Knowledge Bank

Over 100 rotational transitions of the free radicals HCCCS and DCCCS, in both spin components of their $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ electronic ground slate and in several low-lying vibrationally excited states, have been observed by millimeter-wave spectroscopy in the 90-400 GHz region. Prior to this work, HCCCS had only been observed in the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ ground slate spin $component.1$ The spin-orbit constant, determined herd for the first tune, supports the conclusion that, unlike $HCCS,2$ the unpaired electron is localized primarily on carbon, contrary to {ab initio} calculations which predict significant electron density on the sulfur atom. {Ab initio} calculations also predict a bent ground state with $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A\prime$ symmetry at several levels of theory. At the highest level of theory (QCISD) the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ state is only slightly higher $(\sim 450cm-1)$ in energy. Recent progress made in identifying bold isomeric structures and in determining the ordering of the low-lying vibrational states of this interesting radical will be discussed.