The molecule possesses two equivalent methyl rotors whose internal rotation splitting was resolved (in part) in the laboratory. An extensive account on its rotational spectrum was given by
(1) M. Fatima, H. S. P. Müller, O. Zingsheim, F. Lewen, V. M. Rivilla, I. Jiménez-Serra, J. Martín-Pintado, and S. Schlemmer, 2023, Astron. Astrophys. 680, Art. No. A25.
Additional transition frequencies up to 35 GHz were taken from
(2) V. W. Laurie, 1961, J. Chem. Phys. 34, 1516;
from
(3) J. Demaison and H. D. Rudolph, 1975, J. Mol. Struct. 24, 325.
and from
(4) H. S. Gutowski and T. C. Germann, 1991, J. Mol. Spectrosc. 147, 91.
The internal rotation leads to four different internal rotor components which may be labeled as A₁A₁, EE, A₁E, and EA₁. The respective reduced spin-statistical weights are
9 : 16 : 5 : 4
for levels with K_a + K_a being even and
7 : 16 : 3 : 4
for levels with K_a + K_c being odd. Please do note, however, that there is a known bug in erham which recalculates K_c incorrectly under certain circumstances. The spin-statistical weights are correct for the corresponding transition, K_c may need to be adjusted!
Please note also that erham uses two σ values to label each symmetry component. This is incompatible with the VAMDC-CDMS requirements and does not agree with the convention of the classical CDMS to use one state number to label one symmetry species (or one vibrational state etc.). The following relations exist:
0 / 0 0 / A₁A₁,
1 / 0 1 / EE,
2 / 1 1 / A₁E,
3 / 1 2 / EA₁.
Please note also that the partition function refers to the ground vibrational state. Vibrational correction factors for a posteriori corrections were derived in (1) in the harmonic approximation.
The calculation should be sufficient for observational purposes. Frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with substantial caution.
The dipole moment was determined in (2).