Knowledge Bank

The infrared spectrum of natural $TeF6$ and of $99.3\%\mathrm{<mrow\; data-mjx-texclass="ORD">130</mrow>}TeF6$ has been recorded using FTIR and grating spectrometers and tunable diode lasers. The isotopic shift in $\nu 3$ (ca. 0.68 $cm-1/amu$) was measured and rotational structure in the P and R branches was resolved and analyzed. Integrated absorptivities were also measured for the stronger bands. The isotope shifts and Coriolis constants were used to fix the general quadratic symmetry and valence force fields, and the force constants of the Group VI series $SF6,SeF6$, and $TeF6$ will be compared and discussed. A theoretical treatment of the relation between the vibrational frequency $\nu 3$ and centralatom mass m in octahedral $XF6$ molecules yields a semi-empirical formula for the isotope shift $\Delta v3$ (in $cm-1/amu$): $\Delta \nu 3=-4.20\nu 3m-1.75,$ where m is in amu. Frequency shifts obtained from this formula agree with measured shifts for the hexafluorides of S***, Se, Mo, Te, W, and U with a mean error of less than 0.02 $cm-1/amu$. The relative usefulness of $\Delta v3$ and $\Delta v4$ and of the Coriolis constants $\zeta 3$ and $\zeta 4$ in constraining the general quadratic force fields of these molecules will be discussed.