Structure and hydration of the C4H4 •+ ion formed by electron impact ionization of acetylene clusters

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)nclusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured bindingenergy 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–waterinteractions. 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 watermolecule 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

THE USE OF INERT GAS MATRICES IN THE STUDY OF COOPERATIVE PHOTOCHEMICAL PROCESSES

Author Institution: Department of Chemistry, University of RichmondThe photochemistry of weakly bound complexes has been show by a number of authors to sometimes result in anomalous product branching when compared to the product branching due to photochemistry of the uncomplexed reactants. Such anomalous product branching has been labeled cooperative chemistry. Recently, it has been observed that such cooperative chemistry can be observed even in systems where the chromophore in the complex it excited to a purely repulsive excited state. The phenomenon is illustrated by results on photochemistry of complexes of HI with acetylene. An ad-hoc model is presented to explain the results, and allow prediction of the presence or absence of cooperative chemistry. Preliminary tests of the predictions on the photochemistry of hydrogen sulfide with acetylene are reported

Structure of the C₈H₈^•+ Radical Cation Formed by Electron Impact Ionization of Acetylene Clusters. Evidence for a (Benzene^•+·Acetylene) Complex

Here, we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the C₈H₈^•+ ion formed by electron impact ionization of neutral acetylene clusters. The ion dissociates by a dominant low-energy channel involving the loss of an acetylene molecule, leaving behind a stable benzene radical cation. Using equilibrium thermochemical measurements, the enthalpy and entropy changes of association of acetylene to the benzene radical cation are measured as −4.0 ± 1 kcal/mol and −11.4 ± 2.5 cal/(mol K), respectively. Ion mobility measurement indicates the presence of mainly one isomer with an average collision cross section in helium of 61.0 ± 3 Ų, significantly larger than the calculated cross sections of all of the covalently bonded C₈H₈^•+ ions and in excellent agreement with that of the benzene^•+·acetylene complex. The results provide strong evidence that the C₈H₈^•+ ion is predominantly present as an acetylene molecule associated with the benzene radical cation in ionized acetylene clusters

Formation of Covalently Bonded Polycyclic Hydrocarbon Ions by Intracluster Polymerization of Ionized Ethynylbenzene Clusters

Here we report a detailed study aimed at elucidating the mechanism of intracluster ionic polymerization following the electron impact ionization of van der Waals clusters of ethynylbenzene (C₈H₆)_<i>n</i> generated by a supersonic beam expansion. The structures of the C₁₆H₁₂, C₂₄H₁₈, C₃₂H₂₄, C₄₀H₃₀, and C₄₈H₃₆ radical cations resulting from the intracluster ion–molecule addition reactions have been investigated using a combination of mass-selected ion dissociation and ion mobility measurements coupled with theoretical calculations. Noncovalent structures can be totally excluded primarily because the measured fragmentations cannot result from noncovalent structures, and partially because of the large difference between the measured collision cross sections and the calculated values corresponding to noncovalent ion–neutral complexes. All the mass-selected cluster ions show characteristic fragmentations of covalently bonded molecular ions by the loss of stable neutral fragments such as CH₃, C₂H, C₆H₅, and C₇H₇. The population of the C₁₆H₁₂ dimer ions is dominated by structural isomers of the type (C₆H₅)CCCH^<b>•+</b>CH(C₆H₅), which can grow by the sequential addition of ethynylbenzene molecules, in addition to some contributions from cyclic isomers such as the 1,3- or 1,4-diphenyl cyclobutadiene ions. Similarly, two major covalent isomers have been identified for the C₂₄H₁₈ trimer ions: one that has a blocked cyclic structure assigned to 1,2,4- or 1,3,5-triphenylbenzene cation, and a second isomer of the type (C₆H₅)CCC­(C₆H₅)CHCH^<b>•+</b>CH(C₆H₅) where the covalent addition of further ethynylbenzene molecules can occur. For the larger ions such as C₃₂H₂₄, C₄₀H₃₀, and C₄₈H₃₆, the major isomers present involve the growing oligomer sequence (C₆H₅)CC[C­(C₆H₅)CH]_<i>n</i>CH^<b>•+</b>CH(C₆H₅) with different locations and orientations of the phenyl groups along the chain. In addition, the larger ions contain another family of structures consisting of neutral ethynylbenzene molecules associated with the blocked cyclic isomer ions such as the diphenylcyclobutadiene and triphenylbenzene cations. Low-energy dissociation channels corresponding to evaporation of ethynylbenzene molecules weakly associated with the covalent ions are observed in the large clusters in addition to the high-energy channels corresponding to fragmentation of the covalently bonded ions. However, in small clusters only high-energy dissociation channels are observed corresponding to the characteristic fragmentation of the molecular ions, thus providing structural signatures to identify the product ions and establish the mechanism of intracluster ionic polymerization

Formation of Complex Organics in the Gas Phase by Sequential Reactions of Acetylene with the Phenylium Ion

In this paper, we report a study on the reactivity of the phenylium ion with acetylene, by measuring product yield as a function of pressure and temperature using mass-selected ion mobility mass spectrometry. The reactivity is dominated by a rapid sequential addition of acetylene to form covalently bonded C₈H₇⁺ and C₁₀H₉⁺ ions with an overall rate coefficient of 7–5 × 10^–10 cm³ s^–1, indicating a reaction efficiency of nearly 50% at room temperature. The covalent bonding nature of the product ions is confirmed by high temperature studies where enhanced production of these ions is observed at temperatures as high as 660 K. DFT calculations at the UPBEPBE/6-31++G** level identify the C₈H₇⁺ adduct as 2-phenyl-ethenylium ion, the most stable C₈H₇⁺ isomer that maintains the phenylium ion structure. A small barrier of 1.6 kcal/mol is measured and attributed to the formation of the second adduct C₁₀H₉⁺ containing a four-membered ring connected to the phenylium ion. Evidence for rearrangement of the C₁₀H₉⁺ adduct to the protonated naphthalene structure at temperatures higher than 600 K is provided and suggests further reactions with acetylene with the elimination of an H atom and an H₂ molecule to generate 1-naphthylacetylene or acenaphthylene cations. The high reactivity of the phenylium ion toward acetylene is in sharp contrast to the low reactivity of the benzene radical cation with a reaction efficiency of 10^–4–10^–5, confirming that the first step in the cation ring growth mechanism is the loss of an aromatic H atom. The observed reactions can explain the formation of complex organics by gas phase ion–molecule reactions involving the phenylium ion and acetylene under a wide range of temperatures and pressures in astrochemical environments