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. 2021 Jul 29;125(29):6362-6373.
doi: 10.1021/acs.jpca.1c02595. Epub 2021 Jul 15.

Toward an Understanding of the Pressure Effect on the Intramolecular Vibrational Frequencies of Sulfur Hexafluoride

Matteo Boccalini  1 , Roberto Cammi  2 , Marco Pagliai  1 , Gianni Cardini  1 , Vincenzo Schettino  1
Affiliations

Toward an Understanding of the Pressure Effect on the Intramolecular Vibrational Frequencies of Sulfur Hexafluoride

Matteo Boccalini et al. J Phys Chem A. .

Abstract

The structural and vibrational properties of the molecular units of sulfur hexafluoride crystal as a function of pressure have been studied by the Extreme Pressure Polarizable Continuum Model (XP-PCM) method. Within the XP-PCM model, single molecule calculations allow a consistent interpretation of the experimental measurements when considering the effect of pressure on both the molecular structure and the vibrational normal modes. This peculiar aspect of XP-PCM provides a detailed description of the electronic origin of normal modes variations with pressure, via the curvature of the potential energy surface and via the anharmonicity of the normal modes. When applied to the vibrational properties of the sulfur hexafluoride crystal, the XP-PCM method reveals a hitherto unknown interpretation of the effects of the pressure on the vibrational normal modes of the molecular units of this crystal.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Molecular structure of sulfur hexafluoride SF6.
Figure 2
Figure 2
Graphical representation of the normal modes of SF6 with the assignment proposed by Salvi et al. Only one component of the each degenerate modes is shown.
Figure 3
Figure 3
Fit of energy values with both Murnaghan and Birch–Murnaghan equations of state. The procedure has been carried out for XP-PCM calculations with both N = 3 and N = 6.
Figure 4
Figure 4
Comparison between experimental and computed Murnaghan equation of states with volume. Cavity volume and molecular volume have been adopted for calculations and experiments, respectively.
Figure 5
Figure 5
S–F bond length (in Å) with the pressure (in GPa). The results are reported for both PBE and PBE0 exchange and correlation functionals in conjunction with 6-311G(d) basis set. The equations of linear regression are PBE N3, y = −0.00068x + 1.62246; PBE N6, y = −0.00063x + 1.62216; PBE0 N3, y = −0.00070x + 1.58786; and PBE0 N6, y = −0.00055x + 1.58770.
Figure 6
Figure 6
Experimental and computed vibrational frequencies of normal mode ν5(T2g). The XP-PCM calculations have been carried out with both N = 3 and N = 6. The experimental data have been taken by Rademacher et al.
Figure 7
Figure 7
Experimental and computed vibrational frequencies of normal mode ν2(Eg). The XP-PCM calculations have been carried out with both N = 3 and N = 6. The experimental data have been taken by Rademacher et al.
Figure 8
Figure 8
Experimental and computed vibrational frequencies of normal mode ν1(A1g). The XP-PCM calculations have been carried out with both N = 3 and N = 6. The experimental data have been taken by Rademacher et al.
Figure 9
Figure 9
Vibrational analysis in terms of curvature (orange) and relaxation (green) contribution to formula image (blue). The upper panel refers to the results obtained with N = 3, while the lower panel refers to the results obtained with N = 6.
Figure 10
Figure 10
a) Electron density difference between SF6 at 0.0 and 24.6 GPa (cutoff ±3 × 10–5). b) The red and blue colors on the map represent regions with increase and decrease of electron density, respectively. The isosurface cutoff is ±1.5 × 10–5.
See this image and copyright information in PMC

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