Infrared and microwave investigations of interconversion tunneling in the acetylene dimer

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 C_2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C_2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three K_a=0 and three asymmetry-doubled K_a=1 series. The transitions show a clear intensity alternation with K_c with two of the K_a=0 series missing every other line. In addition, the perpendicular band has the same ground-state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T-shaped complex 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, Nandi, and Muenter, verifies the model. The measured microwave splittings yield a tunneling frequency of 2.2 GHz which is consistent with a ∼33 cm^-1 barrier separating the four minima.