Resolving a long-standing ambiguity: the non-planarity of gauche-1,3-butadiene revealed by microwave spectroscopy

The preferred conformation of \textit{cis}-1,3-butadiene (CH$_2$=CH$-$CH=CH$_2$) has been of long-standing importance in organic chemistry because of its role in Diels-Alder transition states. The molecule could adopt a planar \textit{s-cis} conformation, in favor of conjugations in the carbon chain, or a non-planar \textit{gauche} conformation, as a result of steric interactions between the terminal H atoms. To resolve this ambiguity, we have now measured the pure rotational spectrum of this isomer in the microwave region, unambiguously establishing a significant inertial defect, and therefore a \textit{gauche} conformation. Experimental measurements of \textit{gauche}-1,3-butadiene and several of its isotopologues using cavity Fourier-transform microwave (FTMW) spectroscopy in a supersonic expansion and chirped-pulse FTMW spectroscopy in a 4 K buffer gas cell will be summarized, as will new quantum chemical calculations