Cold Snapshot of a Molecular Rotary Motor Captured by High-Resolution Rotational Spectroscopy

We present the first high-resolution rotational spectrum of an artificial molecular rotary motor. By combining chirped-pulse Fourier transform microwave spectroscopy and supersonic expansions, we captured the vibronic ground-state conformation of a second-generation motor based on chiral, overcrowded alkenes. The rotational constants were accurately determined by fitting more than 200 rotational transitions in the 2–4 GHz frequency range. Evidence for dissociation products allowed for the unambiguous identification and characterization of the isolated motor components. Experiment and complementary quantum-chemical calculations provide accurate geometrical parameters for the C₂₇H₂₀ molecular motor, the largest molecule investigated by high-resolution microwave spectroscopy to date.