Rotational spectrum of AHA. (a) Broadband LA-CP-FTMW spectrum of AHA from 6.8 to 14.2 GHz. Strong a-type R-branch (J + 1)_0,J+1 ← J_1,j and (J + 1)_1,J+1 ← J_0,J transitions of rotamer I (Z-AHA, in blue) are shown, as well as some transitions of rotamer II (E-AHA, in red). (b) Zoom-in on the broadband spectrum showing the 1_0,1 ← 0₀,₀ rotational transition for both acetohydroxamic acid species highlighting the A–E doublets due to the internal rotation of the methyl group. The intensity is given in arbitrary units.