Vibrational isotope shifts in hexafluoride molecules

Abstract

Central-atom isotope frequency shifts for the v 3 stretching fundamentals of octahedral hexafluorides are reviewed. Accurate shifts have been measured for the hexafluorides of S, Se, Mo, Te, W and U, and can be calculated from force fields for those of Rh, Ir, Pt, Np and Pu. A theoretical treatment of the relation between the vibrational frequency v 3 and the central-atom mass m establishes the parametric dependence of the isotope shift Δ v 3. This yields a semi-empirical formula Δ v 3(cm −1amu −1) = −4.20 v 3 m −1.75, with v 3 in cm −1 and m in amu. Frequency shifts calculated from this formula agree with measured shifts generally to within 5%, and it promises to be useful in estimating such shifts for Jahn—Teller hexafluorides and for hexafluoride ions. The relative precision of isotope frequency shifts and Coriolis constants in constraining the general quadratic force fields of XF 6 molecules has also been considered. For a given precision in measuring frequency shifts, Δ v 3 is more effective than Δ v 4, by the ratio v 3/ v 4, for determining the off-diagonal symmetry force constant F 34. F 34 is about equally well constrained by the Coriolis constants for all molecules, but the frequency shifts become much less effective for this purpose as the central-atom mass increases.