Pure rotational spectrum of phosphorus pentafluoride observed by microwave Fourier transform spectroscopy

Abstract

Pure rotational transitions of PF 5 (phosphorus pentafluoride) in the vibrational ground state have been measured in the frequency range of 9–25 GHz for the first time. The transitions are forbidden in the rigid rotor limit but become weakly allowed due to a small dipole moment induced by centrifugal distortion. They obey the selection rule Δ k = ±3. The extremely weak absorption lines in the static gas have been recorded with a pulsed microwave Fourier transform spectrometer. Q-branch series for K = ∥ k∥ = 1, 2, 3, and 5 in the lower state have been identified with J ranging from 45 to 94. The series J 6←3 has been found to split into A 1 and A 2 states under the influence of sextic and octic centrifugal distortion nondiagonal in K. In a weighted least-squares fit, the molecular constants ( A- B), D JK , D K , H JK , h 3, and h 3 J have been simultaneously determined from the measured transition frequencies. Combining the difference ( A- B) of the rotational constants with the result for B from recent high-resolution infrared spectra has provided a value for A. A complete r 0 structure has been determined from the rotational constants A and B. It agrees with previous electron diffraction studies and with ab initio calculations. The tensorial constant θ x xx which measures the induced dipole moment has been estimated by comparing the transitions of PF 5 with the “forbidden” transitions of OPF 3. No indication of a splitting larger than the present linewidth of 50 kHz has been found which may be attributed to the Berry pseudorotation.