Microwave Spectrum, Vibration–Rotation Interaction, Molecular Structure, and Dipole Moment of 1-Methylpentaborane (9)

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Abstract
The microwave spectra of a number of isotopic species of 1‐methylpentaborane (9) have been analyzed. All spectra are consistent with a model having free internal rotation and vibration–internal‐rotation interactions. Lack of barrier perturbations leads to an upper limit of 1 cm−1 for the 12‐fold barrier. The theory for the vibration–internal‐rotation interaction is extended to low‐barrier molecules having an N‐fold barrier. The boron framework was determined to be identical with that in pentaborane (9). The dipole moment is 1.93 ± 0.05 D.