ANHARMONIC RESONANCES AMONG LOW-LYING VIBRATIONAL LEVELS OF METHYL ISO-CYANIDE (H$_3$CNC)

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Vibrational levels up to 1000 \wn of H$_3$C-N$\equiv$C are currently studied in FTIR spectra together with rotational transitions within these levels. This investigation comprises the low-lying excited vibrational levels of the CNC doubly degenerate bending vibration $v_{8}=1^{\pm 1}$ (267.3 \wn), $v_{8}=2^{0,\pm 2}$ (524.6 \wn (A), 545.3 \wn (E)), and $v_{8}=3^{\pm 1,\pm 3}$ (792.5 \wn (A1+A2), 833.9 \wn (E)), respectively, and the next higher fundamental level of the C-N valence vibration $v_{4}=1$ (945 \wn). All these vibrational levels exhibit cubic and quartic anharmonic resonances localized to moderate values of the rotational quantum number $K\leq10$. Therefore the system of rovibrational levels has to be treated as a global polyad in order to describe all the available data quantitatively. The ground state constants have been improved considerably by extending the assignments to higher $J/K$ rotational states both in the purely rotational spectra recorded in the ground vibrational level and in the ground state combination differences generated from the wavenumbers assigned in the fundamental ${\nu_4}$ band. Similarities and differences with respect to isoelectronic molecules CH$_3$CN and CH$_3$CCH are discussed.

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Author Institution: J. Heyrovsky Institute of Physical Chemistry, v.v.i.; Academy of Sciences of the Czech Republic; 18223 Prague 8, Czech Republic; Department of Analytical Chemistry, Institute of Chemical Technology; 16628 Prague 6, Czech Republic; Department of Physical Sciences, University of Oulu, Linnanmaa; FIN-90570 Oulu, Finland

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