Hyperfine Resolved Fourier Transform Microwave and Millimeter-Wave Spectroscopy of the Iodomethyl Radical, CH2I ((X)over-tilde2B1)

Fourier-transform microwave and millimeter-wave spectra of the iodomethyl radical, CH2I, have been observed in the ground vibronic state in the frequency ranges 17-38 GHz and 200-610 GHz, respectively. The pi-electron radical was produced either by iodine abstraction from diiodomethane (CH2I2) or by hydrogen abstraction from iodomethane (CH3I). Seventy-three hyperfine resolved lines owing to the two hydrogen and to the iodine nuclei have been detected in the microwave region, including K-a = 0 (ortho species) and Ka = 1 (para species). No hyperfine splitting due to the hydrogen nuclei could be observed for the para species, directly confirming the planarity of the radical and that the ground electronic state is B-2(1). More than 400 a-type transitions that span the values N' <= 35, K-a <= 6, were detected. A global least-squares analysis of the measured lines was conducted and led to the determination of an exhaustive list of molecular constants. In particular, the sign of the Fermi-contact constant of iodine was unambiguously determined to be negative, which is opposite to that of the other monosubstituted halomethyl radicals. This work allowed a systematic comparison of the structural and bonding properties among the CH2X analogues (where X = F, Cl, Br, and I) owing to the hyperfine coupling constants of both the hydrogen and halogen nuclei.