## 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 $\mu $ and of their derivatives $\partial \mu $/$\partial $r, assumed to be vectors directed along the bonds. The results show that values of $\partial \mu $/$\partial $r$_{\text{CH}}$ obtained from the A$_{1}$ class of vibration vary from about 1$\cdot $7 $\times $ 10$^{-10}$ unit (Debye / angstrom) for methyl fluoride to about 0$\cdot $9 $\times $ 10$^{-10}$ for methyl iodide showing a uniform trend along the series. By contrast, the values in the E class range from about 0$\cdot $6 $\times $ 10$^{-10}$ for methyl fluoride to less than 0$\cdot $2 $\times $ 10$^{-10}$ for the iodide. Values of $\mu _{\text{CF}}$ in general lie around 0$\cdot $4 D. $\partial \mu $/$\partial $r$_{\text{C}X}$ increases in value from 1 $\times $ 10$^{-10}$ unit in methyl iodide to the high value of 4$\cdot $7 $\times $ 10$^{-10}$ unit in methyl fluoride. There is some indication that its sign is positive. Values of $\mu _{\text{CF}}$ and of $\mu _{\text{CCl}}$ 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.