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.
