Methyl doubly deuterated acetaldehyde consists of two different conformers, the in-plane conformer and the doubly degenerate out-of-plane conformer, which is about 14.49 cm^–1 or 20.85 K lower in energy than the former. The barrier to tunneling between the two equivalent out-of-plane conformations is sufficiently low to facilitate tunneling, thus lifting the degeneracy upon rotation. Rotation-tunneling interaction is strong between the two out-of-plane conformations, essentially negligible between these and the in-plane conformer.
This documentation is largely based on the one provided in
(1) J. Ferrer Asensio, S. Spezzano, L. H. Coudert, V. Lattanzi, C. P. Endres, J. K. Jørgensen, and P. Caselli, 2023, Astron. Astrophys. 670, Art. No. A177.
By far the most data are from this work. Additional data below 40 GHz were taken from
(2) P. H. Turner and A. P. Cox, 1976, Chem. Phys. Lett. 42, 84;
and from
(3) P. H. Turner, A. P. Cox, and J. A. Hardy, 1981, J. Chem. Soc. Faraday Trans. 2 77, 1217;
for the in-plane and out-of-plane conformers, respectively.
Please note that the lowest frequency is 20 GHz ! Calculations with uncertainties larger than 0.2 MHz should be viewed with caution.
The vibrational identifier 2 refers to the in-plane conformer and 0 and 1 to the symmetric (+) and antisymmetric (–) tunneling states, respectively, of the out-of-plane conformer.
The partition function includes approximate contributions from the lowest torsional mode and from the lowest small amplitude vibration. The partition function should thus be fairly well converged up to about 100 K or slightly higher.
The dipole moment components were derived from those of the main isotopic species.