Rotational spectrum of aminocyclobutane: internal rotation and inversion of the NH2 group in the gauche-equatorial conformer

The microwave spectrum of aminocyclobutane has been investigated by free jet absorption millimeter-wave spectroscopy and conventional microwave spectroscopy in the 60-78 and 26-40 GHz frequency ranges, respectively. The spectrum of the conformer with the amino group in the equatorial position, and the nitrogen lone electron pair gauche with respect to the adjacent hydrogen of the ring, has been assigned. Each rotational transition is split into four component lines, corresponding to the four equivalent minima in the potential energy surface of the internal rotation and inversion of the NH2 group. The potential energy parameters have been calculated from the measured vibrational spacings among the corresponding vibrational sublevels.