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.

High-Resolution Analysis of the ν4 Absorption Band of CH279BrF

“…The specimen of CH 2 79 BrF (90% purity of 79 Br) was synthesized employing the method described in Ref. (3) which was considered the most efficient for microscale preparation.…”

“…In the present work we report the results from the 4 spectral analysis of the isotopically enriched sample of CH 2 79 BrF synthesized in our laboratory. From the least-squares fit of 13 474 ground state combination differences (GSCD) and of 10 209 rovibrational transitions, the rotational constants, all quartic and three sextic centrifugal distortion constants, were determined for both ground and excited vibrational states.…”

“…Early work of Delwaulle and François (1), using the Raman technique, reported the first vibrational measurements of the fundamental modes. Subsequently a normal coordinates analysis, in conjunction with other disubstituted halomethanes belonging to the C s symmetry-point group, were carried out by El-Sabban et al (2). Only recently, the low-resolution FTIR spectrum was studied in detail in our laboratory (3), yielding accurate values for the fundamentals and several overtones, combinations, and hot bands together with the inclusion of few 79/81 Br isotopic shifts.…”

“…It is well known in fact that the natural sample consists of two almost equivalent isotopomers, namely CH 2 79 BrF (50.5%) and CH 2 81 BrF (49.5%), which give rise to overlapped infrared bands since in most cases the bromine isotopic shifts are less than 0.5 cm Ϫ1 (3). Another important reason for using the 79/81 Br atoms is that this molecule was investigated in the microwave region (4) only in the early 1960s, and the available ground state constants of the bromine isotopic forms were confined to their A, B, and C rotational values.…”

“…Bromofluoromethane, CH 2 BrF, is a stable molecule which has been considered very little as a subject of spectroscopic studies. Early work of Delwaulle and François (1), using the Raman technique, reported the first vibrational measurements of the fundamental modes.…”