Collision-Induced Dissociation of Mass-Selected Ethylene Cluster Ions

J. Phys. Chem. 1996, 100, 6427-6433

6427

Collision-Induced Dissociation of Mass-Selected Ethylene Cluster Ions (n ) 2-9) Michael Y. M. Lyktey, Tom Rycroft, and James F. Garvey* Department of Chemistry, NSM Complex, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: December 6, 1995; In Final Form: January 26, 1996X

The structure and chemical identity of ethylene van der Waals cluster ions, (C2H4)n)2-9+, have been studied using triple quadrupole mass spectrometry. The collision-induced dissociation mass spectra reveal that cluster ions with n ) 2-5 may undergo uncatalyzed cationic polymerization to form covalently bonded cationic oligomers (i.e., CnH2n+, n ) 2-5). This has been confirmed by comparison of ethylene cluster ion mass spectra to those of known covalently bonded ions. Cluster ions with n > 5 consist of C10H20+ oligomer cores solvated by an appropriate number of ethylene monomers. Uncatalyzed cationic polymerization of ethylene appears to cease beyond n ) 5 because of a “kinetic bottleneck”. As the proposed structures of the n ) 2-4 cluster ions are either linear or simple cyclic while the n ) 5 ion is proposed to be branched, this kinetic effect is likely due to inhibition of further intracluster polymerization by steric hindrance.

Introduction The polymerization of ethylene has been a topic of continuous and considerable interest for several decades.1-21 This is not surprising since over this time period polyethylene has become a chemical of great importance, finding seemingly innumerable applications in a variety of areas. In addition to catalyzed polymerization,1,2 small pieces of “polyethylene” may be generated by uncatalyzed, or free, cationic polymerization.3-21 This method of ethylene polymerization has been studied extensively for over 30 years. The first research involving free cationic ethylene polymerization was conducted by studying the ion-molecule reactions of ethylene.4-10 Kebarle and co-workers, among others, showed that ionmolecule reactions occurred readily to yield ethylene cluster ions.4-7 More importantly, it was observed that the ethylene cluster ions were generated at ion source temperatures that inhibited the formation of water cluster ions.4 Since water is capable of hydrogen bonding, whereas ethylene possesses weaker intermolecular forces, it was concluded that the ethylene was not forming cluster ions, but rather was forming covalently bonded molecular ions. Ethylene was seen to readily undergo cationic polymerization of up to five monomers via sequential exothermic condensation reactions, but this is where “polymer” growth stopped.5 Kebarle and co-workers found that polymerization was limited because the rate constants for each sequential addition reaction decreased rather dramatically. They attributed the drop in rate constants to the inhibition of further reaction by steric effects within the growing cluster. In recent years the focus of free cationic ethylene polymerization research has shifted from ion-molecule studies to studies of cationic polymerization within van der Waals clusters.11-21 Since intracluster ion-molecule reactions take place with zero collision energy, at low translational, vibrational, and rotational temperatures, and with excellent efficiency, clusters are a suitable environment for the further study of the energetics, kinetics, and bonding of small cationic ethylene polymers, or oligomers. Most of the studies to date have dealt directly or indirectly with the bonding within ethylene cluster ions, that is, covalent versus intermolecular bonding. Among the earliest investigations of ethylene cluster ions were those of Ng and co-workers.11-13 Their energetics were X

Abstract published in AdVance ACS Abstracts, March 15, 1996.

0022-3654/96/20100-6427$12.00/0

consistent with a general scheme in which a true cluster ion (C2H4)n+ rearranges to a long-lived excited complex [C2nH4n+]* prior to unimolecular decomposition for n ) 2 and 3.11,12 Using values from refs 11 and 12, it can be seen that rearrangements of ethylene cluster ions to linear molecular ions are exothermic and involve relatively small energy barriers:

(C2H4)2+ f C4H8+ (1-butene) ∆H ) -35 kcal/mol, ∆Ebarrier ) ∼0.21 eV (1a) (C2H4)3+ f C6H12+ (1-hexene) ∆H ) -46 kcal/mol, ∆Ebarrier ) ∼0.15 eV (1b) As further proof that the cluster ions rearranged to molecular ions, Ng and co-workers compared their (C2H4)3+ results with those of propylene and cyclopropane dimer cations.11,22 The three cluster cations shared the same dissociation reactions with only one exception

(C3H6)2+ f [C6H12+]* f C6H11+ + H (not observed)

(2)

prompting the conclusion that while the distribution of C6H12+ isomers formed by rearrangement was different for each cluster ion, each ion was indeed a covalently bonded species. Buck and co-workers carried out very different experiments to study the fragmentation of ethylene cluster ions.14 They concluded that for cluster ions up to the 8-mer the only molecular ion being formed was the dimer ion (i.e., (C2H4)nC4H8+ clusters are formed) and that the fragmentation of ethylene cluster ions was dominated by the ion-molecule reactions of a single C2H4+ species within the cluster ion. Thus, it appears as though these conclusions are inconsistent with those of Ng and co-workers. Garvey and co-workers attempted to further investigate bonding in these cationic ethylene cluster ions.15-17 An anomalous intensity maximum was observed for the tetramer ion, and a sharp decrease in intensity was noted after the 5-mer ion under conditions of either low temperature (T e 224 K) or high pressure (P g 2.5 atm) and at all electron energies. They inferred that, like those of Kebarle, the cluster ions that they were forming were rearranging to molecular ions up until the 6-mer, and for (C2H4)6+ and larger cluster ions, the structures © 1996 American Chemical Society

6428 J. Phys. Chem., Vol. 100, No. 16, 1996

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Figure 1. Schematic of instrument.

consisted of a molecular ion core solvated by loosely bound ethylene monomers. They rationalized that the anomaly represents a balance of two competing effects: (1) as Kebarle reported, the rate constant for each successive ion-molecule addition reaction decreases and (2) as the cluster size increases, the probability of the growing cationic oligomer “finding” an ethylene monomer increases. Therefore, it can be said that the observed cluster ion distribution anomalies are the result of a kinetic “bottleneck”. In a later study, Garvey and co-workers proposed a mechanism for the oligomerization and suggested that the n ) 3-5 cluster ions are rearranging to substituted cyclopentane and/or cyclohexane ions.16,17 El-Shall and co-workers arrived at similar conclusions in their study of the cationic polymerization of isoprene, C5H8.23 In experiments analogous to those carried out with ethylene, they observed an anomalous intensity maximum at n ) 6 in the cluster ion distribution of isoprene and an especially intense peak corresponding to the formation of C14H21+. The C14H21+ peak was attributed to a set of stable cyclic isomers, and the 6-mer ion was proposed to be the result of optimal clustering conditions favoring this species. Jungwirth and Bally conducted detailed calculations on the structure of the ethylene dimer radical cation (produced by a C2H4+ + C2H4 ion-molecule reaction) and its possible structural rearrangements.18 They concluded that the dominant product of the ion-molecule reaction should be a true ethylene dimer cation but that the rearrangement to a 1-butene cation structure is exothermic by 36.8 kcal/mol and involves an activation barrier of only 5.9 kcal/mol (both at the QCISD(T) level). Subsequent rearrangement to 2-butene cation structures was calculated to be exothermic as well. Their work is quite significant since it agreed very well with experimental data and showed that while the rearrangement of the dimer ion to a molecular ion is energetically feasible, it is not a certainty. Baer and co-workers, who had previously conducted MS/ MS investigations of true ethylene cluster ions,19 recently probed the structures and dissociation dynamics of small ethylene cluster ions using the PEPICO technique.20 Their latest results are especially of great importance to investigations (including the present work) of ethylene cluster ions and ionic rearrangement products formed by dissociative ionization. Baer and coworkers concluded that dimer ions may be formed either by direct ionization of ethylene dimer or by dissociative ionization of larger neutral clusters. It was uncertain whether the structure at all energies is that of a true dimer ion or that of a molecular ion. It was determined, however, that the dimer ion rearranges to a 2-butene structure at certain energies. It was also proposed that the ionization of trimer and tetramer clusters is purely

dissociative, forming primarily the aforementioned dimer ion and a C6H12+ ion with a linear hexene structure, respectively. While there is much that we understand about the chemical nature of ethylene cluster ions, there are still some fundamental questions which need to be addressed. There is general agreement that ethylene does undergo intracluster rearrangement by sequential ion-molecule reactions following ionization to form molecular ions. However, the structures of such ions beyond n ) 3, and the roles they may play in limiting the extent of oligomerization, are not well known. The work presented here is an attempt to gain further insight into the nature of these small ethylene cluster ions, (C2H4)n)2-9+. We report in this paper new results of collision-induced dissociation (CID) studies using a tandem triple quadrupole mass spectrometer in conjunction with an intense continuous source of neutral clusters. The CID spectra for (C2H4)n)3-5+ will be compared to those of selected molecular ion isomers to better understand the nature of the bonding within small ethylene cluster ions. Experimental Section The molecular beam apparatus used for these experiments has been described previously24 and is shown in Figure 1. It consists of a Campargue-type molecular beam source coupled to a triple quadrupole mass spectrometer (Extrel C-50). The Campargue source was operated with a nozzle 250 µm in diameter which was located 5.0 mm from a skimmer. Neutral clusters were formed by expanding a 10% mixture of ethylene in argon (Matheson) at a stagnation pressure of 3.0 atm and a nozzle temperature of 253 K. The resulting beam was skimmed and collimated twice before entering the mass spectrometer chamber. Once in the mass spectrometer chamber, the cluster beam was ionized by electron impact. Q3 and MS/MS mass spectra were acquired for the ethylene/argon expansions. The mass spectra were collected and averaged using a mass spectrum acquisition program (Teknivent Vector One) in the case of Q3 spectra or using a digital storage oscilloscope (Lecroy 9310M) in the case of MS/MS spectra. The Q3 mass spectrum of a 10% mixture of ethylene in argon was acquired at an electron energy of 30 eV, with the pressure in the mass spectrometer chamber between 7 × 10-7 and 1 × 10-6 Torr. For MS/MS spectra, ethylene cluster ions with m/z ) 56, 84, 112, 140, 168, 196, 224, or 252 were selected in the first quadrupole (Q1), and the CID of these ions was studied. The CID experiments were carried out at collision cell pressures of 8 × 10-4 to 1 × 10-3 Torr using He as the collision gas, the electron impact ionization energy was 60 eV, and the collision energy (lab frame) was set to 20 eV. Additional experiments were conducted in which alkenes and cycloalkanes having the same formula weights as selected

Collision-Induced Dissociation of Cluster Ions

Figure 2. Q3 mass spectrum of 10% ethylene in argon acquired using a Campargue source with stagnation pressure ) 3 atm, nozzle temperature ) 253 K, and an electron impact ionization energy of 30 eV.

ethylene cluster ions (m/z ) 84, 112, and 140) were studied in the MS/MS mode in order to compare CID patterns with those of the ethylene cluster ions. For m/z ) 84, cyclohexane, methylcyclopentane, 1-hexene, and 4-methyl-1-pentene were studied. 1,2-Dimethylcyclohexane, 1-octene, and 2,4,4-trimethyl-1-pentene were studied at m/z ) 112, and 1-decene and tert-butylcyclohexane were studied at m/z ) 140. Each of the liquids was placed in a solvent reservoir, and argon was bubbled through the reservoir such that the total stagnation pressure was 3.0 atm. These compounds were analyzed under conditions identical to those of the ethylene clusters. Results A Q3 mass spectrum of ethylene cluster ions is shown in Figure 2. It is consistent with previous cluster ion mass spectra for ethylene in that it displays an anomalous maximum in its cluster ion distribution corresponding to the (C2H4)4+ (n ) 4) species, as well as a sharp intensity drop-off after n ) 5. This observation is significant since it is an indication that we are generating the same cluster ions that were proposed by Garvey and co-workers to undergo intracluster cationic polymerization to form oligomers.15 The bonding and structures of the n ) 2-9 cluster ions were investigated further by studying the CID patterns of these ions. For CID spectra, the collision energy in all cases was set for 20 eV (lab frame), meaning that the ions spent on the order of 10-5 s in the collision cell. Under the above conditions, it is estimated that a C2H4+ ion will undergo an average of 2-3 collisions as it travels through the collision cell.25 Apparent collisional cross sections for the (C2H4)n)2-7+ ions were calculated26 and are given in Figure 3. It should be noted that these are relative cross sections which were calculated using parent and daughter ion intensities and do not take into account loss of parent ions by any means other than dissociation. The CID mass spectra for (C2H4)n)2-5+ are summarized in the Discussion section, as well as in Tables 1-3. The CID mass spectra for (C2H4)n)6-9+ are shown in Figure 4. In order to obtain the most complete picture of the bonding within the (C2H4)n)2-9+ clusters, molecular ions with the same m/z as mass-selected ethylene cluster ions were studied with respect to their CID processes, and their mass spectral features are given in Tables 1-3. Discussion 1. (C2H4)2+. Although the ethylene dimer ion is able to rearrange to a molecular ion, its bonding has not been confirmed

J. Phys. Chem., Vol. 100, No. 16, 1996 6429

Figure 3. Apparent relative collisional cross sections of the (C2H4)n)2-7+ cluster ions calculated from CID mass spectral data.

by CID when ionization is carried out well after the completion of the cluster-forming expansion.11,13-21 The only dissociation channel observed is loss of a CH3 radical

(C2H4)2+ + R f C3H5+ + CH3 + R

(3)

where R is the collision gas, He. The fact that the loss of C2H4 is not evident but the loss of an alkyl radical is may suggest that the dimer ion rearranges to a molecular ion, C4H8+. However, as one reviewer suggested, the loss of CH3 only proves that a molecular dimer ion is generated at some point, which may be immediately following ionization or, alternatively, following an activating collision in Q2. Considering the results of previous related studies, we cannot state conclusively that a molecular ion is formed after ionization.18,20 2. (C2H4)3+. The trimer is the first ethylene cluster ion to show multiple dissociation channels. The following CID processes are all observed

(C2H4)3+ + R f C5H9+ + CH3 + R intensityrel ) 693 (4a) f C4H8+ + C2H4 + R intensityrel ) 1000 (4b) f C4H7+ + C2H5 + R intensityrel ) 185 (4c) f C3H6+ + C3H6 + R intensityrel ) 122 (4d) where R is an “inert” collision gas, in this case He. Initially, it may appear that reaction 4b is indicative of a weakly bound ethylene. However, as can be seen in Table 1, cyclohexane, methylcyclopentane, 1-hexene, and 4-methyl-1-pentene, which are all C6H12+ isomers, display prominent peaks corresponding to loss of C2H4. Comparison of the daughter ion intensities of (C2H4)3+ with those of the known molecular ions reveals that the trimer ion probably rearranges to a molecular ion prior to dissociation, as the CID patterns are similar. This conclusion is consistent with previous studies of the trimer ion.11,20 Least-squares analysis was employed for comparison of the C6H12+ CID mass spectra, and the results are given in Table 1. Because of the numerous differences between the CID mass spectra of 4-methyl-1-pentene and methylcyclopentane and that of the (C2H4)3+ ion, it is highly

6430 J. Phys. Chem., Vol. 100, No. 16, 1996

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TABLE 1: CID Mass Spectrum of (C2H4)3+ Compared to Those of Known C6H12+ Molecular Ionsa m/z

fragment ion

(C2H4)3+ spectrum

cyclohexane

1-hexene

4-methyl-1-pentene

methylcyclopentane

69 68 56 55 43 42 41 ∑n(Icluster - Ii)n2

C5H9+ C5H8+ C4H8+ C4H7+ C3H7+ C3H6+ C3H5+

693 0 1000 185 0 122 0

635 87 1000 83 0 105 50 2.4 × 104

868 0 1000 218 25 161 25 3.4 × 104

456 0 1000 117 57 63 0 6.8 × 104

1000 0 939 117 25 81 25 1.1 × 105

a The most intense daughter ion peak in each spectrum has been arbitrarily normalized to 1000. All compounds, including ethylene, were seeded in argon and expanded at a total stagnation pressure ) 3 atm using a Campargue source. The nozzle temperature was 253 K in the case of ethylene and 293 K for all other compounds. The beam was ionized by EI at energy ) 60 eV, and the m/z ) 84 ion was mass selected by Q1. Helium was used as the collision gas, the total pressure in the collision cell ) 8 × 10-4 to 1 × 10-3 Torr, and the collision energy ) 20 eV (lab frame).

TABLE 2: CID Mass Spectrum of (C2H4)4+ Compared to Those of Known C8H16+ Molecular Ionsa m/z 97 96 84 83 82 70 69 68 57 56 55 42 43 28 ∑n(Icluster - Ii)n2

fragment ion

(C2H4)4+ spectrum

n-propylcyclopentane

cyclooctane

1-octene

trans-4-octene

1,2-dimethylcyclohexane

2,2,4-trimethyl1-pentene

C7H13+ C7H12+ C6H12+ C6H11+ C6H10+ C5H10+ C5H9+ C5H8+ C4H9+ C4H8+ C4H7+ C3H6+ C3H5+ C2H4+

61 0 751 1000 260 332 61 0 0 59 91 0 0 0

132 0 593 1000 731 727 133 0 128 162 57 0 0 0 4.4 × 105

215 0 838 1000 482 779 286 200 102 342 106 88 0 0 4.7 × 105

123 0 591 1000 710 839 137 0 124 183 70 29 0 0 5.3 × 105

95 0 506 1000 614 905 161 0 151 227 78 43 0 43 5.8 × 105

1000 100 183 467 416 266 60 0 68 56 50 0 0 0 1.5 × 106

1000 0 692 181 0 343 816 70 920 319 0 0 54 0 3.1 × 106


unlikely that the ethylene trimer ion rearranges to either of these structures. Of the remaining molecular compounds studied, the CID mass spectrum of cyclohexane is most similar to that of the ethylene trimer ion. Cyclohexane, the second of the cyclic isomers, appears to be a better match than 1-hexene, because the two most intense peaks in the CID spectra of the cyclohexane ion and (C2H4)3+ are of almost equal intensity. As the molecular compounds used for comparison here represent only a few C6H12+ isomers, it would not be prudent to conclude that the (C2H4)3+ species definitely rearranges to a cyclohexane ion only. There is, however, a good possibility the trimer ion may rearrange to the cyclohexane ion and some other C6H12+ isomers. Interestingly, if an average of the intensities of cyclohexane and 1-hexene is taken and compared to the (C2H4)3+ intensities using least squares, the sum ∑n(Icluster - Ii)n2 is 8.1 × 103. This is a considerable improvement upon the results of any one isomer alone. Abramson and Futrell, upon studying the fragmentation pattern of (C3H6)2+, concluded that their cluster ion was rearranging to 3-hexene or possibly 2-hexene.27 Baer and co-workers also concluded that the trimer ion rearranges to a linear structure.20 Thus, it seems probable that the products of the ethylene trimer ion rearrangement may be a distribution of the cyclohexane ion and any of the unbranched hexenes.

3. (C2H4)4+. The most intense species in the cluster ion distribution for ethylene, (C2H4)4+, undergoes numerous CID processes

(C2H4)4+ + R f C7H13+ + CH3 + R intensityrel ) 61 (6a) +

f C6H12 + C2H4 + R intensityrel ) 751 (6b) f C6H11+ + C2H5 + R intensityrel ) 1000 (6c) f C6H10+ + C2H6 + R intensityrel ) 260 (6d) f C5H10+ + C3H6 + R intensityrel ) 332 (6e) f C5H9+ + C3H7 + R intensityrel ) 61 (6f) +

f C4H8 + C4H8 + R intensityrel ) 59 (6g) f C4H7+ + C4H9 + R intensityrel ) 91 (6h) where as before R represents the collision gas, He. The most


J. Phys. Chem., Vol. 100, No. 16, 1996 6431

TABLE 3: CID Mass Spectrum of (C2H4)5+ Compared to Those of Known C10H20+ Molecular Ionsa m/z 112 111 110 98 97 96 85 84 83 82 70 68 56 ∑n(Icluster - Ii)n2

fragment ion

(C2H4)5+ spectrum

C8H16+ C8H15+ C8H14+ C7H14+ C7H13+ C7H12+ C6H13+ C6H12+ C6H11+ C6H10+ C5H10+ C5H8+ C4H8+

668 1000 356 500 0 96 87 183 0 67 72 0 0

1-decene

tert-butylcyclohexane

0 1000 736 779 529 557 0 171 686 371 579 264 0 2.1 × 106

0 0 0 0 0 0 0 0 0 301 0 0 1000 2.9 × 106


intense dissociation channel in the CID spectrum is reaction 6c, loss of C2H5. Besides peaks arising from this dissociation process and reaction 6b, the CID mass spectrum of (C2H4)4+ consists of relatively minor peaks. A least-squares comparison of the CID spectral features of the ethylene tetramer ion with those of some C8H16+ molecular ion isomers is made in Table 2. Since 1,2-dimethylcyclohexane and 2,4,4-trimethyl-1-pentene both dissociate primarily by loss of CH3, rather than by loss of C2H5, they compare least favorably with (C2H4)4+ and can be eliminated immediately as possible rearrangement products. The two octenes studied, trans-4octene and 1-octene, have similar dissociation patterns as there are very few structural differences between them. The features in these CID mass spectra are similar to those observed in the CID spectra of n-propylcyclopentane and cyclooctane. Levsen accounted for the similarities among these four isomer ions when he concluded that partial isomerism occurred between ionization and CID.28 The n-propylcyclopentane ion was observed to possess a CID pattern essentially identical to that of the 1-octene ion, as it has been observed here, indicating either that it isomerizes to the 1-octene structure or that both ions isomerize to a third common structure. The trans-4-octene and cyclooctane were believed to partially isomerize to undetermined structures. While complete isomerization was not observed by Levsen nor is it observed in this work, the possibility of partial isomerization makes the exact assignment of the cluster ion structure difficult. According to least-squares analysis, within the error range of the data, the four aforementioned molecular ions are all fair matches to the tetramer ion, but none is outstanding. Levsen acquired CID mass spectra for an extensive list of commercially available C8H16+ molecular ions,28 and since the other molecular ions studied had CID mass spectra that differed grossly from that of the ethylene tetramer ion, they were immediately eliminated as potential candidates. While no single match is perfect, 1-hexene, trans-4-hexene, n-propylcyclopentane, and cyclooctane ions all have one thing in common: they either initially possess or may easily isomerize to linear structures. It does not appear that the product ions may be branched. As such, further addition reactions may occur without much

Figure 4. CID mass spectra (acquired in MS/MS mode) of (C2H4)n+ clusters for n ) (a) 6, (b) 7, (c) 8, and (d) 9. Cluster ions with m/z ) 28n were mass selected in Q1. A Campargue source generated clusters using 10% ethylene in argon at a stagnation pressure ) 3 atm and nozzle temperature ) 253 K. The electron impact ionization energy was 60 eV. The collision gas employed was helium, the pressure in the collision cell ) 8 × 10-4 to 1 × 10-3 Torr, and the collision energy ) 20 eV (lab frame).

6432 J. Phys. Chem., Vol. 100, No. 16, 1996 inhibition from steric effects. All this information leads to the conclusion that, like the ethylene trimer ion, the tetramer ion rearranges to linear and/or cyclic structures, but not branched products. The octene, n-propylcyclopentane, and cyclooctane ions are all good candidates for the rearrangement products, and multiple products may be generated; however, this is uncertain.

4. (C2H4)5+. The (C2H4)5+ ion undergoes the following CID reactions

(C2H4)5+ + R f C8H16+ + C2H4 + R intensityrel ) 668 (8a) f C8H15+ + C2H5 + R intensityrel ) 1000 (8b) f C8H14+ + C2H6 + R intensityrel ) 356 (8c) f C7H14+ + C3H6 + R intensityrel ) 500 (8d) f C7H12+ + C3H8 + R intensityrel ) 96 (8e) f C6H13+ + C4H7 + R intensityrel ) 87 (8f) f C6H12+ + C4H8 + R intensityrel ) 183 (8g) f C6H10+ + C4H10 + R intensityrel ) 67 (8h) f C5H10+ + C5H10 + R intensityrel ) 72 (8i) where R is He. Table 3 contains relative intensities for the above daughter ions and least-squares analyses for two C10H20+ molecular ions, 1-decene and tert-butylcyclohexane. As C10H20 isomers are not as readily available as isomers of smaller unsaturated hydrocarbons, these two isomers were chosen because they were structurally the most different of the isomers obtainable. Least-squares analysis reveals that both isomers are poor matches for (C2H4)5+. Although the exact structure of the ethylene 5-mer ion cannot be deduced from the CID mass spectrum or by comparison to isomeric ions, its general structure can be determined. The disagreement between the spectra of (C2H4)5+ and the isomeric molecular ions indicates that the 5-mer ion is either a molecular ion structurally different from the 1-decene or tert-butylcyclohexane ions or a species which is not completely covalently bonded (i.e., (C2H4)C8H16+). The latter possibility is not likely since examination of Figure 4c,d shows that the C10H20+ species further dissociates by losing H and H2. Since a (C2H4)C8H16+ ion would not dissociate in this way, it is certain that (C2H4)5+ rearranges to a true molecular ion.

Lyktey et al. Comparison of specific features in the CID spectra shows that there are major differences (i.e., >500 intensity units) between the 5-mer ion and 1-decene ion with respect to the formation of C8H16+, C7H13+, C6H11+, and C5H10+, as well as large discrepancies for almost every other dissociation channel. This proves that the product ion of the 5-mer rearrangement is not similar to any of the decene ions or simple cyclic ions, such as cyclodecane. The structures of the product ions must be those of either branched linear or substituted cyclic ions. This represents a clear break with the pattern seen so far for ethylene cluster ion rearrangement, since the trimer and tetramer ions do not rearrange to branched molecular ions or cyclic ions which may isomerize to branched ions. The general structure of the 5-mer ion is consistent with the previous conclusion that free cationic oligomerization is strongly dependent on kinetics and does not proceed beyond the 5-mer ion. A branched ion will inhibit further addition reactions by presenting an ethylene with considerable steric hindrance, thereby decreasing the rate constant for such a reaction. While the assignment of an exact structure to the (C2H4)5+ rearrangement product would be mere speculation, especially since partial isomerization is a possibility, it seems certain that the structure or structures are those of branched linear and/or cyclic ions. 5. (C2H4)n)6-9+. It is evident from the CID and metastable decay mass spectra of the (C2H4)n+ cluster ions, where n ) 6-9, that there is a clear difference between these ions and those discussed above. The (C2H4)n)6-9+ cluster ions lose only C2H4 as the first dissociation process, showing no evidence of alkyl fragmentation. These larger ethylene cluster ions decompose further by losing only C2H4 monomers with excellent efficiency until the C10H20+ ion is reached

(C2H4)n+ + R f C10H20+ + (n - 5)C2H4 + R

(9)

where, for a given n, a daughter ion peak is observed for each dissociation from 5 to n. The loss of an additional monomer is observed to yield C8H16+, albeit with a dramatic drop in efficiency. Only peaks with very weak intensities are observed for ions smaller than C8H16+. Interestingly, while dissociation other than monomer loss is absent for (C2H4)n)6-9+ ions, dissociation of these species beyond C10H20+ is similar to that discussed in the previous sections. This implies a structural difference between cluster ions of the type (C2H4)n+, where n ) 2-5, and ions for which n g 6. The increase in the calculated apparent cross sections between n ) 5 and 6 in Figure 3 also points to such a conclusion. These data are consistent with the picture that the (C2H4)n)6-9+ cluster ions consist of a C10H20+ molecular core ion solvated by an appropriate number of loosely bound ethylene monomers. 6. Oligomerization Scheme. Using the structures proposed above to be the rearrangement products of the ethylene cluster ions, it is possible to derive a general mechanism for the intracluster free cationic oligomerization (or polymerization) of ethylene, which is given in Figure 5. If the ethylene dimer ion rearranges to a molecular ion, its structure must be linear, since the reaction of the dimer ion with an ethylene yields unbranched products. Upon reaction of the dimer ion with an ethylene, a linear hexene ion is formed. This ion may undergo a ring closure yielding a cyclohexane radical cation. The formation of these trimer ion rearrangement products is consistent with the CID mass spectral observations. At this point, the linear hexyl radical cation likely reacts with an additional ethylene to yield a linear octyl radical cation. Beyond this step, however, it is uncertain which product ions result from rearrangement of


J. Phys. Chem., Vol. 100, No. 16, 1996 6433 Ethylene cluster ions with n g 6 are shown to consist of a C10H20+ core, generated by intracluster ion-molecule reactions and solvated by an appropriate number of loosely bound ethylene monomers. The inhibition of more extensive oligomerization has been attributed to an excessively slow reaction rate between the growing oligomer and additional monomers due to steric hindrance. The formation of a branched 5-mer ion, as well as the possibility of ring structures for smaller cluster ions, as proposed here, would constitute just such a steric effect. Acknowledgment. We gratefully acknowledge the financial support provided by the Office of Naval Research. References and Notes

Figure 5. Proposed general ethylene oligomerization mechanism based on the present experimental observations.

the tetramer ion and what the mechanism of the next addition reaction may be. Conclusions The CID mass spectra of the ethylene cluster ions (C2H4)n)2-9+ have been useful in helping to deduce the nature of the bonding and the structures of these species. From these data it has been determined that the cluster ions with n e 4 are covalently bonded ions with structures that may be linear or cyclic, but not branched. If the dimer ion (which lacks the carbons to form an unstrained ring) rearranges, a linear butene ion will be formed. The trimer ion, upon comparison to known molecular ions, appears to rearrange to either hexene or cyclohexane ions. Since molecular isomers of the tetramer ion can partially isomerize between the ionization and collisioninduced dissociation events, an exact structure for the rearrangement product of this ion could not be determined. Possible structures include linear and unsubstituted cyclic ions, as well as a substituted cyclic ion which isomerizes into a linear structure. Direct comparison of readily available molecular ions isomeric with the 5-mer ion was unsuccessful, but it seems certain that the structure of 5-mer ion is different from those of the smaller cluster ion rearrangement products. It is most likely that the structure is that of a branched linear or branched cyclic ion.

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JP9536413

Collision-Induced Dissociation of Mass-Selected Ethylene Cluster Ions

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