The high-resolution Fourier transform infrared spectrum of nitrogen trifluoride NF3 has been studied in the v1 + v4 perpendicular band region around 1523 cm−1. All experimental data have been refined applying various reduction forms of the effective rovibrational Hamiltonian developed for an isolated degenerate state of a symmetric top molecule. The v1 = v4 = 1 excited state of the 14NF3 oblate molecule was treated with models taking into account ℓ- and k-type intravibrational resonances. Parameters up to sixth order have been accurately determined and the unitary equivalence of the derived parameter sets in different reductions was demonstrated.

The high-resolution Fourier transform infrared spectrum of nitrogen trifluoride NF3 has been studied in the v(1) + v(4) perpendicular band region around 1523 cm(-1). All experimental data have been refined applying various reduction forms of the effective rovibrational Hamiltonian developed for an isolated degenerate state of a symmetric top molecule. The v(1) = v(4) = 1 excited state of the (NF3)-N-14 oblate molecule was treated with models taking into account l- and k-type intravibrational resonances. Parameters up to sixth order have been accurately determined and the unitary equivalence of the derived parameter sets in different reductions was demonstrated.

The high-resolution Fourier transform infrared spectrum of phosphorus trifluoride PF3 have been reinvestigated in the ν4 perpendicular band region around 347cm−1. Thanks to recent pure rotational measurements, 595 new infrared transitions of the ν4 band have been assigned extending the rotational quantum number values up to Kmax=66 and Jmax=67. As a consequence of this extension, a sophisticated model containing a large number of parameters and interaction constants was adopted for the analysis of the IR transitions of the ν4 fundamental band of PF3. A merge of the IR transitions and the reported MW/MM/RF data within the v4=1 excited level yielded an accurate rotational ground state C0 value, 0.159970436 (69)cm−1, which was used to determine an improved GS structure, r0(P–F)=1.56324405 (11)Å and ∡(FPF)=97.752232 (29)°. All experimental data have been refined applying various reduction forms of the effective rovibrational Hamiltonian developed for an isolated degenerate state of a symmetric top molecule. The v4=1 excited state of the PF3 oblate molecule was treated with models taking into account ℓ- and k-type intravibrational resonances. Parameters up to sixth order have been accurately determined and the unitary equivalence of the derived parameter sets in different reductions was demonstrated.

The v(1) + v(4) (E, v(0) = 1523.0407 cm(-1)) perpendicular band of the oblate symmetric top (NF3)-N-14 has been studied by FTIR spectroscopy with a resolution of 2.5 x 10(-3) cm(-1). In total, 1885 rovibrational transitions up to K-max = 70 = J(max) have been assigned. Due to the k(Delta l = 0, Delta k = +/- 6) interaction (h(3) = 2.22 x 10(-13) cm(-1)) within the kl = +4 sublevel, large A(+)/A(-) doublets were identified from J = 24 to J = 33 and large A(+)/A(-) doublets due to l(Delta l = +/- 2, Delta k = +/- 2) rotational resonance (q(4) = 1.730 x 10(-3)cm(-1)) within the kl = +1 level were also observed, while the effect of the expected l(Delta l = +2, Delta k = -/+ 4) interaction was not detected on the Delta K = -1 transitions owing to their vanishing intensity. The same D-reduction Hamiltonian model taking into account intravibrational resonances, recently performed for the v(4) = 1 (E, v(0) = 493.4227 cm(-1)) state of (NF3)-N-14, was employed. Assuming the v(1) = v(4) = 1 vibrational state as isolated, the nonzero weighted experimental data were fitted to 26 free parameters with a standard deviation better than the spectral resolution. The experimental anharmonicity constant x(14) = -2.383 249 (49) cm(-1) was determined.