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

Abstract

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 C(4)H(4)(●+) ion in the cluster beam. The measured average collision cross section of the C(4)H(4)(●+) isomers in helium (38.9 ± 1 Å(2)) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C(4)H(4)(●+) ion [methylenecyclopropene (39.9 Å(2)), 1,2,3-butatriene (41.1 Å(2)), cyclobutadiene (38.6 Å(2)), and vinyl acetylene (41.1 Å(2))]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C(4)H(4)(●+) 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 C(4)H(4)(●+) ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C(2)H(2))(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 C(4)H(4)(●+)(H(2)O)(n) clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C(4)H(4)(●+)H(2)O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene(●+)·H(2)O cluster (41 kJ/mol). The binding energies of the C(4)H(4)(●+)(H(2)O)(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-C(4)H(4)(●+) 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 C(4)H(4)(●+)(H(2)O)(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 C(4)H(4)(●+)(H(2)O)(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 (C(2)H(2))(2)(●+) ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.

Communication: Ion mobility of the radical cation dimers: (Naphthalene)2+• and naphthalene+•-benzene: Evidence for stacked sandwich and T-shape structures

“…[19][20][21][22][23][24][25] The motion of the ion through a buffer gas under the influence of a weak electric field depends on the ion's average collision cross section (Ω, Å 2 ) with the buffer gas, which depends on the geometric shape of the ion. [19][20][21][22][23][24][25] Theoretical calculations of possible structural candidates of the mass-selected ions are then used to compute angle-averaged Ω's at different temperatures for comparison with the measured ones. [19][20][21][22][23][24][25] The experiments were performed using the VCU massselected ion mobility spectrometer (schematic is given in Figure S1, supplementary material).…”

“…[19][20][21][22][23][24][25] Theoretical calculations of possible structural candidates of the mass-selected ions are then used to compute angle-averaged Ω's at different temperatures for comparison with the measured ones. [19][20][21][22][23][24][25] The experiments were performed using the VCU massselected ion mobility spectrometer (schematic is given in Figure S1, supplementary material). 26 The details of the instrument can be found in several publications [20][21][22] and only a brief description of the experimental procedure is given here.…”

“…[19][20][21][22][23][24][25] Ion mobility provides an accurate method for determining collision cross sections of mass-selected ions in a buffer gas. [19][20][21][22][23][24][25] The motion of the ion through a buffer gas under the influence of a weak electric field depends on the ion's average collision cross section (Ω, Å 2 ) with the buffer gas, which depends on the geometric shape of the ion.…”

“…[19][20][21][22][23][24][25] Ion mobility provides an accurate method for determining collision cross sections of mass-selected ions in a buffer gas. [19][20][21][22][23][24][25] The motion of the ion through a buffer gas under the influence of a weak electric field depends on the ion's average collision cross section (Ω, Å 2 ) with the buffer gas, which depends on the geometric shape of the ion. [19][20][21][22][23][24][25] Theoretical calculations of possible structural candidates of the mass-selected ions are then used to compute angle-averaged Ω's at different temperatures for comparison with the measured ones.…”

“…[19][20][21][22][23][24][25] The experiments were performed using the VCU massselected ion mobility spectrometer (schematic is given in Figure S1, supplementary material). 26 The details of the instrument can be found in several publications [20][21][22] and only a brief description of the experimental procedure is given here. The essential elements of the apparatus are jet and beam chambers coupled to an electron impact (EI) ionization source, a quadrupole mass filter, a drift cell, and a second quadrupole mass spectrometer.…”