The Far-Infrared Spectrum of Methoxymethanol (CH₃-O-CH₂OH): A Theoretical Study

Methoxymethanol (CH₃OCH₂OH), an oxygenated volatile organic compound of low stability detected in the interstellar medium, represents an example of nonrigid organic molecules showing various interacting and inseparable large-amplitude motions. The species discloses a relevant coupling among torsional modes, strong enough to prevent complete assignments using effective Hamiltonians of reduced dimensionality. Theoretical models for rotational spectroscopy can improve if they treat three vibrational coordinates together. In this paper, the nonrigid properties and the far-infrared region are analyzed using highly correlated ab initio methods and a three-dimensional vibrational model. The molecule displays two gauche-gauche (CGcg and CGcg') and one trans-gauche (Tcg) conformers, whose relative energies are very small (CGcg/CGcg'/Tcg = 0.0:641.5:792.7 cm^-1). The minima are separated by relatively low barriers (1200-1500 cm^-1), and the corresponding methyl torsional barriers V ₃ are estimated to be 595.7, 829.0, and 683.7 cm^-1, respectively. The ground vibrational state rotational constants of the most stable geometry have been computed to be A ₀ = 17233.99 MHz, B ₀ = 5572.58 MHz, and C ₀ = 4815.55 MHz, at ΔA ₀ = -3.96 MHz, ΔB ₀ = 4.76 MHz, and ΔC ₀ = 2.51 MHz from previous experimental data. Low-energy levels and their tunneling splittings are determined variationally up to 700 cm^-1. The A/E splitting of the ground vibrational state was computed to be 0.003 cm^-1, as was expected given the methyl torsional barrier (∼600 cm^-1). The fundamental levels (100), (010), and (001) are predicted at 132.133 and 132.086 cm^-1 (methyl torsion), 186.507 and 186.467 cm^-1 (O-CH₃ torsion), and 371.925 and 371.950 cm^-1 (OH torsion), respectively.