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The microwave spectra of seven isotopic species of propylene have been studied in order to obtain an accurate molecular structure. The complete rs (substitution) structure has been calculated. The more important parameters are: r(C=C)=1.336±0.004 A,r(C–C)=1.501±0.004 A, ≰CCC=124.3∘±0.3∘. The structure is compared with those of related molecules. It is concluded that no difference can be detected in the double-bond lengths in ethylene, propylene, and the vinyl halides. The CC single-bond length in propylene is indistinguishable from that in acetaldehyde and other acetyl compounds, and is 0.025 A shorter than the CC distance in saturated hydrocarbons. In the =CH2 group in propylene, the CH bond trans to the methyl group appears slightly shorter than the cis CH bond; a similar effect occurs in the vinyl halides.
Lide et al. (Sun,) studied this question.