The v4 = 1 and v4 = 2 rovibrational levels of PF3 revisited: New solutions for old topics

Abstract The high-resolution infrared spectra of trifluorophosphine (PF 3 ) were reinvestigated in the ν 4 fundamental region near 350 cm −1 , and around 690 cm −1 , with the aim to provide a necessary reassignment of the 2 ±2 sublevel of the v 4  = 2 overtone level. The present paper reports on the first complete study of both sublevels of v 4  = 2 (of A 1 and E symmetry, corresponding to l 4  = 0 and ±2, respectively), through the high-resolution analysis of the overtone 2 ν 4 0 band and the 2 ν 4 ± 2 - ν 4 ± 1 hot band. The assignments of the latter were corrected and extended, spanning the rotational states J  ⩽ 82 and −80 ⩽  K ″ · Δ K  ⩽ 48. These new infrared assignments in the v 4  = 2 state were combined with accurate infrared, radiofrequency, centimeter-, millimeter- and submillimeter-wave data of the v 4  = 1 level (Thiessen et al., 2000), together with rotational data in the ground vibrational state (Cotti et al., 1995), in a simultaneous fit. The existence of resonance crossings due to a Δ k  = ±1, Δ l  = ∓2 l -type resonance in the v 4  = 1 state, which generated perturbation-allowed transitions, provided independent values of the C 4 and Cζ 4 constants. Combining these rotational transitions with the wavenumbers of the ν 4 fundamental band enabled us to determine accurately the C 0 axial ground state constant. Moreover, the assignment of a few, very weak r R K transitions in the 2 ν 4 - 2 overtone band and their inclusion in the global least-squares fit allowed also the first accurate experimental determination of D K 0 . The obtained results are (in cm −1 ): C 0  = 0.159970241(29) and D K 0 =1.80457(49) × 10 −7 . Quadratic, cubic, and semidiagonal quartic force fields of PF 3 have been calculated at the CCSD(T) level of theory employing a variety of large correlation-consistent basis sets. These force fields have been used to evaluate spectroscopic constants which are generally found to be in very good agreement with experiment. The present best estimate of the r e structure of PF 3 based on an explicitly correlated coupled-cluster approach (CCSD(T)-F12b) is almost identical with its latest experimental counterpart.