Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 619-625

The article information

Zhao-hui Li, Ye-peng Yu, Xuan Lin, Jun Chen, Hang Zhang, Yan-bo Li, Huan-huan Wang, Qing-hui Meng, Rui-rui Sun, Xiao-bin Shan, Fu-yi Liu, Liu-si Sheng

Experimental and Theoretical Study on Dissociative Photoionization of Cyclopentanone

Chinese Journal of Chemical Physics, 2018, 31(5): 619-625

http://dx.doi.org/10.1063/1674-0068/31/cjcp1804084

Article history

Accepted on: May 18, 2018
Experimental and Theoretical Study on Dissociative Photoionization of Cyclopentanone
Zhao-hui Li, Ye-peng Yu, Xuan Lin, Jun Chen, Hang Zhang, Yan-bo Li, Huan-huan Wang, Qing-hui Meng, Rui-rui Sun, Xiao-bin Shan, Fu-yi Liu, Liu-si Sheng
Dated: Received on April 28, 2018; Accepted on May 18, 2018
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
*Author to whom correspondence should be addressed. Fu-yi Liu, E-mail: fyliu@ustc.edu.cn
Abstract: The dissociative photoionization of cyclopentanone was investigated by means of a reflectron time-of-flight mass spectrometer (RTOF-MS) with tunable vacuum ultraviolet synchrotron radiation in the photon energy range of 9.0-15.5 eV. The photoionization efficiency (PIE) curves for molecular ion and fragment ions were measured. The ionization energy of cyclopentanone was determined to be 9.23$\pm$0.03 eV. Fragment ions from the dissociative photoionization of cyclopentanone were identified as C$_5$H$_7$O$^+$, C$_4$H$_5$O$^+$, C$_4$H$_8^+$/C$_3$H$_4$O$^+$, C$_3$H$_3$O$^+$, C$_4$H$_6^+$, C$_2$H$_4$O$^+$, C$_3$H$_6^+$, C$_3$H$_5^+$, C$_3$H$_4^+$, C$_3$H$_3^+$, C$_2$H$_5^+$ and C$_2$H$_4^+$. With the aid of the ab initio calculations at the $\omega$B97X-D/6-31+G(d, p) level of theory, the dissociative mechanisms of C$_5$H$_8$O$^+$ are proposed. Ring opening and hydrogen migrations are the predominant processes in most of the fragmentation pathways of cyclopentanone.
Key words: Cyclopentanone    Synchrotron radiation    Photoionization and dissociation    ab initio calculations
Ⅰ. INTRODUCTION

The production of excellent alternative clean fuels from raw biomass, which is generally known as lignocelluosic bio-refinery, has attracted more and more attention in view of the urgent demand of sustainable and clean fuels [1-3]. Cyclopentanone, a lignocellulosic platform compound, is one of the selective hydrogenation products of furfural and is regaining attention as a building block for the synthesis of high-density renewable fuels [4-6]. Hence, a better understanding of the energetics for cyclopentanone is clearly desirable. In this work, we report a quantitative study on the photoionization and dissociative photoionization of cyclopentanone.

Wang et al. [7] studied the dissociation dynamics of cyclopentanone in intense 788 nm, 90 fs pulses of radiation using a time-of-flight (TOF) ion mass spectrometer. The ionization rate constants and branching ratios were investigated according to quantum chemical computations. Wu et al. [8] performed a combined experimental and theoretical study on the photoionization/dissociation of cyclopentanone, and proposed several possible reaction channels. Price and co-workers [9] measured the absolute photoionization cross sections of cyclopentanone via a multiplexed photoionization mass spectrometer (PIMS) equipped with synchrotron radiation source in the energy range of 8-11 eV. More recently, Pastoors et al. [10] theoretically and experimentally investigated the photoionization and the thermal decomposition mechanisms of cyclopentanone using an imaging photoelectron photoion coincidence spectroscopy (iPEPICO) apparatus with VUV synchrotron radiation source which was tuned from 8.0 eV to 11.7 eV. For the dissociative photoionization, the fragmentation of the molecule ions is dominated by loss of CO, C$_2$H$_4$, and C$_2$H$_5$ to form C$_4$H$_8^+$, C$_3$H$_4$O$^+$, and C$_3$H$_3$O$^+$, respectively. They calculated possible structures of three main dissociative fragments along with their respective appearance energies and a model for the possible fragmentation mechanism was constructed.

As mentioned above, despite that considerable experimental and theoretical work was performed on neutral and cationic cyclopentanone, the detailed mechanisms for the formation of fragment ions are still not well understood. In the present study, we utilized tunable VUV photoionization TOF mass spectrometry combined with ab initio molecular orbital calculations to investigate the possible reaction mechanism in the dissociative photoionization of cyclopentanone. The ionization energy (IE) of cyclopentanone and the appearance energies (AEs) for major fragment ions were obtained by measuring their photoionization efficiency curves. Additionally, the possible mechanisms of the dissociation pathways are discussed with the aid of ab initio calculations.

Ⅱ. EXPERIMENTS AND CALCULATED METHODS

Experiments were performed using the Atomic and Molecular Physics Beamline (U14A) of National Synchrotron Radiation Laboratory in Hefei, China. Photoionization mass spectra and PIE curves of cyclopentanone for fragment ions were obtained in the energy range of 9.0-15.5 eV. Only a brief summary of the experimental apparatus is given here, the more details of this apparatus have been described elsewhere [11-13]. Synchrotron radiation generated by an undulator at U14 beamline from 800 MeV electron storage ring at NSRL, and a high-resolution spherical-grating monochromator were employed to select the VUV light. The grating was installed in the chamber, covering the energy ranges from 7.5 eV to 22.5 eV with the energies resolving power ($E$/$\Delta E$) about 1000. A Si photodiode was used for measuring the photo flux of synchrotron VUV. The average photo flux was measured to be 5$\times$10$^{13}$ photons per second at the ionization region. Argon (IE=15.759 eV) as the filter gas was utilized for eliminating the higher harmonic produced by the undulator.

Cyclopentanone sample was purchased from Alfa Aesar ($\geq$99% purity), and used directly without further treatment. Cyclopentanone was contained in a stainless evaporator, which is connected to the molecule expansion chamber by a 6 mm diameter stainless steel pipeline. We chose He (purity 99.99%) as the carrier gas and the stagnation pressure was about 0.15 MPa. After the sample was introduced into the beam source chamber, the gaseous cyclopentanone molecules were introduced into the ionization chamber by supersonic expansion through a 70 μm diameter nozzle and one skimmer with diameter of 1.5 mm. Then the cold skimmed molecular beam was injected into the ionized region to perpendicularly intersect the monochromatic VUV radiation. Subsequently, the produced ions were mass-analyzed using a homemade RTOF-MS.

In this study, the high-accuracy ab initio method was utilized for obtaining the most stable configuration of the cyclopentanone and its fragments. Previous studies have indicated that the $\omega$B97X-D [14] functional can significantly reduce self-interaction errors and has been widely used to provide reliable results. The coupled cluster theory with single and double excitations and perturbative estimate of triple excitations CCSD(T) [15] can obtain more accurate single-point energy. Therefore, geometry optimizations of the cyclopentanone, parent cations, transition states (TS), intermediate (INT) ions and fragments ions, were carried out with the $\omega$B97X-D theoretical functional using the 6-31G(d, p) [16] basis set and the reliable sing-point energies were obtained at the CCSD(T) level using the cc-pVTZ basis set. The unscaled $\omega$B97X-D zero-point vibrational energy (ZPVE) was used to correct all CCSD(T) energies. In order to further validate the transition states connecting the desired reactants and products, internal reaction coordinate (IRC) [17] calculations were carried out at the same level with geometry optimization. All these computational calculations were performed using the Gaussian 09 suite of programs on the Supercomputing Center of University of Science and Technology of China [18]. The adiabatic ionization energy (AIE) of cyclopentanone is defined as IE(C$_5$H$_8$O)=$E_0$(C$_5$H$_8$O$^+$)-$E_0$(C$_5$H$_8$O), where $E_0$(C$_5$H$_8$O$^+$) refers the total electronic energy of the C$_5$H$_8$O$^+$ and $E_0$(C$_5$H$_8$O) is the total electronic energy of the C$_5$H$_8$O.

Ⅲ. RESULTS AND DISCUSSION A. VUV photoionization mass spectra

In this work, the photoionization mass spectra were collected continuously by changing the photon energy between 9.0 and 15.5 eV at 298 K. FIG. 1 depicts the typical photoionization mass spectra of cyclopentanone at 15.5, 13.0, and 9.5 eV, respectively. At the low photon energy of 9.5 eV, only the molecular ion C$_5$H$_8$O$^+$ ($m/z$=86) is observed. With photon energy increasing to 13.0 eV, two strong fragments, namely C$_4$H$_8^+$ ($m/z$=56) by CO-loss or C$_3$H$_4$O$^+$ ($m/z$=56) by C$_2$H$_4^-$loss, and C$_3$H$_3$O$^+$ ($m/z$=55) by C$_2$H$_5^-$ loss, were yielded. In addition, more fragment ions at $m/z$ 28, 40, 41, 42, 43, 44, 69, 83 are detected. At the photon energy of 15.5 eV, ionic fragments at $m/z$ 28, 41, 42, 55, 56 have become stronger. Meanwhile, two weak fragments, namely C$_2$H$_5^+$ ($m/z$=29) and C$_3$H$_3^+$ ($m/z$=33) are also observed. All observed fragments are considered to be originated from dissociation of parent ion since no signal at mass greater than that of C$_5$H$_8$O$^+$ ($m/z$=84) is detected. The ion peak of $m/z$=55 is the strongest one in the dissociative photoionization of cyclopentanone, which indicates that C$_5$H$_8^+$ ion is the dominated channel of cyclopentanone ion.

 FIG. 1 Photoionization mass spectra of cyclopentanone at the photon energy of 9.5, 13.0, 15.5 eV.

The photoionization efficiency curves of cyclopentanone cation and its fragment ions C$_5$H$_7$O$^+$, C$_4$H$_5$O$^+$, C$_4$H$_8^+$/C$_3$H$_4$O$^+$, C$_3$H$_3$O$^+$, C$_4$H$_6^+$, C$_3$H$_4$O$^+$, C$_3$H$_3$O$^+$, C$_3$H$_6^+$, C$_3$H$_5^+$, C$_3$H$_4^+$, C$_3$H$_3^+$, C$_2$H$_5^+$, C$_2$H$_4^+$ were obtained by integrating the area of each mass spectral peak at each photon energy. FIG. 2 is the PIE curves of cyclopentanone and its main fragment ions. The appearance energies of all ions were determined from the PIE curves, detailed methods on analyzing the PIE curve have been reported elsewhere previously [19, 20]. Table Ⅰ presents the AEs of all ions and summarizes the calculated energies of related species, as well as possible formation pathways for the dissociation processes. For the parent ion, the measured IE value is (9.23$±$0.03) eV, which is in good agreement with previously reported data of (9.28±0.03) eV [8], (9.30$±$0.05) eV [9], (9.28$±$0.01) eV [21] and (9.25$±$0.02) eV [22]. Cyclopentanone has a nonplanar five membered ring conformation with C$_2$ symmetry. FIG. 3 shows the calculation structures for the ground state neutral and ionized molecules, and it is found that the C1-O distance is shortened from 1.210 Å to 1.189 Å after photoionization. Meanwhile, both the lengths of the $\alpha$-C-C bonds (C1-C2 and C1-C5) connected to the carbonyl group change from 1.524 Å to 1.552 Å. This suggests that the initial ionization result is an electron removing from the $\sigma$ network of the five membered ring. The results are consistent with a previous study by Price et al. [9] who proposed that the initial ionization is caused by removal of an electron from a bonding orbital localized on the $\alpha$ and $\alpha'$ carbons' sigma bonds and antibonding C-O $\pi$ orbital.

 FIG. 2 The PIE curves of parent ion C$_5$H$_8$O$^+$ (a) and the main fragments C$_4$H$_8^+$ (b), C$_3$H$_3$O$^+$ (c), C$_3$H$_6^+$ (d), C$_3$H$_5^+$ (e), and C$_2$H$_4^+$ (f).
Table Ⅰ Experimental and calculated ionization energies (IEs) of cyclopentanone and appearance energies (AEs) of the major fragments.
 FIG. 3 The optimized ground state structures of (a) neutral and (b) ionic cyclopentanone at the $\omega$B97X-D/6-31G(d, p) level.
B. Dissociation mechanisms

With the increasing of photon energy, the parent ion will undergo a series of dissociative reactions to generate fragments. Detailed dissociation pathways of the cyclopentanone cation are established with the aid of calculations at the $\omega$B97X-D/6-31+G(d, p) level. The fragmentation pathways, the relative energies and structures of each species are shown in FIGs. 4-7.

 FIG. 4 The formation pathways for C$_5$H$_7$O$^+$ (P1), C$_4$H$_5$O$^+$ (P2) and C$_3$H$_3$O$^+$ (P5).
 FIG. 5 The formation pathways for C$_4$H$_8^+$ (P3), C$_3$H$_4$O$^+$ (P4), C$_2$H$_4$O$^+$ (P7) and C$_3$H$_6^+$ (P8).
 FIG. 6 The formation pathway for C$_4$H$_6^+$ (P6) and C$_2$H$_4^+$ (P12).
 FIG. 7 The formation pathways for C$_3$H$_5^+$ (P9), C$_3$H$_3^+$ (P10) and C$_2$H$_5^+$ (P11).
1. Formation pathway of C$_5$H$_7$O$^+$ ($m/z$=83)

Intuitively, the C$_5$H$_7$O$^+$ originates from parent ion C$_5$H$_8$O$^+$ by losing one H atom (Reaction (1)). Theoretical calculated AE values for H abstraction from C2 and C3 are 11.81 and 11.82 eV, respectively, both much higher than the experimental AE of C$_5$H$_7$O$^+$ (10.88$±$0.02 eV). Thus, C$_5$H$_7$O$^+$ can not be formed via one-step bond cleavage process and should undergo transition states.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_5\textrm{H}_7\textrm{O}^+ + \textrm{H}$ (1)

The proposed pathway for this reaction is described in FIG. 4. Firstly, parent ion undergoes a ring-open process by C-C bond cleavage via TS1 to form INT1. Afterward, a H atom migration step from C5 to C2 occurs to produce INT2 via TS2 with the energy barrier of 0.44 eV. Then, INT2 undergoes a H migration to produce INT3 via transition state TS3. Subsequently, C$_5$H$_7$O$^+$ is produced via a H atom elimination in C2 atom. The calculated energy barrier for P1, 10.87 eV, matches perfectly with experimental value of (10.88$±$0.05) eV. The most possible configuration of C$_5$H$_7$O$^+$ is CH$_3$CH$_2$CHCHOH$^+$.

2. Formation pathway of C$_4$H$_5$O$^+$ ($m/z$=69)

The formation pathway for C$_4$H$_5$O$^+$ is assumed to remove a methyl directly as reaction (2). The calculated appearance energy for direct dissociation is 10.87 eV, which is lower than the experimental value of (11.03$±$0.06) eV. Then, we scanned the C4-C5 bond length from 1.35 Å to 3.5 Å to search the transition state, and found the TS4 for which energy barrier is 10.97 eV. Finally, the C$_4$H$_5$O$^+$ is generated by breaking the C4-C5 bond in INT3, with a concomitant loss of a methyl radical. In the whole reaction pathway, the highest energy step is TS4 (10.97 eV), which is consistent with experimental value.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_4\textrm{H}_5\textrm{O}^+ + \textrm{CH}_3$ (2)
3. Formation pathways of C$_4$H$_8^+$/C$_3$H$_4$O$^+$ ($m/z$=56)

There are two probable candidate structures, C$_4$H$_8^+$ (P3), and C$_3$H$_4$O$^+$ (P4), that can correspond to the peak at $m/z$=56 according to calculation. For formation of C$_4$H$_8^+$ (P3), firstly, rotation around the C1-C2 bond can transform INT1 into isomer INT4. Secondly, the C$_4$H$_8^+$ is formed by a CO elimination step from INT4. The calculated AE of C$_4$H$_8^+$ is 10.86 eV, which is consistent with the experimental value of (10.64$±$0.03) eV and the other experimental data (10.44 eV [8] and 10.76 eV [10]) as well. However, Price et al. [9] gave the experimental AE value of (9.75$±$0.05) eV for C$_4$H$_8^+$, the reason for this deviations is not known.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_4\textrm{H}_8{^+} + \textrm{CO}$ (3)

The ion C$_3$H$_4$O$^+$ is formed by lossing C$_2$H$_4$ from parent ion (reaction (4)). In this pathway, firstly, INT1 proceeds to undergo bond cleavage of C3-C4 via transition state TS6 and the barrier is located to be 10.90 eV above neutral cyclopentantone. Then C$_3$H$_4$O$^+$ (P4) and C$_2$H$_4$ are produced by the bond fission of C3-C4. The calculated overall energy barrier is 11.01 eV, which is in good agreement with the observed threshold value (11.25$±$0.08 eV). This suggests that C$_4$H$_8^+$ is formed at low energy while the isomer C$_3$H$_4$O$^+$ may be generated at high energy threshold.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_3\textrm{H}_4\textrm{O}^+ + \textrm{C}_2\textrm{H}_4$ (4)
4. Formation pathway of C$_3$H$_3$O$^+$ ($m/z$ =55)

As the dominant dissociation product from C$_5$H$_8$O$^+$, the fragment ion C$_3$H$_3$O$^+$ (P5) is considered to be produced by the direct bond fission of C3-C4 in INT3 (FIG. 4). The corresponding AE is calculated to be 10.89 eV, which is close to the experimental value of (11.18$±$0.04) eV and the previous value (11.14 eV) reported by Pastoors et al. [10] as well.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_3\textrm{H}_3\textrm{O}^+ + \textrm{C}_2\textrm{H}_5$ (5)

It should be noted that, initially we thought that the species at $m/z$=55 was C$_4$H$_7^+$. However, the reaction barrier for this pathway is computed to be at least 11.65 eV, which is higher than the experimental AE of 11.18 eV.

5. Formation pathway of C$_4$H$_6^+$ ($m/z$=54)

Fragment C$_4$H$_6^+$ is assumed to be obtained from further dissociation of C$_4$H$_8^+$ (FIG. 6). First, the C$_4$H$_8^+$, which has a four-membered ring, can produce INT5 via transition state TS7. Then, the INT6 is generated via TS8 with one H-migration step from C2 to C4. Finally, hydrogen atoms of methylene and methyl in INT6 get close to each other, and overcome the barrier of 1.71 eV to generate C$_4$H$_6^+$ (P6). The calculated AE for the reaction (6) is 11.88 eV, which is in reasonable agreement with the observed value 12.05 eV.

 $\textrm{C}_5\textrm{H}_8\textrm{O}^+ + h\nu \rightarrow \textrm{C}_4\textrm{H}_6{^+} + \textrm{H}_2 + \textrm{CO}$ (6)
6. Formation pathways of C$_2$H$_4$O$^+$ ($m/z$=44) and C$_3$H$_6^+$ ($m/z$=42)

For formation of C$_2$H$_4$O$^+$, INT1 can first rearrange via a H-migration to produce INT7 (FIG. 5). The final product C$_2$H$_4$O$^+$ (P7) is yielded by breaking C-C bond in INT7, accompanied with the loss of C$_3$H$_4$. The overall barrier of this process is calculated to be 12.34 eV, which is in excellent agreement with our experimental value of (12.32±0.03) eV.

 $\textrm{C}_5\textrm{H}_8\textrm{O}^+ + h\nu \rightarrow \textrm{C}_2\textrm{H}_4\textrm{O}^+ + \textrm{C}_3\textrm{H}_4$ (7)

C$_3$H$_6^+$ is produced by C$_2$H$_2$O elimination from INT1. As shown in FIG. 5, the reaction (8) is derived from the C-C bond cleavage in INT1. The C2-C3 bond length has been scanned from 1.45 Å to 3.4 Å and no transition state is found. The total barrier of this process is calculated to be 11.73 eV, which is in excellent agreement with the experimental value of (11.78±0.03) eV.

 $\textrm{C}_5\textrm{H}_8\textrm{O} + h\nu \rightarrow \textrm{C}_3\textrm{H}_6{^+}+ \textrm{C}_2\textrm{H}_2\textrm{O}$ (8)
7. Formation pathways of C$_3$H$_5^+$ ($m/z$=41) and C$_3$H$_3^+$ ($m/z$=39)

For C$_3$H$_5^+$, the probable formation pathway is following: C$_5$H$_8$O$^+$+$h\nu\rightarrow$C$_3$H$_5^+$+CH$_3$+CO. Firstly, a 1, 2-H migration from INT6 leads to the formation of INT8 via TS12 with a barrier of 1.28 eV (FIG. 7). Then, the INT8 could dissociate further to form C$_3$H$_5^+$ by losing CH$_3$. The appearance energy calculated is 12.20 eV, which agrees well with the experimental value of (12.32±0.06) eV.

The detailed formation pathway of C$_3$H$_3^+$ is also shown in FIG. 7. First, 2-propenyl cation C$_3$H$_5^+$ undergoes a 1, 2-hydrogen shift to generate the INT9 by overcoming a barrier of 1.22 eV. Then, H migration toward the terminal carbon atom in 2-propenyl cation leads to the formation of intermediate INT10 via transition state TS14 with an energy barrier of 1.47 eV. Finally, two H atoms of methyl group in INT10 get close to each other by passing through a transition state (TS15) located at 14.37 eV, leading to the formation of C$_3$H$_3^+$ and hydrogen molecule (reaction (9)). This pathway is consistent with the literature results [23, 24].

 $\textrm{C}_5\textrm{H}_8\textrm{O}^+ + h\nu \rightarrow \textrm{C}_3\textrm{H}_3{^+} + \textrm{CH}_3 + \textrm{H}_2 + \textrm{CO}$ (9)
8. Formation pathways of C$_2$H$_5^+$ ($m/z$=29) and C$_2$H$_4^+$ ($m/z$=28)

The formation pathway of C$_2$H$_5^+$ is C$_5$H$_8$O$^+$+$h\nu\rightarrow$ C$_2$H$_5^+$+C$_2$H$_3$+CO, as shown in FIG. 7. The ethyl cation is generated with the loss of vinyl radical from 1-butene cation C$_4$H$_8^+$ (P3) via simple C-C bond fission. The final product is situated at 12.96 eV above the ground state of neutral cyclopentone, which is in good accordance with our experimental value of (12.98±0.03) eV.

The C$_2$H$_4^+$ (P12) is produced by further dissociation of C$_4$H$_8^+$ accompanied by formation of neutral ethylene. Detailed formation pathway of C$_2$H$_4^+$ is depicted in FIG. 6. First, INT11 can be generated by the C-C bond cleavage via TS16. Then, INT11 can eliminate ethylene group producing P12 (ethylene cation) with overall barrier of 12.10 eV, which is in accordance with our experimental value of (12.44±0.05) eV. This formation pathway leads to agreement with the results of previous mass spectrometric researches [7, 25].

Ⅳ. CONCLUSION

The photoionization and dissociation of cyclopentanone have been investigated experimentally using reflection time-of-flight mass spectrometer with the tunable vacuum ultraviolet synchrotron radiation as the ionization source. The ionization energy and appearance energies for cyclopentanone and 12 fragment ions are obtained from their PIE curves. The IE and AEs for cyclopentanone and fragments, C$_5$H$_7$O$^+$, C$_4$H$_5$O$^+$, C$_4$H$_8^+$/C$_3$H$_4$O$^+$, C$_3$H$_3$O$^+$, C$_4$H$_6^+$, C$_2$H$_4$O$^+$, C$_3$H$_6^+$, C$_3$H$_5^+$, C$_3$H$_3^+$, C$_2$H$_5^+$ and C$_2$H$_4^+$ are determined to be 10.88, 11.03, 10.64/11.25, 11.18, 12.05, 12.32, 11.78, 12.32, 14.33, 12.98, and 12.44 eV, respectively. The dissociative photoionization mechanisms of C$_5$H$_8$O are proposed with the help of the ab initio calculations at the $\omega$B97X-D/6-31+G(d, p) level. Ring opening and hydrogen migrations are the predominant processes in the fragmentation pathways of cyclopentanone.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.91544105, No.U1532137, No.U1232209, and No.11575178). The authors are grateful to the Supercomputing Center of University of Science and Technology of China for generous allocation of computing resources.

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