Article pubs.acs.org/JPCA
Formation of Covalently Bonded Polycyclic Hydrocarbon Ions by Intracluster Polymerization of Ionized Ethynylbenzene Clusters Paul O. Momoh, Isaac K. Attah, and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States
René P. F. Kanters, John M. Pinski, and Samuel A. Abrash Department of Chemistry, University of Richmond, Richmond, Virginia 23173, United States S Supporting Information *
ABSTRACT: 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)n 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)CC CH•+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)CCC(C6H5)CHCH•+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)CC[C(C6H5)CH]nCH•+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.
I. INTRODUCTION Gas phase organic chemistry plays important roles in flames and combustion processes, particularly for the mechanisms of soot formation and the generation of organic aerosols throughout the atmosphere, as well as in interstellar clouds and solar nebula, particularly for the origin of the observed complex organics.1−15 Insights into the formation mechanisms and the discovery of novel catalytic pathways that lead to the formation of complex organics can be obtained from gas phase and cluster ion experiments.16−29 These experiments can identify the chains of reactions leading to the formation of complex organics such as the PAHs found in soot, combustion, organic aerosols, interstellar clouds, and meteorites. They can also provide detailed information on the kinetics, growth © 2014 American Chemical Society
mechanisms, energy barriers, structures, isomerization, and reactivity of large organic ions as well as their complexes with associated solvent molecules in different environments.24−29 Because acetylene is the smallest organic molecule that can be polymerized, extensive studies have been focused on the ion chemistry of acetylene clusters not only due to the important roles of acetylene in flames and combustion processes including the mechanisms of soot formation but also for the origin of Special Issue: A. W. Castleman, Jr. Festschrift Received: January 29, 2014 Revised: March 28, 2014 Published: April 1, 2014 8251
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the results from mass spectrometry ion mobility and ion dissociation measurements. We also present calculated structures for the C16H12, C24H18, C32H24, C40H30, and C48H36 ions corresponding to the cluster ions of PA from dimers to hexamers, respectively. These structures are used to calculate collision cross sections of the corresponding ions for comparison with the experimental cross sections obtained from the mobility measurements. This comparison together with the measured fragmentation patterns provide the basis for suggesting the likely structures of the covalently bonded polycyclic hydrocarbon ions involved in our experiments.
larger molecular species such as benzene and PAHs in space.1,2,30,31 In fact, 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.32−42 However, the most conclusive evidence for the efficient formation of the benzene radical cation following the electron impact (EI) ionization of large acetylene clusters was provided through a combination of mass-selected ion mobility and ion dissociation experiments coupled with theoretical calculations.37,38 Accordingly, the collision cross section in helium and the fragmentation pattern of the C6H6•+ ion formed in ionized acetylene clusters are identical to those measured for the actual benzene radical cation.37,38 Furthermore, the stepwise hydration energies measured for the acetylene trimer ion are also identical to those measured for the benzene cation.38 Using a similar strategy, the presence of more than one covalent isomer of the C4H4•+ ion in ionized acetylene clusters has been established with the predominant ion being the cyclobutadiene radical cation.42 A similar conclusion has been reached on the basis of the infrared predissociation experiments 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.41 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.41 Both the infrared spectroscopy and ion mobility experiments suggest that the structures of the C4H4•+ and C6H6•+ ions play important roles in the cluster-mediated chemistry in ionized acetylene clusters.41,42 Ethynylbenzene (phenylacetylene, hereafter PA) combines two substructures: the aromatic ring and the ethynyl group (−CC−), which are considered as important intermediates in the formation of aromatics in fuel-rich hydrocarbon flames as well as in the pyrolysis of acetylenic hydrocarbons.43,44 The ionized ethynyl group could serve as a starting point for a second ring condensed to the first or could react with one or two neutral ethynyl groups from a second and third PA molecule to form cyclobutadiene-type or benzene-type cyclic structures, respectively. Nothing is known about this chemistry in the gas phase including the nature of the higher products that could be formed in ionized PA clusters. It is therefore the purpose of this work to investigate intracluster polymerization and characterize the nature and structures of the products formed following the ionization of neutral PA clusters. Herein, we report mass-selected ion mobility and ion dissociation experiments coupled with theoretical studies aimed at elucidating the identity of ionized PA clusters. The results provide, for the first time, conclusive evidence for the formation of covalently bonded polycyclic ions and establish a plausible mechanism that explains the sequential ion−molecule reactions within the ionized PA clusters. The organization of the paper is as follows. In the Experimental Section, we briefly describe the mass-selected ion mobility system and methods used for measuring the mobility and determining the corresponding collision cross section of the mass-selected ions in helium. We also describe the application of ion dissociation by controlling the injection energies of the mass-selected ions into the mobility cell to identify the structures of the selected ions. The computational methods used for the structural calculations are briefly described in section III. In section IV, we present and discuss
II. EXPERIMENTAL SECTION The ion mobility and dissociation experiments were performed using the VCU mass-selected ion mobility spectrometer. The details of the instrument can be found in several publications and only a brief description of the experimental procedure is given here.28,29,42 Figure S1 (Supporting Information) illustrates the essential components of the ion mobility apparatus, which consists of 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. In the experiments, neutral PA clusters are generated in the source chamber by pulsed supersonic adiabatic expansion of a 2% phenylacetylene/helium mixture (He ∼4−5 atm) through a conical nozzle (500 μm diameter) in pulses of 200−300 μs duration at repetition rates of 20−30 Hz. The jet is skimmed and passed into the second chamber, which is maintained at 2 × 10−6 Torr, where the clusters are ionized by 50−70 eV EI ionization. The cluster ions selected by the quadrupole mass filter are focused into the drift cell for the ion mobility and ion dissociation measurements. The injection energies (IEs) used in the mobility experiments (10−15 eV, laboratory frame) are slightly above the minimum energies required to introduce the ions into the cell against the helium outflow from the entrance orifice. Most of the ion thermalization occurs outside the cell entrance by collisions with the helium atoms escaping from the cell entrance orifice. At a cell pressure of 0.2 Torr, the number of collisions that the ion encounters with the He atoms within the 1.5 ms residence time inside the cell is about 104 collisions, which is sufficient to ensure efficient thermalization of the ions. The ion mobility and ion dissociation measurements are described in the results section. III. THEORETICAL SECTION A cluster generation program was used to build 500 initial structures for the weakly bound complexes from the molecular fragments, i.e., PA monomer and/or dimers. In brief, an arbitrary initial orientation of the fragment species was used as an input. A standard random number generator was used to generate three-dimensional displacement vectors, as well as to generate three rotation angles used to position subsequent fragments relative to the already placed ones. Each fragment was defined to have an extra buffer region of 1 Å. This resulted in initial structures with the minimum distances between the fragments 2 Å larger than the contact distance. After optimization of these structures using the PM7 semiempirical model MOPAC2012,45 duplicate structures were removed and the unique structures were reoptimized using the Gaussian03 suite of programs and the B3LYP/6-31G* model chemistry.46 The covalently bonded dimers and trimers were systematically generated on the basis of a PA molecule reacting with a 8252
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Figure 1. (a) Mass spectrum of 70 eV EI ionized phenylacetylene clusters (Pn = (C8H6)n). (b) Time of flight (TOF) mass spectrum of photoionized (193 nm) phenylacetylene clusters.
CH3 and C6H5 radicals. The fragmentation of the trimer ion C24H18+ could also result in loss of a monomer unit thus enhancing the ion intensity of the dimer ion C16H12+ as shown in Figure 1. The appearance of the m/z 637 ion corresponding to the loss of C6H5 from the P7 ion (m/z 714) suggests that the characteristic loss of the phenyl radical occurs from larger ions (Pn) with n ≥ 7 as shown in Figure 1a. Figure 1b displays the mass spectrum obtained by 193 nm two-photon ionization of the neutral PA clusters. The distribution of the cluster ion reveals an enhanced ion intensity at n = 3 and suggests the formation of a stable C24H18+ ion. However, because of the low energy of the two-photon ionization process (12.6 eV), no extensive fragmentation is observed from the C24H18+ ion in comparison to the 70 eV EI ionization shown in Figure 1a. The appearance of specific molecular fragments and the enhancement of the ion intensity of the PA trimer in the EI and photoionization experiments, respectively suggest that exothermic intracluster ion−molecule reactions take place within the ionized PA clusters to form covalently bonded ions that could fragment under excess energy to produce the characteristic fragment ions observed in the mass spectra.16,21,25,35,37 However, these qualitative observations do not provide strong evidence for the conversion of the ionized PA clusters into covalently bonded hydrocarbon ions. In the following sections we present results from mass-selected ion mobility and ion dissociation experiments combined with theoretical calculations to conclusively establish the covalent bonding nature of the product ions and to identify their most likely structures. 2. Mobility and Collision Cross Sections of MassSelected Ions. The mobility K of an ion is defined as42,50
monomer or dimer cation, respectively. The allowed reactions of the PA moiety were the insertion of CC into a C−H bond as well as 2 + 2, 2 + 4, and 2 + 6 cycloaddition reactions. Isomers were selected by considering all possible addition reactions between acetylenic moieties. Then all possible inequivalent conformers of each isomer were identified by running a conformer distribution search using the PM3 semiempirical model in the Spartan06ES program.47 This was followed by a geometry optimization of all conformers of all isomers with the PM3 model using either Spartan or Gaussian03. The resulting conformers where then optimized using Gaussian03 at the B3LYP/6-31G* level. For covalent trimers, all four reaction types were considered for addition of PA to the covalent dimer cation species. For covalent tetramers, pentamers, and hexamers, the large number of possible products prevented us from considering all of the reaction types, so we limited ourselves to building these larger species by considering ladder like insertion of CC into a covalent PA trimer cation with all possible relative orientations of the phenyl substituents (ph, C6H5) in the growing chain (head (H)−tail (T), HH, and TT where H is (−CH) and T is (−Cph). Thus, all final geometries and energies were calculated at the B3LYP/6-31G* level of theory, with frequencies calculated to confirm that geometries were equilibrium geometries. All structural images presented were obtained using Jmol.48 Integral collision cross sections were calculated using the trajectory method.49
IV. RESULTS AND DISCUSSION 1. Mass Spectra of Electron Impact and Photoionized Ethynylbenzene Clusters. Figure 1a displays a typical mass spectrum of the PA clusters obtained following the 70 eV EI ionization of the neutral clusters. In addition to the (C8H6)n ions (Pn), the spectrum shows fragment ions with m/z 291 and 229 corresponding to the C23H15+ and C18H13+ ions. These ions could be produced by fragmentation of the trimer ion C24H18+ through the loss of CH3 and C6H5 (phenyl) groups, respectively. Following the EI ionization of the PA clusters, exothermic ion−molecule addition reactions result in the formation of covalently bonded ions which dissipate their excess energies through the loss of stable fragments such as the
K = υd /E
(1)
where E is the drift field (E = V/z, V is the drift voltage and z is the length of the cell (cm)) and υd is the drift velocity (υd = z/ td, td is the drift time in s). To effectively compare mobility measurements at different cell conditions or different instruments, K is normalized to standard conditions (STP) and referred to as reduced mobility, Ko. 8253
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Figure 2. (a) Arrival time distributions (ATDs) of mass-selected phenylacetylene trimer ions at different applied fields across the mobility cell at 304 K. (b) ATDs of mass-selected phenylacetylene ions (Pn = (C8H6)n+) with n = 1−8. All ATDs correspond to the same cell field (44.1 V), pressure (2.6 Torr He) and temperature (304 K), and hence, the same E/N value (10.6 Td). (c) Maximum arrival time versus P/V plots used to determine the reduced mobility, Ko, for the phenylacetylene ions (Pn).
⎛ 273.15 ⎞⎛ P ⎞ ⎟⎜ ⎟ Ko = K ⎜ ⎝ T ⎠⎝ 760 ⎠
where qe is the ion charge, N is the number density of the buffer gas, and Teff is the effective temperature of the ion (due to the drift field) and is characterized by the energy in the center of mass frame of the colliding ions and neutrals. Mi and Mb are the masses of the ion and buffer gas, respectively, and Ω(1,1) avg is the orientationally averaged collision integral. The increase in Teff due to the drift field is negligible under the lowfield conditions used here).50,51 Figure 2a displays the ATDs for the mass-selected PA trimer ions measured at various cell voltages. Figure 2b shows the ATDs for the mass-selected (PA)n ions with n = 1−8 at a constant P/V ratio. Figure 2c shows plots of td (maximum ATD) versus P/V for the (PA)n ions with n = 1−8 with the solid lines representing the least-squares fittings to the data points. The resulting reduced mobilities K 0 and the corresponding collision cross sections in helium (Ω) are given in Table 1. The measured ATDs of the (PA)n ions with n = 1−4 show narrow profiles consistent with the presence of one structural isomer or a family of multiple structures with very close collision cross sections that could not be resolved with the current mobility resolution. However, for the (PA)n ions with n = 5−8, the experimental ATD is considerably broader with an apparent shoulder present at early times suggesting the existence of more than one family of structures in the cluster beam. The broadened ATDs indicate that the collision cross sections of these families of structures are not sufficiently different to be able to resolve their ATDs in our drift cell where
(2)
Here, T is the buffer gas temperature (K) and P is the buffer gas pressure (Torr). Combination of eqs 1 and 2 gives ⎛ z 2 × 273.15 ⎞⎛ P ⎞ td = ⎜ ⎟⎜ ⎟ + to ⎝ T × 760 × Ko ⎠⎝ V ⎠
(3)
where to is the effective time spent outside the drift cell. A plot of the drift time (td) versus P/V gives a straight line with slope containing Ko and an intercept corresponding to to. In the experiment, a packet of the mass-selected ions of interest is injected into the drift cell and the arrival time distribution (ATD) is collected at varying cell voltages, V (with T and P held constant). All the mobility measurements were carried out in the low-field limit where the ion’s drift velocity is small compared to the thermal velocity and the ion mobility is independent of the field strength (E/N < 6.0, 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).50 The average collision cross section, Ω(1.1) avg of the ion in the helium buffer gas is calculated according to the kinetic theory: 3qe ⎛ 2π ⎞ ⎜ ⎟ 16N ⎝ kBTeff ⎠
1/2
K=
⎛ M i + Mb ⎞1/2 1 ⎜ ⎟ ⎝ M iMb ⎠ Ω(1,1) avg
(4) 8254
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observed of the PA dimer ion (C8H6)2+ (m/z 204). Only when the IE is increased to 40 eV is the m/z 102 fragment ion observed as a result of the loss of a C8H6 neutral fragment. The high IE required to induce this fragmentation suggests that the (C8H6)2+ ion has a covalent structure corresponding to a C16H12 molecular ion that can dissociate by a loss of a C8H6 neutral fragment. For comparison with a noncovalent dimer ion, Figure S2 (Supporting Information) shows the mass spectrum resulting from the dissociation of the mass-selected benzene dimer cation using an IE of 12 eV (lab frame).51 It is clear that the noncovalent (C6H6)2+ dimer ion easily dissociates into the monomer ion C6H6+ at relatively low IE such as 12 eV, as shown in Figure S2 (Supporting Information). This is consistent with the weaker binding energy (17 kcal/mol for (C6H6)2+)52 of the noncovalent dimer ions as compared to the binding of the covalent molecular ions. Similar behaviors to the lack of low-energy dissociation channels of the PA dimer cation have been observed in the dissociation of the acetylene and styrene cluster ions, which were shown to form covalently bonded oligomer ions.37,38,42,53 Increasing the IE to 70 eV (lab) can induce higher energy fragmentations, as shown in Figure 3 where the m/z 203, 202, and 179 fragment ions correspond to the loss of one H atom, two H atoms and a C2H group, respectively, from the C16H12 molecular ion. These results suggest that the C16H12 ion could have a covalent structure consisting of two phenyl rings connected through a C4H2 linear or cyclic link that can be dissociated at higher IEs through the loss of the neutral fragments (C6H5)CCH, C2H, and H. Although the observed fragmentations of the C16H12 ions are consistent with covalently bonded ions, it is informative and also interesting to compare the experimental collision cross sections with those calculated for the noncovalent ion−neutral complexes as well as for the covalently bonded molecular ions. To provide such a comparison, we calculated low-energy structures of the noncovalent (C8H6)2+ dimer ions and the covalent C16H12 molecular ions. Figure 4a shows the noncovalent dimer structures calculated at the B3LYP/6-31G* level along with their binding energies and collision cross sections. These structures can be divided into three groups, namely sandwich (eclipsed) parallel, displaced (or slipped) parallel, and T-shaped. It should be noted that the parallel head-to-head (HH) eclipsed structure (viii) is not a minimum energy structure because it optimized into a covalent isomer (ix) as shown in Figure 4a. As expected, the calculated binding energies of the noncovalent dimer cations shown in Figure 4a are within the normal range of the relatively weak interaction of ion−neutral dimers such as the benzene dimer cation where the experimental binding energy is 17 kcal/mol.52 Clearly, these dimers would not survive the IE of 10−15 eV without significant dissociation into the monomer ion, which is not the case for the results shown in Figure 3 where much higher IE (40 eV) is needed to dissociate the PA dimer ion. Also, the noncovalent dimers exhibit collision cross sections either significantly smaller or significantly larger than the measured value of 89.2 ± 3.8 Å2. The compact HT sandwich parallel structure i has the smallest Ω (84.4 Å2) and the T-shaped vii has the largest Ω (98.2 Å2), both values are outside the range of uncertainty in the measured Ω. The optimized parallel structures with large displacement between the two rings show an increase in Ω and at the largest displacement, Ω’s value (94.8 Å2, structure iii) becomes closer to that of the T-shaped
Table 1. Measured Reduced Mobilities in Helium, the Corresponding Collision Cross Sections of Phenylacetylene Ions, and Calculated Collision Cross Sections Based on the B3LYP/6-31G* Structural Calculations of CovalentlyBonded Molecular Ions n
measured Ko (cm2 V−1 s−1)
1 2 3 4 5 6 7 8
9.39 6.03 4.43 3.59 2.98 2.73 2.55 2.18
measured Ω (Å2)
calculated Ω (Å2) covalent ions
± ± ± ± ± ± ± ±
56.5 85.1−91.9 114.7−125.8 148.1−155.5 171.4−183.6 194.0−205.4
57.7 89.2 120.6 148.8 179.2 195.0 208.7 244.0
2.3 3.8 5.7 6.7 8.1 8.8 9.4 10.9
isomers with less than 5% difference in collision cross section cannot be resolved.42,51 This point will be addressed in discussing the structures of the C32H24, C40H30, and C48H36 ions derived from the EI ionization of phenylacetylene clusters (PA)n with n = 4−6. 3. Mass-Selected Dissociation and Structures of the C16H12 and C24H18 Ions. 3a. Dissociation and Structures of the C16H12 Ions. To provide further evidence for the covalent structures of the ionized PA clusters, we studied the dissociation of the mass-selected ions by increasing the injection energy of the selected ions. In these measurements, the injection energy (IE), which is the energy of ions entering the drift cell, is determined by measuring the difference between the EI ion energy and the cell entrance voltage.29,39,42 This is typically done by increasing the drift cell entrance voltage (and all the voltages after the drift cell entrance correspondingly), because the EI ion energy is usually held constant. Figure 3 shows the dissociation products of the mass-selected dimer ions at different IEs. With 10 eV IE, no dissociation is
Figure 3. Mass spectrum resulting from the injection of mass-selected (C8H6)2+ into the drift cell containing 0.51 Torr of helium at 303 K using injection energies of (i) 10 eV, (ii) 40 eV, and (iii) 70 eV (lab). The small peak observed at m/z 222 is due to the addition of trace water in the mobility cell to the C16H12+ ion. 8255
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Figure 4. (a) Structures and binding energies (ΔE, kcal/mol, with respect to dissociation to a PA radical cation and neutral molecule) of noncovalent C16H12 dimer radical cations calculated at the DFT-B3LYP/6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49 (b) Structures and relative energies (Erel, kcal/mol) of covalently bonded C16H12 radical cations calculated at the DFT-B3LYP/ 6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49 Erel = 0.0 with respect to the total electronic energy of the 2-phenylnaphthalene radical cation, the lowest energy C16H12 ion, which is 44.9 kcal/mol (Table S1, Supporting Information). The radical cations are (i) 1,1′-(1E)-but-1-en-3-yne-1,4-diyldibenzene, (ii) 1,1′-(1Z)-but-1-ene-3-yne-1,4-diyldibenzene, (iii) 1,1′cyclobuta-1,3-diene-1,3-diyldibenzene, and (iv) 1,1′-cyclobuta-2,4-diene-1,2-diyldibenzene.
structure (98.2 Å2, structure vii). Again, these values are significantly larger than the measured Ω for the PA dimer, which suggests that the dimer has a covalent structure consistent with the ion fragmentation study. However, it should be noted that different combinations of noncovalent structures can reproduce the measured collision cross section value of 89.2 ± 3.8 Å2. Therefore, the mobility results alone do not provide clear evidence for the formation of covalently bound PA dimers. In this case, the mass-selected fragmentation data shown in Figure 3 provides the strongest evidence for the
formation of covalently bound dimer. Therefore, the noncovalent structures can be excluded from the likely structures of the C16H12 ions mostly because the measured fragmentations cannot result from noncovalent structures, and partially because of the large difference between the measured Ω and the calculated values corresponding to the noncovalent dimer ions. The B3LYP/6-31G* optimized structures of the ten lowest energy covalent isomers of the C16H12 ions identified by the search procedure described in the calculations section along with their relative energies and calculated collision cross 8256
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Figure 5. (a) (i) Mass spectra obtained following the injection of the mass-selected (C8H6)3+ ions into a drift-tube containing 0.5 Torr He at 303 K using injection energy (IE) of 90 eV (lab). (ii), (iii), and (iv) are 70 eV EI mass spectra of the 1,3,5-, 1,2,4-, and 1,2,3-triphenylbenzene ions obtain from the NIST database.54 (b) Structures and relative energies (Erel, kcal/mol) of covalently bonded C24H18 radical cations calculated at the DFTB3LYP/6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49 Erel = 0.0 with respect to the total electronic energy of the 2-phenyl-6-[(E)-2-phenylvinyl]naphthalene cation, the lowest energy C24H18 ions, which is 8.9 kcal/mol (Table S2, Supporting Information). The radical cations are (i) 1,2,4-triphenylbenzene, (ii) 1,3,5-triphenylbenzene, (iii) 1,2,3-triphenylbenzene, and (iv) 1,1′,1″-(1E,3Z)-hexa-1,3-dien-5-yne-1,4,6-triyltribenzene.
acetylene radical cation to the CC group.32−35,37−39,55 Figure 4b shows four low-energy covalent structures, (i)−(iv) with their calculated Ω values (85.1−91.9 Å2) that agree with the measured value of 89.2 ± 3.8 Å2 of the C16H12 ions. Ions i and ii (similar structures with different orientations of the phenyl groups) are the next lowest energy structures following the 2phenylnaphthalene ion and are the most consistent with the ion dissociation results. For example, these structures can dissociate by the loss of one and two H atoms to produce the phC C+CCHph (m/z 203) and phCCCCph (m/z 202) ions, respectively. The phCCCHCHph ion can also undergo 1,2 H-shift followed by the fission of the CH−CH bond to produce a C8H6 ion (m/z 102). A highenergy process could involve the loss of the C2H group to produce the ph+CCHph ion (m/z 179). In fact, highenergy structures such as 1-ethynyl-2-[(E)-2-phenylvinyl] benzene or 1-ethynyl-3-[(E)-2-phenylvinyl] benzene (55.5 and 57.3 kcal/mol, respectively above the energy of 2phenylnaphthalene radical cation, the lowest energy C16H12 isomer) shown in Table S1 (Supporting Information) can produce the C2H fragmentation channel and they also have collision cross sections comparable to the measured value for the C16H12 ions. However, the formation of theses dimers involves insertion of the CC into a phenyl C−H bond, which is likely to have a barrier and also does not involve typical ion− molecule addition reactions of acetylene.
sections in helium are listed in Table S1 (Supporting Information). The two lowest energy structures 2-phenylnaphthalene (Erel = 0.0) and 1-phenylnaphthalene (Erel = 1.8 kcal/mol) have Ω values (83.4 and 82.6 Å2, respectively) smaller than the measured value of 89.2 ± 3.8 Å2. In addition, these structures are not consistent with the ion dissociation results shown in Figure 3. In fact, the NIST EI mass spectra of both 1- and 2-phenylnaphthalene ions (see Figure S3, Supporting Information) are significantly different from the observed fragmentation of the mass-selected C16H12 ions shown in Figure 3.54 For example, the characteristic loss of a C2H group in the dissociation of the C16H12 ion (Figure 3) is not observed in the EI mass spectra of the phenylnaphthalene ions. Furthermore, although phenylnaphthalenes can be formed in bimolecular neutral reactions by 2−4 cycloaddition of the CC triple bond to another PA molecule followed by a hydrogen shift over three carbon atoms, this chemistry is not expected to occur for the intracluster ion−molecule reactions between the PA radical cation and PA molecule on the basis of the studies of intracluster reactions of methylacetylene.55 Therefore, the naphthalene-type structures were excluded from the possible covalent candidates of the C16H12 ions. Also, structures involving insertion of CC in a phenyl CH bond (Table S1, Supporting Information) were excluded because they are likely to involve barriers associated with 1,3H shift and are not consistent with the ion−molecule chemistry of substituted acetylene which occurs by the addition of the 8257
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covalently bonded terphenyl cation (C18H15+).56 Also, the loss of C7H7 fragment is known to occur from the fragmentation of the 3-benzyl-1,5-diphenylpentane radical cation to produce a carbenium ion which rearranges to a protonated indan derivative. 57 These channels are also observed in the fragmentation of both the 1,3,5- and 1,2,4-trilphenylbenzene radical cations, as shown in Figure 5a. The B3LYP/6-31G* optimized structures of the 13 lowest energy covalent isomers of the C24H18 ions identified by the search procedure along with their relative energies and calculated collision cross sections in helium are listed in Table S2 (Supporting Information). Similar to the C16H12 ions, the lowest energy isomer of the C24H18 ions has a naphthalenetype structure (2-phenyl-6-[(E)-2-phenylvinyl]naphthalene cation, Table S2 Supporting Information). In fact, within an energy range of only 5.8 kcal/mol, nine low-energy isomers with structures involving phenyl- and phenylvinyl-substituted naphthalene ions were identified. As discussed above, the formation of a naphthalene-type structure is not consistent with the ion−molecule reactions of phenylacetylene which are expected to be similar to those of acetylene.32−40 Therefore, we exclude the naphthalene-type ions from the possible candidate structures of the observed PA ions. Obviously, if these structures cannot be formed from the dimer ions as confirmed by the mobility and dissociation results above, their participation in the growth sequence of larger PA ions should not be considered. Therefore, we proceed to discuss other lowenergy covalent structures of the C24H18 ions. Figure 5b shows the four lowest energy structures of the C24H18 ions following the naphthalene-type ions with their relative energies and calculated Ω values. It is interesting to note that the two lowest energy structures are the 1,2,4- and the 1,3,5-triphenylbenzene ions, in excellent agreement with the dissociation results of the PA trimer ions. The next lowest energy ion is the 1,2,3-triphenylbenzene ion that was excluded on the basis of the fragmentation results. Also, this ion has an Ω value of 112.2 Å2 smaller than the measured value for the C24H18 ions (120.6 ± 5.7 Å2); therefore, it can be excluded on the basis of both the ion dissociation and ion mobility results. On the other hand, structure iv in Figure 5b (1,1′,1″-(1E,3Z)hexa-1,3-dien-5-yne-1,4,6-triyltribenzene ion), although higher in energy (47.4 kcal/mol above the lowest energy C24H18 covalent ion as shown in Table S2, Supporting Information), has an Ω value of 124.4 Å2 comparable to experimental value of the PA trimer ion, and it can also explain all the dissociation products shown in Figure 5a. For example, the loss of the C6H5, C7H7, and C8H6 neutral fragments and the sequential loss of the CHx (x = 1,2) units can readily occur from the ion structure iv shown in Figure 5b. Also, this structure is consistent with a growth pattern based on the insertion of a PA molecule to dimer ion i or ii shown in Figure 4b. Therefore, on the basis of the ion dissociation and mobility results coupled with the B3LYP/6-31G* structural calculations, ions i, ii, and iv shown in Figure 5b appear to be the best candidates for the structures of the observed C24H18 ions. Structures i, ii, and iv are consistent with appearance of the C18H13+ fragment by the loss of a phenyl group, whereas structure iv can explain the formation of the fragments C17H11+ (loss of C7H7), C16H12+ (loss of C8H6) and C15H11+ (loss of C9H7) which could have the structures of phCCCC(ph)CH+, phC CCHCH(ph)+, and phCCCH(ph)+, respectively.
Isomers iii and iv shown in in Figure 4b, although slightly higher in energy than isomers i and ii, represent other possible structures for the C16H12 ions because the formation of a cyclobutadiene ring structure from an ionized acetylene dimer ion (C4H4+) has been established by ion mobility and dissociation measurements as well as by infrared spectroscopy of mass-selected acetylene cluster ions.41,42 However, the dissociation results of the C16H12 ions are more consistent with structures i and ii than with structures iii and iv. Therefore, the combination of mobility and dissociation studies indicates that the population of the C16H12 ions is dominated by covalent structures similar to isomers i and ii with some contributions from isomers iii and iv. In fact, the close values of the collision cross sections of ions i−iv suggest that a mixture of these covalent structures could be produced following the ionization and intracluster polymerization of PA clusters. 3b. Dissociation and Structures of the C24H18 Ions. Figure 5a(i) displays the fragmentation mass spectrum of the massselected trimer ions C24H18 (m/z 306) using 90 eV (lab) IE. With this high IE, extensive fragmentation is observed and the most intense fragment ions C23H15 (m/z 291), C18H13 (m/z 229), C17H11 (m/z 215), and C16H12 (m/z 204) correspond to the loss of CH3, C6H5, C7H7, and C8H6 neutral fragments, respectively from the C24H18 ion. In addition, small fragment ions corresponding to sequential loss of CHx (x = 1, 2) units are also observed. The small intensity of the C16H12 ion (m/z 204) excludes the dissociation channel by a loss of a PA monomer unit which would be the predominant channel if the trimer ion had a noncovalent structure or an association complex of a covalent dimer ion with a neutral monomer. Therefore, the ion dissociation experiment confirms the covalent bonding nature of the PA trimer ion. Interestingly, most of the observed fragment ions are similar to the major fragments resulting from the unimolecular decomposition of triphenylbenzene ions.54 In particular, both the 1,3,5- and 1,2,4trilphenylbenzene ions ((ii) and (iii) of Figure 5a, respectively) show fragmentation patterns very close to the observed dissociation of the C 24 H 18 ions (Figure 5a(i)). The fragmentation of the 1,2,3-triphenylbenzene ion (Figure 5a(iv)) (iv)) appears to be quite different from the observed fragments of the C24H18ions (Figure 5a(i)) particularly the lack of fragment ions within the m/z 306−229 and 215−160 regions. This suggests that the 1,2,3-triphenylbenzene ion can be excluded as a reasonable candidate for the PA trimer ion. It is also clear that the relative intensities of the observed fragments from the C24H18 ions are in better agreement with the fragments from the 1,3,5-triphenylbenzene ion than from the 1,2,4-triphenylbenzene ion. The agreement between the fragmentations of the PA trimer ion and the 1,3,5-and 1,2,4triphenylbenzene ions is remarkable because it suggests that polymerization of ionized PA clusters can result in cyclization of three acetylenic units to form a triphenyl-substituted benzene ring similar to acetylene cluster ions where ion mobility, dissociation and hydration experiments confirm the formation of a benzene cation following the ionization of acetylene clusters.37−39 The fragmentation channels involving the loss of CH3 and C7H7 from the mass-selected trimer ions (C24H18) provide further evidence for the covalent nature of the trimer ions. These channels have been observed from complex organic radical cations.56,57 For example, a methyl loss is observed following the ion−molecule reactions of biphenylium ion with benzene which is shown to proceed via the formation of a 8258
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Figure 6. (a) Mass spectrum resulting from the injection of mass selected (C8H6)4+ into the drift cell containing 0.63 Torr of helium at 297 K using different injection energies as indicated. (b) Mass spectrum resulting from the injection of mass selected (C8H6)4+ into the drift cell containing 0.63 Torr of helium at 297 K using 90 eV (lab) injection energy. (c) Structures and relative energies (Erel, kcal/mol) of covalently bonded C32H24 radical cations calculated at the DFT-B3LYP/6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49
4. Mass-Selected Dissociation and Structures of the Covalently bonded C32H24, C40H30, and C48H36 Ions. 4a. Dissociation and Structures of the C32H24 Ions. Figure 6a displays the dissociation products of the mass-selected PA tetramer ions (C32H24, m/z 408) at different IEs. Unlike the dimer and timer ions, the 10 eV IE induces low-energy dissociation resulting in fragments with m/z 306 and 204 corresponding to the C24H18 and C16H12 ions, respectively, as shown in Figure 6a(i). This suggests that some of the C32H24 ions consist of covalently bonded trimer (C24H18) and dimer (C16H12) ions associated with one and two neutral PA molecules, respectively. This is confirmed by the dissociation products observed at higher IEs, shown in (ii) and (iii) of Figure 6a, where the characteristic fragments of the C24H18 and C16H12 ions appear in addition to new fragments from the covalently bonded C32H24 ions. The new fragment ions with m/z 393, 331, and 317 correspond to the loss of the neutral fragments CH3 (methyl), C6H5 (phenyl), and C7H7 from the C32H24 ion, as shown in Figure 6b. It is interesting that at higher IE (90 eV), some trimer fragments such as C17H13 (m/z 217) and C15H11 (m/z 191) appear with much higher intensities (Figure 6b) than in the direct dissociation of the mass-selected trimer (Figure 5a(i)).This suggests that the population of the C32H24 ions consists of two kinds of ions: (1) a covalently bonded C24H18 trimer ion associated with a PA neutral molecule which at a low injection energy can fragment by a loss of a PA neutral molecule (Figure 6a(i)), and at a high injection energy dissociates to give the characteristic fragments of the C24H18 covalent ions (Figure 6b), and (2) covalently
bonded C32H24 tetramer ions that dissociate at high IE into their characteristic fragments including the C24H18 covalent ions. This also implies that there are two different isomers of the covalently bonded C24H18 trimer ions; one that has a blocked cyclic structure where the covalent addition of a PA molecule is hindered due to the lack of an accessible C−C bond, and a second isomer where the covalent addition of a PA molecule is possible. These suggested structures are consistent with the assignments of the triphenylbenzene cyclic structures ((i) and (ii) of Figure 5b) and the triphenylbenzene hexa-1,3dien-5-yne structure (Figure 5b(iv)) for the C24H18 ions. On the basis of the fragmentation results of the PA tetramer ion and the suggested growth mechanism that involves the insertion of PA molecules into the C−C bond of the dimer (Figure 4b, (i) and (ii)) and the trimer ions (Figure 5b(iv)), we optimized all possible structures resulting from the head−tail (HT), head−head (HH), and tail−tail (TT) additions with different relative orientations of the phenyl groups [where H is the (−CH) end and T is the −C(ph) end of the PA molecule and (ph) is the C6H5 group]. The resulting isomers within a relatively small energy range of 10 kcal/mol along with their calculated Ω values are listed in Table S3 (Supporting Information). Most of these isomers have calculated Ω values that lie within the measured value of 148.8 ± 6.7 Å2. Therefore, there are several low-energy isomers that could be reasonable candidates for the structures of the C32H24 ions. Two examples of these isomers representing the HTTH and HHHH additions of the four PA molecules are shown in Figure 6c as isomers i and ii, respectively. 8259
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Figure 7. (a) Mass spectrum resulting from the injection of mass selected (C8H6)5+ into the drift cell containing 0.89 Torr of helium at 302 K using different injection energies as indicated. (b) Structures and relative energies (Erel, kcal/mol) of covalently bonded C40H30 radical cations calculated at the DFT-B3LYP/6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49
energies of 10 kcal/mol and their calculated collision cross sections are listed in Table S4 (Supporting Information). Examples of these isomers are shown in Figure 7b as structures i and ii corresponding the HTTTH and HHHHH additions of the PA molecules, respectively. 4c. Structures of the C48H36 Ions. Continuing with the same growth pattern, low-energy isomers of the C48H36 ions within an energy range of 5 kcal/mol were optimized and their relative energies and calculated collision cross sections are listed in Table S5 (Supporting Information). However, several of these structures resulted in Ω values significantly larger than the measured value of 195.0 ± 8.8 Å2. Examples of the structures with Ω values within the experimental value are shown in Figure 8 where structures a and b correspond to the HTTTTH and HHHHHH additions of the PA molecules (6 units), respectively. The observation of early shoulders in the ATDs of large cluster ions (PA)n with n = 5−8 shown in Figure 2b provides support for the existence of two families of structures: covalently bonded oligomer ions of size n and covalent ions
It is interesting to note that the two fragment ions C17H13 (m/z 217) and C15H11 (m/z 191) add up to the mass of the tetramer ion (C32H24 (m/z 408)). This may suggest that the structure of the tetramer ion is made by the addition of these two fragments. In fact, both structures i and ii in Figure 6c can produce these fragments by simple fission of C−C bonds followed by hydrogen abstractions or by hydrogen atom shifts followed by bond dissociation. Specifically, structure i can dissociate into two fragments: C15H10 [phCCC(ph)] and C17H14 [CHC(ph)CHCHCH(ph)], which after hydrogen abstraction or elimination can produce the C15H11+ and C17H13+ ions, respectively. Similarly, structure ii in Figure 6c can dissociate into the C17H12 [(ph)CCCHC(ph) CH] and C15H12 [phCHCHC(ph)] fragments which could undergo H abstraction or H loss to generate the C17H13+ and C15H11+ ions, respectively. 4b. Dissociation and Structures of the C40H30 Ions. Figure 7a displays the dissociation products of the mass-selected PA pentamer ions (C40H30, m/z 510) at different IEs. Similar to the behavior of the tetramer ions, low-energy dissociation results in fragments with m/z 408, 306, and 204 corresponding to the C32H24, C24H18, and C16H12 ions, respectively, as shown in Figure 7a(i). This is consistent with covalently bonded PA ions associated with neutral PA molecules which can be easily evaporated with low IEs as shown in Figure 7a(i). At higher IEs, the covalently bonded PA ions dissociate into their characteristic fragment ions as shown in (ii) and (iii) of Figure 7a. The repeated pattern of fragmentation of the C32H24 and C40H30 ions confirm their formation by the suggested sequential growth model, which involves the addition of PA molecules into the dimer ions (Figure 4b, (i) and (ii)), then into the trimer structure triphenylbenzene hexa-1,3-dien-5-yne ion (Figure 5b(iv)) to form the tetramer ions C32H24 (Figure 6c, (i) and (ii)) followed by the pentamer ions C40H30. The optimized covalent isomers of the C40H30 ions within relative
Figure 8. Structures and relative energies (Erel, kcal/mol) of covalently bonded C48H36 radical cations calculated at the DFT-B3LYP/6-31G* level of theory and collision cross section (Ω) in helium calculated using the trajectory method.49 8260
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of size n−x associated with x PA molecules. This picture is consistent with the mechanism of intracluster ionic polymerization where the exothermic energy resulting from the covalent addition is dissipated by evaporation of neutral PA molecules from the cluster if the cluster is large enough to provide sufficient evaporative cooling.16,21,23,25 In this way, covalent isomers can be stabilized within the cluster by evaporative cooling, which is analogous to collisional stabilization of the ionic intermediates in the gas phase at high pressures.20,25,29 However, evaporative cooling of the weakly associated molecules is not efficient in small clusters, and the exothermicity of the covalent bond formation is dissipated by fragmentation of the newly formed covalent ions through the loss of their characteristic neutral fragments, thus providing a clear indication of the formation of covalently bonded ions. The observation of characteristic fragmentations from covalently bonded ions in the high-energy dissociation PA dimer and trimer ions (Figures 3 and 5a) and monomer evaporation in the low-energy dissociation of PA tetramer and pentamer ions (Figures 6a and 7a) clearly provide strong support for the mechanism of intracluster ionic polymerization.16,21,25,35,37−40
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.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S3: schematic of the mass-selected ion mobility system, mass spectra of the benzene dimer ion and 1- and 2phenylnaphthalene ions. Tables S1−S5: B3LYP/6-31G* optimized structures of the lowest energy covalent isomers of the C16H12, C24H18, C32H24, C40H30, and C48H36 ions with their relative energies and calculated collision cross sections in helium. This material is available free of charge via the Internet http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*M. S. El-Shall: phone, (804)-828-3518; e-mail, mselshal@vcu. edu. Also affiliated with the Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia.
V. SUMMARY AND CONCLUSIONS In this work, we have studied intracluster ionic polymerization following the electron impact ionization of van der Waals clusters of phenylacetylene (C8H6)n 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)CCCH•+CH(C6H5), which can grow by the sequential addition of phenylacetylene molecules, in addition to some contributions from cyclic isomers such as the 1,3- or 1,4diphenyl 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 1,3,5triphenylbenzne cation, and a second isomer of the type (C6H5)CCC(C6H5)CHCH•+CH(C6H5) where the covalent addition of further phenylacetylene molecules can occur. For the larger ions such as C32H24, C40H30, and C48H36, the major isomers present involve the growing oligomer sequence (C6H5)CC[C(C6H5)CH]n CH•+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 phenylacetylene molecules associated with the blocked cyclic isomers such as the diphenyl cyclobutadiene and triphenylbenzene cations. Therefore, low-energy dissociation channels corresponding to evaporation of the phenylacetylene 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
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0911146) and NASA (NNX08AI46G) for the support of this work. Computational resources at the University of Richmond were provided by NSF MRI (Grant CHE-0958696).
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REFERENCES
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