LETTER pubs.acs.org/JPCL
Structure of the C8H8•+ Radical Cation Formed by Electron Impact Ionization of Acetylene Clusters. Evidence for a (Benzene•+ 3 Acetylene) Complex
Paul O. Momoh, Ahmed M. Hamid, Abdel-Rahman Soliman, Samuel A. Abrash,† and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States
bS Supporting Information ABSTRACT: 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 Å2, significantly larger than the calculated cross sections of all of the covalently bonded C8H8•+ ions and in excellent agreement with that of the benzene•+ 3 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. SECTION: Dynamics, Clusters, Excited States
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he ever-growing discovery of a wide range of organic molecules from acetylene and benzene to polycyclic aromatic hydrocarbons (PAHs) in interstellar clouds, molecular clouds, carbon-rich protoplanetary nebula, and other interplanetary bodies has fueled enhanced interest in the chemistry of interstellar clouds, infant and aging stars, comets, meteorites, planets, and other extraterrestrial bodies.1 4 Gas-phase polymerization, ion molecule and intracluster reactions, and catalysis on nanoparticle grain surfaces are important synthetic pathways for the formation of complex organics in the atmosphere and in space.5 10 Intracluster ion molecule reactions, particularly those that can lead to the formation of covalent ions, are uniquely suited for the discovery of novel catalytic pathways that may lead to the PAHs and other complex organics found in soot, meteorites, and interstellar clouds.11 17 These clusters provide model systems to investigate the reactions that could take place on dust grain surfaces incorporating small organic molecules followed by subsequent desorption of the complex organics by thermal evaporation, cosmic-ray-induced heating, photodesorption, and by the destruction of the dust grains in shock waves.1 6 Because acetylene is the smallest organic molecule that can be polymerized, extensive studies have been focused on the ion chemistry of acetylene clusters to investigate the origin of larger molecular species such as benzene, which represents an intermediate in the formation of the pervasive PAHs believed to be the source of unidentified infrared bands in space.18 22 r 2011 American Chemical Society
The formation of cyclic covalent ions such as cyclobutadiene (C4H4•+) and benzene (C6H6•+) within ionized acetylene clusters, (C2H2)n•+, has been suggested by several cluster studies conducted over many years.23 32 Using a combination of mass-selected ion mobility and ion dissociation experiments coupled with theoretical calculations, we recently provided conclusive evidence for the efficient formation of benzene radical cations following the electron impact (EI) ionization of large acetylene clusters (C2H2)n•+ with n up to ∼50.28,29 Accordingly, ion hydration experiments show that the enthalpy and entropy changes for the stepwise hydration of the C6H6•+ ion formed in ionized acetylene clusters are identical to those of the benzene radical cation.29 Using a similar strategy, we have also provided strong evidence for the presence of more than one covalent isomer of the C4H4•+ ion in ionized acetylene clusters with the predominant ion being the cyclobutadiene radical cation with a small contribution from the vinyl acetylene cation.32 These results are in agreement with the infrared predissociation spectroscopy coupled with harmonic frequency calculations, which showed that ionization of acetylene clusters results in the formation of a covalently bound C4H4•+ “core ion” that most likely has the structure of cyclobutadiene.31 The infrared predissociation spectra suggest the presence of several isomers of the C6H6•+ ion, including a weak absorption attributed to the formation of the benzene cation.31 Both the infrared spectroscopy and ion mobility experiments suggest that the structures of the Received: August 10, 2011 Accepted: August 31, 2011 Published: August 31, 2011 2412
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Figure 1. Mass spectrum of EI ionized (46 eV) neutral acetylene clusters.
C4H4•+ and C6H6•+ ions play important roles in the clustermediated chemistry in ionized acetylene clusters.31,32 Given the strong evidence that the C4H4•+ and C6H6•+ ions in ionized acetylene clusters are cyclobutadiene and benzene cations, respectively, the natural question at this junction is whether the structure of the C8H8•+ radical cation represents another cyclic covalent ion that could be formed by the addition of the acetylene molecule to the benzene cation in ionized acetylene clusters. However, gas-phase reactions of the benzene cation with acetylene show that under ordinary conditions at 300 K, the benzene cation does not react with acetylene.30,33 Only at high temperatures (600 K) has sequential covalent addition of two acetylene molecules onto the benzene cation leading to the formation of naphthalene-type ions been observed.30 Interestingly, at low temperatures (120 130 K), rapid association of acetylene molecules onto the benzene cation was observed, thus forming the clusters C6D6+•(C2H2)n, with n = 1 7.30 The association of a large number of acetylene molecules around the benzene radical cation could induce partial charge transfer from the benzene cation to activate the polymerization of the associated acetylene molecules to form a benzene molecule through an associative charge transfer (ACT) mechanism.30 In ionized clusters, both high- and low-temperature chemistry could take place depending on the excess energy of ionization, the size of the cluster, the exothermicity of intracluster reactions leading to covalent bond formation, and the kinetics of evaporative cooling from the cluster. Therefore, the C8H8•+ ion in ionized acetylene clusters could be produced via the high-temperature covalent addition of acetylene to the benzene radical cation to form a styrene-type radical cation or via the low-temperature ACT mechanism to form a benzene•+ 3 acetylene complex. In this Letter, we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the C8H8•+ ion formed by EI ionization of neutral acetylene clusters. The ion mobility approach requires a comparison of the average collision cross sections calculated for likely structures to those formed in the experiment. We employed density functional theory (DFT) to determine lowest-energy structures of the C8H8•+ potential energy surface. We also report the binding energy and entropy change associated with the formation of the benzene•+ 3 acetylene complex using equilibrium thermochemical measurements at different temperatures. The combination of ion mobility, dissociation,
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Figure 2. Dissociation products resulting from the injection of massselected (C2H2)4•+ ions into the drift cell containing 0.37 Torr of helium at 300 K using IE = 12.3 eV (Lab).
and thermochemical measurements in conjunction with theoretical calculations identifies the structure of the C8H8•+ ion in ionized acetylene clusters. Figure 1 displays a typical mass spectrum obtained by 46 eV EI ionization of neutral acetylene clusters formed by supersonic beam expansion of a 2% acetylene/helium mixture (5 atm, 300 K).28,29 The strong magic number at n = 3 is consistent with previous work on the formation of stable C6H6•+ ions in exothermic processes that can lead to extensive evaporation of neutral acetylene molecules from the cluster.23 26 The other magic numbers such as n = 14, 18, and so forth probably reflect the association of acetylene molecules with the C6H6•+ ion where stable “solvent” shells are formed with specific numbers of the acetylene molecules. As we reported earlier, the (C2H2)2•+ and (C2H2)3•+ massselected ions from the acetylene cluster ions show no fragmentations upon their injections using the 12 eV injection energy, consistent with the formation of stable C4H4•+ and C6H6•+ covalent ions, respectively.28,29 Unlike the (C2H2)2•+ and (C2H2)3•+ clusters, the (C2H2)4•+ cluster shows extensive fragmentation mainly to the (C2H2)3•+ ion. Figure 2 displays the mass spectrum obtained upon injection of the mass-selected (C2H2)4•+ ions into the drift cell containing 0.37 Torr of He at 300 K using the lowest injection energy (IE) (12.3 eV) necessary to inject the ions against the outflow of He gas from the drift cell. Despite the small IE, a significant dissociation of the (C2H2)4•+ is observed due to the loss of one acetylene unit and the observation of the (C2H2)3•+, consistent with a stable benzene cation (C6H6•+). It should be noted that no loss of two acetylene molecules is observed, suggesting that the ion core of the (C2H2)4•+ cluster is a C6H6•+ and not a C4H4•+ ion. The dominant low-energy dissociation channel involving the loss of acetylene from (C2H2)4•+ combined with the stability of the product C6H6•+ ion suggests that the (C2H2)4•+ ion is present as a neutral acetylene associated with a benzene cation. To more conclusively determine the structure of the (C2H2)4•+ ions, we measured the reduced mobility of the mass-selected (C2H2)4•+ ion. Mobility can provide direct structural characterizations of the ions on the basis of their collision integrals (Ω), which depend on the geometric shapes of the ions (Supporting 2413
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Figure 3. (a) Measured ATDs of (C2H2)n•+ with n = 1 4 at room temperature. (b) ATD (open circles) of the mass-selected (C2H2)4•+ and the calcuated ATD (solid line) from the transport theory, assuming a single structure for the ion.
Table 1. Measured Reduced Mobility (K0) and the Corresponding Collision Cross Section (Ω) in Helium for the MassSelected (C2H2)4•+ Ions with n = 1 4 C2H2•+ K0 (cm2 V Ω (Å ) 2
1
s 1)
(C2H2)2•+
(C2H2)3•+
Table 2. Measured Reduced Mobility (K0) and the Corresponding Collision Cross Section (Ω) in Helium for the MassSelected (C2H2)4•+ Ion at Different Temperatures
(C2H2)4•+
T(exp) (K)
P (Torr)
E/N (Td)
K0 (cm2V 1s 1)
Ω (Å2)
19.0
14.2
11.5
9.3
299
1.5
5.7
9.32
58.5
30.2
38.9
47.9
61.0
274
2.0
5.8
9.30
61.2
243
1.0
5.8
9.27
65.2
225
1.2
5.4
9.40
66.9
167
1.1
5.9
10.48
69.7
Information).28,32,34 Theoretical calculations of possible structural candidates of the mass-selected ions are then used to compute angle-averaged Ω's at different temperatures (using the trajectory method)35 for comparison with the measured ones. The agreement between the measured and calculated Ω's of the candidate structures provides reliable assignments of the structures of the ions. Figure 3a displays the arrival time distribution (ATD) for the mass-selected (C2H2)n•+ ions with n = 1 4 measured under similar E/N ratios (E/N = 4.5 4.8, where E is the electric field intensity and N is the gas number density, and E/N is expressed in units of Townsend (Td), where 1 Td = 10 17 V cm2). By varying the applied voltage in the cell and plotting td versus P/V, the reduced mobility K0 can be determined. The results are shown in Table 1 along with the corresponding collision cross sections for the (C2H2)n•+ ions with n = 1 4. A comparison of the ATD of the mass-selected (C2H2)4•+ ions measured under a low E/N ratio of 4.8 Td with the calculated profile using transport theory (Supporting Information, eq 5) is shown in Figure 3b. The comparison of experimental (open circles) and calculated (solid line) ATDs shows a very good fit for the experimental ATD, indicating the presence of one major isomer or multiple isomers with similar mobility. The broadened ATD could indicate the presence of multiple isomers with close structures such that their collision cross sections are not sufficiently different to be able to resolve their ATDs in our drift cell where isomers with less than 5% difference in collision cross section cannot be resolved.34 To increase the resolution, the mobility measurements of the (C2H2)4•+ ions were carried out at low temperatures (resolution is give by (qV/8kBT)1/2, where q is
the charge on the ion).34 However, no multiple peaks could be observed even at a temperature as low as 167 K, suggesting the presence of the dominant isomer. The measured mobilities and the corresponding collision cross sections of the (C2H2)4•+ ions at different temperatures are given in Table 2. To investigate the possibility of the formation of a covalent C8H8•+ ion in the ionized acetylene clusters, 25 lowest-energy covalent structures of the C8H8•+ ion were calculated using the aug-cc-pVDZ basis set, as shown in the Supporting Information (Table S1). The aug-cc-pVDZ basis set was selected because of its accurate description of the structures of covalently bonded molecular ions.28,30,32 The calculated structures were then used to obtain average collision cross sections and mobilities using the trajectory calculations.35 Figure 4 displays the collision cross sections calculated for the C8H8+ covalent isomers and their relative total energies (with the energy of the 1,4-dihydropentalene ion, the most stable C8H8+ isomer, taken as 0). As shown in Figure 4, the most compact structure of the C8H8•+ isomers corresponds to the cubane ion, which has the smallest collision integral of 51 Å2 but also the highest relative total energy of 467 kJ/mol. Aside from the cubane ion, three different structural families with linear energy collision integral relationships can be identified among the other C8H8•+ isomers, as shown in Figure 4. The first family represents tricyclic diene ions with compact structures corresponding to collision cross sections of 52 54 Å2 and a rapid increase in relative energy with cross section (shown in red numbers). The second (blue) and third 2414
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Figure 4. Relative energies and collision integrals of the C8H8•+ covalently bonded isomers. See Table S1, Supporting Information.
(green) families show systematically slower increase in relative energy with cross section. However, all of the calculated C8H8•+ structures exhibit significantly lower collision cross sections than the measured value for the (C2H2)4•+ ion. The only three exceptions are the high-energy structures 13, 21, and 24, which exhibit collision cross sections of 59, 62.6, and 61.8 Å2, respectively, that is, within the experimental uncertainty of the measured cross section of the (C2H2)4•+ ion (61.0 ( 3 Å2). However, the high relative energies of these ions (225.7, 311.9, and 391.2 kJ/mol for structures 13, 21, and 24, respectively) and the lack of reaction pathways from the ionized acetylene tetramer to these ions make them highly unlikely candidates for the (C2H2)4•+ ion formed by EI ionization of the acetylene cluster beam. Therefore, on the basis of the mobility measurement of the (C2H2)4•+ ion, the covalent structures shown in Figure 4 can be excluded from the likely structures of the (C2H2)4•+ ion. As indicated above, multiple evidence exists for the formation of benzene ions within the EI ionized acetylene clusters. First, dissociation using a high IE (62 eV) showed that the observed fragments are C6H5+, C6H4•+, C4H4•+, C4H3+, C4H2•+, and C3H3+ corresponding to m/z of 77, 76, 52, 51, 50, and 39, respectively.29 All of the observed fragment ions from (C2H2)3•+ are identical to the major fragment ions resulting from the unimolecular decomposition of the benzene ion.29 The origin of the fragment C3H3+ is specifically interesting because, unlike the C4H3+ and C4H2•+ fragments, the C3H3+ ion is not produced by any of the known ion molecule reactions of acetylene.29 Second, the measured reduced mobility of the mass-selected (C2H2)3•+ ions was found to be 11.54 ( 0.3 cm2 V 1 s 1, in excellent agreement with the value measured for the benzene cation (11.43 ( 0.4 cm2 V 1 s 1).28 The corresponding collision integrals at 300 K for the (C2H2)3•+ and the benzene ions are 47.4 ( 1.4 and 47.9 ( 1.4 Å2, respectively. In fact, among the cyclic isomers that have collision integrals similar to those measured for the (C2H2)3•+ (fulvene, benvalene, and benzene), only the benzene cation exhibits a fragmentation pattern (including the characteristic C3H3+ fragment) similar to that measured for the (C2H2)3•+ ion.28 Third, the sequential binding energies of the mass-selected (C2H2)3•+ ion to several water molecules are similar to those measured for the benzene cation.29,36 For example, ΔHo for
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the hydration of (C2H2)3•+ and benzene, C6H6•+, ions were measured as 9.0 and 8.8 kcal/mol, respectively.29,36 Similar agreements were found for the higher-order sequential hydration with up to seven water molecules.29,36 Fourth, the sequential entropy changes for hydration of the mass-selected (C2H2)3•+ ion to several water molecules are also similar to those measured for the benzene cation.29,36 Unlike the ΔHo values, which depend on the interaction energy between the ion and the water molecules, the values of ΔSo are very sensitive to the structure of the ion. Therefore, the similarity of ΔSo values for the sequential hydration of the (C2H2)3•+ and benzene radical cations provides concrete evidence that the (C2H2)3•+ ion has indeed the structure of the benzene radical cation. Fifth, vibrational predissociation spectra of the (C2H2)3•+ ions provide evidence, although inconclusive, for the presence of a covalently bound C6H6•+ isomer with a band located in the region expected for the benzene cation.31 However, the major isomer present in these experiments was based on a covalent C4H4•+ molecular core ion solvated with an acetylene molecule. It appears that the differences between the ion mobility and the vibrational predissociation experiments can be attributed to the energetics of the cluster ions prepared in each experiment. The ions produced in the predissociation experiments are grown by sequential condensation of C2H2 molecules onto the C2H2•+ 3 Arn clusters and, therefore, are expected to be significantly cooler than the ions generated by EI ionization of neutral acetylene clusters in the mobility experiments. The efficient formation of benzene cations certainly depends on the amount of excess energy available to the cluster ions in order to overcome the barrier to cyclization.25,28 Given the above evidence for the formation of benzene cations following the EI ionization of acetylene clusters, it is reasonable to assume that large acetylene cluster ions may contain a benzene cation associated with neutral acetylene molecules. This assumption is supported by the IE dissociation of the mass-selected (C2H2)4•+ ion, which indicates that the major low-energy dissociation channel involves a loss of an acetylene molecule, as shown in Figure 2. To establish the nature of the interaction between the benzene cation and acetylene at low temperatures, we measured the enthalpy and entropy changes associated with the formation of a benzene+ 3 acetylene complex using equilibrium thermochemical measurements.29,36 In these measurements, mass-selected benzene ions (C6D6•+) are injected into the drift cell containing 0.5 Torr of purified acetylene vapor. Consistent with previous results,30,33 no association of acetylene with the benzene radical cation is observed at room temperature or even at a temperature as low as 190 K. Under the current conditions, the first association product is observed at 185 K, which indicates a significantly weak interaction between acetylene and the benzene cation (see Figure S2, Supporting Information). As the temperature of the drift cell decreases, the intensity of the C6D6•+(C2H2) complex increases, and higher association products C6D6•+(C2H2)n start to appear (Figure S2, Supporting Information). The ion intensity ratio C6D6•+(C2H2)/C6D6•+ is measured from the integrated peak areas of the ATDs as a function of decreasing cell drift field, corresponding to increasing reaction time, and equilibrium is achieved when a constant ratio is obtained (Figure S2, Supporting Information).29,36 The equilibrium constant is then calculated from K = [I(C6D6•+(C2H2))/I(C6D6•+)P(A)], where I is the integrated ion intensity taken from the ATD and P(A) is the pressure of acetylene in the drift cell in atm. The equilibrium constant measured as a function of temperature yields ΔH° 2415
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Figure 5. (a) Calculated lowest-energy structures BA-1 and BA-2 of the benzene•+ 3 acetylene at the B3LYP/6-311++G(d,p) level and comparison of the experimental ATD of the (C2H2)4•+ ion at 297 K (open circles) and the calculated ATD (solid line) for structure BA-1. (b) Calculated lowest-energy structures BA-3 and BA-4 of the benzene•+ 3 acetylene at the M06-2X/6-311++G(d,p) level and comparison of the experimental ATD of the (C2H2)4•+ ion at 297 K (open circles) and the calculated ATD (solid line) for structure BA-3.
( 4.0 ( 1 kcal/mol) and ΔS° ( 11.4 ( 2.5 cal/(mol K)) from the corresponding van’t Hoff plot (Figure S2, Supporting Information). The measured low binding energy of acetylene to the benzene cation (4.0 kcal/mol) reflects a weak ion-induced dipole interaction consistent with the low-energy dissociation channel of the C8H8•+ ion in ionized acetylene clusters, which supports the assumption that this ion has the structure of the benzene•+ 3 acetylene [C6H6•+(C2H2] complex. To further support this assumption, we calculated the lowest-energy structure of the benzene•+ 3 acetylene complex using DFT calculations at the B3LYP/6-311++G(d,p) level.37 Two isomers with similar energies were found as the lowest-energy structure, as shown in Figure 5a. The calculated binding energies (4.5 kcal/mol for isomer BA-1 and 4.4 kcal/mol for isomer BA-2, both corrected for BSSE using the scheme of Boys and Bernardi counterpoise correction, as described in the Gaussian 03 program37) are in good agreement with the measured ΔH° of 4.0 kcal/mol, suggesting the adequacy of this level of calculations. However, several reports have suggested that the B3LYP functional may be inadequate in describing van der Waals (vdW) and weak ion molecule interactions where multipolar interactions are expected to play an important role in determining the complex minimum-energy structure.38,39 Because an accurate description of the structure of the benzene•+ 3 acetylene complex is critical to reproduce the measured collision cross section, we carried out an additional set of DFT calculations using the M06-2X functional, which is known to be accurate for weak vdW and stacking interactions.40
The two lowest-energy structures calculated at the M06-2X/ 6-311++G(d,p) level, BA-3 and BA-4, shown in Figure 5b, correspond to binding energies (after BSSE corrections) of 5.3 and 3.6 kcal/mol, respectively. For comparison, the calculated binding energies for the four structures calculated using the B3LYP (BA-1 and BA-2) and the M06-2X (BA-3 and BA-4) functions are given in Table 3. In both the B3LYP/6-311++G(d,p) structures BA-1 and BA-2 and the M06-2X/6-311++G(d,p) structure BA-3, the acetylene molecule is located above the plane of the benzene cation with the CtC bond directly above a carbon atom of the benzene cation with distances between the acetylene carbons and the benzene carbon of 2.85 and 3.02 Å in structure BA-1, 2.99 and 2.96 Å in structure BA-2, and 2.78 and 2.92 Å in structure BA-3. The only difference between the three structures is the orientation of the CtC bond of acetylene with respect to the C H bond of benzene. The second-lowest-energy structure (binding energy 3.55 kcal/mol) calculated using the M06-2X functional (BA-4) places the acetylene molecule perpendicular to the plane of the benzene cation with distances between the acetylene carbons and two adjacent carbon atoms of the benzene ring of 3.6 and 3.55 Å. Although the calculated binding energy of the lowest-energy structure BA-3 (5.3 kcal/mol) appears to be significantly higher than the experimental value, the relatively large uncertainty in the experimental value ((1 kcal/mol) precludes any meaningful discussion of the accuracy of the calculated binding energies using the B3LYP and the M06-2X functions. 2416
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Table 3. Binding Energy with and without Counterpoise Correction for the BSSE37,41 a binding energy
binding energy (kcal/mol)
structure
(kcal/mol)
(with BSSE correction)
BA-1 BA-2
4.87 4.80
4.50 4.44
BA-3
5.98
5.34
BA-4
3.89
3.55
a
Structures BA-1 and BA-2 are optimized at the B3LYP/6-311+G(d,p)37 level, while structures BA-3 and BA-4 are optimized using the M06-2X/6311+G(d,p) method.40,41
Table 4. Comparison of the Measured Collision Cross Section Ω in Helium for the Mass-Selected (C2H2)4•+ Ion and the Calculated Ω (Using the Trajectory Method)35 for Structures BA-1 to BA-4 at Different Temperaturesa (K)
Ω(exp) )2 (Å
Ω(cal)BA-1 )2 (Å
Ω(cal)BA-2 )2 (Å
Ω(cal)BA-3 )2 (Å
Ω(cal)BA-4 )2 (Å
299
58.5
59.1.
58.9
58.1
61.6
274
61.2
60.1
60.0
59.1
62.6
243
65.2
61.6
61.5
60.6
64.0
225
66.9
62.6
62.5
61.7
65.0
167
69.7
67.5
67.4
66.5
69.7
T(exp)
a
Structures BA-1 and BA-2 are optimized at the B3LYP/6-311++G(d,p) level,37 while structures BA-3 and BA-4 are optimized at the M06-2X/6-311 ++G(d,p) level.40,41.
It is instructive to compare the lowest-energy structure of the benzene•+ 3 acetylene complex with the corresponding neutral cluster. The binding energy of the neutral system evaluated from multiphoton ionization of the neutral benzene acetylene cluster and the dissociation threshold measurements of the cluster cation has been determined as 2.7 ( 0.2 kcal/mol.38 The small increase in the binding energy of the cluster cation (4.0 ( 1 kcal/mol) reflects the weak interaction between the delocalized charge on the benzene cation and the induced dipole in the acetylene molecule. The structure of the neutral cluster is predominantly governed by electrostatic interaction through the (acetylene) C H/π (benzene) interaction, which gives rise to a T-shaped geometry with acetylene acting as a proton donor to the π system of the benzene molecule. However, this interaction becomes repulsive in the benzene•+ 3 acetylene cluster because of the positive charge on the benzene ring, and therefore, the acetylene molecule rotates 90° to adopt a parallel orientation to the benzene ring with a displacement from the center of the ring, thus interacting predominantly with one carbon atom of the ring, as shown in structure BA-3. The nearly similar structures BA-1, BA-2, and BA-3 reproduce the measured ATD of the (C2H2)4•+ ion at 297 K, as shown in Figure 5. The collision cross sections calculated for the four structures BA(1 4) at different temperatures using the trajectory method are shown in Table 4. Again, the similar structures BA-1, BA-2, and BA-3 exhibit an overall better agreement with the experimentally determined Ω of the (C2H2)4•+ ion at different temperatures than the BA-4 structure. However, at lower temperatures (225 and 167 K), the BA-4 structure shows a better agreement with the experimentally determined Ω than the other three structures. This may suggest that the benzene•+ 3 acetylene
cluster adopts different structures at different temperatures. We note that structure BA-3 can be easily converted into BA-4 by a 90° rotation of the acetylene molecule. Given the weak binding energy, it is reasonable to assume that several closely related structures with comparable energies could exist for the benzene•+ 3 acetylene cluster within the investigated temperature range. In this case, the measured ΔH° reflects an average value of the enthalpy changes of several complexes with different conformations in equilibrium with each other at the temperature range of the experiment. The present results provide new insights on the structures of ionized acetylene clusters. If the association of acetylene molecules with the benzene cation formed by intracluster polymerization continues as a dominant structure feature in large ionized acetylene clusters, then the ACT mechanism could induce partial charge transfer from the benzene cation to activate the cyclization of three associated acetylene molecules to form a second benzene ring.30 In this case, large ionized acetylene clusters may contain a benzene dimer cation C6H6•+(C6H6) or even larger benzene cluster ions. Because the ion mobility and structures of the benzene cluster ions (C6H6)n•+ with n = 1 6 have been determined,34 the comparison of their collision cross sections with those measured for the acetylene cluster ions could provide valuable information on the formation of multiple benzene rings by ACT processes in ionized acetylene clusters. The conversion of large ionized acetylene clusters into benzene clusters or benzenecontaining acetylene clusters could have significant implication on the measured concentration of benzene in space. For example, it is well established that the benzene level measured in space is more than what could be accounted for by the known radical and ionic reactions in space.20,21,42 The ion mobility approach presented here could provide valuable insights on the presence of benzene cluster ions within large ionized acetylene clusters. Both experimental and computational works are currently in progress in our laboratory to identify the structure of the C12H12•+ ion [(C2H2)6•+] in ionized acetylene clusters. In conclusion, ion mobility experiments and theoretical calculations elucidate the identity of the C8H8•+ ion formed by EI 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. The enthalpy and entropy changes of the association of acetylene to the benzene radical cation are determined by equilibrium thermochemical measurements as 4.0 kcal/mol and 11.4 cal/(mol K), respectively, consistent with the weak bonding interaction of acetylene to the benzene radical cation. Ion mobility measurements at different temperatures indicate the presence of mainly one isomer with an average collision cross section in helium of 61.0 ( 3 Å2, significantly larger than the calculated cross sections of all of the covalent C8H8•+ ions and in excellent agreement with the cross section of the benzene•+ 3 acetylene complex. The combination of ion mobility, ion dissociation, binding energy, and theoretical calculations provides strong evidence that the C8H8•+ ion is predominantly present in ionized acetylene clusters as an acetylene molecule associated with a benzene radical cation. Future work will address the question of the presence of multiple benzene ring structures within large ionized acetylene clusters.
’ EXPERIMENTAL SECTION The ion mobility, dissociation, and hydration experiments were performed using the VCU mass-selected ion mobility 2417
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The Journal of Physical Chemistry Letters spectrometer (see Figure S1, Supporting Information). The details of the instrument can be found in several publications, and only a brief description of the experimental procedure is given here.28,32,34,36 In the experiments, acetylene clusters were generated by supersonic expansion of a 2% acetylene/helium mixture (He ≈ 4 atm). The mixture was passed through dry ice and moisture traps to diminish water vapor and acetone impurities (acetone is used as a stabilizing agent for pressurized acetylene). Typical ionization electron energies ranged from 40 to 80 eV.
’ THEORETICAL SECTION Geometries and relative energies for a number of interesting isomers of the empirical formula C8H8•+ were calculated using the unrestricted Perdew, Burke, and Enzerhof exchange and correlation functional (UPBEPBE) and the augmented correlation-consistent polarized valence double ζ basis set (aug-ccpVDZ).43 The aug-cc-pVDZ basis is a 5s2p/3s2p set for H and a 10s5p2d/4s3p2d set for C.43 Likely geometry candidates for the tetramer ions were derived from the known C8H8•+ isomers obtained from the NIST databases.44 All geometry optimizations were followed by vibrational frequency calculations to confirm all minima on the relevant potential energy surface. All relative energies were zero-point energy (ZPE)-corrected. All calculations were performed using the Gaussian 03 suite of programs.37 DFT calculations of the structure of the benzene•+ 3 acetylene cluster were carried out at the B3LYP/6-311++G(d,p) and M062X/6-311++G(d,p) levels using the Gaussian 03 and Gaussian 09 suites of programs, respectively.37,41 The calculated binding energies (with respect to benzene•+ + acetylene) were corrected for BSSE using the scheme of Boys and Bernardi counterpoise correction, as described in the Gaussian programs.37,41 ’ ASSOCIATED CONTENT
bS
Supporting Information. (1) Detailed description of the mass-selected ion mobility system and mobility equations, (2) calculated structures, relative energy, and collision cross section of 25 C8H8•+ covalently bonded ions, and (3) mass spectra, ATDs, and van’t Hoff plot for the C6D6•+ + C2H2 T C6D6•+(C2H2) equilibrium. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Department of Chemistry, University of Richmond, Richmond, VA 23173.
’ ACKNOWLEDGMENT We thank the National Science Foundation (CHE-0911146) and NASA (NNX07AU16G) for the support of this work. ’ REFERENCES (1) Lewis, J. S. Physics and Chemistry of the Solar System; Academic Press: New York, 1997. (2) Fegley, B. Chemical and Physical Processing of Nebula Materials. Space Sci. Rev. 1999, 90, 239–252.
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