STRUCTURE AND TUNNELING DYNAMICS OF gauche-1,3-BUTADIENE

We have recently shown that textit{gauche}-1,3-butadiene is unambiguously non-planar, with a C$=$C$-$C$=$C dihedral angle of about $34^{circ}$, and readily tunnels between two equivalent textit{gauche} structures.footnote{M.-A. Martin-Drumel textit{et al.}, ISMS 2016, MI11} In this talk, subsequent microwave studies of textit{gauche}-1,3-butadiene and its isotopologues as well as the empirical equilibrium structure will be summarized. The experiments have utilized the complementary techniques of cavity enhanced Fourier transform microwave (FTMW) spectroscopy with a supersonic expansion and chirped-pulse FTMW in a cryogenic buffer gas cell. The structural characterization is complicated by the effects of facile tunneling, and full dimensional textit{ab initio} rotational-VMP2 calculations have been performed to address this issue. We will show how the tunneling splitting frequency, which ranges between about $0.5$ and $2.0text{ cm}^{-1}$ (depending on the isotopologue), can be extracted from the experimental spectra by careful examination of tunneling-rotation perturbations