Microwave Spectroscopy and Structures of Several Substituted Acetylenes and Fluorocarbons

The structures of two substituted acetylene compounds have been characterized from their microwave rotational spectra. In the first study, two structures of 6-methyl-3-heptyne have been determined. This compound can be thought of as an ethyl group separated from an isobutyl group by a C≡C spacer. Both structures have the ethyl and isobutyl groups eclipsed, consistent with the dominant interaction determining the orientation about the acetylene axis being the weak dispersion attraction between the end groups. One structure is with the isobutyl group in a symmetric conformation and the other with the isobutyl group asymmetric. In addition, the microwave spectrum of the butane analogue 3,5-octadiyne has been observed. This compound consists of two ethyl groups separated by two C≡C spacers. The study is still in progress, but it appears that the ethyl end groups are freely rotating. Therefore, it seems that the dispersion attractions between the end groups are too weak at this longer distance of about 7 Å. The structures of several fluorocarbons have also been studied by microwave spectroscopy. The structures of perfluoropentane and perfluorohexane have been shown to be helical, like the polymer polytetrafluoroethylene (Teflon©). The structure of perfluoropropane and two conformers of 1H-heptafluoropropane have been determined to be non-helical. It is apparent that the steric and dipole repulsions between fluorine atoms that have been attributed to the helical structure of longer fluorocarbon chains are not sufficient in a three carbon chain to cause a twist in the structures.