Low-energy vibrations of indene

The rotational spectra of the two lowest-energy vibrational satellites of indene have been measured in the frequency range 28–40 GHz. On the basis of relative intensity measurements and from the shifts of rotational constants with respect to the ground state they have been assigned to the butterfly and 1,3-ring-twisting motions. The analysis of the ground state has been extended and the dipole moment has been measured to be 0.62(2) D.