Knowledge Bank

The series of the 1- and 2-halopropenes are well suited to study different influences on the barrier heights of the internal rotation of the methyl group. So far none of the iodine compounds have been studied since the extended quadrupole hyperfine splittings of the iodine makes the analysis difficult. During the investigation of the microwave spectrum of 1-bromopropene, we prepared the computer programs for the exact diagonalization of the rotational and quadrupole interaction energy which are essential for dealing with iodine compounds. It was therefore well justified to start the analysis of the 2-iodopropene spectrum, and the results will be presented. Besides the extension on the iodine compound, a more profound knowledge of the structure of these molecules is needed. All the distances between the hydrogen and halogen atoms are necessary to estimate the contribution from steric factors such as nonbonded interaction to the barrier heights of the halopropenes. Therefore, we studied several isotopic species of 2-chloropropenes and determined the essential ***-structure. The measurements of a compound with an asymmetrically labelled methyl group yielded the absolute conformation of the methyl group in the 2-halopropenes which eclipses the double bond and staggers the halogen as was to be expected.