Fundamental Vibrations and Force Constants in the Partially Deuterated Methyl Halides

Abstract

Gas‐phase infrared spectra of all eight partially deuterated methyl halides have been obtained; samples were of high isotopic purity. Molecular asymmetry, though quite small, was found to yield significant shifts in the perpendicular structure for the two central Q branches. This was most helpful in the choice of band centers. Observed bands were assigned, including the 72 fundamentals and also 35 overtones and combinations. Although most earlier work was done with samples containing two or more isotopic species, agreement with the present results is quite good in many cases. Evidence for Fermi resonance was found, including two examples in which it occurred below 2000 cm⁻¹. Coriolis coupling between fundamentals was detected in several cases for the fluorides, including a strong resonance between a perpendicular and a parallel vibration. Application of the product and sum rules yielded satisfactory agreement. In a few cases substitution of a heavier atom, such as deuterium for hydrogen, was found to result in an increase for one of the vibration frequencies. However, all such instances involved perpendicular bands in which the choice of origin was quite difficult, and the increases were rather small. Anharmonicity corrections for the partially deuterated fundamentals were computed with the aid of a simple expression, using the corrections employed by others for the C_3v molecules. The fundamentals investigated here were compared with values calculated from three different harmonic potential functions for CH₃X and CD₃X molecules; anharmonic corrections were subtracted from the calculated values. The force field of Aldous and Mills yielded substantially better agreement than an earlier one which was also of the general quadratic type. For the chlorides, bromides, and iodides a Urey—Bradley field was also found to give good agreement with the observed frequencies. In the fluorides the deviations for the Urey—Bradley field were larger than for either of the other two potential functions.