Determination of A₀ and D⁰_K of ¹³CH₃F from the Δk = ±2 Forbidden Transitions to the v₅ = 1 Vibrational Level

The Fourier transform spectrum of the vibration-rotational bands υ₂ and υ₅ of gaseous fluoromethane ¹³CH₃F was measured at an unapodized resolution 0.0041 cm^-1. Among the assigned lines, 102 belonged to the Δk = ±2 perturbation-allowed transitions to the doubly degenerate vibrational state v₅ = 1. Combination differences were generated for the ground state of ¹³CH₃F from the wavenumbers of the allowed and forbidden vibration-rotational transitions to the v₅ = 1 level. They were used in a simultaneous least squares fit with the previously measured microwave, submillimeterwave, and far infrared transition frequencies of the pure rotational transitions in the ground state and with the combination differences generated from the allowed transitions to the v₃ = 1 state of ¹³CH₃F (Pracna, Klee, Winnewisser, Papoušek, Yen-Chu Hsu, and Hann-Sen Chen, J. Mol. Spectrosc.156, 217-226 (1992). This made it possible to determine precisely for the first time A₀ and D⁰_K for ¹³CH₃F: A₀ = (5.1824246 ± 0.0000015) cm^-1, D⁰_K = (70.912 ± 0.033) × 10^-6 cm^-1.