b. University of Science and Technology of China, Hefei 230026, China;
c. College of Chemistry and Environment, Minnan Normal University, Zhangzhou 363000, China;
d. School of Mathematics and Physics, Changzhou University, Changzhou 213164, China;
e. School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230026, China;
f. School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230029, China
Predominantly emitted by vegetation, isoprene (2-methyl-1, 3-butadiene, C5H8) is one of the most important biogenic non-methane hydrocarbon emitted into the atmosphere . It is also the basic structural unit of terpenes and sesquiterpenes . For its high abundance and reactivity towards atmospheric radicals, isoprene plays a major role in the oxidative chemistry of the troposphere [3-6], and is also an important precursor for secondary organic aerosol (SOA) [7-10]. Even though the yield is minor, the total contribution of isoprene to entire atmospheric particulate organic matter could be large owing to the great global source strength [11, 12].
Numerous experimental and theoretical investigations have been made on isoprene oxidation mechanism and products detection for reactions of isoprene with hydroxyl radical (OH) [13-17], ozone (O3) [18-22], and the nitrate radical (NO3) [23-26]. However, the dissociation mechanism of isoprene molecule has been relatively little studied.
In the past several decades, various spectroscopic techniques have been used in isoprene and its fragment ions study [27-31]. Although the ionization energy (IE) of isoprene molecule and a part of appearance energies (AEs) of fragment ions have been determined, they are lack of sufficient accuracy.
Synchrotron radiation photoionization mass spectrometry is an excellent method to investigate unimolecular dissociative photoionization [32-36] and photooxidation products [37, 38]. Our group has studied the VUV dissociative photoionization of isoprene in the energy range of~8.5-15 eV, determined the IE of isoprene and the AEs of its fragmental ions, and also calculated their dissociative energy . However, the detailed mechanisms of the fragmentation pathways have not been clarified and certified. In this work, we explored dissociative photoionization of isoprene theoretically and compared the theoretical values with our previous experimental results.II. COMPUTATIONAL METHOD
The CBS-QB3 model, combined general design of the CBS-Q energy calculation with B3LYP DFT optimized geometries and frequencies, is usually faster and slightly more accurate than the G2 model . In the theoretical study, all of the geometry optimizations and energy calculations of the reactants, transition states, intermediates, and products were performed at the CBS-QB3 level. The structures of the transition states and intermediates for dissociative photoionization channels were also identified in this study, and all of the reaction pathways were confirmed with intrinsic reaction coordinate calculations. The quantum chemistry calculations were carried out using the Gaussian 09 software package .III. Results and discussion
Our experimental investigation of isoprene disso-ciative photoionization were described elsewhere . Brie y, the photoionization mass spectrum of isoprene was determined at the photon energy of 17.00 eV. In the spectrum, six major peaks at m/z=67, 53, 42, 41, 40, and 27 are assigned to be C5H7+, C4H5+, C3H6+, C3H5+, C3H4+ and C2H3+ respectively. Three medium peaks at m=z=65, 39 and 26 are assigned to be C5H5 +, C3H3 + and C2H2 + respectively. Also the IE of isoprene molecule and AEs of eight major fragment ions C5H7+, C4H5+, C3H6+, C3H5+, C3H4+, C3H3+, and C2H3+ have been given.
Table I lists the experimental and theoretical values of the IE of parent ion and the AEs of major fragment ions involved in dissociative photoionization of isoprene. The dissociation energies and dissociation channels are also listed in Table I. The dissociation energy (D0) is calculated by subtracting IE of parent molecule from AE of relative fragment ion. Total energies of species involved in the dissociation procedure are listed in Table II.
Parent ion is formed directly via a VUV single photon ionization procedure (C5H8+photon→C5H8++e). When the photon energy goes up, the parent ion undergoes different dissociation procedures. The dissociation mechanisms are discussed as below. In this paper, the products, intermediates and transition states are named with a unique number with the prefix of P (product), INT (intermediate) and TS (transition state) respectively. In the case of isomers, suffix of a, b and c are applied, such as TS1, INT1, P1b, and so on. The results of the optimization structures of the neutral molecule, parent ion are shown in Fig. 1.A. C5H8+→C4H5 ++CH3 & C5H8 +→C2H3 ++C3H5
Mostly, there are two types of dissociative mechanisms, direct bond fragmentation and indirect bond fragmentation via transition states and intermediates. The C4H5+ (P3, m/z=53) can be produced by directly cleaving a methyl group from the parent ion. The direct cleavage energy of C2-C5 bond is 11.53 eV, which is consistent with the experimental value of 11.43±0.02 eV.
The length of C2-C3 bond for C5H8+ is 1.424 Å. It can break to form C2H3- (P8, m/z=27) and C3H5 directly. This is a one-step decomposition process without evident transition states. The reaction coordinate path for the bond dissociation is computed at the B3LYP/6-311G (d, p) level to cover the C-C bond with separation from 1.4 Å to 5.4 Å, with an interval step size of 0.04 Å (shown in Fig. 2). The experimental AE of 13.49±0.02 eV agrees very well with the theoretical one of 13.50 eV.B. C5H8+→C5H7 ++H & C5H8+→C5H5 ++H2+H
It is obvious that there are four nonequivalent H atoms in the parent ion. In view of this we calculated four direct H atom elimination energies initially and compared them with the experimental value (10.71±0.02 eV). The length of C5-H10 bond for C5H8+ is 1.097 Å, which is the longest C-H bond among the parent ion. Our computation results also show it is the lowest AE with H elimination. Even so, the theoretical AE of H10 elimination from C5 in P1a is 11.83 eV, approximately 10% deviating from the experimental one.
There are two alternative isomerization pathways to form C5H7+ (m/z=67) fragment ion. One of them generating P1b is shown in Fig. 3. At first, H11 migrates from C3 to the adjacent C4 forming a methyl group in intermediate INT1 via the transition state TS1. As the C1 close to C3, a three-membered ring in INT2 generates via TS2. In the three-membered ring, H eliminations are from C1 to form P1b via TS3. The highest energy of this dissociation channel is 10.95 eV.
There is another somewhat complicated dissociation channel to generate P1c. For the intermediate INT2, H7 migrates from C1 to C2 gradually and C2-C3 bond begins breaking to form INT3. Similar to H elimination procedure in INT2, H13 elimination in INT3 forms P1c (Fig. 4). The theoretical AE for P1c is 11.66 eV. Compared with the pathways generating P1a and P1c, the one forming P1b has a closer AE value to the experimental AE.
It is evident that the m/z 65 must come from a sequential reaction of m/z 68-67-65 (Fig. 4). With regard to the structure P1c, H8 and H9 detach from C5 gradually and C5-C3 begins bonding simultaneously. As the dissociation energy value is smaller than that relative energy of transition structure TS6, the relative energy of TS6 (13.74 eV) is the reaction barrier of this dissociation channel. It is in good agreement with the experimental value of 13.67±0.02 eV.C. C5H8+→C3H6++C2H2 & C5H8+→C3H5++C2H2+H
Detailed formation pathway of C3H6+ (P4, m/z=42) is depicted in Fig. 5. With the deformation of bond angle C2-C3-C4, H13 is exposed to C2 and transfers from C4 to C2 to form INT5 via TS8 simultaneously. In the structure of INT5, the line of C3-C4 bond is parallel to the plane of C5-C2-C1. For the three-membered ring C1-C3-C4, the length of C1-C3 and C1-C4 bonds are 2.748 and 2.520 Å respectively, easily breaking to form C3H6+ (P4) and C2H2. Theoretical study indicates that the highest energy of this dissociation channel is 11.63 eV (TS8), which agrees with the experimental value 11.50±0.02 eV reasonably.
There are three nonequivalent H atoms in the structure of P4. In consideration of structure stability, only H elimination from C2 and C5 was considered, to generate the corresponding P5a and P5b. Both the theoretical AEs for P5a (13.97 eV) and P5b (13.65 eV) agree with the experimental value 13.73±0.02 eV, but the later may be the dominant way because of its relative lower AE.D. C5H8+→C3H4 ++C2H4 & C5H8 +→C3H3 ++C2H4+H
Figure 4 also shows a C3H4+ (P6a, m/z=40) dissociation pathway in detail. Towards the intermediate INT3, an H atom migrates from C5 to C2 and produces INT4 at the same time. The INT4 can decompose into fragment ion P6a and a neutral molecular C2H4 by breaking the weak C1-C2 bond. The corresponding theoretical AE for P6a is 12.00 eV.
An alternative decomposition channel generating C3H4+ (P6b, m/z=40) is described in Fig. 6. First, a methyl group transfers from C2 to C1 via TS9. Then, isomerization of INT6 generates INT8 by two H migration procedures in sequence: H13 shifts from C4 to C2 and H8 shifts from C5 to C2. After that, the P6b fragment ion is formed by loss of C2H4, with a dissociation energy 12.40 eV.
Based on the above mentioned, the P6b dissociation pathway is expected to make a minor contribution, because its theoretical AE is higher than P6a. As the photon energy rises, however, the structure of P6b forms a three-membered ring (Fig. 7). This leads to a centrosymmetric structure P7 (C3H3+, m/z=39) formed by H loss from C2 in INT9. The theoretical AE for P7 is 13.27 eV, which agrees better with the experimental value 13.48±0.02 eV compared to the previous calculation .IV. Conclusion
Theoretical investigation is performed on electronic energies and geometries of isoprene, its ionic forms, and the molecular species (ionic and neutral fragments) that are eventually produced as a result of the fragmentation channels of isoprene after photoionization. The theoretical IE and AEs are in reasonable consistence with experimental ones. On comparison of theoretical and experimental results, the mechanisms of the dissociation pathways are discussed in detail. Both direct dissociation and isomerization of the parent ion exist during the fragment forming procedure. Most of the dissociation path are multi-steps involving the transition state structure and intermediate, and some of them must be over a reaction barrier during the dissociation process. H shift and ring formation are found to be dominant in the isomerization of the parent ion.5. Acknowledgments
This work was supported by the National Natural Science Foundation of China (No.91544228, No.21307137, No.41575125, No.41375127, No.U1232209) and the Outstanding Youth Science Foundation of Fujian Province of China (No.2015J06009).
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b. 中国科学技术大学, 合肥 230026;
c. 闽南师范大学化学与环境学院, 漳州 363000;
d. 常州大学数理学院, 常州 213164;
e. 中国科学技术大学环境科学与光电技术学院, 合肥 230026;
f. 中国科学技术大学核科学技术学院, 合肥 230026