Interplay of methyl-group internal rotation and fine and hyperfine interaction in a free radical: Fourier transform microwave spectroscopy of the acetyl radical

The fine and hyperfine structure in a free radical containing a methyl group is examined by paying special attention to the effects of the methyl group internal rotation on the fine and hyperfine interactions. The effects are taken into account by expanding pertinent constants describing couplings among angular momenta of electron and nuclear spins, internal rotation and overall rotation in terms of the internal rotation angle. The three-fold symmetry of the methyl internal rotation potential is lifted to some extent by fine and hyperfine interactions. The formulation thus developed is applied to the analysis of Fourier transform microwave spectra of the acetyl radical, and the potential barrier to internal rotation [139.958 (18) cm−1] and fine and hyperfine interaction constants are determined precisely.