Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (3): 303-311

#### The article information

Yu-jie Zhao, You-sheng Zhan, Li Li, Xin Li, Xiang-yu Lian, Pei Huang, Liu-si Sheng, Jun Chen

Theoretical Investigation on Photoionization and Dissociative Photoionization of Toluene

Chinese Journal of Chemical Physics, 2017, 30(3): 303-311

http://dx.doi.org/10.1063/1674-0068/30/cjcp1703044

### Article history

Accepted on: March 27, 2017
Theoretical Investigation on Photoionization and Dissociative Photoionization of Toluene
Yu-jie Zhaoa,b,c, You-sheng Zhana,b, Li Lia,b, Xin Lia,b, Xiang-yu Liana,b, Pei Huanga,b, Liu-si Shengc, Jun Chenc
Dated: Received on March 20, 2017; Accepted on March 27, 2017
a. Engineering Research Center of Nuclear Technology Application(East China University of Technology), Ministry of Education, Nanchang 330013 China;
b. School of Nuclear Science and Engineering, East China University of Technology, Nanchang 330013, China;
c. National Synchrotron Radiation Laboratory, School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230029, China
*Author to whom correspondence should be addressed. Yu-jie Zhao, E-mail:jackzyj@ustc.edu.cn; Liu-si Sheng, E-mail:lssheng@ustc.edu.cn
Abstract: The photoionization and dissociation photoionization of toluene have been studied using quantum chemistry methods.The geometries and frequencies of the reactants, transition states and products have been performed at B3LYP/6-311++G (d, p) level, and single-point energy calculations for all the stationary points were carried out at DFT calculations of the optimized structures with the G3B3 level.The ionization energies of toluene and the appearance energies for major fragment ions, C7H7+, C6H5+, C5H6+, C5H5+, are determined to be 8.90, 11.15 or 11.03, 12.72, 13.69, 16.28 eV, respectively, which are all in good agreement with published experimental data.With the help of available published experimental data and theoretical results, four dissociative photoionization channels have been proposed:C7H7++H, C6H5++CH3, C5H6++C2H2, C5H5++C2H2+H.Transition structures and intermediates for those isomerization processes are determined in this work.Especially, the structures of C5H6+ and C5H5+ produced by dissociative photoionization of toluene have been defined as chain structure in this work with theoretical calculations.
Key words: Quantum chemical calculations    Toluene    Dissociative photoionization mechanism    Density functional theory    Transition states
Ⅰ. INTRODUCTION

Aromatic hydrocarbons such as benzene, toluene, and xylene are major components of volatile organic compounds (VOCs) in urban areas, with toluene being the most abundant aromatic hydrocarbon among them, which play an important role in the formation of secondary organic aerosols [1-4]. Moreover, toluene has strong stimulation to eyes, skin, mucous membrane and respiratory system. Toluene also takes a part in reactions to promote photochemical smog and other local atmospheric effects, which may contribute significantly to ozone formation in the troposphere [5-9]. Therefore, the understanding of the process for dissociative photoionization of toluene is needed for evaluating the risks involved in aromatic compounds.

The photoionization of toluene has been studied by various experimental methods. By using electron impact techniques, Mcloughlin et al. [10, 11] obtained the ionization energy (IE) of toluene and the appearance energy (AE) of C$_7$H$_7$$^+, which are 8.82 and 10.71 eV, respectively. Lifshitz et al. [12, 13] investigated two dissociative photoionization channels of C_7H_7$$^+$, through time resolved photoionization mass spectrometry (TPIMS) by combining with ab initio calculations, in which the AEs of C$_7$H$_7$$^+ being determined to be 11.1 and 10.7 eV at T=0 and T=298 K, respectively. Shaw et al. [14] used three experimental techniques (photoabsorption, photoelectron and photoion spectroscopy), together with many-body Green's function calculations to investigate the spectroscopic and thermodynamic properties of toluene and obtained the IE of the parent ion to be 8.845 eV and the AEs of major fragment ions C_7H_7$$^+$, C$_6$H$_5$$^+, C_5H_5$$^+$ are measured to be 11.8, 14.4, 16.5 eV, respectively. Especially, the obtained AE of C$_5$H$_6$$^+ was 13.5 eV. However, as far as we know, there is few theoretical investigations on dissociative photoionization of toluene. In particular, there is very limited information about the geometries of the parent ion and the main fragment ions in the literatures up to date. Moreover, the dissociative photoionization mechanisms of toluene are still unclear so far. Therefore, in the present work, DFT method is employed to investigate the dissociative photoionization of toluene. The theoretical IE of toluene, AEs for its fragment ions, transition structures (TS) and intermediates (INT) are determined. The mechanisms of the dissociative photoionization pathways are also discussed on the basis of theoretical and experimental data from literature. In addition, the transition and intermediates states involved in the pathways are also obtained by theoretical calculations and described in detail in this work. Ⅱ. QUANTUM CHEMICAL CALCULATIONS In this theoretical study, all the geometry optimizations of the reactants, transition states, intermediates, and other products are done at the B3LYP level with 6-311++G(d, p) basis sets, and harmonic vibrational frequencies are also computed analytically at the same level in order to characterize the optimized geometries as potential minima or saddle points. The structures of transition states (TS) and intermediates (INT) for dissociative photoionization channels are also identified in this study. To confirm that the obtained transition states connect with the right reactants and products, the intrinsic reaction coordinate (IRC) calculations were performed at B3LYP/6-311++G(d, p) level. On the basis of the obtained stationary points, more accurate energies were then obtained by single-point calculations at the G3B3 level. The method of G3B3 has been reported elsewhere [15], and only a brief summary is given here.(ⅰ) Step 1: produce an initial equilibrium structure at the Hartree-Fock level using the 6-31G(d) basis set. Verify that it is a minimum with a frequency calculation and predict the zero-point energy (ZPE). This quantity is scaled by 0.8929.(ⅱ) Step 2: beginning with the final optimized structure from step 1, obtain the final equilibrium geometry using the full MP2 method with the 6-31G(d) basis set. This geometry is used for all subsequent calculations.(ⅲ) Step 3: a series of single-point energies calculations are carried out at higher levels of theory. The first higher level calculation is MP4/6-31G(d). This energy is then modified by a series of corrections from additional calculations.(ⅳ) Step 4: the MP4/6-31G(d) energy and four corrections from step 3 are combined in an additive manner along with a spin-orbit correction, \Delta E(SO), for atomic species only.(ⅴ) Step 5: a "higher level correction" (HLC) is added to take into account remaining deficiencies in the energy calculations:(vi) Step 6: finally, the total energy at 0 K is obtained adding the zero-point energy, obtained from the frequencies of step 1 to the energy. This energy is referred to as the "G3 energy".All calculations above-mentioned are all performed with the Gaussian 03 program. Ⅲ. RESULTS With the theoretical calculation, the total energies of species involved in the study of dissociative photoionization of toluene are obtained at the G3B3 level, which are listed in Table Ⅰ. Especially, the imaginary frequencies of transition states (TS) are given in Tables S3-S17 (see supplementary materials). Because there are lots of different products, intermediates and transition states in the present work, they are named by using their prefix with a unique number, such as P1, INT1 and TS1, which can make them distinguished easily. In the case of isomers, suffixes of a, b, c, etc. are applied (for example, P5a) in the dissociative photoionization channel of C_5H_5$$^+$.

Table Ⅰ Calculated energies of species (neutral toluene and its cation, products, by-produced fragments, intermediates (INT), transition states (TS)) involved in the photodissociation of toluene at the G3B3 level.
Table 3 Transition state's vibrational frequency of TS1
Table 4 Transition state's vibrational frequency of TS2
Table 5 Transition state's vibrational frequency of TS3
Table 6 Transition state's vibrational frequency of TS4
Table 7 Transition state's vibrational frequency of TS5
Table 8 Transition state's vibrational frequency of TS6
Table 9 Transition state's vibrational frequency of TS7
Table 10 Transition state's vibrational frequency of TS8
Table 11 Transition state's vibrational frequency of TS9
Table 12 Transition state's vibrational frequency of TS10
Table 13 Transition state's vibrational frequency of TS11
Table 14 Transition state's vibrational frequency of TS12
Table 15 Transition state's vibrational frequency of TS13
Table 16 Transition state's vibrational frequency of TS14
Table 17 Transition state's vibrational frequency of TS15

It is well known that the parent ion C$_7$H$_8$$^+ can be generated directly by a single-photon ionization [10, 14]. The present calculation using G3B3 method gives an adiabatic IE of 8.90 eV, which is in reasonable agreement with the available experimental value, 8.82 eV [10] and 8.845 eV [14]. The IE of C_7H_8 is calculated as follows:IE(C_7H_8$$^+$)=$E$(C$_7$H$_8$$^+)-E(C_7H_8)=8.90 eV.In the case of possible dissociation channel C_7H_8$$\rightarrow$C$_7$H$_7$$^+ (P1a)+H, the AE of C_7H_7$$^+$(P1a) is obtained from:AE(C$_7$H$_7$$^+(P1a))=E(C_7H_7$$^+$(P1a))+$E$(H)-$E$(C$_7$H$_8$)= 11.15 eV, which is in good agreement with the experimental value of 11.1$\pm$0.1 eV by Lifshitz et al. [12]. As the dissociation energy ($E_\textrm{d}$) can be calculated by subtracting the IE of parent molecules from the AE of the corresponding fragment ion, the $E_\textrm{d}$ of C$_7$H$_7$$^+(P1a) can be expressed in the following form:  \begin{eqnarray}E_\textrm{d}(\textrm{C}_7\textrm{H}_7^+(\textrm{P1a}))\hspace{-0.15cm}&=&\hspace{-0.15cm}\textrm{AE}(\textrm{C}_7\textrm{H}_7^+(\textrm{P1a}))-\textrm{IE}(\textrm{C}_7\textrm{H}_8^+)\nonumber\\&=&\hspace{-0.15cm}2.25 \hspace{0.2cm}\textrm{eV}\end{eqnarray} (1) In addition, the AEs and E_\textrm{d}s of other products are also obtained in the same way, which are listed in Table Ⅱ. Table Ⅱ Theoretical and literature values of the ionization energy (IE), appearance energy (AE), and dissociation energy (Ed, theoretical) of possible dissociative photoionization channels. The optimized geometries of neutral toluene and its parent cation are obtained at the B3LYP/6-311++G(d, p) level, and all their C-C bond lengths are also calculated, which are all shown in the FIG. 1. From FIG. 1, we can know that some C-C bonds of parent cation become shorter, while some others become longer in comparison with those of neutral molecule, which indicates that dissociation of parent cation will undergo different pathways with the photon energy increase.  FIG. 1 The optimized geometries of the neutral toluene and its cation. (a) C_7H_8, (b) C_7H_8$$^+$. Bond length is in unit of Å.
Ⅳ. DISCUSSION

The fragment ions of toluene in the dissociative photoionization have already been discussed elsewhere [10, 12, 16, 17, 18], wherein the main fragmentation channels have been proposed as follows: C$_7$H$_7$$^++H, C_6H_5$$^+$+CH$_3$, C$_5$H$_6$$^++C_2H_2, C_5H_5$$^+$+C$_2$H$_2$+H. However, the detailed dissociative photoionization mechanisms of the fragmentation pathways have not been clarified. In this work, the dissociative mechanisms of C$_7$H$_8$$^+ are discussed based on our theoretical results and available experimental data [10-30]. These dissociative photoionization channels are shown in FIG. 2-4, respectively. In addition, the detailed information on the geometries of the optimized reactants, transition states, intermediates and products are also shown in FIG. 5-7, where the main bond angles and distances are indicated.  FIG. 2 The dissociation channels for toluene cation to produce fragment ions, C_7H_7$$^+$(P1) calculated at the G3B3 level. The energy of neutral toluene is defined to be zero.
 FIG. 4 The dissociation channels for toluene cation to produce fragment ions C$_6$H$_5$$^+(P3) and C_5H_6$$^+$(P4a), C$_5$H$_6$$^+(P4b) and C_5H_5$$^+$, calculated at the G3B3 level. The energy of neutral toluene is defined to be zero.
 FIG. 5 The geometries of the transition states at the B3LYP/6-311++G(d, p) level. Bond length is in unit of Å and bond angel in unit of (°).
 FIG. 6 The geometries of the reactant intermediates at the B3LYP/6-311++G(d, p) level. Bond length is in unit of Å and bond angel in unit of (°).
 FIG. 7 The geometries of the neutral toluene, its cation and its fragments (ions and neutrals) at the B3LYP/6-311++G(d, p) level. Bond length is in unit of Å and bond angel in unit of (°).

As the photon energy increases, the parent ion will undergo different dissociative photoionization pathways. Generally, there are two types of mechanisms for dissociation: direct simple bond cleavage or indirect bond cleavage via transition states and intermediates. For instance, the C$_6$H$_5$$^+ ion are formed by loss of CH_3 from the parent ion (C_7H_8$$^+$). This is one-step dissociation process with no distinct transition states as reported previously [19]. AE of the C$_6$H$_5$$^+ ion is predicted to be 12.72 eV, which is in good agreement with available theoretical value (12.72 eV) [16]. By comparing with the structure of parent cation and neutral molecule (see FIG. 1), we found that the C3-C12 and C2-C1 bond lengths are increased by about 0.0350 and 0.0257 Å, respectively, while the C2-C3 and C5-C6 bond lengths are decreased by about 0.0395 and 0.0287 Å, respectively. Especially, the C3-C12 bond is the longest in the parent ion. Therefore, excited by higher photon energy, the C3-C12 bond can be cleaved easily and dissociated to form C_6H_5$$^+$+CH$_3$, which agrees with the results in Ref.[14].

A. C7H7++H

The structure of C$_7$H$_7$$^+ ion has been investigated by various methods [20-22]. Two important isomers of C_7H_7$$^+$: benzyl (six-membered ring) and tropylium cation (seven-membered ring), are the most important dissociation products of parent ion (C$_7$H$_8$$^+) near the threshold [12, 18, 23, 24]. Therefore, there are two possible formation pathways which can produce C_7H_7$$^+$+H (shown in FIG. 2 and 3).

 FIG. 3 The dissociation channel for toluene cation to produce fragment ion, C$_7$H$_7$$^+(P2) calculated at the G3B3 level. The energy of neutral toluene is defined to be zero. The benzyl ion is formed by direct loss of H from the parent ion. It is obvious that there are four types of hydrogen atoms in the parent ion. We calculated the AEs of four possible fragment ion isomers, and the outcomes are shown in Table Ⅰ and FIG. 2. P1a ((C_7H_7$$^+$) in Table Ⅰ) is formed from the cleavage of C12-H15 bond, which is 1.1064 Å in length. The other three isomers are the H elimination from C1 for P1b, C2 for P1c and C6 for P1d. These C-H bond lengths are 1.0831, 1.0838 and 1.0847 Å correspondingly. And the theoretical AEs for P1a, P1b, P1c and P1d are 11.15, 12.89, 12.95, and 13.04 eV, respectively. It is not surprising to find that the P1a channel is the lowest energy required. There are also possible TS for the other three channels, but their AEs are apparently much higher than the experimental value (11.1$\pm$0.1 eV) [12]. Owing to the former theoretical result is in good accordance with the experimental value, 11.1$\pm$0.1 eV [12], we tend to consider the H eliminates from the C12. The geometry of the fragment ion P1a at the B3LYP/6-311++G(d, p) level is shown in FIG. 7.

The process for Tropylium cation (seven-membered ring, P2) is somewhat complicated, which need to be carried out via TS and INT, as shown in FIG. 3. Firstly, the hydrogen atom H15 of the methyl group is transferred to the C3 of the benzene ring to form INT1 via TS1. Then, because of the steric effect [29] from the ortho C4 of benzene ring, C12-C4 bond reconstructs to form INT2. Thirdly, INT3 is formed through C3-C2 bond cleavage via TS3. Finally, the INT3 tends to produce P2 by further loss of the H radical via TS4. Calculated AE of this channel is 11.03 eV, which is also in agreement with available experimental value (11.1$\pm$0.1 eV) [12].

B. C5H6++C2H2

Further dissociation of C$_7$H$_8$$^+ can produce C_5H_6$$^+$ and C$_2$H$_2$ when the photon energy rises [14, 25]. Flammang and Meyrant et al. [25] have studied the structures of gas phase C$_5$H$_6$$^+ ions, which are generated by direct electron ionization of C_5H_6 isomers or dissociative ionization of other precursor molecules (phenol, thiophenol and so on). They have concluded the geometry of C_5H_6$$^+$ ion can be identified as the ring structure (P4a), which are from phenol and thiophenol.

Similarly, the geometry of C$_5$H$_6$$^+ ion from C_7H_8$$^+$ can also be firstly proposed as the ring structure (P4a), which is shown in FIG. 7. The detailed formation pathway of C$_5$H$_6$$^+ with potential energy is depicted in FIG. 4 (a) and (b). For the ring structure of C_5H_6$$^+$(P4a), the decomposition reaction is initiated by methyl H-migration to an ortho carbon, followed by the decomposition to C$_5$H$_6$$^+ and C_2H_2 from C_7H_8$$^+$. To make it more feasible, the dissociation mechanism is described as follows. (ⅰ) First, a TS7 is proposed and the H15 transfers from C12 to C4 with an energy barrier of 1.52 eV, giving rise to INT6. (ⅱ) Second, one H atom on the C4 transfers to C5 via TS8 to produce INT7. (ⅲ) Third, Because of ortho effect, INT8 can be formed via TS9 with an energy barrier of 3.14 eV. Then, one hydrogen atom on C12 is transferred to C3, forming INT9 before C$_2$H$_2$ elimination. (ⅳ) Finally, the C$_5$H$_6$$^+ is formed by C3-C4 bond cleavage. The theoretical AE in this dissociation pathway is 12.32 eV, which is much lower than the experimental value of 13.50 eV [14]. On the other hand, Li et al. [26] has performed experimental and theoretical studies on the dissociative photoionization of trans-2-methyl-2-butenal. In their study, the structure of C_5H_6$$^+$ ion can be identified as chain structure. According to the above situation, the geometry of C$_5$H$_6$$^+ ion from C_7H_8$$^+$ is tentatively identified as the chain structure (P4b) in the present work, which is also shown in FIG. 7. For the chain structure of C$_5$H$_6$$^+ (P4b), the calculated AE of C_5H_6$$^+$ is determined to be 13.69 eV, which is in agreement with the previous value 13.50 eV [14]. FIG. 4(b) shows that the parent cation undergoes a hydrogen atom shift to C6 from C5 to yield the INT12 via TS13. Next, the hydrogen atom migrates from the C4 to its neighboring C3 through TS14 to produce INT13 by overcoming an energy barrier of 3.17 eV. Then, INT14 is formed through C3-C2 bond cleavage via TS15. Finally, C$_5$H$_6$$^+(P4b) is yielded with loss of C_2H_2 from INT14, and no barrier for this elimination reaction is found at the B3LYP/6-311++G(d, p) level. The reaction barrier for the formation of C_5H_6$$^+$ (P4b) is 4.79 eV (relative to parent ion), which is in agreement with the experimental value, 13.50 eV, obtained by Shaw et al. [14].

C. C5H5++C2H2+H

It is well known that the C$_7$H$_7$$^+(P1a) ion produced from parent ion decomposes into C_5H_5$$^+$ ion by losing C$_2$H$_2$ [27]. However, the structure for C$_5$H$_5$$^+ is controversial. On the one hand, Occolowitz and White have concluded the geometry of C_5H_5$$^+$ ion was identified as the chain structure rather than the ring structure by measuring the heat of formation [28]. On the other hand, in the studies on the dissociative photoionization of $p$-nitrotoluene by Zhang et al. in 2012, the structure of C$_5$H$_5$$^+ ion was identified as ring structure [29]. In this work, both structures are calculated, and we get the TSs and INTs involved in the process using DFT theory. Detailed pathway is described in FIG. 4(c). For the ring structure of C_5H_5$$^+$(P5a), first, in the benzyl ion (P1a), C2-C4 bond reconstructs via TS5 to form INT4. Next, INT4 undergoes a hydrogen atom shift to C12 from C3 to yield INT5 via TS5. The barrier of this step is calculated to be 1.48 eV. Finally, C$_5$H$_5$$^+(P5a) is produced by the fission of C3-C4 bond without any apparent TS. AE of C_5H_5$$^+$(P5a) obtained from our theoretical (15.47 eV) is much lower than the previous value (16.4$\pm$0.2 eV) reported by Tajima et al. [30].

For the chain structure of C$_5$H$_5$$^+(P5b), the benzyl ion (P1a) undergoes a hydrogen atom shift to C4 from C1 to yield INT10 via TS11. The barrier of this step is calculated to be 5.13 eV. The energies of TS11 and INT10 are higher than that of parent ion by 7.38 and 5.16 eV, respectively. The breaking and forming C-H bond lengths at TS11 are 1.3686 and 1.2801 Å, respectively. Next process is from INT10 to INT11, in which the hydrogen atom migrates from C12 to its neighboring C3 through TS12 by overcoming an energy barrier of 1.77 eV. Finally, P5b is generated by the C3-C4 bond broken in INT11, coupled with a C_2H_2 loss. The overall barrier for the formation of C_5H_5$$^+$(P5b) is 16.28 eV (relative to neutral toluene), which is in good agreement with the experimental value (16.4$\pm$0.2 eV) by Tajima et al. [30].

Ⅴ. CONCLUSION

In this work, quantum chemistry methods have been used to study the photoionization and dissociative photoionization of toluene. The present theoretical results provide several new insights into the dissociative photoionization mechanisms of toluene. The energies and possible dissociative channels for fragment ions from toluene have been estimated on the basis of the quantum chemical calculations. Specific information of fragmentation pathways are discussed in detail. Generally speaking, the dissociative photoionization processes of toluene are somewhat complicated, many of them undergo different dissociative photoionization pathways, such as transition structures, intermediates, H-migration and/or H-elimination, except for the channel (C$_6$H$_5$$^++CH_3), which undergoes direct simple bond cleavage. And some dissociative products have different isomers, which are all distinguished in the present work. In particular, according to calculation and comparison, the C_5H_5$$^+$ and C$_5$H$_6$$^+$ can be identified as the chain structure in the dissociative photoionization of toluene. The mechanistic study of dissociative photoionization of toluene will be helpful in understanding the fragmentation.

Supplementary materials: The imaginary frequencies of transition states pertinent to this work are given in Tables S3-S17.

Ⅵ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.11275006, No.U1232209, No.U1232130, No.41275127, No.11575178, No.U1532137), Nuclear Technology Application Engineering Research Center Open Foundation of Ministry of Education (No.HJSJYB2015-6), the Chinese Scholarship Council (No.201608360053), the Graduate Students High-Quality Course Construction Program of Jiangxi Province (No.JXYYK2016-12), the China Postdoctoral Science Foundation (No.2013M531530), the Doctoral Foundation of East China University of Technology (No.DHBK201401) and the Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (No.KJ2012B086).

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a. 东华理工大学, 教育部核技术应用工程研究中心, 南昌 330013;
b. 东华理工大学核科学与工程学院, 南昌 330013;
c. 中国科学技术大学核科学技术学院, 国家同步辐射实验室, 合肥 230029