The microwave spectrum of cis-2-pentene

The microwave spectrum of cis-2-pentene has been shown to originate from molecules in the skew-conformation (dihedral angle φ = 119 ± 3°). From A-E doublet splittings the barrier to internal rotation about the C₁C₂ axis was found to be 280 ± 4 cm^?1; furthermore the dipole moment components and centrifugal distortion constants are reported. In excited states of the C₃C₄ torsion the spectra exhibit further splittings; these are due to tunneling between the two equivalent skew-conformations through a barrier of 210 ± 20 cm^?1.