The Microwave Spectra and Molecular Structures of Trifluorosilane Derivatives

Abstract

Pure rotational spectra have been measured for the molecules Si²⁸F₃H, Si²⁹F₃H, Si³⁰F₃H, Si²⁸F₃Cl³⁵, Si²⁸F₃Cl³⁷, Si²⁸F₃Br⁷⁹, Si²⁸F₃Br⁸¹, and Si²⁸F₃CH₃. Values of B₀ and moments of inertia, I_B, are tabulated. The nuclear quadrupole couplings of Cl³⁵, Br⁷⁹, and Br⁸¹ in these molecules have been determined from their hyperfine structures as −43, 440, and 370 Mc/sec, respectively. Molecular dimensions, calculated by assuming the first parameter mentioned to be within the stated limits, are, for SiF₃H, d_SiH=1.55±0.05A, d_SiF=1.561∓0.005A, ∠FSiF=108°6′∓30′; for SiF₃Cl, ∠FSiF=108°30′±1°, d_SiF=1.560∓0.005A, d_SiCl=1.989±0.018A; for SiF₃Br, ∠FSiF=108°30′±1°, d_SiF=1.560∓0.005A, d_SiBr=2.153±0.018A. The special shortness of silicon‐halogen bonds is discussed and shown to be related to the electronic structure of silicon. The spectrum of SiF₃CH₃ shows lines caused by molecules in excited torsional vibrational states, which show additional splitting caused by the doubling of the torsional vibrational levels. From relative intensities the height of the barrier opposing free rotation about the SiC bond is estimated as 1200 cal/mole. From the variations of B₀ with torsional vibration, the separations of the doublets in the ground, first, and second torsional vibration levels are estimated as 0.1 cm⁻¹, 1.4 cm⁻¹, and 12 cm⁻, respectively.