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The transition frequencies were taken
from
(1) J. M. Riveros and E. Bright Wilson, Jr.,
1967, J. Chem. Phys., 46, 4605;
from
(2) J. Demaison, D. Boucher, J. Burie, and
A. Dubrulle,
1984, Z. Naturforsch. A, 39, 560;
and from
(3) Ivan R. Medvedev, Frank C. De Lucia, and Eric Herbst,
2009, Astrophys. J. Suppl. Ser., 181, 433.
According to (1), the anti conformer is the lowest
conformer with the gauche conformer being higher
in energy by 0.78 ± 0.25 kJ/mol
≡ 65 ± 21 cm–1
≡ 94 ± 30 K. Astronomical
observations are in qualitative agreement.
Data from (1) were assigned an uncertainty of 100 kHz
as has been done on previous occasions. Data from (2)
were ascribed uncertainties of 50 kHz as is
commonly found for data from that lab. New data in (3)
were supposedly uncertain to 150 kHz. That number
appeared to be too conservative, in particular for
the gauche conformer. Therefore, we used a value
of 100 kHz for these data.
According to (3), the ground vibrational state of the
anti conformer appears to be perturbed by a
low-lying excited vibratational state at
Ka = 16 and 17.
No such perturbation were found for the gauche
conformer. However, considerably more spectroscopic
parameters were needed to fit the data accurately.
Therefore, predictions of transitions with uncertainties
larger than 1 MHz should be viewed with great caution.
Individual contributions of the two conformers to the
partition function are given in parentheses.
Note: the statistical weight of the anti and
gauche conformers is 1 : 2 because
of symmetry.
No vibrational corrections are known or given.
However, they are likely to contribute at T > 50 K.
The dipole moment components were determined in (1).
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