E-2-Butenenitrile is lower in energy than Z-2-butenenitrile. The conversion requires a formal rotation around a double bond. Therefore, the two species are considered as separate isomers.
The experimental lines were taken mostly from
(1) A. G. Lesarri, J. Cosléou, X. Li, G. Wlodarczak, and J. Demaison, 1995, J. Mol. Spectrosc. 172, 520.
Also included in the fit were microwave transitions from
(2) S. L. Hsu and W. H. Flygare, 1971, J. Mol. Spectrosc. 37, 92.
Some additional spectroscopic parameters were used, in part estimated. b-type transitions were not calculated because they are weak and apparently affected by substantial internal rotation splitting. Such splitting appears to be negligible for a-type transitions, except maybe for some transitions with low values of J. ¹⁴N hyperfine splitting is also negligible, except at lower J. The calculation should be suitable for most observational purposes. Transition frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with caution.
The a-dipole moment component is from
(3) M. Suzuki and K. Kozima, 1970, J. Mol. Spectrosc. 33, 407.
The b-component is quite uncertain.