Far Infrared Spectrum and Ground State Parameters of ¹²CH₃F

The Fourier transform spectrum of the pure rotational transitions in the ground vibrational state of ¹²CH³F was measured at an unapodized resolution of 0.0019 cm^-1 in the 22-99 cm^-1 spectrum region and 775 lines with J ≤ 58 were assigned. Combination differences were generated for the ground state from the Δk = 0 and Δk = ±1 allowed and the Δk = ±2 "forbidden" transition frequencies of the υ₂/υ₅ bands (Papoušek, Papoušková, Ogilvie, Pracna, Civiš, and Winnewisser, J. Mol. Spectrosc.153, 145-166(1992)) and the frequencies of the Δk = 0 allowed transitions of the υ₃ band (Papoušek, Ogilvie, Civiš, and Winnewisser, J. Mol. Spectrosc.149, 109-124(1991)). These data were fitted simultaneously with the previously measured millimeter-and submillimeter-wave rotational transition frequencies to determine very precisely the ground state parameters of ¹²CH₃F, including A₀ and D⁰_K.