There are two conformers of 4-cyano-2,3-butadiene, also known as 3,4-pentadienenitrile, syn and anti. It should be pointed out that the anti conformer actually has a torsional angle significantly different from 180^o, thus might be better called gauche. Hence, it is conformationally doubly degenerate. There is no indication for tunneling between the two equivalent minima. The energy difference of syn and anti is not known accurately, but the syn conformer may be slightly higher in energy. The two species were treated as isolated species. The two conformers should be found thermally equilibrated in space should their column densities be high enough.
The data were recorded by
(1) M. A. Zdanovskaia, P. M. Dorman, V. L. Orr, A. N. Owen, S. M. Kougias, B. J. Esselman, R. C. Woods, and R. J. McMahon, 2021, J. Am. Chem. Soc. 143, 9551.
One distortion parameters less was used.
The calculations should be sufficiently accurate for all types of radio astronomical observations. Transition frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with caution. Chances to observe this molecule are probably greatest in TMC-1 or other cold sources. The mechanical parameters are sufficiently accurate for these sources, but unfortunately no ¹⁴N hyperfine parameter were determined experimentally. MP3 calculations at the MP2 geometry were carried out by
(2) H. S. P. Müller, 2021, unpublished;
see also
(3) H. S. P. Müller, O. Zingsheim, N. Wehres, J.-U. Grabow, F. Lewen, and S. Schlemmer, 2017, J. Phys. Chem. A, 121 7121.
A separate calculation with hyperfine structure is available up to 72 GHz with J up to 15. Even though the calculated hyperfine parameters are expected to be very close to the experimental ones, some caution is useful for transitions with large splittings.
The dipole moment is from a MP2 quantum-chemical calculation also in (2).