RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND ABINITIO CALCULATIONS FOR 3-IODOPROPENE

The Raman spectra (3200-10 cm-1) of gaseous, liquid and solid and infrared spectra (3200-40 cm-1) of gaseous and solid 3-iodopropene, H2C-CHCH2I, were recorded. The fundamental asymmetric torsional transition of the most stable gauche conformer was observed in the Raman and far-infrared spectra of the gas at 95 cm-1. In the Raman spectrum of the liquid a second C-I stretch and two skeletal bending modes were identified for the less stable cis conformer. From a study of the Raman spectrum of the liquid at various temperatures, a value of 648 +/- 81 cm-1 (1.85 +/- 0.23 kcal mol-1) was determined for the enthalpy difference between the higher energy cis and preferred gauche conformers. These data, along with the dihedral angle for the gauche conformer obtained from an electron diffraction study, combined with results obtained from ab initio calculations, allow for an estimate of the potential function governing internal rotation about the C-C bond. The values for the torsional potential coefficients are V1 = -208 +/- 10, V2 = -532 +/- 10, and V3 = 534 +/- 8 cm-1. This potential is consistent with a dihedral angle (angle CCCI) of 113.0-degrees for the gauche conformer cis to gauche, gauche to gauche and gauche to cis barriers of 160, 904 and 738 cm-1, respectively, and an enthalpy difference between the conformers of 580 +/- 30 cm-1 (1.66 +/- 0.09 kcal mol-1). The asymmetric torsional potential surface, complete equilibrium geometries and vibrational frequencies were calculated using restricted Hartree-Fock calculations with the STO-3G*, 3-21G*/medium, a medium-sized combination basis set suitable for a DZ-type contraction of the valence shell orbitals, and the LANL1DZ basis sets. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.