Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 223–226
Experimental and quantum–chemical studies on dissociative
photoionization of c-C
2
H
4
S
Su-Yu Chiang
a,∗, Yung-Sheng Fang
baNational Synchrotron Radiation Research Center, 101, Hsin Ann Road, Science-Based Industrial Park, Hsinchu 30077, Taiwan bDepartment of Applied Chemistry, National Chiao Tung University, 1001, Ta Hsueh Road, Hsinchu 300, Taiwan
Available online 17 February 2005
Abstract
The dissociative photoionization of thiirane (c-C2H4S) in a region∼9–20 eV was investigated with photoionization mass spectroscopy and calculations of molecular energies with the Gaussian-3 method. An adiabatic ionization energy of 9.051 eV for c-C2H4S agrees with a prediction of 9.07 eV for formation of c-C2H4S+; predictions of energies of formation of ring-opened isomers CH3CHS+, CH2CHSH+and CH2SCH2+ are 8.85, 8.91 and 9.37 eV, respectively. Major fragment ions C2H3S+, C2H2S+, HCS+, H2S+ and C2H3+were observed with their respective appearance energies at 10.71, 13.07, 11.13, 11.96 and 12.58 eV. Based on comparison of determined appearance energies and predicted reaction energies, we established six dissociation channels c-C2H4S+→ CH3CS++ H, HCS++ CH3, H2S++ C2H2, C2H3++ HS, CH2CS++ H2and CHCSH++ H2. Ring-opening and H migration are involved in these processes.
© 2005 Elsevier B.V. All rights reserved.
Keywords: c-C2H4S; Dissociative photoionization; Gaussian-3 method
1. Introduction
Thiirane (c-C2H4S) attracts fundamental interest in its dy-namics of dissociation upon photoexcitation because its large ring strain facilitates ring-opening processes and isomeriza-tion. Photodissociation (PD) and photoionization (PI) of c-C2H4S also produce several important radicals and ions, and thus, offer a powerful means of obtaining information about structures and energetics of these species[1,2].
The ionization energy (IE) of c-C2H4S has been measured by various groups with PI [2,3], optical spectroscopy (S) [4], photoelectron spectroscopy (PES)[5,6]and the electron-impact (EI)[7]technique, but the reported IE from PI, PES and EI methods are scattered over a range 8.9–9.05 eV. Ap-pearance energies (AE) of fragment ions C2H3S+and HCS+ were determined in the gaseous phase with PI[2,3], but both AE values suffer from the effect of thermal energy. Appear-ance energies of fragment ions C2H2S+, CH2S+, H2S+, S+,
∗Corresponding author. Tel.: +886 3 578 0281x7315; fax: +886 3 578 3813.
E-mail address: schiang@nsrrc.org.tw (S.-Y. Chiang).
C2H3+, C2H2+and CH2+were determined with EI[7]; AE values thus determined generally represent a qualitative in-dication of a dissociation threshold.
Our aim in this work is to explore dissociative photoion-ization of molecular-beam-cooled c-C2H4S with a photoion-ization mass spectrometer coupled to a synchrotron radiation as an ionization source and with Gaussian-3 (G3) calcula-tions. We measured photoionization efficiency (PIE) spectra of c-C2H4S and various fragment ions and undertook theoret-ical calculations of molecular structures and energies. Com-paring computed reaction energies with experimental IE and AE values, we are able to identify structures of c-C2H4S+and fragment ions and to propose plausible dissociation mecha-nisms.
2. Experiments and calculations
We performed photoionization mass-spectrometric mea-surements with a molecular-beam/quadrupole mass spec-trometer system on the 1-m Seya-Namioka beamline at the National Synchrotron Radiation Research Center (NSRRC) 0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
224 S.-Y. Chiang, Y.-S. Fang / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 223–226
Fig. 1. Photoionization efficiency curves of c-C2H4S and two major frag-ment ions C2H3S+and HCS+, and the threshold photoelectron spectrum of
c-C2H4S in a region∼9–20 eV.
in Taiwan. The apparatus is described in detail elsewhere [8]. Briefly, a mixture of c-C2H4S/He at a total pressure ∼300 Torr and with a seed ratio of ∼10% was expanded through a nozzle and two skimmers to form a molecular beam. The cooled c-C2H4S molecules were ionized with monochro-matic VUV radiation at a right angle in the ionization cham-ber. Produced ions were mass-analyzed and detected with a quadrupole mass spectrometer. The PIE curves in a region ∼9–20 eV were measured and normalized with respect to the photon flux. The wavelength of the monochromator was calibrated with the photoionization spectra of Ar and He. c-C2H4S (Aldrich,∼98%) was degassed with several freeze-pump-thaw cycles before use and was kept in an ice bath at 0◦C during experiments.
Energies of c-C2H4S, isomers of c-C2H4S+and fragment species at their equilibrium geometries were calculated with
Fig. 2. Photoionization efficiency curves of C2H2S+, H2S+and C2H3+from their respective thresholds to 20 eV.
the G3 method using the Gaussian 98 program[9]. Enthalpies of formation at temperature T (Hfo,T) for plausible prod-ucts were obtained from calculated enthalpy changes of their equations of formation and from experimentalHfo,Tfor iso-lated atoms C, H and S[10]. Li et al. reported that ionization energies for several sulfur- and chlorine-containing species, and oxides predicted with the G2 method agree with experi-mental values typically within±0.15 eV[11].
3. Results and discussion
In the mass spectrum of c-C2H4S excited at 60 nm, five major fragment ions, apart from sulfur isotopic species, at
m/z = 59, 58, 45, 34 and 27 were observed, corresponding to
isomeric structures of C2H3S+, C2H2S+, HCS+, H2S+ and
Fig. 3. Photoionization efficiency curves of c-C2H4S+, C2H3S+and HCS+ near the threshold region.
S.-Y. Chiang, Y.-S. Fang / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 223–226 225
Table 1
Calculated total G3 energies (E0), enthalpies (H298), enthalpies of formation at 0 K (Hfo,0) and 298 K (Hfo,298) for species involved in dissociative photoion-ization of c-C2H4S, with symmetries and electronic states
Species symmetry (state) E0(hartree) H298(hartree) Hfo,0(kcal mol−1) Hfo,298(kcal mol−1)
c-C2H4S C2v(1A1) −476.56145 −476.55711 21.7 19.1 CH3CHS+ Cs(2A) −476.23622 −476.23109 225.8 223.7 CH2CHSH+ Cs(2A) −476.23385 −476.22893 227.3 225.1 c-C2H4S+ C2v(2B1) −476.22832 −476.22382 230.7 228.3 CH2SCH2+ C2v(2A2) −476.21704 −476.21186 237.8 235.8 CH3CS+ C3v(1A1) −475.67411 −475.66937 212.5 211.2 CH2CSH+ Cs(1A) −475.63377 −475.62879 237.8 236.7 CH2CHS+ Cs(1A) −475.62884 −475.62452 240.9 239.3 CH2CHS+ Cs(1A) −475.58588 −475.58105 267.9 266.6 CH2CS+ C2v(2B1) −475.02682 −475.02203 252.7 252.4 CHCSH+ Cs(2A) −474.98173 −474.97663 281.0 280.9 HCS+ C ∝v(1) −436.36196 −436.35848 242.0 242.1 H2S+ C2v(2B1) −398.85461 −398.85082 236.9 236.4 HS C∝v(2) −398.59532 −398.59201 33.6 33.8 C2H3+ C2v(1A1) −77.51350 −77.50923 272.1 271.4 C2H2 D∝h(1g+) −77.27596 −77.27228 55.1 55.1 CH3 D3h(2A2) −39.79330 −39.78905 34.6 34.1 H2 D∝h(1g+) −1.16738 −1.16407 −0.5 −0.5 H (2S) −0.50100 −0.49864
C2H3+, respectively; four minor fragment ions at m/z = 57, 46, 28 and 26 correspond to isomeric structures of C2HS+, CH2S+, C2H4+and C2H2+, respectively.
Fig. 1 shows PIE curves of C2H4S+ and major frag-ment ions C2H3S+and HCS+ from their respective appear-ance onset to 20 eV; threshold photoelectron (TPE) spectrum of c-C2H4S is also presented to show the correlation be-tween electronic excitation and the intensity variations of ions. According to Fig. 1, the major dissociation channel C2H4S+→ C2H3S++ H requires the least energy. The similar features of intensities of C2H4S+, C2H3S+ and HCS+ indi-cate that they are formed through similar paths. PIE curves of other major fragment ions – C2H2S+, H2S+and C2H3+– are shown inFig. 2.
PIE curves near the threshold regions of C2H4S+, C2H3S+ and HCS+, measured with steps of 0.05 nm and shown in Fig. 3(a–c), enable us to determine the AE of each fragment ion. An IE = 9.051± 0.003 eV for c-C2H4S was determined from the first distinct maximum in the first derivative of the PIE curve because of step-like features of ion signals near the threshold region. One additional distinct maximum at 9.185 eV is identified; its interval of 1089 cm−1(0.135 eV) is consistent with a value of 1090 cm−1 for the methy-lene bending mode of C2H4S+obtained from the photoelec-tron spectrum[12]. From rising signals of the PIE curves near the threshold region, AE values of C2H3S+, C2H2S+, HCS+, H2S+and C2H3+were determined to be 10.71± 0.01, 13.07± 0.04, 11.13 ± 0.01, 11.96 ± 0.03 and 12.58 ± 0.03, respectively. As c-C2H4S was cooled under supersonic con-ditions, we ignored effects of thermal energy and collisions on our IE and AE values. The AE values for C2H2S+, H2S+ and C2H3+ determined with PI are reported for the first time.
Geometries of c-C2H4S and species involved in the dis-sociative photoionization were optimized at the MP2(full)/6-31G* level. The calculated G3 energies, enthalpies and stan-dard enthalpies of formation for species pertinent to this work are listed inTable 1; symmetries and electronic states are also indicated. With the aid of these results, we calculated ioniza-tion energies of c-C2H4S to form isomers of c-C2H4S+ and reaction energies of plausible channels of dissociative pho-toionization.
Predicted IE of c-C2H4S to form CH3CHS+, CH2CHSH+,
c-C2H4S+and CH2SCH2+are 8.85, 8.91, 9.07 and 9.37 eV, respectively. The experimental IE at 9.051± 0.003 eV agrees satisfactorily with a prediction of 9.07 eV for forma-tion of c-C2H4S+ despite the energy of CH3CHS+ being 4.9 kcal mol−1 smaller than that of c-C2H4S+; an observa-tion of rapidly rising signals of C2H4S+ near the ionization threshold also reflects a small structural change upon ion-ization. In addition, we established six dissociation chan-nels c-C2H4S+→ CH3CS++ H, HCS++ CH3, H2S++ C2H2, C2H3++ HS, CH2CS++ H2 and CHCSH++ H2 based on comparison of determined AE values and predicted reaction energies E(G3). We discuss the dissociation mechanisms as follows.
To form C2H3S+, loss of an H atom is the most direct way, but four isomers of C2H3S+ are possible. Predicted E(G3) for channels c-C2H4S+→ CH3CS++ H, CH2CSH++ H, c-CH2CHS++ H and CH2CHS++ H are 10.52, 11.62, 11.75 and 12.92 eV, respectively. Only channel
c-C2H4S+→ CH3CS++ H has a predicted E(G3) of 10.52 eV smaller than the experimental AE of 10.71 eV; thus, CH3CS+ is the most likely structure near the threshold. Holmes et al. measured an average kinetic energy release 0.15 eV for for-mation of C2H3S+[13]. A difference of 0.19 eV between
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perimental AE and predicted E(G3) might imply the presence of kinetic shifts and reverse activation energies for this chan-nel. We tried to locate a transition structure for isomerization of c-C2H4S+ to CH3CSH+, but a reaction barrier 1.96 eV is 0.3 eV greater than the experimental AE.
The experimental AE of 11.13 eV for channel c-C2H4S+→ HCS++ CH3 agrees satisfactorily with the pre-dicted E(G3) value, 11.06 eV; a small difference of 0.07 eV implies that the reverse activation energy is smaller than or near that of the products. An observation of kinetic energy release of 0.03 eV at the dissociation threshold by Butler and Baer[2]supports our proposition. The predicted struc-ture of HCS+ at the dissociation threshold is linear; an an-gular structure proposed by Butler and Baer might reflect an underestimateHfo,0(HCS+)≤ 233 kcal mol−1. We derive
Ho
f,0(HCS+) = 243.5± 2 kcal mol−1, according to experi-mental AE = 11.13± 0.01 eV (256.7 ± 0.3 kcal mol−1) and literature values Hfo,0(c-C2H4S) = 22.4± 0.3 kcal mol−1 and Hfo,0(CH3) = 35.6± 0.3 kcal mol−1 [10]. This value agrees with the literature value 243.9 kcal mol−1 [14] and our predicted value of 242.0 kcal mol−1.
At greater ionization energies, two channels
c-C2H4S+→ H2S++ C2H2 and C2H3++ HS are proposed based on comparison of their experimental AE values of 11.96 and 12.58 eV with predicted E(G3) values of 11.73 and 12.16 eV, respectively. The differences of 0.23 and 0.42 eV for both channels reflect increased excess energies involved in dissociation because of more dissociation channels in competition. To form C2H2S+, two channels
c-C2H4S+→ CH2CS++ H2 and CHCSH++ H2 are ener-getically accessible; three-body formation channels with predicted E(G3) values greater than experimental AE are excluded. The former channel is favoured because the predicted energy of 10.00 eV is smaller than 11.23 eV for the other channel.
4. Conclusion
Dissociative photoionization of c-C2H4S to form fragment ions – C2H3S+, C2H2S+, HCS+, H2S+ and C2H3+ was in-vestigated in the region∼9–20 eV. The IE of c-C2H4S and AE of fragment ions were derived from their respective
pho-toionization efficiency curves. Theoretical predictions of IE for c-C2H4S and of reaction energies for formation of ob-served fragment ions and their neutral counterparts were per-formed with the G3 method. With the aid of the latter re-sults, we have established six dissociative photoionization channels.
Acknowledgements
We thank NSRRC and NSC of Taiwan (contract NSC92-2113-M-213-001) for financial support.
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