The E stereo isomer is the lower energy form; the Z form is calculated to be 3.16 kJ/mol higher in energy. The rotational spectrum was recorded between 158 and 470 GHz by
(1) L. Margulès, A. Remijan, A. Belloche, R. A. Motiyenko, B. A. McGuire, C. Xue, H. S. P. Müller, R. T. Garrod, K. M. Menten, and J.-C. Guillemin, 2022, Astron. Astrophys. 663, Art. No. A132.
No hyperfine structure (HFS) splitting was resolved, and it may be too small to be resolved in astronomical observations. The calculations should be sufficiently accurate for all observational purposes. Frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with caution.
The two stereo isomeric forms were treated as separate species in (1) which is certainly appropriate at low temperatures. The partition function below refers to the ground vibrational state only. Vibrational correction factors for a posteriori corrections were derived in (1) in the harmonic approximation.
It may be useful to treat the two forms as being in local thermodynamic equilibrium at higher temperatures. Corresponding vibrational correction factors with contributions for Z for a posteriori corrections of the rotational partition function of E were derived in the harmonic approximation.
The dipole moment components are from a quantum-chemical calculation in (1).