Planarity and low-energy vibrations of catecholborane: a microwave spectroscopic study

The rotational spectra of four isotopomers of catecholborane have been observed and measured in natural abundance in the frequency range 28-40 GHz. The molecule has been shown to be planar. The rotational spectra of three vibrational progressions have been measured for the most abundant isotopic species (B-11). On the basis of the statistical weights and from the shifts of rotational constants with respect to the ground state they have been assigned to the butterfly, 1,3-ring twisting, and ring-puckering motions. The dipole moment has been measured to be 1.26(4) D (1 D almost-equal-to 3.335 64 x 10(-30) C m).