ABSTRACT
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.
ABSTRACT
Spectroscopic and structural properties of methoxymethyl radical (CH3OCH2, RDME) are determined using explicitly correlated ab initio methods. This radical of astrophysical and atmospheric relevance has not been fully characterized at low temperatures, which has delayed astrophysical research. We provide rovibrational parameters, excitations to the low energy electronic states, torsional and inversion barriers, and low vibrational energy levels. In the electronic ground state (X2A), which appears "clean" from nonadiabatic effects, the minimum energy structure is an asymmetric geometry whose rotational constants and dipole moment have been determined to be A0 = 46 718.67 MHz, B0 = 10 748.42 MHz, and C0 = 9272.51 MHz, and 1.432D (µA = 0.695D, µB = 1.215D, µC = 0.302D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A1/A2 and E) corresponding to the three methyl torsion minima. Although the potential energy surface presents 12 minima, at low temperatures, the infrared band shapes correspond to a surface with only three minima because the top of the inversion Vα barrier at α = 0° (109 cm-1) stands below the zero point vibrational energy and the CH2 torsional barrier is relatively high (â¼2000 cm-1). The methyl torsion barrier was computed to be â¼500 cm-1 and produces a splitting of 0.01 cm-1 of the ground vibrational state.
