Infrared Spectroscopy of the Astrochemically Relevant Protonated Formaldehyde Dimer

The protonated formaldehyde dimer (H₂CO)₂H⁺ was generated in an electrical discharge and supersonic expansion of argon saturated with formalin solution vapor. Its infrared spectrum was measured in the region from 900 to 4000 cm^-1 employing infrared laser photodissociation and messenger atom tagging. Comparison of the experiment to quantum chemical computations at the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level reveals that the experimentally observed structure is the head-to-tail dimer and not the more stable proton-bound dimer. This is consistent with the usually observed C-O bond formation upon formaldehyde oligomerization under acidic conditions in solution and resembles the structure of the neutral (H₂CO)₂ dimer in the gas phase. There is no evidence for the formation of other isomers, most notably protonated glycolaldehyde, that could result from covalent bond formation. These findings may be relevant to a proposed carbohydrate formation mechanism in the interstellar medium starting from protonated formaldehyde dimer.