Structure and Spectroscopy of C₂HNO Isomers

Formyl cyanide has been detected toward Sgr B2 making this chemical and its potential isomers of astrophysical interest. We calculated the precise energies of the five most stable structural isomers of formyl cyanide and found that formyl isocyanide and 2H-azirin-2-one are less stable than formyl cyanide by 55 and 125 kJ/mol, respectively. We present our ab initio coupled cluster predictions of the spectroscopic parameters relevant to their gas-phase rotational and vibrational spectroscopy. Parameters include ground vibrational state rotational constants, centrifugal distortion parameters, equilibrium electric dipole moments, and nuclear quadrupole coupling constants. Anharmonic vibrational frequencies together with infrared intensities of fundamental and nonfundamental modes were also calculated. The broader potential energy surface, including transition states and the minima they connect, were explored for these isomers at the B3LYP/aug-cc-pVTZ level of theory. Formyl cyanide can dissociate in an exothermic reaction to form HCN and CO or in an endothermic reaction to form HNC and CO. The activation energies for both processes are close to 260 kJ/mol. The activation barrier for conversion of formyl cyanide into formyl isocyanide is ∼180 kJ/mol. The energetic barrier for the exothermic decomposition of formyl isocyanide into HNC and CO is ∼210 kJ/mol. The barrier for transformation of formyl isocyanide into formyl cyanide is ∼130 kJ/mol. Formyl isocyanide seems to be a stable chemical and could potentially survive in a dense molecular cloud if formed. These data are useful for future identification of members of this family of molecules in a laboratory or in space.