Abstract
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•Microwave spectra observed for c-C3H5SiHFCH3 and c-C3H5SiF2CH3.•Two conformers for c-C3H5SiHFCH3 and one for c-C3H5SiF2CH3 characterized.•Barriers for internal rotation of methyl group measured in all conformers.•Comparison of internal rotation parameters with related silanes and germanes.•Internal rotation barrier decreases upon increasing fluorination of Si.
Rotational spectra for two conformers of c-C3H5SiHFCH3 (cyclopropylfluoromethyl silane, CFMS) and one conformer of c-C3H5SiF2CH3 (cyclopropyldifluoromethyl silane, CDFMS) have been observed by a combination of chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 7.5–16.5GHz range. All rotational transitions were doubled by up to several megahertz into A and E states as a result of methyl group internal rotation, and analysis of measured transitions using a combined axis method yielded internal rotation barriers of ca. 6.0kJmol−1 for the two conformers of CFMS and ca. 5.2kJmol−1 for CDFMS, revealing a systematic decrease in the barrier as fluorine atoms are added to the silicon. Dipole moment information is also available for the CDFMS species.