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Methyl isocyanate is a quasi-symmetric top molecule;
that is an asymmetric top rotor with a low barrier
to linearity of the heavy atoms. In addition, the
internal rotation of the methyl group is almost
not hindered. Data were taken mainly from
(1) J. Cernicharo, Z. Kisiel, B. Tercero, L. Kolesniková,
I. R. Medvedev, A. López, S. Fortman, M. Winnewisser,
F. C. de Lucia, J. L. Alonso, and J.-C. Guillemin,
2016, Astron. Astrophys. 587, Art. No. L4.
Data below 40 :GHz were taken from
(2) J. Koput,
1986, J. Mol. Spectrosc. 115, 131.
Some of the data from (2) were measured with hyperfine
splitting, but hyperfine free frequencies were determined
in that work. The reported transition frequencies should be
reliable to the estimated uncertainties of 50 kHz
because almost all of the lines have been measured
in the laboratory.
The quantum numbers are J, Ka,
Kc, and m, where the latter
refers to the methyl internal rotor states. The quantum
numbers m or Ka may carry a
sign; in the latter case, Kc is
redundant.
NOTE: Because of
the very complex spectrum, data are limited to K
up to 3 for the ground torsional state and excited
torsional states up to m = ±3.
However, data for K = –2 are
missing for m = 1, 2, and –3. Only
selected data are available for m = +3.
On the other hand, data are also given for
K = 0 of m = 4 and 5.
Please note also that there is a separate entry with
data for the first excited bending state.
The partition function takes into account vibrational
states up to about 580 K. Errors are in the range
of 1% around 125 K; they may reach the 10% level
around 200 K.
The dipole moment was measured by
(3) W. Kasten and H. Dreizler,
1986, Z. Naturforsch. 41a, 637.
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