Vibrational Spectroscopy of a Potential Interstellar Ion: Protonated Methyl Formate

Abstract

The abundance of methyl formate (MF, HCOOCH 3 ) 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/N 2 ) 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 OCH 3 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.