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Fourier Transform Infrared Emission Spectra of MgF 2

This study presents the detailed analysis of MgF2 infrared emission spectra along with ab initio calculations, providing insights into the molecular structure and vibrational modes of MgF2. The rotational and vibrational levels, equilibrium constants, and key spectral features are thoroughly examined.

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Fourier Transform Infrared Emission Spectra of MgF 2

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  1. Fourier Transform Infrared Emission Spectra of MgF2 Peter Bernath, Daniel Frohman Department of Chemistry and Biochemistry Old Dominion University, Norfolk, VA Jacek Koput Department of Chemistry, Adam Mickiewicz University, Poznań, Poland

  2. Alkaline Earth Dihalides “The prediction, verification, and explanation of the unexpected bent structure of some alkaline earth dihalides and dihydrides pose extreme demands on experiment and theory and qualify among ‘the most intriguing problems of modern inorganic chemistry’. It must have come as a real surprise when Klemperer et al. showed that CaF2, SrF2, SrCl2, and all barium halides display permanent dipoles in the gas phase and are therefore bent.” von Szentpaly, JPC A106, 11945 (2002) • Be and Mg dihalides are linear • Caand Srdihalides may be either linear or bent • Ba species are always bent • VSEPR model predicts a linear structure; sd hybridization invoked

  3. Alkaline Earth Difluorides Wavenumber (cm-1) BeF2ν3 mode emission BeF2: linear, re=1.373 Å, Frumet al. JCP 95, 1435 (1991), Yu et al. JCP 123, 134304 (2005) MgF2: linear; SrF2: bent; BaF2: bent

  4. Previous MgF2 Studies • 1964, Buchler, Stauffer, and Klemperer – no molecular beam deflection by inhomogeneous electric field: MgF2 is linear • 1966, Snelson - IR matrix isolation in Ne, Ar, and Kr - observed ν2 (254 cm-1) & ν3 (862 cm-1) in a neon matrix • 1968,1969, Baikov– gas phase IR: observed ν2(160 cm-1) & ν3 (825 cm-1) • 1976, Lesiecki& Nibler– IR and Raman in matrices: ν1 (550 cm-1), ν2 (249 cm-1) & ν3 (842 cm-1) in an argon matrix • 1980, Kasparov et al. – gas phase electron diffraction study; recommend rg(Mg-F) = 1.77±0.01 Å, ν1 (540±20 cm-1), ν2(165±10 cm-1) & ν3 (825±20 cm-1) • 2014, this work, ab initio calculation by J. Koput: re(Mg-F) = 1.738 Å, ν1 (569 cm-1), ν2(153 cm-1) & ν3(882 cm-1) ν1 (Σg+) ν3(Σu+) ν2 (Πu) doubly degenerate

  5. MgF2 spectra recorded at University of Waterloo with Bruker IFS 120 HR Fourier transform spectrometer. Solid MgF2 powder was heated in a tube furnace to 1675°C . Resolution was 0.01 cm-1for 700-1300 cm-1 region.

  6. MgF2ν3 Emission 021-020 head 011-010 head 001-000 head Series of band heads 1.6 cm-1 apart Jmax=68 Qvib=615(!)

  7. Ab Initio Calculations • MOLPRO calculations similar to work on BeF2 • re(Mg-F)= 1.7380(5) Å • PES of r(MgF) = 1.4 to 2.4 Å and <FMgF = 80 to 180 °, calculated by CCSD(T)/aug-cc-pCV5Z (Mg), auc-cc-pV5Z (F), with valence and Mg outer core correlation by CCSD(T) • PES used to calculate vibration-rotation energy levels for 24MgF2 • fit the levels to obtain spectroscopic constants for PGOPHER

  8. Ab initio Derived Constants (J=0-12)

  9. Spectrum Overview

  10. Rotational and Vibrational Levels ν3+3ν2-3ν2 ν3+2ν2-2ν2 ν3+ν2-ν2 ν3 l-type doubling via q and qDremoves e/f parity degeneracy

  11. ν3, 001-000 (Σu+ – Σg+) • A 3:1 intensity alternation due to nuclear spin statistics of equivalent fermions (19F with I = ½) • ν3, antisymmetric stretch: 879.0060(24)cm-1 vs 878.7128 cm-1 (ab initio) • Rotational constants: B000of 0.148591(20) cm-1and B001 of 0.147667(20) cm-1vs ab initio values of0.146711 cm-1 and 0.145790 cm-1, respectively

  12. ν3+ν2-ν2, 0111 – 0110 (Πg – Πu) • 0110 (Πu) origin fixed to ab initio value 153.1145 cm-1 • upper state term value 0111 (Πg) fit: 1030.2307(35) cm-1and ab initio: 1029.9399 cm-1. • Rotational constants: B010 and B011 fit: 0.149093(29)cm-1 and 0.148173(29)cm-1 • vs ab initio: 0.147233 cm-1 and 0.146316 cm-1 • l-type doubling observed for 0110 (Πu) and 0111(Πg) q fit: -3.130(73)E-4 cm-1 & -3.096(72)E-4 cm-1 vs ab initio: -3.176E-04 cm-1 and -3.140E-04 cm-1 • qD was held fixed as 1.180E-09 cm-1 and 1.215E-09 cm-1

  13. Rotational and Vibrational Levels

  14. Equilibrium Rotational Constants Bv = Be – α1(v1 + ½) – α2(v2+ 1) –α3(v3+ ½) • a) This value fixed to ab initio value. • b) The ab initio values are based onthe CCSD(T)/aug-cc-pCV5Z; better ab initio value is 1.7380 Å.

  15. Conclusion • The first high resolution infrared emission spectra ofMgF2were analyzed. • Assignments were made for the ν3 fundamental mode and one hot band with the help of high level ab initiocalculations. • Recording spectra with a higher signal-to-noise ratio might help, but the lines will remain overlapped, and assignments challenging.

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