Recent microwave studies of internal rotation and molecular structure

The microwave spectra of several molecules with a sixfold barrier are described. These barriers are expected to be low but the practical method of spectroscopic analysis depends on the reduced barrier height. Molecules with a light internal top such as CH₃NO₂ and CH₃BF₂ are treated in the free-rotor limit. On this basis the spectrum of CH₃BCl₂, a new microwave molecule, has been analysed; the structure and ³⁵Cl quadrupole coupling constants are reported.Molecules possessing a heavy internal symmetric top such as SiF₃BF₂, CF₃NO₂, (CH₃)₃CNO₂ and (CH₃)₃CBF₂ exhibit microwave spectra of intermediate barrier type. Recent barrier and structural results are discussed.