Competitive Gas Phase Reactions for the Production of Isomers C2O2H4. Spectroscopic Constants of Methyl Formate.

New routes for the chemical formation of the C2O2H4 isomers in the gas phase are explored searching for a justification of the prominent astrophysical abundance of methyl formate with respect to the most stable one acetic acid. Kinetic rate constants at low temperatures are provided for eight barrierless reaction pathways. In addition, the spectroscopic parameters are computed using highly correlated ab initio methods for the main isotopologue of methyl formate and for five monosubstituted isotopologues containing 13C, 18O, and deuterium. Accurate rotational constants are obtained at the CCSD(T)-F12 level of theory. The dipole moments components are provided. Centrifugal distortion constants, rovibrational parameters, and Fermi displacements are predicted using second order perturbation theory. A variational procedure of reduced dimensionality is applied to determine band center positions for transitions corresponding to the large amplitude motions. The COC bending mode is considered explicitly as an independent coordinate to evaluate its resonances with the torsional energies. The effect of resonances is proven as negligible.