Cyanamide tunnels between two equivalent positions, just as ammonia does. The experimental data within each tunneling state have been summarized by
(1) Z. Kisiel, A. Krasnicki, W. Jabs, E. Herbst, B. P. Winnewisser, and M. Winnewisser, 2013, J. Phys. Chem. A, 117, 9889.
By far the most lines are from that work. Few lines originate from
(2) J. K. Tyler, J. Sheridan, and C. C. Costain, 1972, J. Mol. Spectrosc., 43, 248;
and from
(3) R. D. Brown, P. D. Godfrey, M. Head-Gordon, K. Wiedenmann, and B. Kleibömer, 1988, J. Mol. Spectrosc., 130, 213.
The present parameter set differs slightly from that in (1). The experimental transition frequencies were reproduced slightly outside the reported uncertainties. This is likely more an issue of unaccounted perturbations rather than too optimistic uncertainties. The a-type transitions are within tunneling states whereas the c-type transitions connect the two tunneling states. The predictions should be sufficiently accurate for all astronomical observaitions, but should be viewed with caution once the predicted uncertainties exceed 0.3 MHz.
The dipole moment components were assumed to agree with those of the main isotopolog:
(4) W. G. Read, E. A. Cohen, and H. M. Pickett, 1986, J. Mol. Spectrosc., 115, 316.