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

Abstract

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 C₂₇ H₂₀ molecular motor, the largest molecule investigated by high-resolution microwave spectroscopy to date.

Keywords: high-resolution spectroscopy; large molecules; microwave spectroscopy; molecular motors; structure elucidation.