The rotational spectrum of ethylamine displays two significant large amplitude motions: the NH₂ internal rotation and the NH₂ inversion, the former being the more important one. The internal rotation of the CH₃ group can be neglected in most instances.
The internal rotation of the NH₂ group leads to three conformers: the anti conformer as the lowest one, and the symmetric and antisymmetric combinations of the two equivalent gauche conformers.
The anti conformer has been characterized first by
(1) E. Fischer and I. Botskor, 1982, J. Mol. Spectrosc.. 91, 116.
No splitting due to NH₂ tunneling was observed, and the spectrum could be reproduced reasonably well with a rigid rotor Hamiltonian.
The gauche conformers have been characterized by
(2) E. Fischer and I. Botskor, 1984, J. Mol. Spectrosc.. 104, 226.
The splitting between the symmetric and antisymmetric combinations of the gauche conformers was found to be of similar magnitude as the splitting between the NH₂ tunneling components. The rather complex spectrum was difficult to fit with moderately complex Hamiltonian models. The gauche conformers were estimated to be about 54 cm^–1 above the anti conformer.
(3) A. J. Apponi, M. Sun, D. T. Halfen, L. M. Ziurys, and H. S. P. Müller, 2008, Astrophys. J.. 673, 1240;
extended the assignments for the anti-conformer to 272 GHz. Tunneling splitting was resolved for some transitions, almost entirely obeying b-type selection rules. The spectrum is sufficiently regular such that it can be reproduced reasonably well with a Hamiltonian model which takes centrifugal distortion and tunneling into account. However, some transitions are perturbed severely. As a consequence, the transitions do fit only within five times the experimental uncertainties even with many transitions weighted out.
Note: The present Hamiltonian model differs somewhat from that in (3). Moreover, transitions beyond those listed in (3) were included and retained in the fit if the assignments appeared to be sufficiently secure. The uncertainties were increased by multiples of 1 MHz if the residuals were deemed to be too large. 99 MHz were added to transitions whose assignments were less certain. Assignments, which are more likely incorrect, have been excluded from the fit. They are in a file called unclear.lin. These categorizations should be viewed with caution. Attributed uncertainties should be appropriate for the most part, though some very small uncertainties of millimeter wave transitions may be slightly optimistic. All lines from the final fit were merged, irrespective of the residuals.
Predictions should be viewed with great caution for all transitions with J ≥ 20, even if some assignments extend thus far. Transitions with K_a = 4 and J ≤ 12 should be viewed with caution. The same applies to the a-type transitions with K_a = 9 and J ≥ 12 and all with K_a ≥ 10, as well as b-type transitions with K_a ≥ 5. The prediction were limited in J to 40 and in K_a to 12 and 6 for a- and b-type transitions, respectively, and to 500 GHz.
The ¹⁴N hyperfine splitting may be resolved for transitions covering a variety of quantum numbers. Therefore, separate predictions with hyperfine splitting are available. The partition function takes into account the spin multiplicity g_I = 3 of the ¹⁴N nucleus !
No vibrational state and no other conformer were considered in the calculation of the partition function.
The dipole moment was determined in (1).