INFRARED AND MICROWAVE INVESTIGATIONS OF INTERCONVERSION TUNNELING IN THE ACETYLENE DIMER
Loading...
Date
1988
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
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 $et al.^{1}$ and the perpendicular band to a $C_{2h}$ complex by Bryant $et al.^{2}$ 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 $K_{a}=O$ 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 $et al.^{1}$. verifies the model. The measured microwave splittings yield a tunneling frequency of 2.2 GHz for $K_{a}=0$ which is consistent with a $-33 cm^{-1}$ barrier separating the four minima.
Description
$^{1}$ D. G. Prichard, R. N. Nandi, and J. S. Muenter, J. Chem. Phys. (to be published). $^{2}$ G. W. Bryant. D. F. Eggers, and R. O. Watts. J. Chem. Soc. Farad. II. (to be published).
Author Institution: Department of Chemistry, University of Rochester; Molecular Spectroscopy Division, National Bureau of Standards
Author Institution: Department of Chemistry, University of Rochester; Molecular Spectroscopy Division, National Bureau of Standards