Anomalous band shifts in the 14 μm infrared absorption spectra of rare gas–BF3 complexes

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Abstract
High resolution infrared absorption spectra of rare gas (Rg)–BF3 van der Waals complexes are studied in the 14 μm region near the ν2 band of BF3 monomer. Spectroscopic constants are determined for the 20Ne–11BF3, Ar–10,11BF3, 82–84,86Kr–11BF3, and 84Kr–10BF3 complexes. The observed redshifts from the monomer band origin correlate linearly with the rare gas polarizabilities. These shifts are about three times as large as those measured previously near the monomer ν3 band. This mode dependence of the shifts cannot be reproduced in a consistent manner with the instantaneous vibrational dipole–induced dipole interaction model, and indicates much greater enhancement of bonding energy by the excitation of ν2 vibration. The band shifts are discussed on the basis of electrostatic interaction between rare gas atom and point charges on BF3. The anomalous band shifts for the ν2 band are successfully accounted for by the interaction of vibrational dipole moment with the static induced dipole moment on the rare gas atom, which is parallel to the direction of vibrational motion. The isotope shifts observed for the Kr–BF3 complexes and the band shifts due to the modification of force field by vdW bonding are discussed with a linear triatomic molecular model.

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