Far-infrared spectra conformational stability and barriers to internal rotation of some substituted 3-chloropropenes

摘要

The far-infrared spectrum (375 to 30 cm$+$MIN@1$/) of gaseous 2,3- dichloropropene has been recorded at a resolution of 0.06 cm$+$MIN@1$/. The fundamental asymmetric torsional mode is observed at 91.9 cm$+$MIN@1$/ with eight excited states falling to low frequency for the s-cis (chlorine atom eclipsing the double bond) conformer. Utilizing the s-cis torsional frequency, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in cm$+$MIN@1$/): V$-1$/ $EQ 1047 $POM 96, V$-2$/ $EQ $MIN@619 $POM 68, V$-3$/ $EQ 1216 $POM 50, and V$-4$/ $EQ 26 $POM 22, with an enthalpy difference between the more stable s-cis and gauche conformers of 245 $POM 30 cm$+$MIN@1$/ (0.70 $POM 0.09 kcal/mol). Similarly, the fundamental asymmetric torsion for the more stable gauche conformer of trans-1,3-dichloropropene has been observed in the far infrared spectrum of the gas at 84.0 cm$+$MIN@1$/. The corresponding mode for the s-cis conformer gives rise to a series of well defined C-type Q branches beginning at 129.8 cm$+$MIN@1$/. From these data the potential function governing internal rotation of the asymmetric top has been determined and the potential coefficients are: V$-1$/ $EQ $MIN@558 $POM 7, V$- 2$/ $EQ $MIN@38 $POM 10, V$-3$/ $EQ 637 $POM 9, V$-4$/ $EQ 174 $POM 7, V$-6$/ $EQ $MIN@16 $POM 4 cm$+$MIN@1$/. From studies of the Raman and infrared spectra of the gas at variable temperatures, the conformational enthalpy difference has been determined to be 342 $POM 36 cm$+$MIN@1$/ (978 $POM 103 cal/mol). All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing either the 3-21G$+*$/ or 6-31G$+*$/ basis sets.