419

M. Cao et al., Eur. J. Mass Spectrom. 20, 419–428 (2014) Received: 11 January 2015 n Accepted: 22 January 2015 n Publication: 17 February 2015

EUROPEAN JOURNAL OF MASS SPECTROMETRY

Dissociative photoionization of b-pinene: an experimental and theoretical study Maoqi Cao,a Jun Chen,a Wenzheng Fang,a,b Yuquan Li,a Shaolin Ge,a Xiaobin Shan,a Fuyi Liu,a Yujie Zhao,a,c* Zhenya Wangd and Liusi Shenga* a National Synchrotron Radiation Laboratory, School of Nuclear Science and Technology, University of Science and Technology of China Hefei 230029, China. E-mail: [email protected]; [email protected] b

Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, China

c

School of Nuclear Engineering and Geophysics, East China Institute of Technology, Nanchang, Jiangxi 330013, China

d

Laboratory of Environmental Spectroscopy, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China

We investigated the photoionization and dissociation photoionization of the b-pinene molecular using time-of-flight mass spectrometry with a tunable vacuum ultraviolet source in the region from 8.00 eV to 15.50 eV. The experimental ionization energy (IE) value is 8.60 eV using electron impact as the ionization source, which is not in good agreement with theoretical value (8.41 eV) with a G3MP2 method. We obtained the accurate IE of b-pinene (8.45 ± 0.03 eV) derived from the efficiency spectrum, which is in good agreement with the theoretical value (8.38 eV) of the CBS-QB3 method. We elucidated the dissociation pathways of primary fragment ions from the b-pinene cation on the basis of experimental observations in combination with theoretical calculations. Most of the dissociation pathways occur via a rearrangement reaction prior to dissociation. We also determined the structures of the transition states and intermediates for those isomerization processes. Keywords: b-pinene, photoionization mass spectra, VUV synchrotron radiation, CBS-QB3

Introduction Chemical processes occurring in the troposphere have a direct impact on both global and local climatic changes. Such processes are strongly influenced by the presence of volatile organic compounds (VOCs). b-pinene is one of the most important VOCs and are used to denote the entire set of vaporphase atmospheric organics, excluding CO and CO2. As a large group of atmospheric chemicals, VOCs have a wide variety of anthropogenic and natural sources. Anthropogenic sources1–5 comprise organics such as alkanes, alkenes, aromatics and carbonyls, while biogenic sources6–11 include organics such as isoprene (C5H8), monoterpenes (C10H16) and sesquiterpenes (C15H24). It has been well established that biogenic volatile organic compounds (BVOCs) play a major role in atmospheric ISSN: 1469-0667 doi: 10.1255/ejms.1304

chemistry from the urban to the global scale.12–15 In 1960, Went first proposed that natural foliar emissions of VOCs from trees and other vegetation could have a significant effect on the chemistry of the Earth’s atmosphere. 16 The worldwide emission of BVOCs is about 1150 Tg year–1, which exceeds the estimated anthropogenic emissions by about one order of magnitude.17–21 The most abundant naturally formed VOCs are isoprene and isoprene chains called monoterpenes, the latter making up more than half (54%) of the total natural VOCs production,22 while b-pinene (23%) is one of the most abundant monoterpenes in the atmosphere. It has been identified in aerosol precursors, which can be quite reactive under atmospheric­conditions, and is also the precursor for © IM Publications LLP 2014 All rights reserved

420

Dissociative Photoionization of b-Pinene: an Experimental and Theoretical Study

the creation­of many terrestrial secondary organic aerosols (SOAs). The mass spectrometry (MS) of b-pinene has been studied only by Kubala et al., in 2009, using electron impact as the ionization source.23 The ionization energy (IE) of b-pinene and most of the appearance energies (AEs) of its fragments were found by Kubala et al.23 However, the mechanism of fragmentation from parent ion were not given in their paper.23 The mass spectra of b-pinene obtained at lower energies are also useful to analyze the products of SOAs formed during the photooxidation and photodecomposition of b-pinene.9 As a result, in this research the photoionization and dissociation photoionization of b-pinene was studied by a tunable vacuum ultraviolet (VUV) mass spectrometer, in which the ion time-of-flight (TOF) mass spectra are recorded as a function of the photon energy in the range 8.0 eV to 15.50 eV. Quantum chemical calculations with the methodology of CBS-QB3 were also performed to elucidate the dissociation pathways. In order to understand the photoionization and dissociation photoionization of molecules, two processes should be considered:

M + hv → M+ + e  (direct ionization)

(1)

M + hv → X+ + M–X + e  (dissociative ionization)

(2)

and

M represents the parent molecule and M–X represents the neutral molecule or the radical. The parent cation can be formed directly by VUV singlephoton ionization. When the photon energy increases, the parent undergoes various dissociation reactions to form fragments. In the case of dissociative photoionization, the fragments are generally considered to be formed by ionization of the parent molecule followed by the spontaneous dissociation of the parent ion. We have measured the photoionization efficiency (PIE) spectrum directly by scanning the photon energy of the monochromatized synchrotron radiation (SR) in order to determine the AEs of the most abundant fragment ions.

Experimental apparatus and computational calculations The apparatus is described in detail elsewhere.24,25 Briefly, the monochromatic VUV SR, the supersonic molecular beam and the reflectron TOF are mutually perpendicular. SR from an undulator-based U14-A beamline of an 800 MeV electron-storage ring at the National Synchrotron Radiation Laboratory in Hefei, China, was dispersed with a 6 m length monochromator, which is equipped with three gratings (370, 740 and 1250 grooves mm –1). In this experiment, only the 370 grooves mm–1 grating was used and its energy resolving power (E/ΔE) is above 2000 when the widths of the entrance and exit slits are adjusted to 80 μm. For instance, the energy resolution of photons at 15.9 eV is measured to be 6 meV (full

width at half maximum) which is much higher than the energy resolution of about 140 meV of the electron beam energy formed in a trochoidal electron monochromator, as used in Kubala’s experiment.23 The monochromator SR was focused by a toroidal mirror into the photoionization chamber. A raregas harmonic filter was inserted into the beamline to overcome the contamination of higher-order harmonics in the energy range. The absolute energy of the monochromator was precisely calibrated with the known IEs of inert gases. The sample of b-pinene was expanded to form a continuous supersonic molecular beam seeded in the carrier gas through a stainless-steel nozzle with a diameter of 70 μm and two skimmers with diameters of the orifice of 1 mm. Ion TOF mass spectra were obtained while the monochromator was scanned with energy increments of 30 meV. The data acquisition time for each point was 60 s. To normalize the ion signals, the photon intensity was monitored simultaneously with a silicon photodiode. PIE curves can be extracted for each mass after correction for photon flux. The liquid sample b-pinene, purity of 98%, was purchased from J&K CHEMICA and was used without further purification. All the calculations were performed with the Gaussion 09 program package26 at the Supercomputing Center of University of Science and Technology of China. The candidate structures of the parent molecular, parent ion, fragments, neutral molecules, intermediates and transition states were calculated with the CBS-QB3 method,27,28 which is a composite method starting from a geometry optimization at level of B3LYP/6311G(2d, d, p) followed by a series of high-level single-point energy corrections. The stationary points were identified with frequency calculations at the same level to verify that minima and transition state structures had either zero or only one imaginary frequency, respectively. The intrinsic reaction coordinate calculations at the same level as the geometry optimization were used to confirm the connection between the designated transition states and the reactant or products.28,29 In the dissociation pathways, the energy corresponding to the neutral molecule of b-pinene is defined to be zero. The AE of the ionic fragment is defined as EAE = EMax – E0, where EMax refers to the highest energy involved in the formation pathway of the corresponding ionic fragment and E0 is the absolute energy of the neutral precursor.

Results and discussion Results

Figure 1 shows the optimized lowest-energy geometries of neutral b-pinene and the b-pinene cation. The optimized C2–C5 distance, 2.246 Å, of b-pinene cation is much longer than the C2-C5 distance, 1.583 Å, of the neutral b-pinene, suggesting that the b-pinene cation is formed through opening the ring, which is also in agreement with the conclusion of Kubala’s work applying the G3MP2 method.23 Figure 2 shows the photoionization mass spectra of b-pinene at different photon energies. Many more fragment ions can

M. Cao et al., Eur. J. Mass Spectrom. 20, 419–428 (2014) 421 Fig 2. (a) Photoionization mass spectrum of β-Pinene at photon energy of 14.50 eV. (b) Photoionization mass spectrum of β-Pinene from 8.00 to 15.00 eV at integral energies. Fig 1. Optimized lowest energy geometries of β-Pinene and β-Pinene cation. Arabic numbers represent marked carbon atoms and hydrogen atoms. The numbers of the labeled carbons in all the transition states (TS) and intermediates (INT) remain unchanged

Figure 1. Optimized lowest energy geometries of b-pinene and the b-pinene cation. Arabic numbers represent marked carbon atoms and hydrogen atoms. The numbers of the labeled carbons in all the transition states (TS) and intermediates (INT) remain unchanged in the paper.

.

be observed with increasing photon energy, especially in the case of polyatomic molecules such as b-pinene. Comparing with the data (measured using electron impact at an energy of 70 eV) recorded in the NIST database, basically the same fragment ions are observed in both cases. However, a larger contribution of fragment ions is observed in the photoionization mass spectrum, indicating a softer fragmentation of the molecule using a single photon as the ionization source. As no signals at a mass greater than that of the b-pinene cation, m/z 136, were detected in our experiment, shown in Figure 3, all the detected fragment ions are considered to be from the dissociation of the parent ion. Figure 4(a) shows the dissociation pathways of the b-pinene cation to produce fragment ions m/z 121. Figure 4(b) shows the dissociation pathways of b-pinene cation to produce ­fragment ions m/z  107, m/z  94 and m/z  93. Stoichiometric identification of the ions from b-pinene with a given m/z is relatively easy because they are formed only from carbon and hydrogen atoms. Such mass spectrometric measurements provide little information on the structure of the fragment ions. Therefore, we calculated the structures and energies of the primary neutral molecules and radicals using the

Figure 2. (a) Photoionization mass spectrum of b-pinene at a photon energy of 14.50 eV. (b) Photoionization mass spectrum of b-pinene from 8.00 eV to 15.00 eV at integral energies.

methodology­of CBS-QB3. Detailed information on the geometries and ­energies of the optimized reactants, transition states, intermediates and products involved in the dissociation are shown in Figures 5 and 6, and Table 1. Dissociation following the ionization of b-pinene results in the formation of large numbers of fragment ions. The list of the AEs for all the measured ions are presented in Table 2.

Discussion

Accurate IE of b-pinene

We measured the photoionization mass spectra of the abundant fragments and derived their AEs from PIEs using the nonlinear least-squares fitting technique in the threshold region (Table 1). Considering the cooling effect of the molecular beam,30 the discrepancies of the measured ionization thresholds caused by the hot-band ionization are typically less than 0.03 eV for species with strong signal-to-noise (S/N) ratios and 0.06 eV for species with poor S/N ratios. The experimental IE value of b-pinene is 8.45 eV, which is in good agreement with the theoretical calculation of the adiabatic IE of 8.38 eV using the CBS-QB3 method in this paper and of 8.41 eV with the G3MP2 method in Kubala’s paper,23 while an experimental IE value of 8.60 eV was obtained by Kubala’s group

422 Fig

Dissociative Photoionization of b-Pinene: Study 3. (a)-(d) are the normalized photoionization efficiency cures anofExperimental β-Pinene and m/zTheoretical 136 and fragment ions of m/z 121, 93 and 69.

Figure 3. Normalized PIE cures of b-pinene m/z 136 (a) and fragment ions m/z 121 (b), m/z 93 (c) and m/z 69 (d).

in 2009 using electron impact as the ionization source. The reason for the higher IE obtained by them is, perhaps, because of the poor energy resolution used in their experiment and the weak ionization­cross-section at energies near the threshold value. As a result, the experimental IE of ours is much more reasonable. Many PIE curves of the fragment ions from b-pinene were also obtained, but they provide little information on the structure of the fragment cations. Therefore, in order to clarify the mechanism of the primary fragmentation pathways, we calculated the structures and energies of the neutral molecules and radicals of several positive ions.

Products m/z 121, m/z 69 and m/z 67 from the b-pinene cation

The AE of the product at m/z 121 is 9.47 eV in our experiment. It is considered to be from the channel C10H16+ → C9H13+ + CH3. Three candidates (P121-1, P121-2, P121-3) are involved in the proposed possible dissociation channels shown in Figure 4(a). There are two direct dissociation channels of b-pinene cation to form the fragment ion m/z 121 via the direct elimination of the methyl radical located at C1–C2 and C2–C3. The AEs of P121-1 and P121-2 are calculated to be 10.15 eV and 10.25 eV, which are higher than the observed experimental value 9.47 eV.

Upon comparison of the experimental result of 9.47 eV with the theoretical result of 9.58 eV, one dissociation channel of b-pinene ion forms m/z 121. The channel is described as b-pinene cation → TS1 → INT1 → TS2 → INT2 → P121-3. The b-pinene cation undergoes a hydrogen-atom shift to yield intermediate INT1 via the transition state TS1. The barrier of this step is calculated to be 0.96 eV. Then, INT1 undergoes a methyl radical shift to yield INT2 via transition TS2. The barrier of this step is calculated to be 1.46 eV. P121-3 is generated by breaking the C–C bond in the intermediate INT2 coupled with a methyl radical loss. The experimental AE of m/z 121 is 9.47 eV, which is in good agreement with the calculated value of 9.58 eV, while the experimental AE of the same ion is 9.99 eV in Kubala’s work.23 Combining the experimental and theoretical results, we think the result of ours is much more reasonable and the P121-3 pathway is the most probable channel for product m/z 121 from the dissociation of the b-pinene cation. The m/z  69 (C 10 H 16 +  → C 5 H 9 +  + C 5 H 7 ) and m/z  67 (C10H16+ → C5H7+ + C5H9) can be generated by breaking the C–C bonds in the intermediate INT2. However, breaking two bonds at the same time needs much more energy. As a result, the channel of products m/z 69 and m/z 67 should undergo other transition states after the INT2 structure. Considering the complexity of the pathways, further transition states are not

(the units is eV). (a) The dissociation pathways of β-Pinene cation to produce fragment ions of m/z 121 (the units is eV) (b) The dissociation pathways of β-Pinene cation to produce fragment ions of m/z 107, 94 and 93 M. Cao et al., Eur.units J. Mass Spectrom. 20, 419–428 (2014) 423 (the is eV).

Figure 4. (a) Dissociation pathways of the b-pinene cation to produce fragment ions m/z 121. (b) Dissociation pathways of the b-pinene cation to produce fragment ions m/z 107, m/z 94 and m/z 93.

given. The experimental AEs of products m/z 69 and m/z 67 are 11.63 eV and 12.08 eV, respectively. By comparing the present mass spectra of the isomers with those of a-pinene, b-pinene and limonene, the main difference is the p ­ resence of the ion with m/z 69 in the mass spectra of b-pinene, which is totally absent in the mass spectra of a-pinene and limonene.23,31 Thus, this ion may be used as a marker for the b-pinene ­molecule.

Products m/z 107 and m/z 93 The ion of m/z 93 is the most abundant fragment ion which can be seen in the mass spectra of b-pinene. The ion m/z 93 is assigned to C7H9+and the mechanism of the formation for C7H9+is loss of the propyl group of the b-pinene cation.

The b-pinene cation does not possess the geometry of the propyl group, so the channel producing m/z  93 should be via some rearrangement reactions prior to dissociation. The structure of m/z 93 is suggested to be a form of protonated toluene.23 Comparing the results of the experiments and the theoretical calculations, there are pathways that produce the fragment ions m/z 107 (C10H16+ → C8H11+ + C2H5) and m/z 93 (C 10 H 16 +  → C 7 H 9 +  + C 3 H 7 ), respectively, which are shown in Figure 4(b). Intermediate INT2 undergoes a hydrogenatom shift to yield intermediate INT3 via transition TS3, with a ­calculated barrier of 0.48 eV. Then, INT3 undergoes a hydrogen-atom shift to yield intermediate INT7 via transition state TS7. P107 is generated by breaking the C–C bond in INT7 coupled with a radical ethyl loss. The AE of product m/z 107 is

424

Dissociative Photoionization of b-Pinene: an Experimental and Theoretical Study

Fig 5. The geometries of the neutral β-Pinene, β-Pinene  cation, possible fragments (ion, radical and neutral molecule) optimized at level of B3LYP/6-311G(2d, d, p).

Figure 5. Geometries of the neutral b-pinene, b-pinene cation and possible fragments (ion, radical and neutral molecule) optimized at the level of B3LYP/6-311G(2d,d,p).

Fig 6. The possible geometries of the transition states (TS) and intermediates (INT) optimized at level of B3LYP/6-311G(2d, d, p).

Figure 6. Possible geometries of the TSs and INTs optimized at the level of B3LYP/6-311G(2d,d,p).

M. Cao et al., Eur. J. Mass Spectrom. 20, 419–428 (2014) 425

Table 1. Experimental, theoretical (CBS-QB3) and literature values of the IE and AEs of fragments and the possible channels.

Mass (amu)

IE or AE (eV), CBS-QB3

IE or AE (eV), experimental

Literature values

Possible dissociation channels

136

 8.38

8.45 ± 0.03

8.60 ± 0.0323

C10H16 → C10H16 + + e

121

 9.58

9.47 ± 0.06

9.99 ± 0.0423

C10H16 + → C9H13 + + CH3

107

 9.58

9.47 ± 0.06

10.22 ± 0.0623

C10H16 + → C 8H11+ + C2H5

105

11.78

11.73 ± 0.06

13.03 ± 0.0823

C10H16 + → C 8H9+ + C2H5 + H2

 94

 9.59

9.50 ± 0.03

 93

 9.70

 92

9.53 ± 0.03

C10H16 + → C7H10 + + C 3H6 9.13 ± 0.04

23

C10H16 + → C7H9+ + C 3H7 C10H16 + → C7H8 + + C 3H8

8.87 ± 0.06

 91

12.18

 80

11.95 ± 0.03

12.17 ± 0.0523

C10H16 + → C7H7+ + C 3H7 + H2 C10H16 + → C6H8 + + C 4H8

9.05 ± 0.06

 79

10.29

10.45 ± 0.03

10.48 ± 0.0523

C10H16 + → C6H7+ + C 3H6 + CH3

 77

12.88

12.80 ± 0.03

12.49 ± 0.0523

C10H16 + → C6H5 + + C 3H8 + CH3

 69

11.63 ± 0.03

11.17 ± 0.0423

C10H16 + → C5H9+ + C5H7

 67

12.08 ± 0.03

11.16 ± 0.2023

C10H16 + → C5H7+ + C5H9

 55

11.60 ± 0.06

 43

11.62 ± 0.06

 41

13.07 ± 0.06

9.58 eV, which is in good agreement with the theoretical value of 9.62 eV. INT2 can undergo another hydrogen-atom shift to yield intermediate INT4 via transition TS4. P93 is generated

13.34 ± 0.0523 by breaking the C–C bond in intermediate INT4 coupled with a radical propyl loss. The AE of product m/z 93 is 9.53 eV, which is consistent with theoretical result of 9.70 eV.

Table 2. Total energies of species (ions, neutrals, transition states and intermediates) involved in the dissociative photoionization of b-pinene calculated with CBS-QB3.

Species b-pinene (C10H16) b-pinene cation (C10H16 +) +

P1-1 (C9H13 ) +

E0(CBS-QB3) (Hartree) (at 0 K )

Species

E0(CBS-QB3) (Hartree) (at 0 K)

–389.838511

P79 (C6H7+) +

–232.069295

–389.530490

P69 (C5H9 )

–195.189811

–349.720265

(C5H7+)

–193.963093

P67

P1-2 (C9H13 )

–349.741532

69 (C5H9)

–195.463486

P1-3 (C9H13 +)

–349.717042

67 (C5H7)

–194.256868

P107 (C 8H11+) P2-1 (C7H10 +) P2-2 (C7H10 +) P93 (C7H9+) P92 (C7H8 +)

–310.530515

44 (C 3H8)

–118.855864

–271.822516

43 (C 3H7)

–118.196311

–271.880537

29 (C2H5)

–78.971555

–271.285408

15 (CH3)

–39.744795

–270.692795

2 (H2)

–1.166083

TS1

–389.495362

INT1

–389.546905

TS2

–389.493475

INT2

–389.045139

TS3

–389.484978

INT3

–389.541102

TS4

–389.488652

INT4

–389.547642

TS5

–389.486899

INT5

–389.535655

TS6

–389.485876

INT6

–389.550379

TS7

–389.486164

INT7

–389.495723

TS8

–310.433300

TS9

–271.194831

426

Dissociative Photoionization of b-Pinene: an Experimental and Theoretical Study

Products m/z 94 and m/z 79 from the b-pinene cation The ion of m/z  94 is not a primary fragment ion that can be seen from the mass spectra at different energies from Figure 2(b). In Kubala’s work,23 the IE of ion m/z  94 is not given, perhaps because of the poor S/N resulting from electron impact as the ionization source. However, the AE of m/z 94 (9.50 eV) was first obtained in our experiment with SR as ionization source. The product m/z 94 can be generated from the channel C10H16+ → C7H10+ + C3H6. P94-1 can be generated by breaking the C–C bond in intermediate INT2. The calculated AE is 10.06 eV, which is not in accordance with the experimental value 9.50 eV, implying that there are other possible pathways for the product m/z 94, one of which is shown in Figure 4(b). INT2 undergoes a hydrogen-atom shift to yield intermediate INT5 via transition TS5, with a calculated barrier of 0.42 eV. Then, INT5 undergoes another hydrogen-atom shift to yield intermediate INT6 via transition TS6. P94-2 is generated by breaking the C–C bond in intermediate INT6 coupled with the loss of a molecular of propylene, C3H6. The calculated AE is 9.59 eV which is in good agreement with the experimental AE of 9.5 eV. P79 (C10H16+ → C6H7+ + C3H6 + CH3) is generated by loss of the methyl from P94-2 directly. The calculated AE of P79 is 10.29 eV, which is in good agreement with the experimental value of 10.50 eV.

Products m/z 105 and m/z 91 from the ions m/z 107 and m/z 105

The products of m/z  105 (C10H16+ → C8H9+ + C2H5 + H2) and m/z 91 (C10H16+ → C7H7+ + C3H7 + H2) can be generated by loss of a molecule of H2 from the ions at m/z 107 and m/z 93. The appropriate transition states connecting the reactants and products are obtained. The theoretical AEs are 11.78 eV and 12.18 eV, which are consistent with the experimental values of 11.49 eV and 11.95 eV.

Other products from the b-pinene cation

Other products, such as m/z 108, 92, 80, 55 and 41, are shown in Figure 2(a). Most of these are not primary fragment ions and some have multiple possible dissociation channels, such as C3H7+(m/z 43) → C3H5+(m/z 41) + H2, C5H7+(m/z 67) → C3H5+(m/ z 41) + C2H2. As a result, the possible channels of these products are not discussed in detail in our paper. However, all their experimental AEs are given in Table 1 and some of the AEs of the fragment ions were obtained for the first time in this work, such as m/z 94, m/z 92 and m/z 80.

Conclusions We reported the experimental study of one of the most abundant monoterpenes, b-pinene, using TOF mass spectrometry with a tunable VUV source. We obtained an accurate IE of b-pinene, 8.45 eV, and AEs from most fragment ions derived

from PIE curves; some of the AEs are given here for the first time. We reported, for the first time, the dissociation pathways of the primary fragment ions from b-pinene with the help of theoretical calculations. The majority of the fragmentation pathways involve hydrogen-atom shifts. The isomerization processes lead to stable products and are helpful in understanding the fragmentation pathways of b-pinene.

Acknowledgments This work is supported by the National Natural Science Foundation of China (NOS. 11075165, 11275006, 41275127, 10979048, U1232209 and U1232130) and the China Postdoctoral Science Foundation (Grant No. 2013M531530). The authors would like to thank the Supercomputing Center of University of Science and Technology of China.

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Dissociative Photoionization of b-Pinene: an Experimental and Theoretical Study

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Dissociative photoionization of β-pinene: an experimental and theoretical study.

We investigated the photoionization and dissociation photoionization of the β-pinene molecular using time-of-flight mass spectrometry with a tunable v...
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