Proton inversion tunneling in the rotational spectrum of acetone cyanohydrin

[Display omitted] •First reported high resolution microwave spectrum of acetone cyanohydrin.•First characterization of Coriolis coupling and hyperfine structure.•Spectral complexity due to large amplitude motions. Rotational transitions of gauche-acetone cyanohydrin are measured in the microwave range between 5 and 23 GHz. Results from quantum chemical calculations indicate that the hydrogen atom of the hydroxyl group undergoes a tunneling motion connecting two equivalent structures. The observed signals are assigned to the two lowest tunneling substates 0+ and 0-, which belong to either a- or c-type transitions exhibiting Coriolis splittings of a few MHz. Additional hyperfine structure arises from the quadrupole coupling 14N nucleus. The energy separation ΔE between the 0+ and 0- states is predicted to be around 50 GHz. The molecular structure and internal dynamics are discussed in terms of their spectral signatures.