C6H5 Reactions - The Journal of

ARTICLE pubs.acs.org/JPCA

Novel Products from C6H5 þ C6H6/C6H5 Reactions Bikau Shukla,*,† Kentaro Tsuchiya,‡ and Mitsuo Koshi§ †

Department of Aerospace and Mechanical Engineering, The University of Southern California, Los Angeles, California, United States Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan § Institute of Engineering Innovation, The University of Tokyo, Bunkyo-ku, Tokyo, Japan ‡

bS Supporting Information ABSTRACT: To date only one product, biphenyl, has been reported to be produced from C6H5 þ C6H6/C6H5 reactions. In this study, we have investigated some unique products of C6H5 þ C6H6/C6H5 reactions via both experimental observation and theoretical modeling. In the experimental study, gasphase reaction products produced from the pyrolysis of selected aromatics and aromatic/acetylene mixtures were detected by an in situ technique, vacuum ultraviolet (VUV) single photon ionization (SPI) time-of-flight mass spectrometry (TOFMS). The mass spectra revealed a remarkable correlation in mass peaks at m/z = 154 {C12H10 (biphenyl)} and m/z = 152 {C12H8 (?)}. It also demonstrated an unexpected correlation among the HACA (hydrogen abstraction and acetylene addition) products at m/z = 78, 102, 128, 152, and 176. The analysis of formation routes of products suggested the contribution of some other isomers in addition to a well-known candidate, acenaphthylene, in the mass peak at m/z = 152 (C12H8). Considering the difficulties of identifying the contributing isomers from an observed mass number peak, quantum chemical calculations for the above-mentioned reactions were performed. As a result, cyclopenta[a]indene, asindacene, s-indacene, biphenylene, acenaphthylene, and naphthalene appeared as novel products, produced from the possible channels of C6H5 þ C6H6/C6H5 reactions rather than from their previously reported formation pathways. The most notable point is the production of acenaphthylene and naphthalene from C6H5 þ C6H6/C6H5 reactions via the PAC (phenyl additioncyclization) mechanism because, until now, both of them have been thought to be formed via the HACA routes. In this way, this study has paved the way for exploring alternative paths for other inefficient HACA routes using the PAC mechanism.

’ INTRODUCTION The self-reactions of benzene/phenyl are crucial to understand several unresolved but extremely important issues in which benzene/phenyl serves either as a starting precursor or is produced as an active intermediate. Some issues include the understanding of pollutant emission from combustion and pyrolysis, formation of large polycyclic aromatic hydrocarbons (PAHs) followed by soot nucleation,14 and the chemistry of chemical vapor deposition (CVD) growth of carbon nanomaterials like fullerenes,57 carbon nanofibers,8 and carbon nanotubes.9,10 In spite of such great importance and vigorous research on benzene decomposition in shock tubes,1124 flow systems,25,26 and others systems,27,28 most of the studies are limited to an estimation of the order and rate of benzene decay. Besides this, high temperature (>1500 K) ring-opening reactions15,1719,25,27,2932 and studies focusing on particular reactions like C6H6/C6H5 þ H = products,28,33 C6H6 þ C = products,34 and C6H5 þ H2 = C6H6 þ H35 have also been reported. However, rather less attention has been given to the selfreactions of benzene/phenyl resulting in mass growth useful in explaining the formation mechanism of phenyl-substituted species like phenyl-PAHs and their derived products.3638 In general those phenyl-substituted products are produced via a chain reaction r 2011 American Chemical Society

carried out by benzene/phenyl. Although understanding of those chain reactions directly depends on the rate and products of the chain-initiating step phenyl þ benzene (1) and phenyl þ phenyl (2) reactions, these reactions also have not been explored adequately.

Until now, all experimental and theoretical studies27,31,3948 have reported biphenyl as the only product of reactions 1 and 2. Additionally, Manion et al.46 has reported the rate constant for the reverse reaction (1). In this way, to our knowledge, previous studies have overlooked other possible channels of reactions 1 and 2. Our recent experimental results49 attracted our attention toward reactions 1 and 2. Received: October 3, 2010 Revised: April 6, 2011 Published: May 05, 2011 5284

dx.doi.org/10.1021/jp201817n | J. Phys. Chem. A 2011, 115, 5284–5293

The Journal of Physical Chemistry A

ARTICLE

Above limitations and our experimental indication stimulated us to explore other possible products of reactions 1 and 2 that might be useful in different combustion and pyrolytic conditions. To meet this necessity, the present study is aimed to focus on the elucidation of all possible routes of C6H5 þ C6H6/C6H5 with sufficient reliability to explore the novel products. On the experimental side, mass spectrometric analysis of gas-phase products, produced from the pyrolysis of benzene with and without addition of acetylene and of toluene with/without addition of benzene, were performed. To validate the experimental suggestions and to identify the most probable isomers of C12H8, quantum chemical calculations were also performed. All the possible reaction routes and their products were theoretically determined through B3LYP hybrid density functional method. As a result of experimental and theoretical studies, new products from C6H5 þ C6H6/C6H5 reactions have been obtained.

’ EXPERIMENTAL SECTION Details of the experimental setup have been described elsewhere.49 Briefly, the apparatus consists of a quartz reaction tube wrapped with a tungsten heater placed into a source chamber aligned with the linear TOFMS. Typical temperature profiles of the pyrolysis zone recorded by using a K-type thermocouple are shown in Supporting Information Figure S1, which reflects that it is hard to maintain a steady temperature along the length of reaction tube; it means that only a semiquantitative measurement can be expected. Thus, in this study only the distribution of products and their formation pathways have been targeted rather than extracting the rate constant for a specific reaction. During the experimental procedure, the head of the thermocouple was kept in front of the pinhole for recording the reaction temperature. To ionize the resulting species, a UV laser pulse (355 nm) was loosely focused into the frequency tripling cell filled with xenon (pressure = ∼7.4 Torr) to generate vacuum ultraviolet (VUV) photons at 118 nm (10.5 eV). The UV beam was separated and removed by a LiF crystal prism in order to avoid the (1 þ 1) ionization by 355 and 118 nm photons. A premixture of 20% benzene in He, 20% toluene in He, 20% benzene þ 20% toluene in He, or 20% benzene/toluene þ 20% acetylene in He was supplied through a mass flow controller into the quartz reaction tube. The gaseous product molecules were continuously sampled through a pinhole and were collimated by a 1.0 mm orifice skimmer mounted at 3.0 mm from the pinhole. The molecular beam was introduced into the ionization region of the TOFMS, and the molecular species were ionized by the 118 nm photons. The ionized species were accelerated by electrodes, electronic lenses, and deflectors through the fieldfree drift tube to the MCP (multichannel plate). Mass spectra were recorded at different temperatures and at a constant pressure of ∼10.0 Torr with a constant residence time of ∼0.6 s in each experiment (specific values are shown in the captions of Figures 13). ’ RESULTS AND DISCUSSION The mass spectra of gas-phase products of pyrolysis of hydrocarbons, observed at similar experimental conditions, are compared to each other in Figures 13. Figure 1 includes the products of pyrolysis of toluene only and of toluene þ acetylene mixture, Figure 2 consists of products of pyrolysis of toluene with and without addition of benzene, and Figure 3 shows the products of pyrolysis of only benzene and of benzene þ acetylene mixture.

Figure 1. Mass spectra of gas-phase products of pyrolysis of only toluene (a) compared to toluene þ acetylene (b), at five different temperatures and at constant pressure of 10.12 Torr and residence time of 0.56 s. (c) Temperature dependence of signal intensities of selected species, with their structural formula, showing the comparison of their production from the pyrolysis of toluene with and without acetylene addition. Bold lines represent their production from the mixture of toluene þ acetylene, while dotted lines represent their production from pure toluene.

It is worth mentioning that all observed mass spectra have been normalized by the VUV intensities recorded at each temperature only by supplying He before feeding the actual reactants into the reactor. In these mass spectra only the most probable candidates for observed mass peaks have been assigned. Temperature-dependent variation in signal intensities of some specific species observed in are 5285

dx.doi.org/10.1021/jp201817n |J. Phys. Chem. A 2011, 115, 5284–5293


Figure 2. Mass spectra of gas-phase products of pyrolysis of only toluene (a) compared to toluene þ benzene (b) at five different temperatures and at a constant pressure of 10.02 Torr and residence time of 0.56 s. (c) Temperature dependence of signal intensities of selected species, with their structural formula, showing the comparison of their production from the pyrolysis of toluene with and without benzene addition. Bold lines represent their production from the mixture of toluene þ benzene, while dotted lines represent their production from pure toluene.

shown in Figures 1c3c. In those figures, the peak intensities of species in the case of hydrocarbon mixtures (bold lines) have been compared with their peak intensities in the case of pure hydrocarbons (dotted lines) under same experimental conditions. At first, the origination of keen interest on a particular mass peak at m/z = 152 (C12H8) and assignment of the most probable species suited for different reaction conditions will be discussed. Since mass spectra cannot confirm the isomers with certainty, to

ARTICLE

Figure 3. Mass spectra of gas-phase products of pyrolysis of only benzene (a) compared to benzene þ acetylene mixture (b) at five different temperatures and at a constant pressure of 10.18 Torr and residence time of 0.61s. (c) Temperature dependence of signal intensities of selected species, with their structural formula, showing the comparison of their production from the pyrolysis of benzene with and without acetylene addition. Bold lines represent their production from the mixture of benzene þ acetylene, while dotted lines represent their production from pure benzene.

explore the experimental suggestions in detail, theoretical/computational results will be discussed. Finally, as a result of both experimental and theoretical outputs, novel products of reactions 1 and 2 will be suggested. The mass spectra of the gas-phase products of toluene pyrolysis49 are shown in Figure 1a. A sequence of mass peaks at m/z = 78, 102, 128, 152, and 176 is noticeable. On the basis of continuous mass growth by mass number 24/26 corresponding to C2 addition, the most likely candidates produced via hydrogen 5286


The Journal of Physical Chemistry A abstraction and acetylene addition5053 (HACA) routes 3 are benzene, phenylacetylene, naphthalene, acenaphthylene, and pyracylene, respectively.

On the basis of spontaneous conversion of each species into its subsequent species by the same chain carrier, C2H2, as can be seen in reaction 3, one might expect a good correlation in the mass peaks of these sequential products. However, in spite of sufficient production of acetylene (the main chain carrier of reaction 3), from the decomposition of benzyl (C6H5CH2) and cyclopentadienyl radicals (C5H5)49 around 1300 K and above, the observed variation in the intensities of their mass peaks is remarkable (Figure 1a). The unexpected correlation in the intensities of mass peaks for phenylacetylene (m/z = 102) and naphthalene (m/z = 128) is easy to explain as the production of naphthalene via a non-HACA path, benzene/phenyl þ C4H3/C4H4 (reaction 4),3638,49,54 which has already been reported.

But the cause of unexpected variation in the intensities of mass peaks at m/z = 128, 152, and 176, corresponding to naphthalene, acenaphthylene and pyracylene, has not yet been explored. Briefly, a rather weak peak of acenaphthylene against the significant peak of naphthalene around 1300 K, and a very weak peak of pyracylene against a significant peak of acenaphthylene at high temperature indicates that conversions of naphthalene into acenaphthylene and further into pyracylene via reaction 3 are inefficient. Thus, similarly to naphthalene production via a nonHACA path 4, there is a strong probability of the production of acenaphthylene, pyracylene, and/or their isomers via non-HACA paths in addition to the HACA route 3. However, until now no non-HACA path has been suggested for these products. On the other hand, the surprising trend of increasing intensity of mass peak at m/z = 152 against the suppression of the mass peak for biphenyl (m/z = 154) with increasing temperatures (Figure 1a,c) strongly suggests that the mass peak at m/z = 152 should be contributed by the product(s) produced directly from the reactions 1 and 2 or from biphenyl (C12H10) and/or its corresponding radicals (C12H9). Thus, the mass peak at m/z = 152 is only assigned to C12H8 rather than acenaphthylene. On the basis of these observations, unexpected correlation in abovementioned HACA products, despite the presence of C2H2, and significant enhancement of a specific peak at m/z = 152 and its close correlation with biphenyl peak at m/z = 154, the possible contributing species in the mass peak at m/z = 152 (C12H8) and their formation pathways can be speculated as follows: (A) Acenaphthylene and other isomers of C12H8, such as 1-ethynylnaphthalene, biphenylene, or cyclopentaindenes produced via their previously known reaction pathways.

ARTICLE

(B) All species listed in case A, or some of them, most probably produced directly from C6H5 þ C6H6/C6H5 reactions or from biphenyl. According to a very recent study,55 production of 1-ethynylnaphthalene even directly from 1-naphthyl radical þ C2H2 requires a comparatively high energy barrier versus acenaphthylene production. Similarly, from the interesting trend of mass peaks m/z = 152 and 154 (biphenyl), an expected candidate is biphenylene, produced either by the dehydrocyclization of biphenyl via reaction 5 or by the direct dimerization of benzyne via reaction 6.56 However, the elimination of hydrogen molecules for enabling cyclization (reaction 5) involves a high energy barrier, and thus, biphenylene also seems to be energetically less preferable. On the other hand, since in any case a mass peak for benzyne at m/z = 76 was not observed, reaction 6 cannot be a source of biphenylene production or a source contributing to the mass peak at m/z = 152. The contribution of cyclopentaindene is hard to imagine, since its precursors have not been observed significantly in the mass spectra. Thus, case A is difficult to prove.

At this point, on the basis of the observed trend of mass peaks at m/z = 152 and 154, scenario B, the alternative way of producing acenaphthylene and its isomers, seems to be more probable, but further experimental and theoretical verifications are required. Following this need, in consecutive experiments both acetylene, an accelerating species for reaction 3, and benzene, an accelerating species for reactions 1, 2, and 5, were added to toluene separately, and the mass spectra of resulting products are shown in Figures 1b and 2a,b. Interestingly the mass peak at m/z = 152 (C12H8) was found to be enhanced in both cases. Noticeably, even in the presence of sufficient acetylene that backed up the HACA routes 3, the mass peak at m/z = 152 (C12H8) was very weak, while the peak for naphthalene (m/z = 128, C10H8) was significant up to 1214 K (Figure 1b,c). Contrarily, on addition of benzene, a small peak at m/z = 152 (C12H8) associated with biphenyl (m/z = 154, C12H10) was observed even at 1242 K, where naphthalene was absent (Figure 2b,c). On the other hand, a relatively significant peak of m/z = 152 (C12H8) at 1305 K against the weak peak of naphthalene together with a weak peak for m/z = 176 (pyracylene) was observed (Figure 2b,c). The presence of an extremely weak peak at m/z = 176 (pyracylene) in both cases (Figures 1b and 2b) indicates the inefficiency of the HACA route 3 for converting acenaphthylene into pyracylene. Moreover, a particularly interesting correlation in mass peaks at m/z = 152 (C12H8) and m/z = 154 (C12H10) seen in Figure 2b,c, similar to Figures 1a and 2a, shows closeness with scenario B, indicating the formation of acenaphthylene and its isomers via an alternative path, probably from C6H5 þ C6H6/C6H5 reactions 1 and 2 in addition to the HACA routes 3. For further confirmation, pyrolysis of benzene with and without acetylene addition was performed, and resulting gas-phase products were observed as shown in Figure 3a,b. A significantly 5287



ARTICLE

enhanced mass peak at m/z = 152 (C12H8) against the weak peaks of naphthalene and pyracylene even in the case of benzene only pyrolysis (Figure 3a) at 1242 K was observed. Once again, the similar trend of mass peaks at m/z = 152 and 154 (Figure 3c) was noticed. The observation of phenyl radical contributed products and benzene as a major product in the above-mentioned systematic experimental observation confirms the presence of benzene and phenyl radicals in the reaction mixture. Considering their presence together with other active radicals, and on the basis of observed close corelation in the mass peaks at m/z = 152 and 154, the important sequence leading to the production of the experimentally observed species C12H8 (m/z = 152) can be expected as follows: At first, the stable species benzene is converted to phenyl radicals via hydrogen abstraction by active radicals 7 C6 H6 þ R f C6 H5 þ RH

ðR ¼ H, CH3 , C6 H5 CH2 , etc:Þ

ð7Þ Then, C6H5 so produced accelerates reactions 1 and 2 to produce sufficient biphenyl (C12H10) that was confirmed by the observation of biphenyl as the most abundant product in the case of pure benzene pyrolysis (Figures 3). Certainly, because of the higher concentration of benzene, reaction 1 would be more likely to take place compared to reaction 2. The above discussed experimentally observed enhancing trend of C12H8 (m/z = 152) against the reduction of biphenyl (m/z = 154, C12H10) at least qualitatively suggests that C12H8 is produced from biphenyl (C12H10). Because of the presence of hydrogen atoms (produced initially from the self-thermal decomposition of benzene and other products) and other radicals in the reaction mixture, further hydrogen abstraction reaction is most likely to produce biphenyl radicals (8a) which subsequently progress through many intermediate steps to be converted to C12H8 (8b), or biphenyl might be directly changed to C12H8 via direct hydrogen elimination reaction 9: C12 H10 þ R f C12 H9 þ RH

ð8aÞ

C12 H9 f C12 H8 þ H

ð8bÞ

C12 H10 f C12 H8 þ H2

ð9Þ

It is worth mentioning that the rapid drops in yield of products of high mass numbers at high temperatures (>1450 K) are expected to be due to (a) further decomposition of a significant fraction of products at low temperature into smaller fragments, most probably C1C4 species, and (b) the further conversion of the remaining fraction of products to higher PAHs and soot. The basis of the former expectation is the observation of only C2H4 and C4H2 species at high temperatures, while the cause of the absence of CH4 and C2H2 species, which are the most abundant products in general pyrolysis and combustion processes, is their higher IPs than the IP of the single photon ionization (SPI) technique (10.5 eV) used in this study. Similarly, the basis of the later expectation is the observation of tar on the inner wall of the reactor in the downstream zone, where temperature was quite low.

’ COMPUTATIONAL METHODS, RESULTS, AND EXPLANATION Although the sequence of reactions 7, 1, 2, 8a, 8b, and 9 are plausible, since the mass spectra cannot distinguish the isomers,

to determine the most probable species contributing to the mass peak at m/z = 152 (C12H8) and their formation routes, a detailed computational calculation was also performed. Quantum chemical calculations were performed with the Gaussian 03 software package.57 Geometries of reactants, products, intermediates, and transition states (TSs) were optimized by the B3LYP hybrid density functional method58,59 and 6-311G(d,p) basis set. More accurate energies were further calculated by the CBS-QB3 method,60,61 except for singlet minima and transition states that showed biradical characters in the hybrid density functional calculations. It is worth mentioning here that for calculating the energies of species with diradical characters, the CBS-QB3 method is inefficient, as it encounters the problem of correcting parameters that account for spincontamination. To identify whether a particular singlet minima or transition state is biradical or not, its Æs2æ (actual eigenvalue of the s2 operator) values obtained from UB3LYP calculations were considered. For example, the singlet state with Æs2æ values equal to zero was considered as closed-shell and the CBS-QB3 energy was calculated at the optimized geometry, while the singlet state with nonzero Æs2æ values was considered to be open-shell/biradical for which no further CBS-QB3 calculation was conducted. Hence, to obtain the potential energy surfaces of reactions based on the CBS-QB3 calculations, the energy (Ebi) of a singlet minima or transition state with the biradical characters was estimated in reference to the nearest closed-shell singlet one as follows Ebi ¼ Ebi ðUB3LYPÞ Eref ðRB3LYPÞ þ Eref ðCBS-QB3Þ where Ebi(UB3LYP) is the UB3LYP energy of a singlet intermediate or transition state with biradical character, and Eref(RB3LYP) and Eref(CBS-QB3) are respectively RB3LYP and CBS-QB3 energies of the referred nearest closed-shell singlet species. In typical cases, where the character of a transition state is an open-shell singlet and the two minima connecting the transition state are closed-shell singlets, the one side minimum with the closet B3LYP energy to that of the transition state was selected as a reference. Following the general rule, if one side minimum was a closed-shell singlet and the other side minimum was an open-shell singlet, the former was chosen as the reference. Depending on the state of the reference, the magnitude of the Ebi is inherently ambiguous. The energy of an open-shell singlet state obtained by the above-mentioned procedure involves relatively larger error than that of the energy of a closed-shell singlet obtained directly by the CBS-QB3 method; thus, an asterisk mark (*) has been inserted on the right shoulder of some numerals in Figure 4a,b to distinguish the energy of the open-shell from the closed shells. Some choices of reference states that were out of the general rule have not been explained here but instead they have been specified in the figure captions. Calculated potential energy surfaces (PESs) of the phenyl (C6H5) þ phenyl (C6H5) reactions are shown in Figure 4a,b. For the carboncarbon bond dissociation energy (experimentally 496 ( 8 kJmol1) and the reaction of biphenyl to two phenyl radicals, the UB3LYP/6-311G(d,p) calculations underestimated the experimental values by ca. 8%, while the composite CBS-QB3 method overestimated the experimental values by ca. 5%. Figure 4a displays PESs of singlet, phenyl þ phenyl reactions progressing via benzyne þ benzene, and Figure 4b shows the 5288



ARTICLE

Figure 4. (a) Schematic potential energy diagram of phenyl þ phenyl reactions via o-benzyne þ benzene. As for the magnitude of enthalpy with an asterisk on the right shoulder of figures, the reference state of TS0 is phenyl þ phenyl. The reference state of TS3, TS4, TS5, TS6 and intermediates between these transition states is o-benzyne þ benzene. 2a,8b-Dihydrocyclobuta[a]naphthalene is reference state to transition states and intermediates in its reaction channel to naphthalene þ acetylene. Similarly, cyclobuta[b]naphthalene is that to transition states and intermediates in its reaction channel to naphthalene þ acetylene. See the text for an explanation of the asterisk on the right shoulder in the figure. See Figure S2 of the Supporting Information for the geometry of transition states (TS0TS13). (b) Schematic potential energy diagram of phenyl þ phenyl reactions via biphenyl. Dihydrocyclopenta[a]indenes, spiro[H-pentalene-cyclopentadiene]s, dihydro-as-indacenes, and dihydro-sindacenes are drawn in a single well, respectively. In the Supporting Information, see Figure S3 for AC on the right shoulder, Figure S4 for D and E and channels F and G, Figure S5 for J and channel H, and Figure S6 for K and channel I. Relative enthalpy of m-biphenyl þ H and p-biphenyl þ H are 31 and 28 kJ mol1, respectively. As for the magnitude of enthalpy with an asterisk on the right shoulder of figures, the reference of a transition states and an intermediate in the reaction channel from biphenyl to phenylfulvene is the first transition state with 126 kJ mol1 of enthalpy. See the text for details.

reactions progressing via biphenyl. In Figure 4a the maximum point of a curve shows singular TS connecting two wells on both sides, while in Figure 4b a curve sometimes represents a reaction channel via plural TSs and intermediates. The maximum height of potential barrier for the energetically most preferable reaction channels is denoted with a letter, AK, on the right shoulder in Figure 4b. Details of those reaction channels are shown in the

Supporting Information (Figures S2S6). Figure 4a clearly shows that besides the formation of biphenyl as a major product via barrier-free C6H5 þ C6H5 reactions, there is strong probability of the formation of o-benzyne and benzene as secondary products via an exothermic (144 kJ/mol) process by passing through a transition state with a minimum energy barrier of 4 kJ/mol. When benzene þ benzyne was considered as the reference state, 5289



ARTICLE

many successive channels were originated out of which only two channels appeared to be the exit channels with a low energy barrier. Moreover, these two channels ended in the same surprising products, naphthalene þ C2H2, demonstrating their formations via a novel route (reaction 10). Until now, only reactions 3 and 4 are known for the formation of naphthalene, while reactions 11a and 11b6264 are known for the production of acetylene from benzene and phenyl.

C6 H6 ¼ C4 H4 þ C2 H2

ð11aÞ

C6 H5 ¼ C4 H3 þ C2 H2

ð11bÞ

Briefly, naphthalene and acetylene are produced by passing through an intermediate species, 1,4-dihydro-1,4-ethenonaphthalene, as well as via other intermediate species, (5Z,7Z,9Z)-benzo[8]annulene and 1,2-dihydro-1,2-ethenonaphthalene (Figure 4a). Another remarkable point is the high energy potential barrier (82 kJ/mol) for the formation of biphenylene relative to former channels, and thus, its minor contribution in the mass peak m/z = 152 can be expected. The details of transition states TS0TS13 can be seen in the Supporting Information (Figure S2). Figure 4b reflects that biphenyl, the most preferable product, may undergo isomerization with energies less than the dissociation energy of CC bond which link two benzene rings and/or CH bonds of biphenyl. As a result of isomerization, six novel products were produced from the biphenyl. Briefly, biphenyl can be changed to phenylfulvene (cyclopenta-2,4-dien-1-ylidenemethylbenzene), which further isomerizes to dihydrocyclopenta[a]indene. Dihydrocyclopenta[a]indene itself has 16 closedshell isomers produced as a result of the active roles of two hydrogen atoms that rapidly interchange their positions through 1,5-hydrogen shifts reactions. Dihydrocyclopenta[a]indene initiates three different reaction channels out of which one continues the chain of isomerization by converting it into spiro[Hpentalene-cyclopentadiene]s (having six isomers), while the other two channels initiate fission reactions that ended with the production of cyclopenta[a]indene and acenaphthylene (for details, see Supporting Information, Figure S2 and S3). Following the sequence, spiro[H-pentalene-cyclopentadiene]s further isomerizes to dihydro-as-indacenes (having 36 isomers). On the same pattern, dihydro-as-indacenes initiates five reaction channels out of which one involves its isomerization into dihydro-sindacenes (having 13 isomers). Interestingly, the other four reaction channels followed fission reactions that produce asindacene þ H2, acenaphthylene þ H2, naphthalene þ C2H2, and biphenylene þ H2, respectively. Although many pathways connect to these two wells at dihydrocyclopenta[a]indene and dihydro-as-indacenes, the details of some specific channels that involve the lowest potential barrier have been presented in the Supporting Information: in particular, Figure S3 shows the preferred routes for the production of cyclopenta[a]indene, Figure S4 shows the preferred production of acenaphthylene, Figure S5 shows pathways producing naphthalene þ C2H2 and as-indacene þ H2, and Figure S6 shows reaction routes producing s-indacene þ H2. The production of these six new products—cyclopenta[a]indene, acenaphthylene, as-indacenes,

s-indacenes, biphenylene, and naphthalene—can be shown schematically as in reaction 12.

To explore the unknown features of phenyl þ benzene (C6H5 þ C6H6) reactions 1, detailed computations were performed, and the resulting PESs are shown in Figure 5a,b. It can be clearly seen in Figure 5a that biphenyl formation proceeds through the formation of a thermally unstable intermediate, C12H11 (phenyl cyclohexadienyl radical), by an addition/elimination mechanism. Similarly, decomposition of the excited intermediate cyclohexadienyl adduct65 to produce biphenyl has been proposed in other studies.4448 Of course, energetically, the most preferable product is biphenyl, but as discussed before in reactions 79, because of the presence of hydrogen atoms (produced initially from the self-thermal decomposition of benzene and other products) and other active radicals in the reaction mixture, further hydrogen abstraction reaction is most likely to produce biphenyl radicals which subsequently progresses through many intermediate steps. It is worth mentioning here that the energies of m- and p-biphenyl radicals are almost same to that of o-biphenyl radical. Similarly, at the employed level of theory, enthalpies of m- and p-biphenyl radicals relative to that of o-biphenyl radical are 1 and þ1 kJ/mol, respectively. Thus, considering the reaction complexities and computational time, only o-biphenyl radical has been considered in this study. o-Biphenyl radicals, further produce three different stable products—biphenylene, cyclopenta[a]indene, and acenapthylene (Figure5b)—by passing through many intermediate steps. Among them, cyclopenta[a]indene and acenaphthylene are energetically preferred over biphenylene; however, the energy barrier difference in these channels is only 30 kJ/mol. Since the reaction channel that can produce biphenylene involves a large number of densities of states that allows a greater contribution of vibrational entropy, biphenylene might be the dominant product at high temperatures (>1500 K). The details of reaction channels involved in the conversion of o-biphenyl radical into acenaphthylene are presented in the Supporting Information (Figure S7). The final result of figure 5b can be presented as in reaction 13.

Among these conversion routes of o-biphenyl radicals into three different products (Figure5b), the production of biphenylene þ H involves the fewest number of intermediate steps. Thus, the rate constant k14 for the specific reaction 14 that involves the cyclization of o-biphenyl radicals into biphenylene via H elimination was calculated at the high pressure limit using the conventional 5290



ARTICLE

Figure 5. (a) Potential energy diagram of phenyl þ benzene reactions showing the production of o-biphenyl radical via the production of a thermally unstable intermediate C12H11 (phenyl cyclohexadienyl radical) by an addition/elimination mechanism. (b) Schematic potential energy diagram of the isomerization and decomposition of o-biphenyl radical energies are given relative to phenyl þ benzene H2 in kJmol1. Monohydrocyclopenta[a]indenes and monohydro-as-indacenes are drawn in a single well, respectively. See the Supporting Information (Figure S7) for the details of channels A, B, C, and D.

transition state theory. These calculations were solely based on the information of the o-biphenyl radicals (indicated by “13” in Figure5b) and the transition state of this CH fission followed by a cyclization reaction (indicated by “222” in Figure 5b).

Similarly, the high-pressure limiting rate constant k15 for the isomerization of o-biphenyl radicals to hydrocyclopenta[a]indenes was also calculated based on biphenyl and the transition state of isomerization (indicated by “192” in Figure5b). k15 is assumed to be representative of rate constants for the reaction pathways to cyclopenta[a]indene þ H, as-indacene þ H, and acenaphthylene þ H.

Temperature dependencies of rate constants k14 and k15 are shown in Figure 6. Due to the potential barrier difference (ca. 30 kJ/mol), k15 is much larger than k14 at lower temperatures. However, k14 dominates at high temperatures above 1900 K due to large vibrational entropy contributions. Thus, the production of biphenylene from o-biphenyl radicals might be an effective reaction pathway, particularly in the higher temperature range of the present study. Finally, the whole discussion supports case B, the contribution of many isomers of acenaphthylene in the mass peak at m/z = 152 (C12H8). At this point, it is not difficult to understand the cause of the unexpected enhancement in the intensity of mass peak at m/z = 152 (that appeared to be contributed by acenaphthylene and its isomers), compared to its closest HACA products; naphthalene (m/z = 128) and pyracylene (m/z = 176). Regarding the significance of this study, it has identified several new products of reactions 1 and 2. Namely, these novel products are acenaphthylene, cyclopenta[a]indene, as-indacene, 5291



ARTICLE

(1) Novel products of phenyl/benzene reactions, C6H5 þ C6H6/C6H5, have been explored, and they are acenaphthylene, cyclopenta[a]indene, as-indacene, s-indacene, biphenylene, and naphthalene. (2) The unexpected enhancement in the mass peak at m/z = 152 (C12H8) was found to be caused by the contribution of many isomers of acenaphthylene produced from phenyl/benzene reactions 1 and 2. (3) Alternative paths for the formation of acenaphthylene and naphthalene via C6H5 þ C6H6/C6H5 reactions in addition to their production by the HACA have been proposed. (4) Acenaphthylene was found to be formed as a fission product during progressive isomerization of biphenyl in C6H5 þ C6H5 reactions while as an abstraction/cyclization product in C6H5 þ C6H6 reactions. (5) Acetylene was found to be produced from C6H5 þ C6H6/ C6H5 reactions in addition to its production from the direct decomposition of benzene/phenyl.

’ ASSOCIATED CONTENT

bS

Figure 6. Temperature dependencies of rate constants k14 and k15 estimated at the high-pressure limit using conventional transition state theory. k14 stands for the cyclization of o-biphenyl radicals into biphenylene via H elimination, and k15 represents the conversion of obiphenyl radicals into hydrocyclopenta[a]indene; details of conversion phenomena can be seen in Figure 5b.

s-indacene, and naphthalene. Moreover, it has explored the alternative paths for the formation of all the above-mentioned products. Additionally, it has answered a long-standing question in the field of combustion chemistry: why is acenaphthylene produced as the most abundant product among the cyclopentafused PAHs (CPPAHs) under most conditions of combustion6668 and pyrolysis?69,70 The most reliable answer here is, besides the usual HACA route, acenaphthylene is additionally produced from C6H5 þ C6H6/ C6H5 reactions, and its precursor benzene/phenyl is produced abundantly in all conditions of combustion and pyrolysis. This paper has explored an alternative path of acenaphthylene production to its main route of HACA. It is expected that the calculations reported here will be useful for kinetic modeling of combustion and pyrolysis of hydrocarbons under different conditions. As acenaphthylene plays key roles in the formation chemistry of soot71,72 and fullerenes73 in flames, because of its unique capabilities furnished by cyclopentafused structure that allows isomerization involving intramolecular rearrangement74,75 over other kinds of PAHs, this investigation will greatly help in understanding the growth mechanism of soot and fullerenes. Additionally, this study has paved the way for exploring the efficient alternative paths for other HACA routes that seem to be inefficient. Finally, the authors suggest direct experimental studies on nitrosobenzene and o-nitrosobiphenyl and kinetic modeling by including the reactions reported here together with reverse reactions to explore this issue in more detail in future research.

’ CONCLUSIONS The above-discussed in situ mass spectrometric study of pyrolysis of hydrocarbons and related quantum chemical calculations can be summarized as follows:

Supporting Information. Temperature profiles of the pyrolysis zone of the reaction tube; geometries of transition states TS0TS13; details of the reaction channels involved in the isomerization of biphenyl into cyclopenta[a]indene þ H2, conversion of dihydrocyclopenta[a]indenes, dihydro-as-indacenes, and spiro[H-pentalene-cyclopentadiene]s into acenaphthylene þ H2; conversion of dihydro-as-indacenes into naphthalene þ acetylene and as-indacene þ H2; conversion of dihydro-as-indacenes into dihydro-s-indacenes followed by dissociation to s-indacene þ H2; and the conversion of o-biphenyl radical to acenaphthylene þ H. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected]. Tel: (þ1) 213-740-4332. Fax: (þ1)213-740-7774).

’ REFERENCES (1) Graham, S. C.; Homer, J. B.; Rosenfeld, J. L. J. Proc. R. Soc. London, Ser. A 1975, 344, 259–285. (2) Bohm, H.; Jander, H.; Tanke, D. Proc. Combust. Inst. 1998, 27, 1605–1612. (3) Frenklach, M.; Clary, D. W.; Gardiner, W. C.; Stein, S. E. Proc. Combust. Inst. 1986, 21, 1067–1076. (4) Richter, H.; Benish, T. G.; Mazyar, O. A.; Green, W. H.; Howard, J. B. Proc. Combust. Inst. 2000, 28, 2609–2618. (5) Petcu, S.; Cauchetier, M.; Armand, X.; Voicu, I.; Alexandrescu, R. Combust. Flame 2000, 122, 500–507. (6) Howard, J. B.; McKinnon, J. T.; Johnson, M. E.; Makarovsky, Y.; Lafleur, A. L. J. Phys. Chem. 1992, 96, 6657–6662. (7) Hebgen, P.; Goel, A.; Howard, J. B.; Rainey, L.; Vander Sande, J. B. Proc. Combust. Inst. 2000, 28, 1397–1404. (8) Ritschel, M.; Taschner, C.; Selbmann, D.; Leonhardt, A.; Graff, A.; Fink, J. Proc. Am. Inst. Phys. 2000, 544, 423–426. (9) Cheng, H. M.; Li, F.; Su, G.; Pan, H. Y.; He, L. L.; Sun, X.; Dresselhaus, M. S. Appl. Phys. Lett. 1998, 72, 3282–3284. (10) Tian, Y.; Hu, Z.; Yang, Y.; Wang, X.; Chen, X.; Xu, H.; Wu, Q.; Ji, W.; Chen, Y. J. Am. Chem. Soc. 2004, 126, 1180–1183. (11) Knorre, V. G.; Tanke, D.; Thienel, Th.; Wagner, H. Gg. Proc. Combust. Inst. 1996, 26, 2303–2310. 5292


The Journal of Physical Chemistry A (12) Bauer, S. H.; Aten, C. F. J. Phys. Chem. 1963, 39, 1253–1260. (13) Maryasin, L.; Nubutovsky, Z. A. Kinet. Catal. 1969, 10, 983–987. (14) Richardson, W. B.; Lin, S. H.; Evans, D. L. J. Chem. Soc. Faraday Trans., 2 1982, 78, 1–15. (15) Sivaramakrishnan, R.; Brenzinsky, K.; Vasudevan, H.; Tranter, R. S. Combust. Sci. Technol. 2006, 178, 285–305. (16) Bohm, H.; Jander, H. Phys. Chem. Chem. Phys. 1999, 1, 3775–3781. (17) Kern, R. D.; Singh, H. J.; Esslinger, M. A.; Winkeler, P. W. Proc. Combust. Inst. 1982, 19, 1351–1358. (18) Singh, H. J.; Kern, R. D. Combust. Flame 1983, 54, 49–59. (19) Vaughn, S. N.; Lester, T. W.; Merklin, J. F. Proceedomgs of the 13th International Symposium on Shock Tubes and Waves; Niagara Falls, NY, 1981; pp 860865. (20) Asaba, T.; Fujii, N. Proc. Combust. Inst. 1972, 13, 155–164. (21) Banerjee, D. K.; Matei, V.; Vantu, V. Rev. Roum. Chim. 1982, 27, 621–628. (22) Kiefer, J. H.; Mizerka, L. J.; Patel, M. R.; Wei, H.-C. J. Phys. Chem. 1985, 89, 2013–2019. (23) Rao, V. S.; Skinner, G. B. J. Phys. Chem. 1984, 88, 5990–5995. (24) Mead, F. C.; Burk, R. E. Ind. Eng. Chem. 1935, 27, 299–303. (25) Slysh, R. S.; Kinney, C. R. J. Phys. Chem. 1961, 65, 1044–1045. (26) Hou, K. C.; Palmer, H. B. J. Phys. Chem. 1965, 69, 863–868. (27) Smith, R. D.; Johnson, A. L. Combust. Flame 1983, 51, 1–22. (28) Mebel, A. M.; Lin, M. C.; Chakraborty, D.; Park, J.; Lin, S. H.; Lee, Y. T. J. Chem. Phys. 2001, 114, 8421–8435. (29) Ingold, K. U.; Lossing, F. P. Can. J. Chem. 1952, 31, 30–33. (30) Spokes, G. N.; Benson, S. W.; Recent Development in Mass Spectrometry; Ogata, K., Kayakawa, T., Eds.; University Park Press: Baltimore, MD, 1970; p 1146. (31) Laskin, A.; Lifshitz, A. Proc. Combust. Inst. 1996, 26, 669–675. (32) Wang, H.; Laskin, A.; Moriarty, N. W.; Frenklach, M. Proc. Combust. Inst. 2000, 28, 1545–1555. (33) Giri, B. R.; Bentz, T.; Hippler, H.; Olzmann, M. Z. Phys. Chem. 2009, 223, 539–543. (34) Kaiser, R. I.; Vereecken, L.; Peeters, J.; Bettinger, H. F.; Schileyer, P. V. R.; Schaefer, H. F. Astron. Astrophys. 2003, 406 (2), 385–391. (35) Park, J.; Dyakov, I. V.; Lin, M. C. J. Phys. Chem. A 1997, 101, 8839–8843. (36) Shukla, B.; Susa, A.; Miyoshi, A.; Koshi, M. J. Phys. Chem. A 2008, 112, 2362–2369. (37) Shukla, B.; Miyoshi, A.; Koshi, M. J. Combust. Soc. Jpn. 2008, 50 (151), 8–14. (38) Shukla, B.; Koshi, M. Phys. Chem. Chem. Phys. 2010, 12, 2427–2437. (39) Bassanelli, R.; Perez, G.; Possagno, E.; Polcaro, C. J. Chromatogr. 1983, 280, 142–145. (40) Brooks, C. T.; Peacock, S. J.; Reuben, B. G. J. Chem. Soc. Faraday Trans. 1 1979, 75, 652–662. (41) McKinnon, J. T.; Howard, J. B. Proc. Combust. Inst. 1992, 24, 965–971. (42) Fahr, A.; Mallard, W. G.; Stein, S. E. Proc. Combust. Inst. 1988, 21, 825–831. (43) Brioukov, M. G.; Park, J.; Lin, M. C. Int. J. Chem. Kinet. 1999, 31 (8), 577–582. (44) Fahr, A.; Stein, S. E. Proc. Combust. Inst. 1989, 22, 1023–1029. (45) Scaiano, J. C.; Stewart, L. C. J. Am. Chem. Soc. 1983, 105, 3609–3614. (46) Manion, J. A.; Tsang, W.; Proceedings of the Chemical and Physical Processes in Combustion; Fall Technical Meeting, 1996; pp 527529. (47) Louw, R.; Lucas, H. J. Recl. Trav. Chim. 1973, 92, 55–59. (48) Park, J.; Burova, S.; Rodgers, A. S.; Lin, M. C. J. Phys. Chem. 1999, 103, 9036–9041. (49) Shukla, B.; Susa, A.; Miyoshi, A.; Koshi, M. J. Phys. Chem. A 2007, 111, 8308–8324.

ARTICLE

(50) Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173–221. (51) Frenklach, M.; Clary, D. W.; Gardiner, W. C.; Stein, S. E. Proc. Combust. Inst. 1985, 20, 887–901. (52) Frenklach, M.; Wang, H. Proc. Combust. Inst. 1991, 23, 1559–1566. (53) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1067–1073. (54) Frenklach, M.; Taki, S.; Durgaprasad, M. B.; Matula, R. A. Combust. Flame 1983, 54, 81–101. (55) Lifshitz, A.; Tamburu, C.; Dubnikova, F. J. Phys. Chem. A 2009, 113, 10446–10451. (56) Porter, G.; Steinfeld, J. I. J. Chem. Soc. (A) 1968, 877–878. (57) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr. J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (58) Becke, D. J. Chem. Phys. 1993, 98, 5648–5652. (59) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (60) Montgomery, J. A., Jr.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 1999, 110, 2822–8827. (61) Montgomery, J. A., Jr.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 2000, 112, 6532–6542. (62) Kern, R. D.; Wu, C. H.; Skinner, G. B.; Rao, V. S.; Kiefer, J. H.; Mizerka, J. A. Proc. Combust. Inst. 1984, 20, 789–797. (63) Hou, K. C.; Palmer, H. B. J. Phys. Chem. 1965, 69, 863–868. (64) Fujii, N.; Asaba, T. Proc. Combust. Inst. 1973, 14, 433–437. (65) Mebel, A. M.; Lin, M. D.; Yu, T.; Morokuma, K. J. Chem. Phys. 1997, 101, 3189–3196. (66) Marsh, N. D.; Wornat, M. J.; Scott, L. T.; Necula, A.; Lafleur, A. L.; Plummer, E. F. Polycyclic Aromat. Compd. 2000, 13, 379–402. (67) Lafleur, A. L.; Taghizadeh, K.; Howard, J. B.; Anacleto, J. F.; Quilliam, M. A. J. Am. Soc. Mass Spectrom. 1996, 7, 276–286. (68) Lafleur, A. L.; Howard, J. B.; Plummer, E. F.; Taghizadeh, K.; Necula, A.; Scott, L. T.; Swallow, K. C. Polycyclic Aromat. Compd. 1998, 12, 223–237. (69) Wornat, M. J.; Vriesendorp, F. J. J.; Lafleur, A. L.; Plummer, E. F.; Necula, A.; Scott, L. T. Polycyclic Aromat. Compd. 1999, 13, 221–240. (70) Wornat, M. J.; Vernaglia, B. A.; Lafleur, A. L.; Plummer, E. F.; Taghizadeh, K.; Nelson, P. F.; Li, C.-Z.; Necula, A.; Scott, L. T. Proc. Combust. Inst. 1998, 27, 1677–1686. (71) Benish, T. G.; Lafleur, A. L.; Taghizadeh, K.; Howard, J. B. Proc. Combust. Inst. 1996, 26, 2319–2325. (72) (a) McEnally, C. S.; Pfefferle, L. D. Combust. Sci. Technol. 1998, 131, 323–344. (b) Violi, A.; Sarofim, A. F.; Truong, T. N. Combust. Flame 2001, 126, 1506–1515. (73) Lafleur, A. L.; Howard, J. B.; Taghizadeh, K.; Plummer, E. F.; Scott, L. T.; Necula, A.; Swallow, K. C. J. Phys. Chem. 1996, 100, 17421–17428. (74) Scott, L. T.; Roelofs, N. H. J. Am. Chem. Soc. 1987, 109, 5461–5465. (75) Howard, J. B.; Longwell, J. P.; Marr, J. A.; Pope, C. J.; Busby, W. F., Jr.; Lafleur, A. L.; Taghizadeh, K. Combust. Flame 1995, 101, 262–270. 5293


C6H5 Reactions - The Journal of

Recommend Documents