Outermost and Inner-Shell Electronic Properties of ClC(O)SCH2CH3

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Outermost and Inner-Shell Electronic Properties of ClC(O)SCH2CH3 Studied Using HeI Photoelectron Spectroscopy and Synchrotron Radiation Lucas S. Rodríguez Pirani,† Mauricio F. Erben,*,† Mariana Geron
es,† Chunping Ma,‡ Maofa Ge,‡ Rosana M. Romano,† Reinaldo L. Cavasso Filho,|| and Carlos O. Della V
edova*,†,§ †

)

CEQUINOR (CONICET-UNLP), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. C. 962 (1900) La Plata, Rep
ublica Argentina § Laboratorio de Servicios a la Industria y al Sistema Científico, LaSeISiC (UNLP-CIC-CONICET), Camino Centenario e/505 y 508, (1903) Gonnet, Rep
ublica Argentina ‡ State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, Peoples Republic of China Universidade Federal do ABC, Rua Catequese, 242, CEP: 09090-400, Santo Andr
e, S~ao Paulo, Brazil

bS Supporting Information ABSTRACT: A study of valence electronic properties of S-ethyl chlorothioformate (S-ethyl chloromethanethioate), ClC(O) SCH2CH3, using HeI photoelectron spectra (PES) and synchrotron radiation is presented. Moreover, the photon impact excitation and dissociation dynamics of ClC(O)SCH2CH3 excited at the S 2p and Cl 2p levels are elucidated by analyzing the total ion yield (TIY) spectra and time-of-flight mass spectra acquired in multicoincidence mode [photoelectron
photoion coincidence (PEPICO) and photoelectron
photoion
photoion coincidence (PEPIPICO)]. The HeI photoelectron spectrum is dominated by features associated with lone-pair electrons from the ClC(O)S— group, the HOMO at 9.84 eV being assigned to the nπ(S) sulfur lone-pair orbital. Whereas the formation of C2H5þ ion dominates the fragmentation in the valence energy region, the most abundant ion formed in both the S and Cl 2p energy ranges is C2H3þ. Comparison with related XC(O)SR (X = H, F, Cl and R = —CH3, —C2H5) species reveals the impact of the alkyl chain on the photodissociation behavior of S-alkyl (halo)thioformates.

1. INTRODUCTION S-alkyl thioformates, HC(O)SR, were widely studied in the past decade by Flammang, Nguyen, and co-workers,1
3 and their behaviors upon electron impact were investigated using tandem mass spectrometry. The prominent loss of a SH• radical was observed to form hydroxycarbenium ions1 by collisional-activation spectra and consecutive collisional-activation experiments.2,3 Moreover, [C2H4OS]•þ radical cations generated upon dissociative ionization of these species were characterized by tandem mass spectrometry. The use of a combination of collisional activation, neutralization
reionization, and ion
molecule reaction experiments led to the identification of distonic radical cations (species in which the charge and radical sites are formally separated)4 and ion
molecule complexes.1 In particular, the •CH2CH2SH2þ βdistonic radical cation was observed as the product of decarbonylation of HC(O)SCH2CH3 in the 70 eV electron-impact mass spectra.2 The interest in this species arises because its well-known oxygenated analogue, the β-distonic radical cation of ethanol, r 2011 American Chemical Society

CH2CH2OH2þ, was calculated to be about 42 kJ/mol lower in energy than the classical radical cation.5 Experimental evidence has further confirmed that the fragment ion C2H6O•þ of 1,3-propanediol has a distinct structure6 and reactivity7 from that of the conventional ionized ethanol, the features being attributed to the β-distonic isomeric nature of the former species. Our research group has reported studies concerning the electronic properties of shallow- and inner-core levels in sulfenylcarbonyl compounds with general formula XC(O)SCl. Thus, we studied FC(O)SCl,8,9 ClC(O)SCl,10 thioacetic acid [CH3C(O)SH],11 and CH3OC(O)SCl12 species using synchrotron radiation in the 100
1000 eV range and analyzed their ionic fragmentations after electronic decay. We succeeded in analyzing also the electronic structure and the ionic dissociation induced by

•

Received: December 22, 2010 Revised: March 31, 2011 Published: May 10, 2011 5307

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The Journal of Physical Chemistry A photon absorption in the outermost valence region of these compounds by using a combined experimental approach that includes HeI (21.2 eV) photoelectron spectroscopy and photoionization under the action of synchrotron radiation in the 12.0
21.5 eV region.13,14 Very recently, we focused our attention on the photoionization of S-methyl halothioformates, XC(O)SCH3 (X = F15 and Cl16) irradiated with vacuum ultraviolet (VUV) photons. As a prominent aspect, the high stability of the HCSþ ion can be observed over the whole range of photon energies analyzed. As a logical extension of these studies, we are now interested in the S-ethyl XC(O)SCH2CH3 (X = F, Cl) derivatives. The resemblance with the S-ethyl thioformate molecule [HC(O) SCH2CH3] is also apparent. Relevant studies have been reported on the ground-state properties of the title molecule. In a low-resolution microwave spectroscopy study of ethyl thioesters,17 three spectroscopically distinct species were observed. Moreover, the IR spectra of ClC(O)SCH2CH3, both as a vapor and trapped in a solid N2 or Ar matrix, together with the Raman spectrum in the liquid phase, have already been reported.18 Two conformers were found to exist at room temperature, called syn-gauche and synanti (CdO double bond syn with respect to the S—C single bond and C—S single bond gauche and anti with respect to the C—C single bond, respectively), with the syn-gauche form being the most stable one.18 In this work, the electronic structure of S-ethyl chlorothioformate [S-ethyl chloromethanethioate (IUPAC) or (ethylthio) carbonyl chloride], ClC(O)SCH2CH3, in both the valence and inner-shell regions was investigated using synchrotron-based experimental techniques and HeI photoelectron spectroscopy (PES). Total ion yield (TIY) spectra and multicoincidence techniques [photoelectron
photoion coincidence (PEPICO) and photoelectron
photoion
photoion coincidence (PEPIPICO)] time-of-flight (TOF) mass spectrometry were applied.

2. EXPERIMENTAL SECTION The synchrotron radiation was used at the Laborat
orio Nacional de Luz Síncrotron (LNLS), Campinas, Sao Paulo, Brazil.19 Linearly polarized light monochromatized with a toroidal grating monochromator (available at the TGM beamline in the range of 12
310 eV)20 intersects the effusive gaseous sample inside a high-vacuum chamber at a base pressure in the range of 10
8 mbar. During the experiments, the pressure was maintained below 2  10
6 mbar. The resolution power is better than 400 in the TGM beamline at the LNLS. The photon energy resolution from 12 to 21.5 eV is E/ΔE = 550. The energy calibration was established by means of the S 2p f 6a1 g and S 2p f 2t2 g absorption resonances in SF6.21 The intensity of the emergent beam was recorded by a light-sensitive diode. The ions produced by the interaction of the gaseous sample with the light beam were detected by means of a time-of-flight (TOF) mass spectrometer of the Wiley
McLaren type for both PEPICO and PEPIPICO measurements.22,23 This instrument was constructed at the Institute of Physics, Brasilia University, Brasilia, Brazil.24 The axis of the TOF spectrometer is perpendicular to the photon beam and parallel to the plane of the storage ring. Electrons are accelerated onto a multichannel plate (MCP) and recorded without energy analysis. This event starts the flight time determination process for the corresponding ion, which was consequently accelerated to another MCP. High-purity vacuum-ultraviolet photons

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were used. The problem of contamination by high-order harmonics was suppressed by the gas-phase harmonic filter installed at the TGM beamline at the LNLS.25
27 We calculated the average kinetic energy release (KER) values of the fragments from the coincidence spectra by assuming that the fragments were distributed isotropically, that they were perfectly space-focused, and that the electric field applied in the extraction region was uniform.28 Under these conditions, the KER in the fragmentation process can be determined from the peak width following the method suggested by Pilling et al.29 Deviations from ideal conditions always increase the peak width, and thus, the values calculated are upper bounds. Santos et. al30 measured the argon TOF spectrum under very similar experimental conditions and achieved a peak width value of 0.05 eV for the Arþ ion. Because the broadening in argon can be the result of only thermal energy and instrument broadening, this value represents a good estimation for the instrumental resolution. The HeI PE spectrum of ClC(O)SCH2CH3 was recorded on a double-chamber UPS-II instrument built specifically to detect transient species at a resolution of about 30 meV, as indicated by the Arþ(2P3/2) photoelectron band.31
37 Experimental vertical ionization energies (Iv, eV) were calibrated by adding a small amount of argon or iodomethane to the sample. OVGF (outer valence Green's function) calculations38 using the 6-311þþG(d,p) basis set and the B3LYP/6-311þþG(d,p)optimized geometry of the most stable conformer were performed on ClC(O)SCH2CH3 in its ground electronic state using the Gaussian 03 program suite.39 The dissociation energies of the ClC(O)SCH2CH3•þ parent radical ion into possible fragments were calculated at the UB3LYP/6-311þþG(d,p) level of approximation. S-ethyl chlorothioformate was synthesized using triphosgene and ethanethiol in presence of triethylamine, according to the equation40 Cl3 COCðOÞOCCl3 þ 2CH3 CH2 SH f Cl3 COCðOÞSCH2 CH3 þ ClCðOÞSCH2 CH3 þ 2HCl

ð1Þ

The liquid products were purified by fractional distillation and subsequently purified several times by fractional condensation at reduced pressure to eliminate volatile impurities. The final purity of the compound in both the vapor and liquid phases was carefully checked by IR (vapor) and Raman (liquid) spectroscopies.18

3. RESULTS AND DISCUSSION 3.1. Valence Electron Region. Ionization energies in the valence region are known to be strongly dependent on the molecular conformation.41,42 Very recently, Morini et al.43 showed that the inclusion of several conformers of n-hexane is needed for the correct interpretation of the photoelectron and electron momentum spectra. As noted before, the most stable conformation of ClC(O)SCH2CH3 in the ground electronic state is the syn-gauche conformer.17,18 From matrix isolation studies, a syn-anti form was determined to be only 0.3 kcal/mol (ΔH
) less stable. By taking into consideration the ΔS
value computed at the B3LYP/6-311þþG(p,d) level of approximation [ΔS
= 1.59 cal/(mol K)] and degeneracy values (m = 2 and 1 for the syn-gauche and syn-anti forms, respectively), from the Boltzmann distribution, a relative abundance of 40% of the syn-anti 5308

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Figure 1. HeI photoelectron spectrum of ClC(O)SCH2CH3.

Table 1. Experimental and Calculated Ionization Energies (eV) and MO Characters for ClC(O)SCH2CH3 calcd (eV)a PES (eV)

syn-gauche

syn-anti

MO

character

9.84

9.74 (0.90)

9.83 (0.90), a00

32

nπ(S)

10.74

10.82 (0.90)

10.84 (0.89), a0

31

nσ(O)

11.84

11.44 (0.91) 11.61 (0.90)

11.53 (0.90), a00 11.64 (0.89), a0

30 29

nπ(Cl) nσ(Cl)

12.35

12.19 (0.90)

12.30 (0.90), a00

28

π(CdO)

13.18

13.39 (0.91)

13.43 (0.91), a0

27

σC—H(—CH3)

14.13

13.91 (0.90)

13.81 (0.89), a00

26

σC—H(—CH3)

14.72

14.84 (0.89)

14.56 (0.90), a0

25

σC—H(—CH3)

15.24

15.22 (0.87)

15.20 (0.85), a0

24

σC—H(—CH2)

a

Values calculated at the OVGF/6-311þþG(d,p) level of approximation with B3LYP/6-311þþG(d,p)-optimized geometries.

form is expected at 298 K. Although the resolution of the present method is not sufficient for the experimental observation of minor differences arising from conformation specificity even when it is present, better qualitative agreement with experimental data is obtained when both conformers are considered. The most stable form belongs to the C1 point group, whereas the syn-anti form belongs to the Cs symmetry point group. For the latter, all of the canonical molecular orbitals of type a0 are σ orbitals lying in the molecular plane, whereas those of type a00 are π orbitals. Their 36 valence electrons are arranged in 18 doubly occupied orbitals in the independent particle description. The photoelectron spectra, as well as the dissociative photoionization and the photoion branching ratios of ClC(O)SCH2CH3, are conveniently discussed with reference to these ground-state configurations. 3.1.1. Photoelectron Spectra. The PE spectrum of ClC(O)SCH2CH3 is presented in Figure 1. The experimental and theoretical ionization energies and pole strengths are listed in Table 1, along with the character assignments of each occupied molecular orbital. The assignments of the PE spectrum bands to photoionization processes from specific molecular orbitals were made with reference to the results of the OVGF/6-311þþG(d,p) calculations [syn-gauche and syn-anti conformers optimized at the

B3LYP/6-311þþG(d,p) level of approximation]. Very similar ionization values were computed for the two conformers, and the characters determined for the molecular orbitals were the same at the level of theory used in this work (see Table 1). Thus, the characters of the highest occupied molecular orbitals of the most stable syn-gauche conformer of ClC(O)SCH2CH3 are shown in Figure 2. In the PE spectrum, well-defined bands associated with ionization processes from occupied molecular orbitals can be observed. From the overall analysis of the experimental values and results derived from Green's function calculation, the welldefined bands observed in the PES below 14.0 eV originate from the removal of nonbonded electrons on the sulfur, oxygen, and chorine atoms. Indeed, the first ionization band appearing in the spectrum at 9.84 eV can be assigned with confidence to the ionization process from the highest occupied molecular orbital (HOMO) [nπ(S) orbital], which can be visualized as a lone pair formally localized on the sulfur atom. This value agrees with the first ionization potential (IP) values previously determined for sulfenylcarbonyl compounds. For example, the first vertical ionization energies for the related species FC(O) SCl,44 ClC(O)SCl,45 ClC(O)SCH3,15 CH3C(O)SH,46 CH3C(O)SCH2CH3,46 and CH3C(O)SCH314 are 10.68, 10.36, 10.11, 10.00, 9.60, and 9.53 eV, respectively. The influence of the electronegativity of the atoms or groups bonding to the — SC(O)— skeleton is apparent.47 The substitution of highly electronegative atoms such as F and Cl by electron-releasing alkyl groups results in a decrease of the IP value of the HOMO (see Scheme 1). The second ionization potential observed at 10.74 eV is assigned to the ionization process of an electron ejected from the nσ(O) orbital. The third and fourth ionization potentials are observed as an intense signal around 11.84 eV and are assigned to ionization processes of the lone-pair electrons located at the chorine atom, more specifically at the nπ and nσ orbitals, respectively. Computed values [OVGF/6-311þþG(p,d)] for the nπ and nσ ionization potentials are very close in energy, at 11.44 and 11.61 eV, respectively. The last well-defined band at 12.35 eV is associated to the ionization process from the bonding π orbital of the carbonyl group. Above 14.00 eV, the bands in the photoelectron spectrum become less defined, so their assignment to different ionization processes of the ethyl group cannot be clearly done. It is worth noting that, in one-particle Green’s function calculations performed using a diagonal self-energy (quasiparticle approximation) such as the OVGF method included in the Gaussian 03 program suite, pole strengths larger than 0.80 are assumed to validate the one-electron picture of ionization.48 However, Deleuze and co-workers demonstrated that pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital picture of ionization,49
53 compared with the results of the more reliable ADC(3) calculations, which properly handle configuration interactions in the final state. For the title species, despite the obvious limitations of the 6-311þþG** basis set, the computed OVGF one-electron ionization spectrum is in good agreement with the available He(I) photoelectron spectrum, up to binding energies of ca. 16 eV. The picture of the valence electronic structure emerging from the current joint analysis of the HeI photoelectron spectrum and quantum chemical calculations is further complemented by an experimental and theoretical study of the structural properties of the title species, published in a separate work.54 5309

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Figure 2. Characters of the five highest occupied orbitals of ClC(O)SCH2CH3.

Scheme 1. Energies of the HOMOs [nπ(S)] for a Series of Related —SC(O)— Compounds as Determined by PES

Table 2. Branching Ratios (%) for Fragment Ions Extracted from PEPICO Spectra Taken at Selected Photon Energies in the Valence Region of ClC(O)SCH2CH3 photon energy (eV) m/z (amu/q)

Further calculations [UB3LYP/6-311þþG(d,p)] were performed with the primary aim of analyzing the nature of the cation formed in the first ionization process. The results demonstrate that atomic charges (Mulliken and natural atomic charges) are delocalized over the entire molecule, with an appreciable fraction localized at the S atom (Supporting Information, Table S1). The partial positive charges of the S and Cl are increased by 0.43 and 0.28 units, respectively, after ionization. Changes in the geometrical parameters are also observed, with the C—S bond length and the — Cl—CdO and — OdC—S angles being the geometrical parameters that are most influenced by ionization (see Table S2, Supporting Information). 3.1.2. Photoionization and Photodissociation Processes in the Valence Region. The main goal of this work was the study of different ionization channels and their consequent dissociation molecular processes in the photon energy range involving the first ionization potentials observed in the photoelectron spectrum. The lowest energy delivered by the TGM beamline at the LNLS (11.10 eV) is higher than the first ionization potential of the ClC(O)SCH2CH3 molecule (9.84 eV). Therefore, ionization processes can already be observed at the very first stage of the experiment. The PEPICO spectra measured for this molecule at selected photon energies are shown, together with the fragment assignment of the signals, in Figure 3. In Table 2, the branching ratios for ion production are also given. The PEPICO spectrum measured at 12.00 eV is dominated by three intense signals. The molecular ion is clearly observed with the natural occurring isotopomeric ratio at m/z = 124 and 126 amu/q for the 35Cl and 37Cl isotopomers, respectively. One of the most important contributions to this spectrum is the C(O)SCH2CH3þ (89 amu/q) fragment, derived from the loss

ion

27

C2H3þ

28

C2H4þ/COþ

29

CH3CH2þ

12.0

33.1

12.4

43.8

13.4

17.0

20.0

9.6

10.5

1.7

4.5

5.0

53.6

31.8

26.7

32

Sþ

2.8

1.6

35

Clþ

5.2

6.2

44

CSþ

1.6

2.0

45

HCSþ

2.8

4.2

59 60

C2H3Sþ C2H4Sþ/OCSþ

1.8

1.2

3.3 3.0

3.0 3.9

61

CH3CH2Sþ

1.2

1.7

63

ClC(O)þ

89

CH3CH2SC(O)þ

38.3

CH3CH2SC(O)Clþ

16.8

124

2.5

1.8 3.1

6.2

7.4

1.7

4.5

5.7

31.6

23.2

12.0

10.4

4.6

8.6

5.6

4.6

of Cl atom (Mþ
35). This signal amounts to 38% of all of the fragments generated at this energy (see Table 2). The third intense signal appearing at m/z = 29 amu/q corresponds to the CH2CH3þ ionic fragment. This cation is observed with high intensity at all measured energies in the valence region, reaching a contribution of 53% for an incident photon energy of 13.40 eV. At this energy, the depletion of molecular ion together with the C(O)SCH2CH3þ fragment can be observed in Table 2. Thus, it is plausible that the CH2CH3þ fragment is formed by the inductive loss of an OCS molecule from the C(O)SCH2CH3þ cation, as shown in Scheme 2. Other ionization channels are opened when the incident photons reach an energy of ca. 17.0 eV. Thus, the ClC(O)þ (63 amu/q) and CH3CH2Sþ (61 amu/q) ions are observed in the spectrum, probably originating from the R rupture of the C—S bond, as shown in Schemes 3 and 4. It should be mentioned that the potential energy surface of the [C2H5Sþ] ion was studied with the G2 method by Chiu et al., and four stable isomers were identified, together with the isomerization and unimolecular dissociation barriers.55 The m/z = 61 amu/q ion is stable on the time scale here used and is clearly observed in our experiments. In the following discussion, it is represented as CH3CH2Sþ, although it might, in fact, correspond to another isomer. To better understand the competition between different ionization channels, at least in a qualitative way, the theoretical energy profile for ionic dissociation was calculated at the UB3LYP/6-311þþG(d,p) level of approximation. A schematic representation of some of these dissociation channels is shown in 5310

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Figure 3. PEPICO spectra of ClC(O)SCH2CH3 at selected irradiation energies.

Scheme 2

Scheme 3

Scheme 4

Figure S1 in the Supporting Information. The calculations predict that the channel affording C(O)SCH2CH3þ and Cl• is the energetically most favored, whereas the channel to form Clþ ion appears at higher energies. This is in agreement with the behavior observed in the PEPICO spectra, where C(O)SCH2 CH3þ already appears at 12.0 eV, whereas much higher energies are needed to form the Clþ ion. Also, the homolytic rupture of the S—C2H5 bond favors the formation of CH3CH2þ ions over ClCOSþ fragments, in agreement with the experimental results.

3.2. Inner-Shell S 2p and Cl 2p Electron Region. 3.2.1. Total Ion Yield (TIY) Spectra. TIY spectra were obtained by recording

the count rate of the total ions as the photon energy was scanned. At high photon energy, corresponding to shallow-shell electronic levels, the TIY spectroscopy is a powerful method to be used as a complement to absorption spectroscopy.56 The TIY spectrum of ClC(O)SCH2CH3 following S 2p excitation (from 162.0 to 174.0 eV photon energy) is shown in Figure 4. Below the S 2p threshold, the spectrum is dominated by a group of four signals centered at 164.6, 165.9, 167.2, and 168.4 eV, and the ionization edge is located at approximately 170.9 eV. Complex electronic processes occur at resonant energies below the S 2p ionization edge, which could be interpreted as due to resonant transitions corresponding to dipole-allowed transitions that involve excitations of a 2p electron to antibonding molecular orbitals. ClC(O)SCH2CH3 in its ground state belongs to the C1 symmetry group, and the dipole selection rules are completely relaxed in the transition processes. Quantum chemical calculations at the B3LYP/6-311þþG(d,p) level of approximation for neutral ClC(O)SCH2CH3 in its ground state predict that these unoccupied orbitals should be mainly the lowest unoccupied molecular orbital (LUMO) π*CdO and the σ*Cl—C and σ*S—C antibonding orbitals (see Figure 5). The low energy computed for the LUMO π*CdO can be associated with the resonance interactions of this unoccupied orbital with lone pairs of both sulfur and chlorine atoms. Moreover, it is expected that a spin
orbital split occurs in the excited species for the 2p term of sulfur atom in 2p1/2 and 2p3/2 levels. In the case of the simplest sulfide, H2S, this splitting was reported to be 1.201 eV.57 Thus, the first resonances at 164.6 and 165.9 eV were tentatively assigned to the S 2p f π*CdO transition. It should be noted, however, that the expected intensity ratio of 2:1 for the 2p1/2 and 2p3/2 levels was not observed, suggesting the presence of overlapping transitions. Such an effect was already noted when the joint assignment of the electron loss and TIY 5311

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Figure 4. Total ion yield spectra of ClC(O)SCH2CH3 close to the S 2p (right) and Cl 2p (left) regions.

Figure 5. Characters of the four lowest-energy unoccupied molecular orbitals for ClC(O)SCH2CH3 calculated at the B3LYP/6-311þþG(d,p) level of approximation.

spectra of the CH3SCN species was proposed.58,59 It is probable that excitations to the σ*Cl—C and σ*S—C vacant orbitals overlap, giving rise to the intense 165.9 eV resonance. Based on quantum chemical calculation results, the well-resolved 168.4 eV resonance below the ionization edge is tentatively assigned to the S 2p f σ*C—S transition. It is worth mentioning that the σ*CdO antibonding orbital was computed as a very high-energy vacant orbital, corresponding to the LUMO þ 10 depicted in Figure 5. Thus, the S 2pσ f σ*CdO transition is expected to appear near the S 2p ionization edge. This is also in agreement with the inner-shell excitation spectra of carbonate species,60 which show an appreciable energy separation between transitions involving both π*CdO and σ*CdO orbitals. The TIY spectrum measured near the Cl 2p region is also shown in Figure 4. The Cl 2p threshold is located at approximately 210.5 eV, and only two broad and poor-resolved signals can be observed at 201.0 and 202.5 eV. By comparison with previous studies on chlorinated species,61 these signals could be assigned to unresolved transitions involving the spin
orbital split of the 2p term in the 2p1/2 and 2p3/2 levels of the excited species toward the LUMO þ 1 (σ*Cl—C) vacant orbital.62 3.2.2. PEPICO Spectra. Several PEPICO spectra were recorded by setting the photon energy at the resonant values obtained in the TIY spectrum around both the S 2p and Cl 2p levels. To identify the role of resonant processes in the fragmentation, spectra were also measured at photon energy values below (typically 10 eV) and above (typically 50 eV) the ionization edge. The spectra correspond to the arrival of only one ion during the open time window. They therefore contain contributions of a single ionization process and also include the contribution of dissociative multiple ionization where only the lighter ion is detected. PEPICO spectra at the main resonance values observed in the TIY spectrum at 165.8 (S 2p) and 201.0 eV (Cl 2p) are shown in

Figure 6. In Table 3, the corresponding branching ratios are collected for the main fragment ions. The molecular ion is clearly observed throughout the whole S 2p and Cl 2p regions, along with the typical isotopic distribution due to the presence of 35Cl and 37Cl isotopes. These isotopic distributions are clearly observed even at these high photon energies because of the suitable mass resolution attained in the experiments. The most abundant ion formed in both the S and Cl 2p energy ranges is C2H3þ (approximately 10
14%), which can be formed by the rupture of the S—C single bond from the molecular ion with subsequent elimination of a H2 molecule from the CH2CH3þ species. Indeed, H2 elimination from the CH2CH3þ ion has been studied by different experimental techniques,63
66 and theoretical studies have also been performed.67 These investigations concluded that H2 elimination could be interpreted as 1,1eliminations proceeding through a symmetry-allowed mechanism involving back-donation from the HOMO of C2H3þ to the LUMO of the H2 molecule. This hydrocarbon species plays a fundamental role as an intermediate in hydrocarbon combustion chemistry,68 and both •C2H3 and C2H3þ are believed to participate as reaction intermediates in interstellar space.69,70 Other prominent ions in the spectra with relative abundances between 4% and 10% are Hþ (m/z = 1), COþ (m/z = 28), CH2CH3þ (m/z = 29), Sþ (m/z = 32), Clþ (m/z = 35), HCSþ (m/z = 45), and ClC(O)þ (m/z = 63), whereas the fragments OCSþ and SCH2CH3þ and the molecular ion are formed with relative abundances below 3%. When the PEPICO spectra taken at different energies are compared (see Table 3), small changes in the peak intensities become evident. Both an increase in the peak intensities corresponding to the lighter ions Hþ, CH3þ, Sþ, Clþ, ClC(O)þ and a diminution in the intensity of the C2H3þ, CH3CH2þ, SCH2CH3þ, C(O)SCH2CH3þ, and Mþ ion signals are observed upon going from the S 2p to the Cl 2p region. This 5312

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Figure 6. PEPICO spectra of ClC(O)SCH2CH3 recorded at photon energies of 165.8 and 201.0 eV.

is a quite expected behavior as atomization processes become dominant when the photon energy is increased. Upon comparison of the PEPICO branching ratio with those obtained for the related S-methylated halothioformates,15,16 a noticeable diminution is observed in the production of the HCSþ ion, which amounts ca. 17% of the ions produced in the S 2p ionized XC(O)SCH3 species.15 Thus, the length of the alkyl chain seems to play a relevant role in the photodissociation of S-alkylated (halo)thioformates. From the PEPICO peak shape, the kinetic energy release (KER) was determined for each ion, and these values are reported in Table 3 for selected photon energies. A broadening in the peak widths is clearly observed above the S 2p edge (171.8 eV) and in the Cl 2p region, where all of the ions shown higher KER values. This effect becomes apparent in the PEPICO spectra shown in Figure 6. It is well-known that the decay of core
shell-excited species normally leads to the formation of doubly charged parent ions in several excited states, which could then dissociate, releasing much of their internal energy as kinetic energy (KER) of the fragment ions.28 As mentioned below, the PEPICO spectra in this energy range have contributions from both single and multiple ionization processes. Thus, singly charged ions are observed in the spectrum, and many of them could derive from doubly charged species. The heaviest fragment detected, the C(O)SCH2CH3þ (m/z = 89) ion, could be produced only from the singly charged species ClC(O)SCH2CH3•þ. The peak shape corresponding to this ion is clearly symmetric, and only slight variations are observed in the KER values. The relatively low KER value for this ion, below 0.2 eV, suggests the extraction of neutral fragments (chlorine atom) from singly charged ClC(O)SCH2CH3þ, in good agreement with the proposed rupture mechanisms shown in Schemes 2 and 3. ClCðOÞSCH2 CH3 •þ f CðOÞSCH2 CH3 •þ þ Cl• ðKER ¼ 0:15
0:17 eVÞ

ð2Þ

Another ion observed with relatively low KER values throughout the whole energy range is the CH2CH3þ fragment, which

involves the rupture of the S—C bond. Moreover, the ClC(O)Sþ cation is not detected in the PEPICO spectra, in qualitative agreement with the low ionization potential for the CH2CH3 radical.63
66 The following simple mechanism explains the experimental observations ClCðOÞSCH2 CH3 •þ f CH2 CH3 •þ þ ClCðOÞS• ðKER ¼ 0:19
0:74 eVÞ

ð3Þ

The singly charged molecular ion is observed throughout the whole range of photon energies studied, and the KER values determined for this ion are close to the “thermal” value of 0.05 eV.30 The clear observation of the singly charged molecular ion and the determination of some of its main dissociation pathways evidence the importance of participant or spectator Auger processes in the electronic decay of the excited species below the ionization thresholds. As noted before, the •CH2CH2SH2þ β-distonic radical cation is observed in the 70 eV electron -impact mass spectra of HC(O)SCH2CH3.2 The decarbonylation assumes the formation of a sulfurane ion as intermediate. A hydrogen-bridge complex is assumed to be involved in the β-distonic radical cation, as shown in Scheme 5.2,71 For the title species, the (M
28)þ fragment was not observed in our spectra in either the valence or innershell region. Thus, decarbonylation events are precluded when hydrogen [HC(O)SCH2CH3] is exchanged for a chlorine atom in ClC(O)SCH2CH3. It is likely that migration of heavy atoms such as chlorine disfavors the formation of the sulfurane ion. 3.2.3. PEPIPICO Spectra. In addition to the importance of singly charged species discussed in the previous section, very high KER values were determined for other cationic fragments, likely produced from the doubly charged molecular species, manifesting that normal Auger processes dominate the electronic decay after the photoelectron is ejected. In effect, in the core ionization continuum, normal Auger processes explain the enhancement of double-ionization channels at the expense of single-ionization ones, giving rise to the observation of lighter fragments. Thus, the 5313

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Table 3. Branching Ratios (%) and Kinetic Energy Release Values (eV) for Fragment Ions Extracted from PEPICO Spectra Taken at Photon Energies around the S 2p and Cl 2p Energies for ClC(O)SCH2CH3a,b photon energy (eV) off resonance m/z

ion þ

S 2p

off resonance

Cl 2p

off resonance

155.0

164.8

165.8

167.0

168.4

170.8

190.0

201.0

203.1

209.5

250.0

1

H

2.6

3.5

3.8/6.3

3.8

4.4

4.4/8.6

6.2

6.6/8.3

6.8

6.9/8.9

9.3

12

Cþ

6.8

2.0

1.1/3.3

1.6

1.8

3.8/2.0

1.4

1.7/4.4

1.6

1.5/4.6

2.0

14 15

CH2þ CH3þ

2.5 0.9

1.8 1.5

1.6/4.4 1.5/3.6

1.6 1.5

2.0 1.6

1.8/4.5 1.8/4.9

1.8 2.5

1.9/6.2 2.4/6.3

1.8 2.4

1.7/6.8 2.7/6.8

1.9 2.6

16

S2þ/Oþ c

14.3

3.0

1.2/5.7

2.1

2.4

7.4/3.6

1.1

1.4/8.3

1.1

1.1/9.8

1.2

25

C2Hþ

0.5

1.1

1.3/1.1

1.2

1.4

0.9/5.3

1.6

1.8/2.0

1.7

1.7/2.5

2.0

26

C2H2þ

2.2

4.7

5.8/1.6

5.6

5.7

3.6/3.7

5.7

6.0/3.9

5.8

5.5/4.3

5.8

27

C2H3þ

6.4

12.3

14.0/0.6

13.2

12.0

10.0/1.9

10.7

11.0/2.5

10.5

10.1/3.5

9.7

28

C2H4þ/COþ

8.4

5.7

5.0/0.8

5.2

5.3

5.5/0.5

4.3

4.4/3.3

4.4

4.2/4.3

4.5

29

CH3CH2þ

6.5

9.4

10.3/0.2

9.7

7.7

7.1/0.6

5.1

5.2/0.7

4.9

5.8/2.4

5.1

32 33

Sþ SHþ

2.7 0.5

5.7 1.0

5.7/1.2 1.2/0.4

6.2 1.1

6.0 1.2

3.6/1.3 0.8/1.7

6.6 1.2

7.2/2.7 1.2/1.2

7.1 1.2

6.8/2.7 1.2/1.6

7.9 1.2

35

Clþ a

3.4

6.5

7.0/1.2

5.9

5.8

6.0/0.6

8.1

8.4/3.0

8.4

9.0/3.6

10.2

44

CSþ c

45

HCSþ

2.3

4.2

4.2/0.7

4.2

4.8

3.6/0.8

4.5

4.6/2.9

4.4

3.9/2.6

3.7

46

H2CSþ

1.0

1.8

1.7/1.0

1.7

1.7

1.8/1.4

2.0

1.8/6.2

1.9

2.1/2.8

1.9

57

SC2Hþ

0.6

1.1

1.0/1.0

1.2

1.5

d

d

d

d

d

d

58

SC2H2þ

0.9

1.7

1.7/0.6

1.8

2.2

d

d

d

d

d

d

59 60

SC2H3þ SC2H4þ/SCOþ

0.8 1.1

1.5 2.2

1.5/0.7 2.6/0.5

1.6 2.6

1.7 2.4

d d

d 3.1

d 3.0/3.5

d 2.9

d 4.2/3.7

d 3.8

61

SCH2CH3þ

0.9

2.1

1.8/0.5

1.6

1.2

d

d

d

d

d

d

63

ClC(O)þ a

1.9

3.5

4.2/1.0

3.6

4.0

3.8/6.8

7.6

6.6/2.9

6.9

5.7/10.8

5.0

67

ClSþ

0.4

0.8

1.3/0.8

1.0

0.8

0.5/1.9

1.0

0.9/2.7

1.0

1.2/5.0

1.0

89

C(O)SCH2CH3þ

2.8

3.2

3.0/0.2

3.0

3.0

1.6/0.2

1.1

1.0/0.2

1.0

0.7/0.2

0.5

ClC(O)SCH2CH3þ a

0.9

1.1

1.1/0.02

1.0

1.0

0.3/0.02

0.4

0.2/0.02

0.3

0.2/0.02

0.1

124 a

Peaks for the corresponding naturally occurring isotopomers observed. b Kinetic energy release values determined at selected energies given in italics. c Possible contamination with CO2 observed. d Overlapping of peaks observed.

Scheme 5

fragmentation dynamics and the final fate of the ClC(O)SCH2CH32þ ion are discussed in the context of the twodimensional PEPIPICO multicoincidence technique. In these spectra, one electron and two positive ions were recorded in a correlated way at several photon energies near the S 2p and Cl 2p edges. A multicoincidence measurement allows the identification of the various ions produced in the same photoionization event. Thus, analysis of PEPIPICO spectra is useful for identifying several two-, three-, and four-body dissociation mechanisms that essentially follow Auger decay mechanisms.22,72,73 The double-coincidence branching ratios extracted from the PEPIPICO spectra at several photon energies are given in Table 4. The more important coincidences observed in this

energy range are C2H3þ/Sþ (4
7%), C2H3þ/Clþ (2
4%), Hþ/Sþ (3
4%), Hþ/Clþ (2
4%), and C2H3þ/ClC(O)þ (3
4%). Consequently, the contributions of both C2H3þ and Hþ ions to the double-coincidence spectra are very high, with the coincidence between C2H3þ and Sþ ions being the most intense island, with a relative abundance of 7.1% at 165.8 eV. Coincidences involving the arrival of C2H3þ ions as either the lighter or heavier ion represent as much as 23.8% of the double coincidences generated when 165.8 eV photons impact gaseous ClC(O)SCH2CH3. The dynamics of the fragmentations are analyzed from the perspectives introduced by Eland74 and Simon et al.,73 in which the shape and slope of the coincidence island, along with the kinetic energy released in the process, are analyzed to elucidate the mechanism of the fragmentations. Very similar PEPIPICO spectra were obtained at the several photon energies studied in this work, so the following discussion refers only to the data determined from the PEPIPICO spectrum taken at a photon energy of 165.8 eV. Coincidence between Ions with m/z Values of 27 amu/q (C2H3þ) and 32 amu/q (Sþ). This coincidence is the most intense island observed in the spectrum as a well-defined parallelogram with a slope of
0.8 (see Figure 7). In principle, 5314

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Table 4. Relative Intensities of Double-Coincidence Islands Derived from the PEPIPICO Spectra of ClC(O)SCH2CH3 as a Function of the Photon Energya photon energy (eV) ion 1 þ

165.8

170.8

201.0

209.5

H

C

1.4

0.9

1.2

1.3

Hþ

C2Hþ

1.6

0.9

1.2

1.2

Hþ

C2H2þ

2.9

3.2

2.2

2.1

Hþ

C2H3þ

1.4

2.4

1.2

1.1

Hþ Hþ

C2H4þ Sþ

1.4 3.8

0.8 2.9

1.1 2.9

1.2 3.0

Hþ

Clþ

3.3

7.0b

2.8

3.5

H

CS

1.3

1.0

0.9

0.7

Hþ

HCSþ

0.9

1.0

0.7

0.7

C

Sþ

0.6

CH3þ

HCSþ

1.4

CH3þ

SCH2þ

1.8

C2H C2H2þ

Sþ COþ

1.1 1.2

0.3

C2H2þ

Sþ

5.6

C2H2þ

Clþ

4.0

C2H3þ

COþ

C2H3þ C2H3þ C2H3þ C2H3þ C2H3þ C2H4þ C2H4þ

þ

þ

þ

1.0

0.7

1.3

1.4

1.2

1.9

1.6

2.2

0.9

0.7

3.4

3.6

3.2

1.5

2.6

2.7

2.0

1.1

1.1

1.2

Sþ

7.1

4.9

4.3

4.4

Clþ

5.7

3.5

3.3

4.1

OCS þ

2.8

3.1

2.0

2.7

ClC(O)þ SClþ

3.5 1.3

4.2 0.5

3.7 0.8

3.1 1.0

Sþ

2.3

1.2

2.1

1.9

Clþ

1.4

0.3

1.0

1.3

CH2CH3þ

Sþ

1.5

0.3

0.7

1.1

CH2CH3þ

Clþ

1.1

2.4

0.8

1.8

CH2CH3þ

C(O)Sþ

1.1

1.5

1.0

2.9

CH2CH3þ

ClC(O)þ

0.7

0.8

1.1

0.9

þ

þ

a

ion 2 þ

þ

S Clþ

Cl HCSþ

2.1 1.4

0.4 0.5

2.0 1.1

2.3 1.1

HCSþ

ClC(O)þ

1.7

1.9

2.1

1.6

Figure 7. Enlargement of the PEPIPICO spectrum of ClC(O)SCH2CH3 obtained at a photon energy of 165.8 eV in the range of m/z 25
37 amu/q in the T1 (2000
2400 ns) and T2 (2000
2400 ns) domains.

hydrogen extraction increases (see Figure 7), suggesting that similar mechanisms are involved. Coincidence between Ions with m/z Values of 27 amu/q (C2H3þ) and 35 amu/q (Clþ). This is the second most important coincidence observed at 165.8 eV, amounting to ca. 7% and 5% of all double coincidences for the S 2p and Cl 2p regions, respectively. This pair of ions can also originate by assuming that the four-body mechanism called secondary decay after deferred charge separation (SDDCS) takes place, for which the calculated slope is
1.1, in very good agreement with the experimental value of
1.1. The coincidence island for these two fragments is also shown in Figure 7. ClCðOÞSCH2 CH3 2þ f ClCH2 CH3 2þ þ CðOÞS

ð7Þ

ClCH2 CH3 2þ f Clþ þ CH2 CH3 þ

ð8Þ

CH2 CH3 þ f C2 H3 þ þ H2

ð9Þ

b

Only coincidences with intensities >0.5% given. Contamination due to HCl could affect this value.

several mechanisms could be proposed to explain this coincidence. The experimental shape and slope displayed by this coincidence are better interpreted by the occurrence of the deferred charge separation (DCS) mechanism given by ClCðOÞSCH2 CH3 2þ f ClCðOÞSC2 H3 2þ þ H2

ð4Þ

ClCðOÞSC2 H3 2þ f ClCðOÞ þ SC2 H3 2þ

ð5Þ

SC2 H3 2þ f Sþ þ C2 H3 þ

ð6Þ

The expected slope for the C2H3þ/Sþ island is equal to
1, which is higher than the experimental value. The occurrence of other dissociative mechanisms acting in competition cannot be ruled out. The coincidences concerning the series of CHxCHyþ ions (x = 0
2, y = 0
3) show slight changes in the slope when the

Assuming that the time scales for stepwise processes are large enough, dissociation energies can be defined, and kinetic momentum conservation can be applied at each step. Simon et al. determined analytically the peak shape in the PEPIPICO spectrum for deferred charge separation processes.73 The first step can be considered as a step producing an isotropic kinetic momentum to a doubly ionized molecule that dissociates following a secondary decay. The peak shape is a parallelogram with a defined slope given by the relative masses of the involved ions. Coincidence between Ions with m/z Values of 27 amu/q (C2H3þ) and 63 amu/q [ClC(O)þ]. The mechanism proposed for this coincidence also involves the molecular hydrogen loss in the first dissociative step.

5315

ClCðOÞSCH2 CH3 2þ f ClCðOÞSC2 H3 2þ þ H2

ð10Þ

ClCðOÞSC2 H3 2þ f ClCðOÞþ þ SC2 H3 þ

ð11Þ

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Figure 8. Enlargement of the PEPIPICO spectrum of ClC(O)SCH2CH3 obtained at a photon energy of 165.8 eV in the ranges m/z 25
29 amu/q in the T1 domain (2000
2300 ns) and m/z 60
65 amu/q in the T2 (3000
3300 ns) domain.

SC2 H3 þ f S þ C2 H3 þ

ð12Þ

The calculated slope for the SDDCS mechanism (
2.18), is in very good agreement with the experimental slope,
2.1, as shown in Figure 8. Coincidence between Ions with m/z Values of 27 amu/q (C2H3þ) and 60 amu/q (OCSþ). A plausible mechanism for this coincidence is four-body secondary decay in competition (SDC), in which the kinetic energy release corresponding to neutral ejection is neglected. The calculated slope for this mechanism is
0.67, in very good agreement with the experimental value of
0.7, as shown in Figure 8. ClCðOÞSCH2 CH3 2þ f ClCðOÞSþ þ CH2 CH3 þ

ð13Þ

ClCðOÞSþ f OCSþ þ Cl

ð14Þ

CH2 CH3 þ f C2 H3 þ þ H2

ð15Þ

3.2.4. Coincidences Involving the Hþ Ion. The double coincidences with this ion are important in the photodissociation of the title molecule, giving rise to very intense islands in the S 2p and Cl 2p spectra from both Hþ/Clþ and Hþ/Sþ pairs of ions, reflecting the importance of atomization processes in the dissociation mechanisms in these energy ranges. These ions could originate from several multibody dissociation events, making the analysis of these coincidence islands ambiguous. However, the photoion
photoion coincidence (PIPICO) technique offers a convenient way to analyze this type of coincidence qualitatively. In this technique, the ion pairs originating in the same photodissociation event are identified according to their time-of-flight differences. An enlargement of the PIPICO projection taken at 165.8 eV is shown in Figure 9. The progression of the doublecoincidence series of ions Hþ/CHxCHyþ (x, y = 0
2), Hþ/Sþ, Hþ/Clþ, and Hþ/HxCSþ (x = 0
2) is clearly observed, denoting the importance of double coincidences involving Hþ as the lighter ion. This is in agreement with the photofragmentation behavior reported previously for methylated species excited

Figure 9. Enlargement of the PIPICO projection spectrum of ClC(O)SCH2CH3 recorded at a photon energy of 165.8 eV. The spectrum corresponds at the double coincidences between Hþ and several series of ions.

at inner-shell levels.12,75 As proposed recently by Montenegro et al., these processes can be characterized as “evaporation” (eliminating light H0 neutral atoms) or “fission” (ejecting H þ ions) or as the molecule breaking up into two or more charged species.76

4. CONCLUSIONS A detailed study of the ionic fragmentation of the ClC(O)SCH2CH3 molecule in the gas phase following valence and shallow-core (S 2p and Cl 2p) excitations has been performed using multicoincidence techniques based on time-of-flight mass spectrometry and synchrotron radiation as the photon source. Moreover, the PES spectrum was analyzed, and the valence electronic structure was determined. The first four bands at 9.84, 10.74, 11.84, and 12.35 eV are associated with ionizations from electrons formally located at the nπ(S), nσ(O), n(Cl), and πCdO orbitals, respectively, demonstrating the importance of the — SC(O)— group in the outermost electronic properties. Dissociation mechanisms have been proposed to explain the ionic fragmentation decay for singly and doubly charged excited species. In the valence region, the PEPICO spectra show the formation of three main ions. The parent (Mþ) and C(O)SCH2 CH3þ (89 amu/q) ionic fragments dominate the production of ions at low photon energies. The intensity of a third fragment identified as CH2CH3þ increases strongly when the incident photon energy is higher than 13.40 eV. PEPICO spectra indicate that this fragment is formed by the inductive loss of a neutral OCS molecule from the C(O)SCH2CH3þ cation. When the photon energy is high enough for ionization of the S 2p and Cl 2p core
shell electrons, the ionic fragmentation of the doubly charged parent species produces the C2H3þ ion, which participates in several ionization channels on the shallow-core excited molecule. The mechanism for the formation of C2H3þ possibly involves the loss of a H2 molecule from the ethyl group of the excited species. In effect, the analysis of the shape and slope of the islands in the PEPIPICO spectra allows the postulation of a series of dissociation mechanisms in which the C2H3þ ion is formed together with H2. Thus, four-body dissociation mechanisms have 5316

dx.doi.org/10.1021/jp112182x |J. Phys. Chem. A 2011, 115, 5307–5318

The Journal of Physical Chemistry A been identified, including the following schemes: deferred charge separation (C2H3þ/Sþ), secondary decay after deferred charge separation [C2H3þ/Clþ and C2H3þ/ClC(O)þ,] and secondary decay in competition (C2H3þ/OCSþ). This fragmentation picture is rather different from that of the related CIC(O)SCH3 molecule,16 evidencing the relevance of the alkyl chain in the fragmentation of CIC(O)SR (R = alkyl group) species.

’ ASSOCIATED CONTENT

bS

Supporting Information. Atomic charges for the molecular and cation-radical forms of ClC(O)SCH2CH3. Geometrical parameters and dissociation energy diagram computed for the radical cation forms. Explanation of the four-body dissociation mechanisms invoked in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (C.O.D.V.), erben@ quimica.unlp.edu.ar (M.F.E.).

’ ACKNOWLEDGMENT L.S.R.P. is a doctoral fellow of CONICET, and M.F.E., M.G., R.M.R., and C.O.D.V. are members of the Carrera del Investigador of CONICET. This work was largely supported by the Brazilian Synchrotron Light Source (LNLS). The authors thank Arnaldo Naves de Brito and his research group for fruitful discussions and generous collaboration during their several stays in Campinas and the TGM beamline staffs for their assistance throughout the experiments. They also are indebt to the Agencia Nacional de Promoci
on Científica y Tecnol
ogica (ANPCyT), Consejo Nacional de Investigaciones Científicas y T
ecnicas (CONICET), and the Comisi
on de Investigaciones Científicas de la Provincia de Buenos Aires (CIC), Rep
ublica Argentina, for financial support. They also thank the Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Rep
ublica Argentina, for financial support. ’ REFERENCES (1) Le, H. T.; Nguyen, T. L.; Lahem, D.; Flammang, R.; Nguyen, M. T. Phys. Chem. Chem. Phys. 1999, 1, 755. (2) Flammang, R.; Lahem, D.; Nguyen, M. T. J. Phys. Chem. A 1997, 101, 9818. (3) Flammang, R.; Nguyen, M. T.; Bouchoux, G.; Gerbaux, P. Int. J. Mass Spectrom. 2000, 202, A8. (4) IUPAC Compendium of Chemical Terminology, 2nd ed.; McNaught, A. D., Wilkinson, A., Eds.; Blackwell Scientific Publications: Oxford, U.K., 1997; XML on-line corrected version available at http:// goldbook.iupac.org. (5) Bouma, W. J.; Nobes, R. H.; Radom, L. J. Am. Chem. Soc. 1983, 105, 1743. (6) Terlouw, J. K.; Heerma, W.; Dijkstra, G. Org. Mass Spectrom. 1981, 16, 326. (7) Stirk, K. G.; Kenttamaa, H. I. J. Phys. Chem. A 1992, 96, 5272. (8) Erben, M. F.; Romano, R. M.; Della V
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