The Structure of gauche-1,3-Butadiene: Further Insights from the Centimeter-Wave, Millimeter-Wave, and Far-Infrared High Resolution Spectra.

Polyenes such as butadiene are important feedstocks in organic synthesis. This work presents an expanded spectroscopic investigation of the high-energy conformer of 1,3-butadiene, gauche-1,3-butadiene, using a combination of centimeter-wave, millimeter-wave, and far-infrared high-resolution spectroscopy. Building upon earlier microwave studies, we report new rotational and rovibrational data that significantly extend the experimental characterization of this nonplanar conformer. These results provide a more complete description of the potential energy surface governing its conformational dynamics, with the experimental determination of the ground state tunneling splitting. A refined semiexperimental equilibrium structure is derived through analysis of multiple isotopologues and comparison with high-level quantum chemical calculations. The combined data set enables a modest improvement in the determination of structural parameters. This comprehensive study highlights the power of combining multiple spectroscopic techniques to resolve subtle structural features and provides enhanced insight into tunneling processes in flexible organic molecules.