Knowledge Bank

A sub-Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C-H stretching fundamental using an optothermal molecular-beam color-center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed-nozzle Fourier-transform microwave spectrometer. In the infrared spectrum both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T-shaped complex by Prichard $etal.1$ and the perpendicular band to a $C2h$ complex by Bryant $etal.2$ The parallel band exhibits three $Ka=0$ and three asymmetry-doubled $Ka=1$ series. The transitions show a clear intensity alternation with $Kc$. with two of the $Ka=O$ series missing every other line. In addition, the perpendicular band has the same ground-state combination differences as the parallel band. To explain these anomalies in the spectrum, we invoke a model consisting of a T-shaped equilibrium configuration with interconversion tunneling between four isoenergetic hydrogen-bonded minima. In this picture, the parallel and perpendicular bands arise from excitation of the acetylene units parallel and perpendicular to the hydrogen bond. The observation of rotation-inversion transitions in the microwave spectrum, in addition to the pure rotation transitions of Prichard $etal.1$. verifies the model. The measured microwave splittings yield a tunneling frequency of 2.2 GHz for $Ka=0$ which is consistent with a $-33cm-1$ barrier separating the four minima.