High-resolution FTIR and MMW study of the v4 = 2 (A1, E) excited state of 14NF3 near 985 cm−1: the axial ground state rotational constants derived by the “loop-method”

Abstract The two substates v 4  = 2 0  ( A 1 , 983.702 cm −1 ) and v 4  = 2 ±2  ( E , 986.622 cm −1 ) of the oblate symmetric top molecule, 14 NF 3 , have been studied by high-resolution (2.5 × 10 −3  cm −1 ) infrared spectroscopy of the 2 ν 4 0 and 2 ν 4 ± 2 overtones and 2 ν 4  −  ν 4 hot bands. Transitions of the 2 ν 4 ± 2 overtone, the 2 ν 4 ± 2 - ν 4 ± 1 hot band, and the previously measured v 4 ± 1 fundamental band were combined to yield 585 ground state combination differences differing in K by ±3, with K max  = 36. Using the “loop-method,” a fit (standard deviation σ  = 0.320 × 10 −3  cm −1 ) provided a complete set of the hitherto not experimentally known axial ground state constants. In units of cm −1 these have the following values: C 0 = 0.19499250 ( 44 ) , D K 0 = 3.4343 ( 88 ) × 10 - 7 and H K 0 = - 1.0335 ( 481 ) × 10 - 12 . Upper state parameters were determined using a vibrationally isolated model. Considering l  (2, 2) and l  (2, −1) interactions between the v 4  = 2 0 and v 4  = 2 ±2 substates and effects accounting for the l  (4, −2) interactions within the kl  = −2 levels, 25 upper state parameters were obtained by fitting 2747 IR data (1842 transitions, 905 deduced energies, J max  = 42,  K max  = 39) with σ IR  = 0.353 × 10 −3  cm −1 . Moreover, millimeter-wave spectroscopy furnished 86 transitions ( J max  = 16, K max  = 13) measured on the v 4  = 2 excited state. A merged fit, refining 24 parameters using the described model gave σ IR  = 0.365 × 10 −3  cm −1 and σ MMW  = 0.855 × 10 −6  cm −1 (26 kHz). The anharmonicity constants (in cm −1 ) are x 44  = −0.84174 (2) and g 44  =  + 0.73014 (1). In addition to this model, the D , Q , and L reductions of the rovibrational Hamiltonian were tested. Standard deviations σ IR  = 0.375 × 10 −3  cm −1 and σ MMW  = 0.865 × 10 −6  cm −1 were obtained for both D and L reductions, and σ IR  = 0.392 × 10 −3  cm −1 and σ MMW  = 0.935 × 10 −6  cm −1 for Q reduction. The unitary equivalence of the majority of the 18 tested relations between the derived parameters was satisfactorily fulfilled. This confirms that the v 4  = 2 excited vibrational state can be considered in reasonable approximation to be isolated.