The intensities of absorption bands in the methyl halides

Abstract

Abstract The infra-red intensities of the fundamental vibration bands of the four methyl halides have been measured, using the method involving pressure broadening of bands described by Wilson. The force constants have been determined on the basis of a force field previously suggested by Noether, and the transformations from normal to symmetry co-ordinates have been calculated. After allowing for the effects of the overlapping of bands and of Fermi resonance, the intensities have been interpreted in terms of bond moments μ and of their derivatives ∂μ/∂r, assumed to be vectors directed along the bonds. The results show that values of ∂μ/∂rCH obtained from the A1 class of vibration vary from about 1⋅7 x 10-10 unit (Debye/Å) for methyl fluoride to about 0⋅9 x 10-10 for methyl iodide showing a uniform trend along the series. By contrast, the values in the E class range from about 0⋅6 x 10-10 for methyl fluoride to less than 0⋅2 x 10-10 for the iodide. Values of μCH in general lie around 0⋅4 D. ∂μ/∂rCX increases in value from 1 x 10-10 unit in methyl iodide to the high value of 4⋅7 x 10-10 unit in methyl fluoride. There is some indication that its sign is positive. Values of μCF and of μCCl are obtained which are considerably higher than the total molecular dipole moment. Some tentative suggestions are put forward to account for the magnitudes of the polar properties found.