Knowledge Bank

Pure rotational transitions of $C2H2$, a three-membered carbene ring that is a ubiquitous interstellar molecule, were detected in the laboratory and in space in $1985.1$ Following the recent observation of the $\nu 2$ mode at. $1277cm-1$ by infrared $spectroscopy,2$ we observed millimeter-wave rotational transitions of the 1/3 mode and three other vibrational modes (one with B symmetry and two with A symmetry) in a dc glow discharge through allene and helium. The key to the identification of the vibrationally excited states was detection of closely spaced ortho-para doublets near 184 GHz within 3 GHz of the ground state transitions. Subsequently, further rotational lines in the range 150 to 385 GHz were identified, and accurate rotational and centrifugal constants for all four modes were determined. Although we are presently engaged in assigning these modes from comparison of inertial defects from experimental and ab initio force constants, conclusive assignment will require infrared spectroscopy.