Vibrational spectrum, ab initio calculations, conformational equilibria and torsional modes of 1,3-dichloropropane

Abstract The infrared and Raman spectrum of 1,3-dibromopropane is reported in the crystalline, liquid and gaseous states. These measurements are compared to the results of ab initio calculations carried out using the 6-31+g * Gaussian basis set for a restricted Hartree–Fock computation. The calculation was repeated using second order Moeller–Ploesset perturbation theory to accommodate electron correlation using the 6-31g * basis set. The three most stable conformers are GG (C 2 ), AG (C 1 ) and AA (C 2v ), where A and G stand for anti and gauche orientations of the bromomethyl group relative to the plane of the carbon atoms. The point group symmetry of each structure is given in parentheses. The fourth conformer, G′G (C s ) is of such high energy that it is not observed experimentally in isotropic media in either the infrared or Raman spectrum. In the crystalline state, comparison of the infrared and Raman spectrum with that calculated for the C 2 conformer shows that only the GG (C 2 ) conformer survives, and the doublet structure of many of the bands in the spectrum indicates at least two molecules per unit cell. The ab initio calculations predict and the temperature dependence of the Raman spectrum of the liquid confirms that the stability order is C 2 1 2v  ≪ C s . These data show that in the liquid the C 1 conformer lies 220 ± 30 cm −1 above the lowest energy C 2 conformer, while the C 2v is 435 ± 60 cm −1 above it. A nearly complete assignment of fundamentals is possible for the C 2 conformer, and a number of bands can be confidently identified for the C 1 . However, only two weak Raman bands with very weak infrared counterparts could be confidently assigned to the C 2v conformer. A complete scaled normal coordinate calculation was conducted. The spectrum calculated from the results agrees very well with the observed. The average error for the C 2 conformer could be reduced to as little as 0.6% (6.8 cm −1 ) by appropriate scaling.