Normal-Coordinate Analysis and Molecular Properties of Four CH2XY Halomethanes (X = F or I; Y = Cl or Br)

Abstract

A normal-coordinate treatment for the series CH2XY halosubstituted methanes (X = F or I; Y = Cl or Br) was carried out with a 709 digital computer and a perturbation program supplied by Schachtschneider of the Shell Development Company. A reasonable set of potential constants was obtained for each molecule that reproduced exactly the observed fundamentals. Trends that were noticed among the potential constants of the obtained sets were demonstrated and discussed. The potential-energy distributions for each molecule were calculated. The effect of the redundancy on the quadratic potential constants as well as an estimate of the first-order potential energy constants were obtained according to the method of Crawford and Overend. In the present work, the assignments of the ν3 and ν8 fundamentals of the two iodinated compounds, as given by Pitzer and Gelles, were interchanged and the argument for this change was given. A new value for the unperturbed ν5 of the fluorobromomethane, which is undergoing a Fermi resonance with 2ν6, was adopted. This new value, higher than that adopted by Pitzer and Gelles, is backed by the results of the normal-coordinate treatment. Mean-square amplitudes of vibration for bonded and nonbonded atoms for the four molecules, as well as the Coriolis coupling coefficients were calculated. With the adopted values of the fundamentals, the usual ideal-gas thermodynamic properties were calculated for the four compounds based on the latest values of the physical constants and the carbon−12 atomic mass scale.