Vibrational Spectroscopy of a Potential Interstellar Ion: Protonated Methyl Formate

The abundance of methyl formate (MF, HCOOCH₃) in star-forming regions of the interstellar medium (ISM) suggests the presence of protonated MF (H⁺MF). However, no spectroscopic data exist for isolated H⁺MF. Here, we address the vibrational properties of H⁺MF and its H⁺MF-L_n≤2 clusters (L = Ar/N₂) by infrared photodissociation (IRPD) spectroscopy and density functional theory calculations. Protonation of MF occurs at the CO oxygen, resulting in four different isomers arising from the syn/anti (s/a) and cis/trans (c/t) orientation between OCH₃ and the excess proton. H⁺MF photofragments into protonated methanol by CO elimination. The IRPD spectrum exhibits redshifted OH stretch bands of the most stable H⁺MF(t/s) and H⁺MF(c/a) conformers because of the high internal energy required for dissociation. Tagging of H⁺MF with inert ligands drastically reduces both the internal energy and the dissociation threshold. The resulting higher-resolution IRPD spectra allow determination of the most stable H⁺MF rotamers as (t/s) and (c/a). In the cold H⁺MF-L dimers, the ligand forms an OH...L hydrogen bond, while bonding to the positively charged 2p_z orbital of the carbonyl C atom is less favorable. The latter allows estimation of the free OH stretch fundamental of the most stable H⁺MF(t/s) rotamer as 3545 ± 5 cm⁻¹. While for neutral MF the more stable syn rotamer MF(s) dominates the population in both the laboratory and the ISM (>99%), the anti conformer is substantially populated for H⁺MF (∼30%), which is rationalized by protonation-induced isomerization. This mechanism may lead to an enhanced abundance of MF(a) in certain regions of the ISM.