FOURIER-TRANSFORM MICROWAVE SPECTROSCOPY OF THE DEUTERATED ACETYLENE DIMERS - THE INTERCONVERSION TUNNELING MOTIONS OF (DCCD)2, (DCCH)2, DCCD-DCCH, DCCH-DCCD, HCCH-DCCD, AND HCCH-DCCH

Microwave spectra of the deuterated acetylene dimers, produced in a molecular beam at 1 K from samples of HCCH, DCCH, and DCCD, were observed using a Fourier transform microwave spectrometer. We observed all variations of deuterated acetylene dimers in which a deuterium atom participates in the hydrogen bond; i.e., (DCCD)2, (DCCH)2, DCCDDCCH, DCCHDCCD, HCCHDCCD, and HCCHDCCH. Deuterated acetylene dimers with the hydrogen atom located in the hydrogen bond could not be detected. Precise molecular constants were determined for each species. Among the dimers identified, (DCCD)2, (DCCH)2, DCCDDCCH, and DCCHDCCD showed evidence of an interconversion tunneling motion like the tunneling observed for (HCCH)2. The tunneling potential of (DCCD)2 was analyzed using a one-coordinate model and the potential depth was determined to be V4 = 35.577 cm-1, which is 2.371 cm-1 deeper than that of (HCCH)2 studied by Fraser et al. (J. Chem. Phys. 89, 6028-6045 (1988)). A one-coordinate model was also applied to the other deutered acetylene dimers by adopting a further assumption of a composite potential. © 1991.