Knowledge Bank

The far infrared spectra of gaseous 3-bromopropene $(H2CCHCH2Br)$ and 1-bromo-2-tluoroethane $(BrCH2CH2F)$ have been recorded at a resolution of $0.1cm-1$ in the region of 350 to $35cm-1$. For 3-bromopropene, the fundamental asymmetric torsional frequencies of the more stable gauche (dih angle $\sphericalangle CCCBr=120.50\circ$) and high energy cis (dih angle $\sphericalangle CCCBr=0\circ$) conformers have been observed at 99 and $133.31cm-1$ in the Raman and infrared spectra of the gas, respectively, each with several excited states. For 1-bromo-2-fluoroethane, the fundamental asymmetric torsional frequencies of the more stable trans (two halogen atoms oriented trans to one another) and high energy gauche conformers have been observed at 125.3 and $111.3cm-1$, respectively, each with excited states falling to lower frequency. From these respective data, the asymmetric torsional potential function governing internal rotation about the C-C bonds in both molecules have been determined. From studies of the Raman spectrum of 1-bromo-2-fluoroethane at variable temperature the conformational energy difference has been determined to be $383\pm 36cm-1(1.1\pm 0.1kcal/mol)$ and $300\pm 46cm-1(0.9\pm 0.1keal/mol)$ for the gaseous and liquid phases, respectively. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the $STO-3G\ast$ basis set. Complete equilibrium geometries have also been determined for both conformers in both molecules.