffraction analysis. The Raman spectra of the solid display an extra line which indicates side splitting. A tetramer of POF3 molecules was calculated as a model for such interactions. The results of the calculation suggest that the additional vibration mode x1 is caused by intermolecular interactions of the molecules, which were confirmed by the crystal structure. This effect may be also present in the homologous phosphoryl halogenides described in literature.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Synthesis and sample handling was performed by employing standard Schlenk techniques and using a stainless-steel vacuum line. Reactions were carried out in 1 L stainless steel reactors, which were closed with a stainless-steel valve. The matrix isolation device consists of a standard high vacuum pumping system and a gas mixing unit. For condensation of the gas-mixtures the cryostat (ARS 202B) was cooled by a closed-cycle helium refrigerator to 10 K. The cold tip attached to the cryostat consists of a silver coated copper block. The matrix layers were prepared by condensation of 6 mmol gas at a continuous flow rate of 6 mmol·h–1 (130 cm3·h–1). An argon ion laser (Stabilite 2017, Spectra-Physics, 514.5 nm) was used as light source with an angle of 55°. The Raman spectra were detected with a CCD Raman spectrometer (Jobin-Yvon T64000). The low-temperature X-ray diffraction of POF3 was performed with an Oxford XCalibur3 diffractometer equipped with a Spellman generator (voltage 50 kV, current 40 mA) and a KappaCCD detector, operating with Mo-Kα radiation (λ = 0.7107 Å). Data collection at 173 K was performed using the CrysAlis CCD software,14 the data reductions were carried out using the CrysAlis RED software.15 The solution and refinement of the structure was performed with the programs SHELXS16 and SHELXL-9717 implemented in the WinGX software package18 and finally checked with the PLATON software.19 Selected data and parameters of the X-ray analysis are given in Table 2.

Table 2. X-ray data and parameters of POF3.
 POF3
  1. a

    a) R1 = Σ||Fo|–|Fc||/Σ|Fo|. b) wR2 = [Σ[w(F02Fc2)2]/Σ[w(F0)2]]1/2; w = [σc2(Fo2)+(xP)2+yP]–1; P = (Fo2 + 2Fc2)/3. c) GoF = {Σ[w(Fo2Fc2)2]/(n–p)}1/2 (n = number of reflections; p = total number of parameters).

FormulaPOF3
Mr /g·mol–1103.97
Cryst. size /mm30.25 × 0.12 × 0.1
Crystal systemtrigonal
Space groupPequation imagem1
a5.4159(2)
b5.4159(2)
c6.1301(5)
α90.0
β90.0
γ120.0
V3155.72(2)
Z2
ρcalcd /g·cm–32.217
μ /mm–10.767
λMoKα0.71073
F(000)100
T /K173(2)
hkl range–7:6; –7:7;–7:6
Refl. measured523
Refl. unique159
Rint0.0210
Parameters14
R(F) / wR(F2) a) (all reflexions)0.0271/ 0.0613
Weighting scheme b)0.0253/ 0.0611
S (GoF)c)1.183
residual density /e·Å–30.177/ –0.334
Device typeOxford XCalibur
Solution/ refinementSHELXS-97/ SHELXL-97

Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository number CCDC-952338 http://www.ccdc.cam.ac.uk/data_request/cif (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).

Synthesis of POF3: Phosphoryl trichloride POCl3 (1.25 mol, 196.6 g) was consecutively condensed to antimony(III) fluoride SbF3 (1.82 mol, 326 g) into a 1 L reactor (steel) cooled to –196 °C. Afterwards, the reactor was slowly warmed up to 110–120 °C. The reaction starts during the heating (p = 43 bar). After 3 h the reactor was cooled to room temperature (p = 5–8 bar). The raw product was condensed in a second 1 L reactor with antimony(III) fluoride SbF3 (89 g, 0,49 mol, 87 g). To remove the remaining raw product, which is trapped in SbCl3, the reactor was heated up to 120 °C (p = <2 bar), then cooled to room temperature, and also condensed into the second reactor (20 g). Afterwards, this reactor was heat up to 120 °C for 3–5 h. After cooling to room temperature, the product was condensed in a 0.5 L lecture bottle (101 g). To purify the product POF3 from the more volatile byproducts (HCl or PF5) the lecture bottle was cooled to –70 °C and evacuated to 100 mbar. 31P NMR: δ = –33.8 ppm. 19F NMR: δ = –94.2 ppm

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Financial support of this work by the Ludwig-Maximilian University of Munich (LMU) and the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.

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