Positive molecular ions and ion-neutral complexes in the gas phase: Structure and stability of C2H4O2+· and C2H4O22+ isomers

The structure and stability of several C2H4O2+· and C2H4O22+ isomers were studied by quantum chemical methods in an attempt to elucidate their most stable molecular arrangements and infer their contribution to the ion chemistry of astrophysical environments. The geometry optimizations were performed at B3LYP/6-311G** and MP2/6-311G** level. Single-point calculations at equilibrium geometries were performed at left eigenstate completely renormalized coupled cluster method, CR-CCL(2,3). Ninety minimum structures for the ions at the MP2 level were found: 38 related to C2H4O2+·, 33 to C2H4O22+, and 19 to ion-neutral complexes. The more stable C2H4O2+· isomers have enolic structures, in agreement with the McLafferty rearrangement mechanism of straight chain aliphatic acids. In contrast, the most stable C2H4O22+ isomer has an acyclic OCCO nonplanar linear chain structure, with terminal +OH2 and CO+ linkages. In addition, acetic acid and methyl formate analogue structures were found only for the monopositive ion, with higher energies though. Both global minima are significantly different from the more stable neutral isomers and, therefore, could not be found in a geometry optimization procedure starting from the neutral structures. Dissociative recombination reaction pathways of C2H4O2+· and its application to the formation of astrophysical neutral molecules are also discussed.