The Infra-Red Absorption Spectra of the Methyl Halides

The molecules are symmetrical tops having one low moment of inertia. Two types of bands are expected, one arising from vibration parallel to the axis of symmetry, ∥ bands, and the other from vibration perpendicular to the axis of symmetry, ⊥ bands; the appearance of each type is described. There are seven bands for each of the four compounds. The twenty-eight bands fall into seven series, $A$ to $G$ inclusive. Two of the four bands of series $A$ lie beyond the range of observation. Series $A$, $C$, $E$ and $F$ are made up of bands of the ∥ type, and series $B$, $D$, and $G$, of bands of the ⊥ type. The series converge towards the known bands of methane with decrease in the atomic weight of the halogen in the halide. The envelopes of the bands of the ∥ type demonstrate the existence of $P$, $Q$, and $R$ branches. The doublet separations for the various bands of the same compound appear to be the same; the chloride may however present an exception to this rule. In passing from compound to compound, the doublet separations increase with decrease in the atomic weight of the halogen in the halide. The band of series $A$ for methyl fluoride has been resolved, and it is revealed that the $P$ and $R$ branches consist of a single series of lines. Strong convergence in the spacing is present. The bands of the ⊥ type have all been resolved. Every third line is more intense than the others. The spacing converges in series $B$ toward lower frequencies, and in series $D$ and $G$ toward higher frequencies. The mean frequency intervals from band to band of the same compound are found to vary markedly. The explanation of this variation presents a theoretical problem of interest.