The E-isomer of C-cyanomethanimine is higher in energy than the Z-isomer. Nevertheless, thus far only the former has been detected in space. The experimental data were taken from
(1) S. Takano, M. Sugie, K. Sugawara, H. Takeo, C. Matsumura, A. Masuda, and K. Kuchitsu, 1990, J. Mol. Spectrosc., 141, 13.
They derived the E-isomer to be higher in energy by 310 (70) K as suggested by the context. (They wrote that it were lower in energy.)
Additional data with extensive ¹⁴N hyperfine splitting were obtained in the context of
(2) D. P. Zaleski, N. A. Seifert, A. L. Steber, M. T. Muckle, R. A. Loomis, J. F. Corby, O. Martinez, Jr., K. N. Crabtree, P. R. Jewell, J. M. Hollis, F. J. Lovas, D. Vasquez, J. Nyiramahirwe, N. Sciortino, K. Johnson, M. C. McCarthy, A. J. Remijan, and B. H. Pate,
2013, Astrophys. J., 765, Art. No. L10.
With respect to the first entry from Feb. 2014, additional millimeter and submillimeter data were included in the fit. They were reported by
(3) M. Melosso, A. Melli, C. Puzzarini, C. Codella, L. Spada, L. Dore, C. Degli Esposti, B. Lefloch, R. Bachiller, C. Ceccarelli, J. Cernicharo, and V. Barone, 2018, Astron. Astrophys., 609, Art. No. A121.
Quantum-chemical calculations by
(4) C. Puzzarini, 2015, J. Phys. Chem. A, 119, 11614
suggest the energy difference may be about half the value determined in (1). The barrier between the two supports the idea of two different isomers rather than two conformers, even if formally no bond needs to be broken.
D. P. Zaleski is thanked for the unpublished details of their fit. That fit considered only a subset of the data in (1), so the spectroscopic parameters obtained here are different from those in (2). Moreover, the hyperfine treatments is somewhat different, as it was in the first version. The mechanical parameters are the same as in (3), and the values are very similar. The frequency of the J" = K_a" = 9 transition of (1) was omitted because of large residuals. Predictions should be viewed with caution once the predicted uncertainties exceed 0.3 MHz.
The ¹⁴N hyperfine splitting was at least partially resolved in astronomical observations. Therefore, a separate hyperfine calculation is provided up to J" = 18, K_a" = 6, and up to 260 GHz along with appropriate partition function values.
Isomerization in the gas phase between E- and Z-isomer can only take place at very high temperatures. Therefore, the molecules are treated as two independent ones.
The dipole moment components were determined in (1).