Infrared Absorption Intensities and Bond Moments in BF3 and Related Molecules

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.