Theoretical Spectroscopic Characterization at low Temperatures of C2H3NO and its Isomers

Abstract

The astrophysical relevant molecules of empirical formula C2H3NO are characterized using explicitly correlated coupled cluster methods CCSD(T)-F12/AVTZ-F12). Rovibrational parameters and far infrared transitions are provided for five isomers: methyl isocyanate (CH3NCO), methyl cyanate (CH3OCN), methyl fulminate (CH3ONC), acetonitrile N-oxide (CH3CNO). A CH3CON transition state is inspected. The equilibrium rotational constants of acetonitrile N-oxide, Ae =160322.4 MHz, Be = Ce = 3909.39 MHz, correspond to a linear C3v structure. The remaining isomers present a bend skeleton and non-rigidity caused by internal rotation. The recently detected methyl isocyanate, the most stable isomer, contains a rotating methyl group that can be considered a free rotor given the very low torsional energy barrier (V3=16.2 cm-1). The ground vibrational state rotational constants have been calculated to be A0 =76164.32 MHz, B0 =4411.89 MHz and C0 =4254.58 MHz. Divergences between ab initio data and previous parameters fitted from observed lines, are related to the large rovibrational interaction. For methyl cyanate (V3= 364.84 cm-1, A0 =39089.80 MHz, B0 = 5314.78 MHz and C0 = 4816.35 MHz), the ab initio rotational constants are in a very agreement with the experimental ones. Torsional and rotational parameters are computed for methyl fulminate (V3=821.7 cm-1, A0 =37004.74 MHz, B0 =5696.21 MHz, C0 =5083.48 MHz).