SPECTROSCOPIC CHARACTERIZATION OF THE ELUSIVE GAUCHE-ISOPRENE BY HIGH RESOLUTION MICROWAVE SPECTROSCOPY

The microwave spectrum of isoprene has been investigated in a cryogenic (4-7\,K) He buffer gas cell (12-26.5 GHz) and in a room temperature waveguide (8.7-26.5\,GHz). Rotation about the single C-C bond converts the lower-energy $trans$ rotamer to the higher-energy $gauche$-conformer $via$ an unstable, planar, $cis$ transition state. As in butadiene, it is counterintuitive that $gauche$ is more stable than $cis$ because it has long been believed that planarity is required for $\pi$ electron delocalization, a factor that often imparts greater stability. \begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.8]{isoprene_conformers.eps} \end{wrapfigure} In standard jet experiments, observation of the higher-energy $gauche$ conformer has proven challenging. However in the buffer gas cell, collisions with cold He result in rapid but gentle conformational cooling, allowing for straightforward observation of the $gauche$-isoprene rotamer. The rotational spectrum of $gauche$ is complex owing to the combined effects of rotational line splitting (0$^+$ / 0$^-$ from $gauche$-$gauche$ inversion) and A/E splitting of the methyl rotor. On the basis of new theoretical calculations, steric hindrance of methyl rotation is predicted to be lower in the $gauche$ than in the $trans$ conformer ($V_3$ barrier approximately 653 cm$^{-1}$ or 7.8 kJ/mol). In addition to the on-going spectroscopic analysis, efforts are now underway to better characterize the energetics of C-C bond rotation in isoprene, with the goal of ultimately understanding the factors that result in the greater stability of the $gauche$ relative to the $cis$ isoprene rotamer