High resolution infrared absorption spectra of BCl3 and BCl2F dissolved in solid argon and solid krypton

Abstract

High-resolution (≈0.035 cm−1) infrared absorption spectra of BCl3 and BCl2F dissolved in solid argon and krypton have been obtained. The solids were grown from liquid solutions, were 1.4 cm thick, and were reasonably clear optically. The ν2, ν3, ν1+ν4, ν3+ν4, ν1 +ν3, and 2ν3 bands of BCl3 were found, and the positions of the ν1 and ν4 bands of BCl2F were inferred. An observed perturbation of the ν2 band of 10BCl3 is explained by invoking a lattice-induced distortion of BCl3 from planarity. The observed spectral lines are very narrow, so the absorption bands of different boron and chlorine isotopomers are well resolved. The observed full spectral width of the absorption lines at half maximum absorbance is <0.04 cm−1 for the ν2 lines of BCl3 and ∼0.08 cm−1 for the other fundamentals. The wave number ṽ of a given line depends on solid solvent and on temperature T; dṽ/dT is within the range 0.014⩽dṽ /dT⩽0.09 cm−1/°C for all bands. Anharmonicities and harmonic frequencies have been obtained from the data, and the anharmonicity constants lie between −1.8 and 0.8 cm−1. Energies have been estimated for the first excited states of the ν1 and ν4 modes of BCl3 and also for the A′1 and A′2 levels of the v3=v4=1 state of BCl3. The usual theory of harmonic oscillators having D3h symmetry does not adequately describe the ν3 and ν4 band positions and isotopic splittings.