Knowledge Bank

The infrared absorption intensities of the fundamental vibration bands of $BF3$ have been measured and vibrational bond moments $\mu$ and their derivatives $\delta \mu /\delta r$ determined. In the $A2\prime \prime$ class, $\mu BF=1.7D$; in the $E\prime$ class, $\mu BF=0.9$ or $2.6D,\delta \mu /\delta r=-6.1$ or $+4.0D/A$. Inspection of the form of the bending modes suggests that deviations from the bond moment approximation occur such that (static) ). The solution $\mu (E\prime )=2.6D$ is therefore preferred. Re-examination of the data of Schatz and Hornig on $CF4$, $SiF4$, and $SF6$ in the light of the present results shows that the vibrational bond moments in these molecules are: $\mu CF=2.4,\mu SiF=3.3,\mu SF=2.7D$. These moments are roughly equal to $\Delta r0$, where $r0$ is the bond length and $\Delta$ the electro-negativity difference between the atoms in the bond. The magnitudes of the static bond moments in these molecules remain uncertain.