Raman spectra and internal rotation of 1,1-dimethylcyclopropane and 1,1-dimethyl-d6-cyclopropane

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Abstract
The Raman spectra of gaseous and crystalline 1,1‐dimethylcyclopropane and 1,1‐dimethyl‐d6‐cyclopropane are reported for the frequency region between 100 and 600 cm−1. Assignment of the Raman shifts at 251 and 228 cm−1 for solid 1,1‐dimethylcyclopropane to the b2 and a2 torsional modes is supported by the spectral data recorded for the deuterium analog. In addition, the 2←0 transition for the b2 torsional mode is observed at 460 and 462 cm−1 in the spectra of gaseous and liquid 1,1‐dimethylcyclopropane. Whereas the internal rotational barrier for solid 1,1‐dimethylcyclopropane is calculated to be 3.7 kcal/mole on the basis of the low temperature frequency data, it is estimated the barrier is on the order of 3.2 kcal/mole for the gaseous state. The internal rotational parameters determined for solid and gaseous 1,1‐dimethylcyclopropane are compared with barrier heights determined previously for molecules with C(sp2)–C(sp3) and C(sp3)–C(sp3) single bonds.