ARTICLE pubs.acs.org/JPCA
Positive and Negative Photoion Spectroscopy Study of Monochlorothiophenes Yun-Feng Xu,† Shan Xi Tian,*,† Liuli Chen,† Fu-Yi Liu,‡ and Liusi Sheng‡ †
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
bS Supporting Information ABSTRACT: Photolysis dynamics of monochlorothiophenes (2- and 3-chlorothiophenes) is investigated using positive and negative photoion mass spectrometry combined with the synchrotron vacuum ultraviolet radiation. A dozen of the daughter cations are observed in the time-of-flight mass spectra, and their appearance energies are determined by the photoion efficiency spectroscopy measurements. At the energetic threshold, the concerted process rather than a stepwise reaction for C4H3SCl+ f C2HSCl+ + C2H2 and the ring-open isomers of the dehydrogenated thiophene cations (C4H3S+ and C4H2S+) formed in C4H3SCl+ f C4H3S+ + Cl and C4H2S+ + HCl are proposed on the basis of the B3LYP/6-311+G(3df,3pd) calculations. The chlorine anion (Cl) is observed as the product of the photoion-pair dissociations in the energy range of 10.7022.00 eV. A set of valence-to-Rydberg state transitions 12a0 f np (n = 6, 7, 8, 9, 10, etc.) and several series of vibrational excitations are tentatively assigned in the Cl spectrum of 2-chlorothiophene in the lower energy range of 10.9012.00 eV.
1. INTRODUCTION The gas-phase chemistry of an ion is a fundamental area of research, providing the basic thermochemical data of molecules (e.g., bonding energies, ionization potentials, etc.) and an abundant of information on the related reaction dynamics. Unimolecular dissociation is an important way to produce the fragment ions, in particular, induced by vacuum ultraviolet (VUV) photons.1 Positive photoions are normal products in the photoionization dissociations, while negative photoions as well as the correlative positive ions are the unique yields in the photoion-pair dissociations.2 In this work, the photoionization and photoion-pair dissociation dynamics of monochlorothiophenes (2-chlorothiophene, noted as 2-Cl-Th and 3-chlorothiophene, noted as 3-Cl-Th) is investigated by means of the positive and negative photoion mass spectrometry. The photochemistry of thiophene and its derivatives received considerable attention due to the possible photoinduced rearrangements of the neutral38 and the positive ion.912 On the structure of thiophene cation C4H4S+, the structural rearrangement from the planar structure of the neutral to the Ladenburg conformation9,10 is queried by a series of studies.11,12 In the more recent work, Rennie et al.13 reexamined the dissociation dynamics of thiophene cation, C4H4S+ f C2H2S+ + C2H2, and concluded that this was a concerted dissociation process (i.e., by the concerted bond breakings of CC and CS) rather than through the rearrangement of C4H4S+ to so-called Ladenburg9,10 or linear structure.11 Monochlorothiophenes as the substituted thiophene are suitable for clarifying the above arguments. The different chlorine-substituted positions in 2-Cl-Th and r 2011 American Chemical Society
3-Cl-Th may influence the dissociation dynamics of the parent ions. Meanwhile, the valence isomerization between 2- and 3-substituted thiophenes8 should be considered in the analysis of the photoionization dissociations of monochlorothiophenes. On the other hand, the cross sections of photoion-pair dissociation are usually much lower than those of photoionizaition dissociation, especially for the polyatomic molecules and above the molecular ionization threshold (also known as the adiabatic ionization potential, IPa).2 However, the yields of the photoion-pair dissociation of the polyatomic molecule, namely, the neutral radical and the positive and negative ions, are extremely active in chemistry and exceedingly possible to initiate more subsequent reactions. The photoion-pair dissociation of a triatomic molecule ABC may be, ABC þ hν f A þ BCþ , f A þ Bþ þ C or f A þ B þ Cþ , etc:
ð1Þ
The appearance energy (AE) of a certain cation, for example, C+ in ABC + hν f A + B + C+ + e, is determined by recording the photoion efficiency (PIE) curve of C+, but the same experimental route, that is, the collection of the C+ signal, cannot be applied for the investigation of the photoion-pair dissociation ABC + hν f A + B + C+, because the C+ signal of the photoionization dissociation ABC + hν f A + B + C+ + e will have serious Received: June 9, 2011 Revised: August 30, 2011 Published: September 05, 2011 10920
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interference with that produced in the photoion-pair dissociation. Therefore, for a certain channel in (1), for example, ABC + hν f A + B + C+, its energetic threshold can be reasonably determined by the AE value of the negative ion A (i.e., detecting A using the anion mass spectrometry), AEðA Þ g EThreshold ðA Þ IPa ðABCÞ þ D0 ðA þ B þ Cþ Þ EAðAÞ
ð2Þ
where EThreshold(A ) is the thermodynamic threshold, D0 is the dissociation energy of (ABC)+ f A + B + C+, and EA is the electron affinity of fragment A (usually the positive value for halogen atom). IPa and D0 values can be determined by the quantum chemistry calculations or as the experimental AE values of (ABC)+ and C+, respectively. If there is a dynamic energy barrier in ABC + hν f A + B + C+, the AE(A) determined in the anion mass spectrum is usually higher than the theoretical EThreshold(A). In this work, the chlorine anion Cl as the product in the ion-pair dissociation of monochlorothiophenes is observed in a wide photon energy range of 10.7022.00 eV. The thermodynamic values in eq 2 are required in the interpretations to the ion-pair anion efficiency spectra (IPAES) of Cl,1416 because a lot of dissociation paths may be involved in such wide energy range. Moreover, some electronic and vibrational excitations may couple with the ion-pair dissociations, showing the discrete structures or peaks in the IPAES.1418 Therefore, the interpretations to the IPAES and the spectral assignments heavily depend on the other spectral sources such as photoabsorption, photoelectron, and photoion mass spectra. However, only photoelectron spectra (PES) of monochlorothiophenes are available;19 thus, the present PIE spectra of monochlorothiophenes will play an essential role in the interpretations to the IPAES.
2. EXPERIMENTAL TECHNIQUES AND THEORETICAL CALCULATIONS The production efficiency spectra of the positive and negative photoions were measured at the atomic and molecular physics end-station of the National Synchrotron Radiation Facility at Hefei, China. The apparatus used in the present ion-pair photodissociation study was described previously in detail.1416 Briefly, the VUV photon beam was monochromatized with the undulator-based spherical-gratings (made by Horiba Jobin Yvon) then focused onto the reaction region. In detection of the positive photoions, a supersonic molecular beam (buffered with Ar gas) installed perpendicularly to the plane containing the VUV photon beam and the time-of-flight (TOF) tube was used. Because of the much lower efficiency of the photoion-pair dissociations, it was critical to make an alignment between the molecular beam and the VUV photon beam. In the anion mass spectrometer, a diffuse molecular beam was introduced with a stainless steel tube (its diameter: 0.5 mm), and the outlet was less than 2 mm away from the VUV photon beam. The cationic or anionic fragments were collected with a homemade reflectron TOF mass spectrometer. Packets of photoions were pushed periodically from the reaction region by a pulsed repeller ((165 V voltage, 1.52.0 μs pulse width, and 18 000 Hz repetition) into an acceleration region. The pulsed ions were focused and transferred through the drift area and then reflected by the retarding lenses. At last the photoions were detected with two zig-zag stacking microchannel plates. The wavelength of VUV beam was calibrated and the energy resolution was determined
Figure 1. Mass spectra of 2-Cl-Th (top) and 3-Cl-Th (bottom). The red asterisk, *, and blue pound sign, #, represent the S and Cl isotopic cations, respectively.
(E/ΔE = 5000) before the present experiments. The energy scanning step and the signal accumulation time at each step were typically 0.02 eV and 60 s, respectively. To normalize the photoion signals, the photon flux was monitored with a silicon photodiode (SXUV-100, International Radiation Detectors, Inc.). The commercial samples 2-Cl-Th and 3-Cl-Th (purity >99.9%) were used after several freezepumpthaw cycles. Because of lack of the formation enthalpy values of the molecules and ions, standard ab initio calculations were performed with the Gaussian 03 program.20 The geometries of the neutral and singly charged species were optimized, and the harmonic vibrational frequencies were calculated at the B3LYP/6-311+G(3df,3pd) level. The choice of this computational method was based on two points: this method was successfully applied in the theoretical studies of photoionization dissociations of thiophene and pyrrole;12,13 to explore the Cl-substitution effect on the photodissociation dynamics, the same level of theory was needed for comparison of the calculated results between thiophene12,13 and monochlorothiophenes. The threshold energy of a certain dissociation channel (neutral + cation or cation + anion) was calculated including the zero-point vibrational energy corrections. The experimental EA value, 3.6144 eV, of Cl21 was used to estimate the EThreshold values in eq 2.
3. RESULTS AND DISCUSSION In the photoionization dissociations, the parent cation and about 14 daughter cations are detected at the photon energy of 10921
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Figure 2. PIE curves of the sulfo-chloro cations (left for 2-Cl-Th and right for 3-Cl-Th). The AE values are given with arrows.
18.00 eV, as shown in Figure 1. The natural isotope abundance ratio 35Cl/37Cl is about 3:1, thereby the cations containing the rare 37Cl isotope are also observed (labeled with a pound # in the upper panel of Figure 1). The natural isotope abundances of 32S, 33S, 34S, and 36S are 95.02%, 0.75%, 4.21%, and 0.02%, respectively. The cations containing the rare 34S can be observed (labeled with an asterisk, *, in the upper panel of Figure 1). The cation with the mass-charge ratio (m/z) of 81 could be either 34 SC35Cl+ or 32SC37Cl+, but here it is labeled as the rare 37Cl isotopic cation 32SC37Cl+ (labeled with a pound sign, #) according to the following analysis in the text. The PIE spectra of the cations m/z = 118, 92, 83, 82, 79, 73, 69, 58, 57, 45, and 39 are depicted in Figures 2, 4, and 6. These cations can be classified as four types: the sulfo-chloro, the sulfo, the chloro, and the
hydrocarbon cations. In the photoion-pair dissociations, only Cl (35Cl and 37Cl) are detected, and no isotopic effect is observed in the IPAES of these two Cl anions. The production efficiency curves of 35Cl are depicted in Figure 7 for 2-Cl-Th (the upper panel) and 3-Cl-Th (the below panel). In the Supporting Information, the structures and the total energies of the molecules and the fragments that may be produced in the photolysis are shown in Figures S1S5. Positive Photoions (m/z = 118, 92, and 79). Besides the parent cation C4H3SCl+ (m/z = 118), two daughter sulfo-chloro cations (C2HSCl+ and CSCl+) are detected, and their PIE curves are shown in Figure 2b, c, e, and f. The IPa value of 2-Cl-Th was previously determined to be 8.694 eV,10 which is slightly higher than the present value 8.58 ( 0.04 eV (shown with an arrow in 10922
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Table 1. Appearance Energies and the Theoretical Thresholds of the Positive Photoions appearance energy of positive photoion/eV m/z +
118(C4H3SCl ) 92(C2HSCl+)
3-Cl-Th
8.58 ( 0.04
8.72 ( 0.04
11.65 ( 0.16
11.94 ( 0.12
dissociation path +
C4H3SCl
2-Cl-Th
3-Cl-Th
8.47
8.66
C2HSCl+ (1) + C2H2
11.60
11.71
C2HSCl+ (2) + C2H2
12.48
12.58
C2HSCl+ (3) + C2H2
12.34
12.45
83(C4H3S+)
11.80 ( 0.04
11.90 ( 0.04
C4H3S+ + Cl
13.11a
12.75a
82(C4H2S+)
12.55 ( 0.20
12.82 ( 0.10
C4H2S+ (1) + HCl
13.31
13.42
C4H2S+ (2) + HCl
12.05
12.16
C4H2S+ (3) + HCl C4H2S+ (4) + HCl
11.16 10.48
11.27 10.59
12.72 ( 0.10
+
79(SCCl ) +
73(C3H2Cl )
13.18 ( 0.10
12.76 ( 0.10 13.25 ( 0.10
C4H2S+ (5) + HCl
10.00
10.11
SCCl+ + CH2CCH
12.05
12.16
SCCl+ + cyclo-C3H3
13.53
13.63
cyclo-C3H2Cl+ + HCS
12.36
12.47
CHClCCH+ + HCS
13.04
13.14
CH2CCCl+ + HCS
13.05
13.16
69(C3HS )
12.45 ( 0.50
12.65 ( 0.40
C3HS+ (1) + CH2Cl C3HS+ (2) + CH2Cl
11.89 15.15
12.00 15.26
58(C2H2S+)
12.24 ( 0.10
12.46 ( 0.10
C2H2S+ (3) + HCCCl
11.76
11.87
C2H2S+ (2) + HCCCl
12.52
12.62
C2H2S+ (1) + HCCCl
12.91
13.02
C2HS+ (1) + CH2CCl
14.87
14.98
C2HS+ (2) + CH2CCl
15.39
15.50
C2HS+ (3) + CH2CCl
16.97
17.08
HCS+ + CHClCCH HCS+ + CH2CCCl
12.54 12.72
12.65 12.83
+
57(C2HS+)
45(CHS+)
39(C3H3+) a
2-Cl-Th
theoretical threshold/eV
16.23 ( 0.50
13.15 ( 0.06
12.85 ( 0.08
15.81 ( 0.50
13.17 ( 0.06
13.00 ( 0.08
HCS+ + cyclo-C3H2Cl
13.97
14.08
cyclo-C3H3+ + SCCl
12.13
12.24
CH2CCH+ + SCCl
13.20
13.31
Cations of 2-dehydrothiophene and 3-dehydrothiophene are used, respectively. More discussions can be found in the text.
Figure 2a). The present experimental IPa of 3-Cl-Th, 8.72 ( 0.04 eV (see Figure 2d), is a little higher than that of 2-Cl-Th. This is in line with the order of the present theoretical IPa values, namely, 8.66 eV (3-Cl-Th) > 8.47 eV (2-Cl-Th). 2-Cl-Th and 3-Cl-Th (Cs symmetry) have the same electron configurations at the ground state, (1a0 )2 (2a0 )2 (3a0 )2 (4a0 )2 (5a0 )2 (6a0 )2 (7a0 )2 (8a0 )2 (9a0 )2 (10a0 )2 (1a00 )2 (11a0 )2 (2a00 )2 (12a0 )2 (3a00 )2 (4a00 )2 X1A0 . The first IPa value is the energy of the electron promotion from the highest occupied molecular orbital (HOMO) 4a00 (π3), and the IPa difference for two isomers is arising from the different stabilities of the neutral or cations. As shown in Figure S1 of the SI, the neutral 3-Cl-Th is 0.108 eV more stable than 2-Cl-Th, while the cation (3-Cl-Th)+ is slightly higher (0.080 eV) in energy than (2-Cl-Th)+. In Figure 2b and 2e the AE difference of C2HSCl+ (m/z = 92) for two molecules is clearly shown. Three dissociation channels for this daughter cation are considered in this work. As shown in Figure S2 of the SI, there are three isomers of C2HSCl+, that is, thiolchloroketen CHClCS+ (1), chloroethynethiol ClCCSH+ (2), and cyclo-CHCClS+ (3). In Table 1, the theoretical thresholds of these dissociation pathways, C4H3SCl + hν f C2HSCl+ (1, 2, or 3) + C2H2, are listed. The last two channels to produce C2HSCl+ (2) and C2HSCl+ (3) can be ignored due to their higher thresholds with respect to the experimental AEs. Only the
EThreshold values of C4H3SCl + hν f C2HSCl+ (1) + C2H2 are much closer to the experimental AEs, but the EThreshold difference between 2-Cl-Th and 3-Cl-Th is much smaller than that of the experimental AEs. As mentioned above, such deviations may be due to their variant photodissociation dynamics. In the previous study of the photoionization dissociation of thiophene,12,13 the corresponding cation C2H2S+ produced near the threshold was hypothesized to be the linear thiolketen CH2CS+. Moreover, the concerted dissociation process (i.e., by the concerted bond breakings of C3C4 and C5S) rather than through the rearrangement of C4H4S+ to so-called Ladenburg9,10 or linear structure11 was proposed.12,13 Therefore, we conjecture that it should be in a similar way to produce C2HSCl+ (1). In Figure 3, the stepwise dynamic processes from the parent cations to C2HSCl+(1) + C2H2 are plotted on the basis of the B3LYP/6-311+G(3df,3pd) calculations. The corresponding process of C4H4S+ f C2HSCl+(1) + C2H2 was cited from ref 13 for comparison. One can find that the energies of both transition states (TS's) and the intermediates (X and Y) are enhanced for the monochlorothiophenes with respect to those for thiophene. The distinct energy difference of TS1 for two monochlorothiophenes is 0.243 eV, and the TS1 state of 3-Cl-Th is higher in energy. This is in good agreement with the observations of C2HSCl+, the lower AE of 11.65 ( 0.16 eV for 2-Cl-Th, while the higher AE of 11.94 ( 0.12 eV for 3-Cl-Th. 10923
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Figure 3. Minimum energy reaction pathways for loss CHClCS+ from the ionized 2- and 3-Cl-Th. The values for the ionized thiophene are cited from ref 13.
These errors include the bandwidth of our monochromator and ionization threshold uncertainty on the PIE spectra. The uncertainties estimated with the onset of PIE spectra are strongly dependent on the spectral profiles; in particular, the larger uncertainties are usually estimated for the gradually rising profiles.22 It is noted that, in the process from the parent cation C4H3SCl+ to X, the mechanisms are significantly different; namely, the chlorine shifts from C2 to C3 positions in 2-Cl-Th+, while the hydrogen shifts from C2 to C3 positions in 3-Cl-Th+. Moreover, the concerted dissociation (X f products) proceeds more easily for monochlorothiophenes than thiophene because the energetic differences between X and the products (CHClCS+ + C2H2) are less than 0.5 eV while that for thiophene is more than 1.0 eV. In Figure 1, we observe two peaks around m/z = 80. They are identified as 32SC35Cl+ (m/z = 79) and 32SC37Cl+ (m/z = 81) based on the following two points: the intensity ratio I(m/z=79)/ I(m/z=81) ≈ 3:1, which is equal to the natural isotope abundance ratio 35Cl/37Cl; as shown in Figure 2c, the AEs of these two cationic fragments for 2-Cl-Th are almost same, that is, 12.72 ( 0.10 eV. It is interesting that SCCl+ for 3-Cl-Th has a similar AE value, 12.72 ( 0.10 eV. It is obvious that the formation of this cationic fragment from 2-Cl-Th+ should be much easier in dynamics than 3-Cl-Th+ because the S, C2, and Cl atoms are in vicinity in the former while the Cl atom is a little far from S atom in the latter. However, the similar AE values of SCCl+ for these two molecules imply that SCCl+ could not be formed by the direct bond cleavages; that is, via the C2C3 and C5S bond breaks. Two mechanisms are postulated here: in the precursor 3-Cl-Th+, after the valence isomerization from 3- to 2-substituted thiophenes,8 the C2C3 and C5S bonds are broken to form SCCl+; alternatively, in the rearrangement of C4H3SCl+ to the so-called Ladenburg intermediate,9 SCCl+ may be formed in a concerted dissociation. According to the theoretical thermodynamic energies listed in Table 1, we find that the formation of the neutral propargyl radical CH2CCH is much more favorable than the cyclo-C3H3 in the dissociation; thus, the dissociation to SCCl+ + cyclo-C3H3 is not accessible near the photon energy of the AE value. In contrast to the present case, the corresponding dissociation
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C4H4S (thiophene) + hν f CHS+ + cyclo-C3H3 has the experimental threshold of 13.013.19 eV.12 Here the theoretical thresholds of the dissociations to SCCl+ + CH2CCH are 12.05 and 12.16 eV for 2-Cl-Th and 3-Cl-Th, respectively. These values are smaller than their respective AEs, implying that the energy barriers in the indirect dissociations (as postulated above) should be overcome. Positive Photoions (m/z = 83, 82, 69, 58, 57, and 45). As shown in Figure 4 and Table 1, the AE values of C4H3S+ (m/z = 83), C4H2S+ (m/z = 82), C3HS+ (m/z = 69), C2H2S+ (m/z = 58), C2HS+ (m/z = 57), and CHS+ (m/z = 45) are determined by the experiment and theoretical calculations. In this photon energy range, these sulfo cations together with the S+ species were detected in the photoionization dissociation of thiophene.12 In the photoionization dissociations of monochlorothiophenes, the AEs of C4H3S+ are 11.80 ( 0.04 eV and 11.90 ( 0.04 eV for 2-Cl-Th and 3-Cl-Th, respectively. To our surprise, both of these two values are distinctly different from their respective theoretical thresholds, 13.11 eV (2-C4H3S+ + Cl) and 12.75 (3-C4H3S+ + Cl). The theoretically optimized structures of 2-/3-C4H3S+ can be found in Figure S2 of the SI. It is noted that the AE of C4H3S+ for thiophene is experimentally determined to be ca. 13.05 eV, but without any theoretical identifications of C4H3S+.12 In the present case, the deviations for C4H3S+ between the experimental AE and the theoretical EThreshold may be due to that the 2-/3-C4H3S+ in Figure S2 of the SI are not the global minima on the potential energy surface of C4H3S+; in other words, the fivenumbered ring structure of C4H3S+ is potentially opened. This speculation is further proved for C4H2S+. In Figure 4b, the AEs of C4H2S+ are a little different between two monochlorothiophenes, implying the possible different dynamic processes in the hydrogen and chlorine eliminations. In the previous study of thiophene, it was hypothesized that both C4H3S+ and C4H2S+ cations might undergo a hydrogen loss metastable transition.12 In the present case, the direct hydrogen loss to form C4H3S+ or C4H2S+ is not favorable in energy, because their EThreshold values of C4H3S+ and C4H2S+(1) are much higher than their experimental AE values. As shown in Figures 5 and S3, the other four isomers of C4H2S+ (25) may be produced near the experimental AEs. More interestingly, the corresponding neutral radicals C4H2S 3 , as well as their transition dynamics from 2,5-didehydrothiophene, have been investigated with the high-level ab initio calculations23 and the matrix IR spectroscopy.24 The cumulene CH2CCCS 3 is the most stable isomer, and the ring-structure 2,5-didehydrothiophene is the highest-lying isomer.24 This energetic order is maintained for their corresponding cations; the butatrienethione cation C4H2S+ (5) is the lowest-lying, while the 2,5-dihydrothiophene cation C4H2S+ (1) is much higher in energy. These ring-opening C4H2S+ cations near the AE values are hardly formatted via C4H2S+ (1) which is produced by the direct hydrogen eliminations. At the photon energies of 12.5512.82 eV, the electrons of 11a0 (3p lone-pair of S) can be shaken off.19 The five-numbered ring may be opened together with the structural rearrangement24 during the electron promotion from the 11a0 orbital, and subsequently the C4H2S+ (25) can be produced. In the PIE curves of C3HS+ (m/z = 69), see Figure 4c, the AE values are 12.45 ( 0.50 eV and 12.65 ( 0.40 eV, where the large uncertainties are due to the very gradual increases of the ion signals. Two possible isomers of C3HS+ are depicted in Figure S4 of the SI, and only C4H3SCl + hν f C3HS+(1) + CH2Cl is favorable near the AE (see Table 1). As shown in Figure S4, there 10924
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Figure 4. PIE curves of the sulfo cations (the black circles for 2-Cl-Th and the red circles for 3-Cl-Th). The AE values are given with arrows.
are three isomers of C2H2S+, which has been studied in the PIE experiment of thiophene.12 In Figure 4d, two PIE curves show the different AE values of C2H2S+, implying the possible different dynamic processes for 2- and 3-Cl-Th. As listed in Table 1, only the EThreshold of the dissociation path C4H3SCl + hν f C2H2S+(3) + HCCCl is lower than the AE values, while the other two having the products C2H2S+ (1) and C2H2S+ (2) can be discounted since their EThreshold values are higher than the AE values. As mentioned by Rennie et al., the formations of thioketene cation C2H2S+ (1) and cyclo-C2H2S+ (2) would necessitate a more significant rearrangement of the molecular structure.12 Therefore, the C2H2S+ (3) yield is quite preferred both in energy and dynamics. In Figure 4e and 4f, one can find that the AE values of C2HS+ (m/z = 57) and CHS+ (m/z = 45) are higher than those of C2H2S+ (m/z = 58). The relative high AE of C2HS+ (m/z = 57)
17.41 ( 0.08 eV was also found for thiophene.12 In the present case, as listed in Table 1, two pathways, C4H3SCl + hν f C2HS+(1) + CH2CCl and f C2HS+(2) + CH2CCl, can be accessed at the AEs. See Figure S5 of the SI; one can find the structural similarities between C2HS+ and C2H2S+ (as shown in Figure S4), and thus the C2HS+ cations may be produced via the sequential dehydrogenation of C2H2S+. However, this direct dehydrogenation process is not energetically favorable; for instance, the threshold of the cascade path, C4H3SCl + hν f C2H2S+(3) + HCCCl f C2HS+(1) + HCCCl + H for 2-Cl-Th, is 16.91 eV which is a little higher than the AE value of 16.23 eV. The mechanism that C2HS+ is directly originated from C2H2S+12 can be ruled out, while the possible path to C2HS+ may be the rearrangement reaction of the parent cation or the photoion collision reactions. The photoion collisions usually result in the lower AE and more small fragments.22 The AEs of CHS+ 10925
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(m/z = 45) are almost same for 2- and 3-Cl-Th and a little higher than that (12.81 ( 0.05 eV) of CHS+ for thiophene.12 In the present case, the dissociation to CHS+ and the cyclo-chloropropenyl radical (cyclo-C3H2Cl) is discounted near the AE, due to its higher EThreshold values. On the contrary, the other two paths to produce the chloropropargyl radicals (CHClCCH and CH2CCCl) show the lower thresholds. Positive Photoions (m/z = 73 and 39). Only one chloro cation C3H2Cl+ is observed in Figure 1, and its PIE curve is plotted in Figure 6a and 6c for 2-Cl-Th and 3-Cl-Th, respectively. As shown in Table 1 and Figure S4 of the SI, three isomers of C3H2Cl+ may be formed near the AEs. In contrast to the
+
corresponding neutral radicals, the cyclo-chloropropenyl cation (cyclo-C3H2Cl+) is much more stable than the other two chloropropargyl cations (CHClCCH+ and CH2CCCl+). In Figure 1, there are at least four hydrocarbon cations produced in the photoionization dissociations, that is, C3H2+ (m/z = 38), C3H3+ (m/z = 39), C4H2+ (m/z = 50), and C4H3+ (m/z = 51). The PIE curves of C3H3+ (m/z = 39) are shown in Figure 6b and 6d while the others are not given due to their much low signals. The C4H+ (m/z = 49), C3H4+ (m/z = 40), C3H+ (m/z = 37), C2H3+ (m/z = 27), and C2H2+ (m/z = 26) that have been observed in the previous study for thiophene12 are not prominent for monochlorothiophenes. It is noted that the C3H4+ (m/z = 40) cation may appear in Figure 1, but it may be screened from the Ar+(m/z = 40) signals (here the argon is used as the buffer gas). The AE of C3H3+ (m/z = 39) for 3-Cl-Th is about 0.15 eV higher than that for 2-Cl-Th; however, both of them support the favorable dissociation path C4H3SCl + hν f cyclo-C3H3+ + SCCl near the AE. Photoion-Pair Dissociation Dynamics. In Figure 7, two IPAES of Cl recorded in the photon energy range of 10.70 22.00 eV show the similar profiles, namely, a low shoulder in the photon energy range of 12.014.0 eV, a big band at ca. 15.60 eV, and a diffuse band at ca. 17.20 eV. In Figure 7a, the vertical ionization potential (IPv) values for the valence ionizations of 2-Cl-Th19 are labeled as the short lines, indicating the possible valence-to-Rydberg state transitions converged to the respective IPv values,14,1618 ER ¼ IPv
+
Figure 5. Five isomers of C4H2S by the loss of HCl from C4H3SCl .
R ðn δÞ2
ð3Þ
Figure 6. PIE curves of C3H2Cl+ and C3H3+ (left for 2-Cl-Th and right for 3-Cl-Th). The AE values are given with the arrows. 10926
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Figure 8. Tentative assignments with the valence-to-Rydberg state transitions and the vibrational progressions (cited from ref 19) to the Cl IPAES of 2-Cl-Th.
possible multibody fragmentations are estimated as: 2-C4 H3 SCl þ hν f C2 HSþ þ HCCH þ Cl ðexperimental : 12:62 eV ( 0:50 eVÞ 3-C4 H3 SCl þ hν f C2 HSþ þ HCCH þ Cl ðexperimental : 12:20 eV ( 0:50 eVÞ
Figure 7. IPAES of Cl. The red lines (a) represent the IPv positions cited from ref 19. The statistic uncertainties are selectively presented.
where ER is the Rydberg excitation energy, R is the Rydberg constant (13.60569 eV), n is the principal quantum number, and δ is the quantum defect resulting from the penetration of the Rydberg orbital into the core. If these valence-to-Rydberg state transitions couple with the ion-pair dissociation paths, the resonant peaks can be observed in the IPAES.14,1618 However, as shown in Figure 7, such resonant coupling may be quite weak at the higher photon energies. On the other hand, the multibody fragmentations formulated in eq 2 may be predominant in the higher energy range.15,16 According to the thermodynamic values in Table 1 and using eq 2, the thresholds of Cl can be deduced as: 2-C4 H3 SCl þ hν f 2-C4 H3 Sþ þ Cl ðtheoretical : 9:50 eV; experimental : 8:19 eV ( 0:04 eVÞ 3-C4 H3 SCl þ hν f 3-C4 H3 Sþ þ Cl ðtheoretical : 9:14 eV; experimental : 8:19 eV ( 0:04 eVÞ
These two thresholds cannot be directly determined in the present IPAES because of the much low Cl signals at the lower photon energies. Above large deviations between the theoretical thresholds and the experimental values may also be due to the ring-opening mechanism of C4H3S+ as mentioned before. In the energy range shown in Figure 7, the thermodynamic thresholds of the other
Here at the AEs of C2HS+ (see Figure 4e) the cation C2HS+ is assumed to be produced together with acetylene HCCH and Cl. For the polyatomic molecules, the cross sections of ionpair dissociation are usually much smaller (on the order of 0.1% or less of the total photoabsorption cross sections2). Therefore, the production of C2HS+ in the ion-pair dissociations could be ignorable with respect to that of the photoionization dissociation C4H3SCl + hν f C2HS+ + CH2CCl. However, the experimental AEs of C4H3SCl + hν f C2HS+ + HCCH + Cl are difficult to determined by the present Cl PIAES, because there may be lots of the other channels to produce Cl. There are still huge challenges toward the understanding of the photoion-pair dissociation dynamics of the complex polyatomic molecules, especially at the higher photon energies. At the lower photon energies, the spectral assignments to the IPAES may be applicable. The valence-to-Rydberg state transitions as well as the vibrational excitations may couple strongly with the ion-pair dissociation channels. In Figure 8, the low-energy Cl IPAES of 2-Cl-Th is tentatively assigned with reference to the high-resolution PES19 and the spectral fittings with eq 3. In the high-resolution PES, several vibrational progressions were observed around B2A0 and C2A00 ionic states.19 In the photon energy range of 11.5012.00 eV, the significant fluctuations are observed in the IPAES although the smaller scanning step 0.01 eV is used. Three types of vibrational modes, v13, v14, and v15 (X-sensitive), are activated in the electron promotions from 12a0 (Cl: in-plane 3p orbital) and 2a00 (Cl: out-of-plane 3p orbital).19 Their vibrational overtones and combination modes may be involved in the couplings with the ion-pair dissociations; thus they are tentatively assigned to the IPAES (here the superscript + represents the corresponding vibrational modes of the parent cation). In the lower energy range of 10.5011.51 eV, a set of valence-to-Rydberg 10927
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Table 2. Energies of the Vibrational Excitations and Valence to Rydberg State Transitions (Quantum Defect δ = 1.30) That Are Possibly Coupling with Photoion-Pair Dissociations for 2-Cl-Th vibrational statesa 2 0
BA
vibrational energies/meV
+
11.511
0
v14+
11.552
41
v13+
11.584
73
v13+ + v14+
11.622
111
2v13+
11.652
141
11.685 11.712
174 201
0
+
2v13 + 3v13+ C2A00
ionization potentials/eV
v14+
0+
11.848
0
v15+
11.874
26
v14+
11.890
42
2v15+
11.899
51
v14+ + v15+
11.917
69
11.929
81
2v14
+
Rydberg Statesb 0
12a f
6p 7p
10.895 11.092
8p
11.208
9p
11.282
10p
11.331
etc.
etc.
B2A0 a
11.511 (IPv)a b
Cited from ref 19. Estimated in this work.
transitions, 12a0 f np (n = 6, 7, 8, 9, 10, etc.), are proposed by fitting the spectrum with eq 3. The experimental IPv of B2A0 (12a0 1) ionic state at 11.511 eV is adopted from the strongest peak 0f0 vibrational transition in the PES,19 and our fitted quantum defect δ is 1.30. The vibrational and Rydberg progression energies are summarized in Table 2.
4. CONCLUSION Two monochlorothiophenes are investigated by means of the positive and negative photoion mass spectrometry combined with a synchrotron radiation facility. The AE values of the parent and daughter cations are determined by recording the PIE curves. With the help of B3LYP/6-311+G(3df,3pd) calculations, the dissociation dynamics is discussed, for instance, as the concerted process rather than a stepwise reaction for C4H3SCl+ f C2HSCl+ + C2H2, and the ring-opening isomers of the singly and doubly dehydrated cations (C4H3S+ and C4H2S+) formed in C4H3SCl+ f C4H3S+ + Cl and C4H2S+ + HCl. Isomerism, namely, the different chlorine-substituted positions in the molecules, is found to slightly influence the photoionization dissociations. The IPAES of Cl are recorded in the energy range of 10.7022.00 eV, and the thermodynamic thresholds of the possible multibody ion-pair dissociations are discussed on the basis of the PIE measurements. Two IPAES of Cl are similar to each other for 2- and 3-Cl-Th. A valence to Rydberg state transitions 12a0 f np (n = 6, 7, 8, 9, 10, etc.) and several series of vibrational excitations are tentatively assigned in the Cl spectrum in the energy range of 10.9012.00 eV.
’ ASSOCIATED CONTENT
bS
Supporting Information. Structures and energies (in hartree units) of the neutral and cationic fragments. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the NSFC (Grant No. 10775130), MOST (Grant No. 2011CB921401), USTC-NSRL association funding, and the Fundamental Research Funds for the Central Universities (Grant No. WK2340000012). We thank Gen-Bai Chu and Xiao-Bin Shan for their help in the experiments. ’ REFERENCES (1) Ng, C. Y. Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters; World Scientific: Singapore, 1991. (2) Berkowitz, J. In VUV and Soft-Ray Photoionization; Becker, U., Shirley, D. A., Eds.; Plenum: New York, 1996; p 263. (3) Wynberg, H.; van Driel, H. J. Am. Chem. Soc. 1965, 87, 3998. (4) Wynberg, H.; Kellogg, R. M.; van Driel, H.; Beekhuis, G. E. J. Am. Chem. Soc. 1966, 88, 5047. (5) Wynberg, H.; van Driel, H.; Kellogg, R. M.; Buter, J. J. Am. Chem. Soc. 1967, 89, 3487. (6) Kellogg, R. M.; Wynberg, H. J. Am. Chem. Soc. 1967, 89, 3495. (7) Wynberg, H.; Beekhuis, G. E.; van Driel, H.; Kellogg, R. M. J. Am. Chem. Soc. 1967, 89, 3498. (8) Wynberg, H.; Kellogg, R. M.; van Driel, H.; Beekhuis, G. E. J. Am. Chem. Soc. 1967, 89, 3501. (9) Siegel, A. S. Tetrahedron Lett. 1970, 47, 4113. (10) de Jong, F.; Sinnige, H. J. M.; Janssen, M. J. Org. Mass Spectrom. 1970, 3, 1539. (11) Butler, J. J.; Baer, T. J. Am. Chem. Soc. 1980, 102, 6764. (12) Rennie, E. E.; Holland, D. M. P.; Shaw, D. A.; Johnson, C. A. F.; Parker, J. E. Chem. Phys. 2004, 306, 295. (13) Rennie, E. E.; Cooper, L.; Shpinkova, L. G.; Holland, D. M. P.; Shaw, D. A.; Mayer, P. M. Int. J. Mass Spectrom. 2010, 290, 142. (14) Tian, S. X.; Xu, Y. F.; Wang, Y. F.; Feng, Q.; Chen, L. L.; Sun, J. D.; Liu, F. Y.; Shan, X. B.; Sheng, L. S. Chem. Phys. Lett. 2010, 496, 254. (15) Chen, L. L.; Xu, Y. F.; Feng, Q.; Tian, S. X.; Liu, F. Y.; Shan, X. B.; Sheng, L. S. J. Phys. Chem. A 2011, 115, 4248. (16) Chen, L. L.; Tian, S. X.; Xu, Y. F.; Chu, G. B.; Liu, F. Y.; Shan, X. B.; Sheng, L. S. Int. J. Mass Spectrom. 2011, 305, 20–25. (17) Mistuke, K.; Hottori, H.; Yoshita, H. J. Chem. Phys. 1993, 99, 6642. (18) Suzuki, S.; Mistuke, K.; Imamura, T.; Koyano, I. J. Chem. Phys. 1992, 96, 7500. (19) Trofimov, A. B.; Schirmer, J.; Holland, D. M. P.; Karlsson, L.; Maripuu, R.; Siegbahn, K.; Potts, A. W. Chem. Phys. 2001, 263, 167. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; 10928
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