Knowledge Bank

The absolute intensities of infrared absorption bands of species $B1$ and $B2$ have been determined for the halogenated methanes $CH2F2,CH2Cl2,CH2Br2$, and $CF2Cl2$. For the three methylene halides, the bond moments $\mu CH$ and $\mu CX$ and the bond derivative $\delta \mu CX/\delta rCX$ have been determined. The ambiguity in sign of the bond derivative $\delta \mu CH/\delta rCH$ has not been resolved. The bond moments $\mu CX$ are all assumed to be negative; their numerical values vary in a nearly linear manner with the electronegativities of the halogens in question. The bond moments $\mu CH$ are all positive but differ from molecule to molecule being larger in $CH2Br2$ than in $CH2F2$. The sign of the bond derivatives $\delta \mu CX/\delta rCX$ is opposite that of the associated $\mu CX$ and in each case represents an effective charge of approximately 0.7e. The simplified model does not yield consistent results when applied to bands of different species in $CF2Cl2$.