MOLECULAR SPECTROSCOPY AND APPLICATIONS

(Department of Optics and Matter-Radiation Interaction)

  

Institut Carnot de Bourgogne
UMR 5209 CNRS-Université de Bourgogne, 9 av. A. Savary, BP 47 870, F-21078 DIJON Cedex, FRANCE

Transition-metal hexafluorides, hexacarbonyls and other molecular open-shell systems

The transition metal hexafluorides form a very interesting group of molecules, since they are the only compounds of these heavy elements with substantial vapor pressures at room temperature. This implies several industrial applications, such as isotope separation processes (especially with UF6, NpF6 and PuF6) or chemical vapor deposition of thin metal layers on a substrate (WF6, MoF6 for integrated circuits ; ReF6, IrF6 for anti-corrosion layers).

Among these molecules, there is a very interesting group which is traditionally called "colored hexafluorides". They are caracterized by the fact that they have an incomplete electronic subshell. This leads to low energy electronic levels, corresponding to transitions in the visible or near infrared regions (see figure below). Some of the "colored hexafluorides" also have the property of possessing an odd number of electrons (TcF6, RhF6, ReF6, IrF6, NpF6, AmF6).

Difference between the absorption spectra of XY6 closed- and open-shell systems.

Their specificities of these molecules are that :

We have already recorded and analyzed low-resolution absorption and Raman spectra of IrF6, OsF6 and WF6 and obtained low-resolution absorption spectra of PtF6.

High-resolution spectra of WF6 and ReF6 using infrared spectroscopy (Fourier transform and diode laser) in a supersonic expansion jet were obtained in the Group of Professor Martin Quack (LPC-ETH Zürich). The analysis of the spectra of ReF6 requires a tensorial formalism adapted to rovibronic couplings and to the presence of half-integer angular momenta in octahedral symmetry. This work is still in progress

Transition-metal hexacarbonyls (Cr(CO)6, Mo(CO)6, W(CO)6, ...) are also interesting octahedral systems although this familly of molecules is much smaller that that of hexafluorides. There also exist an open-shell hexacarbonyl system with an odd number of electrons, namely V(CO)6 (vanadium hexacarbonyl).

Such studies could be extended to other open-shell systems like ReOF4 or to molecular ions or radicals of interest in astrophysics (CH3O, ...) or atmospherics physics.

Table of the known and expected hexafluorides

(n is the number of valence electrons of the central M6+ ion of MF6.)

Example 1 : The NIR-Visible-UV absorption spectrum of some colored hexafluorides

Spectra from M. Rotger and V. Boudon. The letters (a, b, c, ...) indicate the different electronic states.

Example 2 : The NIR-Visible-UV absorption spectrum of some IrF6

Spectra from M. Rotger and V. Boudon. The letters (a, b, c, ...) indicate the different electronic states (with symmetry in parentheses).

Example 3 : Jet + diode laser high-resolution spectrum in the v3 band of WF6
The case of a non-degenerate electronic ground state

Spectra from V. Boudon, M. Rotger, Y. He, H. Hollenstein, M. Quack and U. Schmitt
Journal of Chemical Physics, 117, 3196–3207 (2002)

Example 4 : Jet + FTIR and diode laser high-resolution spectrum in the v3 band of ReF6
The case of a fourfold degenerate electronic ground state

Spectra from V. Boudon, M. Rotger, Y. He, H. Hollenstein, M. Quack and U. Schmitt
Journal of Chemical Physics, 117, 3196–3207 (2002)

Example 5 : Jet + FTIR spectrum in the v6 band (C-O stretch) of V(CO)6
The case of a threefold degenerate electronic ground state

Spectrum from P. Asselin, L. Manceron and P. Soulard / LADIR, Paris, France
Simulation calculated by Michael Rey
Journal of Chemical Physics, 114, 10773-10779 (2001)

 More references about our group's work in this field can be found in Vincent Boudon's home page.

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