Infrared Spectrum, Force Constants, and Thermodynamic Functions of SiF2

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Abstract
The infrared spectrum of gaseous SiF2 has been recorded from 1050 to 400 cm−1. Two bands centered at 872 and 855 cm−1 are, respectively, assigned to the antisymmetric stretching (ν3) and symmetric stretching (ν1) frequencies of the molecule. The symmetric bending frequency (ν2) as determined from the earlier ultraviolet absorption studies lies at 345 cm−1 and could not be observed because this was well below the low‐frequency limit of the spectrometer used. The observed vibrational frequencies are combined with the available microwave data to obtain the following values of the quadratic force constants:fr=5.019 mdyn/Å, frr=0.310 mdyn/Å, fα/r2=0.440 mdyn/Å, fαr/r=0.123 mdyn/Å. Two cubic potential constants have been evaluated, K222=−9.5 cm−1 and K122=9.9 cm−1. The average structure of SiF2 in its ground state is computed and found to be 〈r〉=1.5947 Å, 〈α〉=100°53′, where r is the Si–F bond distance and α the F–Si–F angle. A table of thermodynamic functions of SiF2 is presented.