Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (3): 306-312

The article information

Ye-peng Yu, Zhao-hui Li, Xuan Lin, Jun Chen, Hang Zhang, Yan-bo Li, Huang-huang Wang, Rui-rui Sun, Qing-hui Meng, Xiao-bin Shan, Fu-yi Liu, Liu-si Sheng
余业鹏, 李照辉, 林烜, 陈军, 张航, 李淹博, 王欢欢, 孙瑞瑞, 孟庆慧, 单晓斌, 刘付轶, 盛六四
Photoionization and Dissociation Study of 2-Methyl-2-propen-1-ol: Experimental and Theoretical Insights
2-甲基-2-丙烯-1-醇的光电离解离研究
Chinese Journal of Chemical Physics, 2019, 32(3): 306-312
化学物理学报, 2019, 32(3): 306-312
http://dx.doi.org/10.1063/1674-0068/cjcp1805089

Article history

Received on: May 3, 2018
Accepted on: June 3, 2018
Photoionization and Dissociation Study of 2-Methyl-2-propen-1-ol: Experimental and Theoretical Insights
Ye-peng Yu , Zhao-hui Li , Xuan Lin , Jun Chen , Hang Zhang , Yan-bo Li , Huang-huang Wang , Rui-rui Sun , Qing-hui Meng , Xiao-bin Shan , Fu-yi Liu , Liu-si Sheng     
Dated: Received on May 3, 2018; Accepted on June 3, 2018
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
Abstract: The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultraviolet synchrotron radiation in the photon energy region of 8.0$-$15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C$_4$H$_8$O$^+$, C$_4$H$_7$O$^+$, C$_3$H$_5$O$^+$, C$_4$H$_7$$^+$, C$_4$H$_6$$^+$, C$_4$H$_5$$^+$, C$_2$H$_4$O$^+$, C$_2$H$_3$O$^+$, C$_3$H$_6$$^+$, C$_3$H$_5$$^+$, C$_3$H$_3$$^+$, CH$_3$O$^+$, CHO$^+$ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d, p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO.
Key words: 2-Methyl-2-propen-1-ol    Synchrotron radiation    Photoionization    Vacuum ultraviolet    Mass spectrometry    
Ⅰ. INTRODUCTION

The volatile organic compound (VOC) inventory prepared by the U.S. Environmental Protection Agency (2005), shows that more than 1.7 million tons of oxygen-containing volatile organic compounds (OVOCs) are discharged per year from anthropogenic sources in the USA [1-3]. Incomplete combustion of fossil fuels is the major source of the emissions: off-highway sources (15%), vehicular emissions (30%), and emissions from solvent use (30%). OVOCs are active organic compounds that are emitted into the atmosphere through a number of natural and anthropogenic sources, among which marine air is an important source [4]. Unsaturated alcohols are an important class of OVOCs. Previous investigations [5-8] show the main removal processes of gas-phase unsaturated alcohols (including MPO) in the atmosphere are through the reactions with the atmospheric oxidants, such as Cl atom, OH, O$_3$, and NO$_3$ radicals. Besides, MPO plays an important role in industry, where it is applied as a monomer for the synthesis of polymers and as a starting material in the synthesis of pharmaceuticals, pesticides and other allyl compounds [9, 10].

Previous work has been principally concentrated on the removing mechanism and reaction rate coefficients of MPO with OH radical or Cl atom in the atmosphere [8, 11]. Despite the extensive studies performed, some physicochemical properties of MPO, for instance, its IE of parent molecule and AEs of photo-dissociative fragment ions, are not definitely ascertained yet. To the extent of our knowledge, there have been only two studies [12, 13] on the ionization of MPO wherein the IE of MPO has been determined to be 9.24 and 9.28 eV, respectively, and only one fragment ion has been identified as C$_2$H$_5$$^+$. The thorough dissociative photoionization of MPO, in experiment or in theory, is still less comprehended. For this purpose, we perform a study on dissociative photoionization process with vacuum ultraviolet (VUV) source ranging over 8.0-15.5 eV. From photoionization efficiency curves, the IE of MPO and AEs of related cations from fragmentation are obtained. With aid of theoretical calculations, our group proposes the fragmental mechanism for the photoionization process of MPO.

Ⅱ. EXPERIMENTAL METHOD

The experimental apparatus has been depicted comprehensively elsewhere [14-16], and only a brief description is given here. The dissociative photoionization of MPO has been obtained by using reflectron time-of-flight (TOF) spectrometer with supersonic expansion molecular beam. As the ionization source of mass spectrometer, the synchrotron radiation from the 800 MeV electron storage ring of NSRL is designed to be in the 7.5-124 eV photon energy range, monochromized with a grazing incidence spherical grating monochromator, and then filtered through a differentially pumped noble gas filter to remove the high-energy harmonics produced by undulator. Argon (6 Torr) is used as the filter gas to restrain higher order of harmonics.

2-Methyl-2-propen-1-ol (Aladdin Corp.) with purity of 99% is used directly without further treatment. Ar (99.995%) is used as carried gas (1 atm) to carry the sample into a continuous beam nozzle (70 $μ$m in diameter). In order to keep vacuum difference and achieve the formation of ultrasonic jet, the mixed gas (sample and Ar) must go through two 1-mm-diameter skimmers and then reach ionization chamber. For detection, the ions formed by the ionization of synchrotron radiation are then extracted by the repulsive electric field into reflectron-TOF mass spectrometer. All mass spectra of fragment ions can be collected by a multiscaler P7888 (FAST ComTek, Germany). PIE curves are deducted from the mass spectra and then normalized with the photocurrent. The turning point of PIE curve is determined as AEs which has been used widely [17, 18].

Ⅲ. THEORETICAL METHOD

In this theoretical inquiry, all the geometries of related species are optimized for the possible minima and saddle points by the hybrid density functional theory, B3LYP method with 6-31+G(d, p) as basis set, and the vibrational frequencies are also calculated at this level for zero-point correction. In pursuance of exact energies, calculations at the CCSD(T) [19] level employing the correlation consistent triple $\zeta$ basis set cc-pVTZ [20], which has been tested to achieve good agreement between computational and experimental results [21-24], were used to calculate the single-point energy of the sample. All theoretical calculations are carried out at the Supercomputing Center of USTC, Hefei, China, using the Gaussian 09 [25] suite of program.

Ⅳ. RESULTS AND DISCUSSION A. Time-of-flight mass spectra

The tunable nature of synchrotron VUV photoionization allows a soft ionization process near threshold energy, which can prevent the interference of fragment ions. The mass spectra of MPO under energies of 9.5, 12.5, and 15.5 eV, respectively, are demonstrated in FIG. 1. At low energy of 9.5 eV, TOF mass spectrum shows only the parent ion ($m/z$$=$72), formed by losing one electron of the olefinic bond of MPO. With increasing the energy to 12.5 eV, more fragment ions: C$_4$H$_7$O$^+$ ($m/z$$=$71), C$_3$H$_5$O$^+$ ($m/z$$=$57), C$_4$H$_6$$^+$ ($m/z$$=$54), C$_2$H$_4$O$^+$ ($m/z$$=$53) and C$_2$H$_3$O$^+$ ($m/z$$=$43) are emerged. At the high energy of 15.5 eV, other smaller fragment ions of 55 (C$_4$H$_7$$^+$), 44 (C$_2$H$_6$$^+$), 41 (C$_3$H$_5$$^+$), 39 (C$_3$H$_3$$^+$), 31 (CH$_3$O$^+$), and 29 (CHO$^+$) have been identified. The distribution of the fragments is basically similar to statistics that NIST provided [26]. All observed fragments are believed to be born from parent ion C$_4$H$_8$O$^+$ since no signal at mass greater than that of C$_4$H$_8$O$^+$ ($m/z$$=$72) could be detected. The peak of C$_3$H$_5$O$^+$ ($m/z$$=$57) has the strongest intensity in the energy ranges studied here, which indicates that C$_3$H$_5$O$^+$ is the dominated dissociation pathway of MPO by removing a methyl group.

FIG. 1 Photoionization mass spectra of MPO at 9.5, 12.5, and 15.5 eV.

The PIE curves for mother ion C$_4$H$_8$O$^+$ as well as fragment ions C$_4$H$_7$O$^+$, C$_3$H$_5$O$^+$, C$_4$H$_7$$^+$, C$_4$H$_6$$^+$, C$_2$H$_4$O$^+$, C$_2$H$_3$O$^+$, and C$_3$H$_6$$^+$, are obtained and shown in FIG. 2. The AE values of these fragment ions are determined for the first time by this method. The experimental IE of mother molecule, (9.20$\pm$0.03) eV, is in good agreement with the experimental values of 9.28 eV [12] by electron ionization and 9.24 eV by photoionization method [13]. The calculated IE value of parent molecule, 8.94 eV, however, is slightly smaller than the experimental result. The experimental AEs of fragments (C$_4$H$_7$O$^+$, C$_3$H$_5$O$^+$, C$_4$H$_7$$^+$, C$_4$H$_6$$^+$, C$_4$H$_5$$^+$, C$_2$H$_5$O$^+$, C$_2$H$_4$O$^+$, C$_2$H$_3$O$^+$, C$_3$H$_6$$^+$, C$_3$H$_5$$^+$, C$_3$H$_3$$^+$) are presented in Table Ⅰ, as well as the calculated IE and AEs of the fragment ions.

FIG. 2 The normalized PIE curves for fragment ions from dissociative photoionization of MPO.
Table Ⅰ Ionization energy and appearance energies for main fragment ions from dissociative photoionization of MPO, as well as calculation value.
B. Dissociation mechanisms

As the increment of photon energy, the fate of different dissociation mechanisms will fall on the parent ions. The geometries of the neutral MPO (P0) and parent ion (P) in the ground states are shown in FIG. 3. Generally, two types of mechanisms for dissociation come in our perspective: direct simple bond fission and indirect bond cleavage via transition states and intermediates. The dissociation mechanisms, including both direct and indirect cleavages, will be discussed in detail. The geometries of the related species, products, transition states (TSs), and intermediates (INTs) before and after, involved in the discussion below are shown in FIG. 4 and FIG. 5.

FIG. 3 The geometries of the neutral MPO (P0) and parent ion (P) in the ground states.
FIG. 4 Geometries of transition states and intermediates obtained under B3LYP method with 6-31+G(d, p) as basis set.
FIG. 5 Geometries of related products optimized with B3LYP/6-31+G(d, p).
1. The formation pathways of C$_{4}$H$_{7}$O$^{+}$ (${m/z}$$=$71) and C$_{4}$H$_{7}$$^{+}$ (${m/z}$$=$55)

H atom elimination is firstly taken into consideration to form C$_4$H$_7$O$^+$ from parent ion C$_4$H$_8$O$^+$. The relative energies of C$_4$H$_7$O$^+$ calculated at the B3LYP/6-31+G(d, p) level are 10.09 eV (H removing from C5), 11.49 eV (H removing from C9) and 12.54 eV (H removing from O12), respectively. The experimental AE of (10.49$\pm$0.09) eV is close to the calculated AE of 10.09 eV (H elimination from C5). Comparing the experimental and calculated results, the pathway of C$_4$H$_7$O$^+$ could be hydrogen elimination from C5 in molecule ion.

The fragment C$_4$H$_7$$^+$ ($m/z$$=$55) can be formed by direct cleavage of C9-O12 bond to lose a OH group. The calculated AE of 11.03 eV using the hybrid density functional theory at B3LYP/6-31+G(d, p) level is in excellent agreement with the experimental value (11.06$\pm$0.03) eV, which indicates that the formation pathway of C$_4$H$_7$$^+$ is direct fission without barrier.

2. The formation pathways of C$_{3}$H$_{5}$O$^{+}$ (${m/z}$$=$57) and CH$_{3}$O$^{+}$ (${m/z}$$=$31)

For the formation of C$_3$H$_5$O$^+$ ($m/z$$=$57), direct cleavage of C2-C5 bond is at beginning taken into account by losing CH$_3$ group. The calculated AE for this cation (CH$_2$CCH$_2$OH$^+$) is 11.77 eV, which does not agree with experimental value (11.06$\pm$0.03) eV. The proposed pathway for the formation of C$_3$H$_5$O$^+$ is shown in FIG. 6 via transition state TS1 where H transfers from C9 to C2 with a barrier of 0.94 eV. The calculated AE of C$_3$H$_5$O$^+$ is 9.88 eV, which is in reasonable accordance with the experimental result of (9.74$\pm$0.03) eV. The most possible configuration of C$_3$H$_5$O$^+$ is CH$_2$CHCHOH$^+$. What is more, this reaction pathway for C$_3$H$_5$O$^+$ ion is the dominant dissociative photoionization pathway of MPO.

CH$_2$OH$^+$ is initially assumed to come from direct cleavage bond C2-C9 of parent ion by losing neutral CH$_2$CCH$_3$. However, the calculation AE value (11.30 eV) is intensively smaller than the value of (12.2$\pm$0.03) eV in this work. First, the direct fission of the bond C2-C5 of parent ion forms CH$_2$CCH$_2$OH$^+$$+$CH$_3$ by crossing an energy barrier of 2.83 eV. Second, an H atom on the C1 transfers to C2 passing through a transition state (TS2) to form INT2. Then, the INT15 (CHCHCHOH) loses acetylene to produce CH$_2$OH$^+$. Formation pathway of CH$_2$OH$^+$ is shown in FIG. 6 and the calculated AE of 12.42 eV is consistent with the experimental value (12.23$\pm$0.03) eV.

FIG. 6 Formation pathways of C3H5O+ and CH3O+.
3. The formation pathways of C$_{4}$H$_{6}$$^{+}$ (${m/z}$$=$54), C$_{4}$H$_{5}$$^{+}$ (${m/z}$$=$53), and C$_{3}$H$_{3}$$^{+}$ (${m/z}$$=$39)

For the formation of the fragment C$_4$H$_6$$^+$ ($m/z$$=$54), the suggested pathway is described in FIG. 7. First, the CHOH group of INT1 transfers to the C1 across TS3 to form INT3. Second, by overcoming an energy barrier of 0.74 eV, H3 transfers to the C9 by passing through a transition state (TS4) and forms an intermediate (INT4). Then, the INT4 transfers H7 to O12 to form INT5. Due to that the bond C9$=$O12 of INT5 is much weaker, the CH$_2$CHCHCH$_2$$^+$ can be produced by directly breaking this bond. This calculated AE of C$_4$H$_6$$^+$ ($m/z$$=$54) is 9.61 eV, which is in good agreement with experimental value (9.74$\pm$0.03) eV.

FIG. 7 Formation pathways of C4H6+ and C4H5+.

For the production of C$_4$H$_5$$^+$ ($m/z$$=$53), the parent ion initially loses H$_2$O to form CH$_2$CHCHCH$_2$$^+$ from MPO. The next step, one atomic hydrogen located on the middle carbon of C$_4$H$_6$$^+$ (F4) is eliminated to produce the C$_4$H$_5$$^+$ (F5). The theoretical AE of C$_4$H$_5$$^+$ (12.39 eV) is in excellent agreement with the experimental value (12.35$\pm$0.03) eV.

For the formation pathway of C$_3$H$_3$$^+$ ($m/z$$=$39), H4 of C1 in INT2 firstly transfers to C9 via a transition state (TS6) to form INT6. Secondly, H10 travels to C1 via a transition state of TS7 to form INT7 by overcoming the barrier of 1.4 eV. Thirdly, H10 and H3 have equal chance to contact with O12 to form INT8. As the C9-O12 bond is directly cleaved, the other $m/z$$=$54 structure (CH$_3$CCHCH$_2$$^+$) can be obtained. This structure of C$_4$H$_6$$^+$ (CH$_3$CCHCH$_2$$^+$) further loses a methyl group to form C$_3$H$_3$$^+$ ($m/z$$=$39). Then, one hydrogen shifts from the CH$_2$ group to the adjacent hydrogen-less carbon coupled with a ring closure process through a transition state (TS9) to form INT9 by overcoming an energy barrier of 1.25 eV. The AE of this conformation of C$_3$H$_3$$^+$ is 11.61 eV, which is in rough agreement with the experimental value (11.75$\pm$0.03) eV (see FIG. 8).

FIG. 8 Formation pathway of C$_3$H$_3$$^+$.
4. The formation pathways of C$_{2}$H$_{4}$O$^{+}$ (${m/z}$$=$44) and CHO$^{+}$ (${m/z}$$=$29)

For the production of C$_2$H$_4$O$^+$ ($m/z$$=$44), the C$_2$H$_4$O$^+$ can proceed by only eliminating CH$_2$CH$_2$ group through transferring of one hydrogen atom of methyl of INT3 to adjacent carbon by overcoming an energy barrier of 1.55 eV to generate an intermediate (INT10) where the bond C1-C2 is weak (see FIG. 9). Compared the experimental result of (10.10$\pm$0.03) eV with the theoretical calculation value (10.17 eV), the pathway for C$_2$H$_4$O$^+$ formation is reasonable.

FIG. 9 Formation pathways of C$_2$H$_4$O$^+$ and CHO$^+$.

For the formation of CHO$^+$, a hydrogen of hydroxyl in C$_2$H$_4$O$^+$ (F6) migrates to the methylene through TS11 with an energy barrier of 2.23 eV to produce CH$_3$CHCO$^+$ (INT11), and direct methyl group elimination forms CHO$^+$. The AE of CHO$^+$ is calculated to be 12.15 eV, which is consistent with experimental value (12.20$\pm$0.03) eV.

5. The formation pathway of C$_{2}$H$_{3}$O$^{+}$ (${m/z}$$=$43)

The roughly linear (excluding hydrogen) fragment C$_2$H$_3$O$^+$ ($m/z$$=$43) is produced by the pathway: P$\rightarrow$INT1$\rightarrow$INT3$\rightarrow$INT12$\rightarrow$INT13$\rightarrow$INT14$\rightarrow$C$_2$H$_3$O$^+$ +C$_2$H$_5$ (FIG. 10). One hydrogen on the C9 (INT2) transfers to C2 through TS11 with an energy barrier of 1.51 eV to form INT12, wherein the C1-C2 bond length is elongated from 1.49 Å to 1.59 Å. The process of shifting ethyl in the INT11 to the C9 needs to overcome an energy barrier of 0.94 eV to form TS12 which can then convert into INT13. Subsequently, one hydrogen of the hydroxyl in INT13 transfers to yield 2-butanone cation (INT14) via transition state TS14. C$_2$H$_3$O$^+$ is generated by C-C bond cleavage in INT14. The highest barrier along the pathway of formation of acetyl cation is 10.38 eV (relative to neutral parent), which is greater than the experimental AE (9.61 eV) of C$_2$H$_3$O$^+$. The reason is not known for the large discrepancy between the theoretical and experimental inquires.

FIG. 10 Formation pathway of C$_2$H$_3$O$^+$.
6. The formation pathways of C$_{3}$H$_{6}$$^{+}$ (${m/z}$$=$42) and C$_{3}$H$_{5}$$^{+}$ (${m/z}$$=$41)

The fragment of CH$_3$CHCH$_2$$^+$ is suggested via the pathway: P$\rightarrow$INT1$\rightarrow$INT3$\rightarrow$INT4$\rightarrow$INT15$\rightarrow$ CH$_3$CHCH$_2$$^+$$+$CH$_2$O (FIG. 11) wherein the first three steps are described above in detail. The migration of hydrogen atom on hydroxyl in INT4 toward the C1 leads to the formation of intermediate INT15 via the transition state TS15 with an energy barrier of 2.09 eV. The bond length of C1-C9 in INT15 is elongated from 1.46 Å to 1.70 Å, making it possible to dissociate fragment C$_3$H$_6$$^+$ ($m/z$$=$42). The calculated AE of 11.09 eV at the B3LYP/6-31+G(d, p) level, which agrees with the experimental value (11.24$\pm$0.03) eV. The fragment ion C$_3$H$_5$$^+$ ($m/z$$=$41) can be generated with the loss of one hydrogen in methyl from the above fragment (C$_3$H$_6$$^+$). The calculated AE of this ion is 12.35 eV, which is closed to experimental value (12.53$\pm$0.03) eV.

FIG. 11 Formation pathways of C$_3$H$_6$$^+$ and C$_3$H$_5$$^+$.
Ⅴ. CONCLUSION

The dissociative photoionization of MPO was carried out by photoionization mass spectrometry with synchrotron radiation as ionization source. The ionization energy of MPO and appearance energies for fragment ions were derived from their PIE curves. With help of theoretical calculation on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d, p) level, the formation pathways of the dissociation products were explored and twelve possible pathways were proposed—R1: C$_4$H$_7$O$^+$$+$H, R2: C$_4$H$_7$$^+$$+$OH, R3: C$_3$H$_5$O$^+$$+$CH$_3$, R4: CH$_2$OH$^+$$+$CHCH$+$CH$_3$, R5: C$_4$H$_6$$^+$$+$H$_2$O, R6: C$_4$H$_5$$^+$$+$H$+$H$_2$O, R7: C$_3$H$_3$$^+$$+$CH$_3$$+$H$_2$O, R8: C$_2$H$_4$O$^+$$+$C$_2$H$_4$, R9: CHO$^+$$+$CH$_3$$+$C$_2$H$_4$, R10: C$_2$H$_3$O$^+$$+$C$_2$H$_5$, R11: C$_3$H$_6$$^+$$+$CH$_2$O, R12: C$_3$H$_5$$^+$$+$H$+$CH$_2$O. The dominant processes in most of the dissociative photoionization of MPO are intramolecular hydrogen migrations.

Ⅵ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.91544105, No.U1532137, No.275127, No.U1232130, No.U1232209, and No.U11575178) and the Supercomputing Center of USTC is also acknowledged.

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2-甲基-2-丙烯-1-醇的光电离解离研究
余业鹏 , 李照辉 , 林烜 , 陈军 , 张航 , 李淹博 , 王欢欢 , 孙瑞瑞 , 孟庆慧 , 单晓斌 , 刘付轶 , 盛六四     
中国科学技术大学国家同步辐射实验室,合肥 230029
摘要: 利用可调谐真空紫外同步辐射和分子束实验装置在8.0$\sim$15.5 eV的光子能量范围内,研究2-甲基-2-丙烯-1-醇的光电离解离.测出母体离子和碎片离子:C$_4$H$_8$O$^+$、C$_4$H$_7$O$^+$、C$_3$H$_5$O$^+$、C$_4$H$_7$$^+$、C$_4$H$_6$$^+$、C$_4$H$_5$$^+$、C$_2$H$_4$O$^+$、C$_2$H$_3$O$^+$、C$_3$H$_6$$^+$、C$_3$H$_5$$^+$、C$_3$H$_3$$^+$、CH$_3$O$^+$和CHO$^+$的光电离效率曲线,并获得母体分子的电离能和碎片离子的实验出现势.在B3LYP/6-31+G(d, p)理论水平上,计算光电离过程中母体分子、过渡态和中间体的稳定结构.采用CCSD(T)/cc-pVTZ耦合簇方法计算零点能,得到母体电离能和碎片离子的出现势.通过实验和理论研究,提出2-甲基-2-丙烯-1-醇的光解离路径,分子内氢转移是其中大部分解离途径中的主要过程.
关键词: 2-甲基-2-丙烯-1-醇    同步辐射    光电离    真空紫外线    质谱