Infrared Absorption Intensities and Bond Moments in BF3 and Related Molecules

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Abstract
The infrared absorption intensities of the fundamental vibration bands of BF3 have been measured and vibrational bond moments μ and their derivatives ∂μ/∂r determined. In the A2″ class, μBF=1.7 D; in the E′ class μBF=0.9 or 2.6 D, ∂μ/∂r = —6.1 or +4.0 D/A. Inspection of the form of the bending modes suggests that deviations from the bond moment approximation occur such that μ(A2″) <μ(static) <μ(E′). The solution μ(E′) = 2.6 D 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: μCF∼2.4, μSiF∼3.3, μSF∼2.7 D. These moments are roughly equal to Δr0, where r0 is the bond length and Δ the electronegativity difference between the atoms in the bond. The available data on μ and ∂μ/∂r for the C–F bond in various molecules are compared and briefly discussed.