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The molecule displays internal rotation of the
methyl group. The experimental data were reproduced
quite well with the erham program up to 661 GHz;
see below for more details.
The present analysis is based on the data from
(1) L. Margulès, T. R. Huet, J. Demaison, M. Carvajal, I. Kleiner,
H. Møllendal, B. Tercero, N. Marcelino, and J. Cernicharo,
2010, Astrophys. J. 714, 1120.
The analysis takes into account lower frequency data
up to 377 GHz mainly from
(2) L. C. Oesterling, D. W. Ferguson, E. Herbst, and
F. C. DeLucia,
1995, J. Mol. Spectrosc. 172, 469.
Also included were few rest frequencies from
(3) R. F. Curl, Jr.,
1959, J. Chem. Phys. 30, 1529.
Additional higher frequency data, up to 1200 GHz,
extending also to higher quantum numbers, were published
more recently by
(4) C. Duan, M. Carvajal, S. Yu, J. C. Pearson, B. J. Drouin,
and I. Kleiner,
2015, Astron. Astrophys. 576, Art. No. A39.
However, these data could not be included into the fit satisfactorily
and were thus omitted. Deficiencies in the erham program or in the
knowledge of the contributor are likely explanations. Nevertheless,
the calculations should be sufficiently accurate for all
observational purposes. Transitions with calculated uncertainties
exceeding 0.1 MHz should be viewed with some caution.
Please note that state number 0 and 1 refer to the A and E
symmetry components, respectively. This differs slightly from
the standard erham notation. Some lines were not merged
because of weakness and some because Kc
quantum numbers were evaluated incorrectly by the erham program.
The partition function refers to the complete ground vibrational state.
The dipole moment components were determined in (1).
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