Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C4H4•+ ion in the cluster beam. The measured average collision cross section of the C4H4•+ isomers in helium (38.9 ± 1 Å2) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C4H4•+ ion [methylenecyclopropene (39.9 Å2), 1,2,3-butatriene (41.1 Å2), cyclobutadiene (38.6 Å2), and vinyl acetylene (41.1 Å2)]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C4H4•+ ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C4H4•+ ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C2H2)2•+[R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys.131, 114305 (2009)]

. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C4H4•+(H2O)n clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C4H4•+H2O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene•+·H2O cluster (41 kJ/mol). The binding energies of the C4H4•+(H2O)n clusters change little from n = 1 to 5 (39–48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water–C4H4•+ and water–water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C4H4•+(H2O)5 cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C4H4•+(H2O)5 cluster presumably consisting of a cyclobutadiene•+ cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C2H2)2•+ ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.

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