Ab initio spectroscopic characterization of the radical CH$_3$OCH$_2$ at low temperatures

Spectroscopic and structural properties of methoxymethyl radical (CH$_3$OCH$_2$, 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 the astrophysical searches. 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 (X$^2$A), which appears "clean" from non-adiabatic effects, the minimum energy structure is an asymmetric geometry which rotational constants and dipole moment have been determined to be A$_0$=46718.6745 MHz, B$_0$=10748.4182 MHz, and C$_0$=9272.5105 MHz, and 1.432 D ($\mu_A$=0.6952 D, $\mu_B$=1.215 D, $\mu_C$=0.3016 D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A$_1$/A$_2$ 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 V$^{\alpha}$ barrier at ${\alpha}=0^{\circ}$ (109 cm$^{-1}$) stands below the zero point vibrational energy and the CH$_2$ torsional barrier is relatively high ($\sim$2000 cm$^{-1}$). The methyl torsion barrier was computed to be $\sim$500 cm$^{-1}$ and produces a splitting of 0.01 cm$^{-1}$ of the ground vibrational state.