Microwave Spectrum and Structure of Perrhenyl Fluoride

Abstract

Rotational absorption lines of the symmetric‐top molecule ReO₃F have been observed in the K‐band regions of 21 400 and 28 500 Mc, with a Stark modulation spectrometer designed specially to accommodate this and other chemically reactive molecules The spectrum exhibits a hyperfine structure arising from interaction of the Re nuclear electric quadrupole moment with the surrounding charge. The hfs pattern establishes the Re nuclear spin as 5/2 and yields a quadrupole coupling constant eqQ (Re¹⁸⁷) = (—48.4±1.3) Mc for the ground vibration state. Additional groups of lines associated with molecular states of vibrational excitation were also present. Of the six normal vibration frequencies, three were sufficiently low to contribute an observable fraction of molecules to their excited states: one non‐degenerate, ν₃ (a₁), and two doubly degenerate, ν₅ (e) and ν₆ (e). The corresponding rotation‐vibration and l type doubling constants were measured to be α₃ = (+12.30±0.06) Mc, α₅ = (—10.91±0.06) Mc, ql₅ = (16.31±0.25) Mc, α₆ = (+2.52±0.06) Mc, ql₆ = (5.00±0.24) Mc. Strong dependence of quadrupole coupling on molecular vibration was observed, the values of eqQ (Re¹⁸⁷) being —27, —35, and —58 Mc respectively in the singly‐excited states ν₃, ν₅, and ν₆. Comparative measurements of line intensities relative to those of the ground vibration state yield the following approximate values for the three lowest normal vibration frequencies: ν₃ = (325±40) cm^—1, ν₅ = (410±25) cm^—1, ν₆ = (345±40) cm^—1. Measurements of the Stark effect in ReO₃F give as the dipole moment of this molecule a value μ = (0.85±0.05) debye. Precise determination of molecular structure was effected from frequency measurements of lines belonging to the slightly asymmetric top ReO₂¹⁶O¹⁸F. The results give Re–O = (1.692±0.003) A, Re–F = (1.859±0.008) A, F–Re–O = 109° 31′±16′. Comparison is made between these and the known values of two other structurally similar molecules, ReO₃Cl and MnO₃F.