Isocyanamide tunnels between two equivalent minima as does the isoelectronic cyanamide, but the tunneling splitting is much smaller. Perturbations associated with the tunneling are smaller in isocyanamide, whereas vibration-rotation interaction is more pronounced. Selection rules are of a symmetry within the tunneling states and of c symmetry between the states.
The transition frequencies were taken from
(1) E. Schäfer, M. Winnewisser, and J. J. Christiansen, 1981, Chem. Phys. Lett., 81, 380;
and from
(2) E. Schäfer and M. Winnewisser, 1982, Ber. Bunsenges. Phys. Chem., 86, 780.
Some of the transitions omitted in (1) or (2) were used in the present fit. Transitions with a-type selection rules were omitted for K_a = 5 to 7 because of perturbations. The calculations include K_a = 5, but these are lower than observed by a about 0.2 MHz at low J increasing to about 0.65 MHz near 425 GHz. All other transition frequencies should be reliable. Calculations of c-type transitions were restricted to K_a = 1 ↔ 0. The available data should be sufficient for all astronomical observations.
The dipole moment components were calculated by
(3) H. S. P. Müller, 2018, unpublished.