VIBRATIONAL-SPECTRA, ABINITIO CALCULATIONS AND NORMAL COORDINATE ANALYSIS FOR 3-METHYL-3-VINYLCYCLOPROPENE

The Raman spectra (3400-10 cm-1) of liquid and solid 3-methyl-3-vinylcyclopropene, C3H2(CH3)CHCH2, and its isotopomer C3D2(CH3)CHCH2 were recorded. The temperature dependence of some band intensities and qualitiative depolarization ratios show that the thermodynamically preferred conformation is s-trans. The structural parameters and conformational stability of s-trans and gauche rotamers were obtained from ab initio calculations by employing both 3-21G and 6-31G* basis sets. Barriers to internal rotation and fundamental vibrational frequencies were calculated utilizing the 3-21G basis set. The ab initio results are consistent with the experimentally indicated greater stability of the s-trans relative to the gauche conformer.