10min:

P. PRACNA, J. URBAN, J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic; S. URBAN, J. VARGA, Department of Analytical Chemistry, Institute of Chemical Technology, 16628 Prague 6, Czech Republic; V.-M. HORNEMAN, Department of Physical Sciences, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland.

Vibrational levels up to 1000 cm^-1 of H₃C-NC 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 v₈=1^{\pm 1} (267.3 cm^-1), v₈=2^{0,\pm 2} (524.6 cm^-1 (A), 545.3 cm^-1 (E)), and v₈=3^{\pm 1,\pm 3} (792.5 cm^-1 (A1+A2), 833.9 cm^-1 (E)), respectively, and the next higher fundamental level of the C-N valence vibration v₄=1 (945 cm^-1). 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 ₄ band. Similarities and differences with respect to isoelectronic molecules CH₃CN and CH₃CCH are discussed.