Photoelectron Spectroscopy and Ab Initio Calculations of CS3

Article pubs.acs.org/JPCA

Photoelectron Spectroscopy and Ab Initio Calculations of CS3− Isomers: Carbon Trisulfide and Carbon Disulfide S‑Sulfide Anions Ryuzo Nakanishi,* Shugo Kato, Yasushi Matsuyama, and Takashi Nagata* Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Carbon sulfides are known as a class of binary compounds that can exist in various isomeric and/or polymeric forms. As for a sulfur-rich compound with the composition formula CS3, two possible constitutional isomers have been proposed experimentally or theoretically for the neutral species and its corresponding radical cation and anion. Although the previous studies claim that one isomer has a carbon trisulfide (CS3) C-centered configuration and the other has a carbon disulfide S-sulfide (SCSS) chain configuration, they have not yet been fully identified by a spectroscopic method. In this study, we have prepared the anions of those isomers in the gas phase by employing two types of reactions: dissociative electron attachment to 1,3-dithiole-2-thione for CS3− formation and the S− + CS2 ion−molecule reaction for SCSS−. Photoelectron spectroscopic measurements reveal that the reactions result in the production of two anionic species that can be well distinguished by their vertical detachment energy. With the aid of ab initio calculations, they are identified distinctively as the anions of carbon trisulfide and carbon disulfide S-sulfide.

1. INTRODUCTION Carbon sulfides show a variety of compositions and morphologies by virtue of the energetically favored bond formation of C−C, C−S, and S−S linkages, which eventually enriches the chemistry of those binary compounds.1−3 Several experimental and theoretical studies have discovered that such is the case even with a small carbon sulfide such as CS3.4−8 Sülzle et al. have demonstrated by neutralization−reionization mass spectrometry that carbon trisulfide, CS3, and its corresponding radical cation and anion are viable stable species in the gas phase.4 Their isotope-labeling studies suggest that CS3+ and CS3− take on either the C2v or D3h form whereas ab initio calculations at the HF/6-31G* level favor the C2v form for CS3, CS3+, and CS3−. The labeling studies have also revealed the existence of another cationic isomer that is assigned as the cation radical of carbon disulfide S-sulfide, SCSS+. Froese and Goddard theoretically investigated the potential energy surfaces relevant to the reactions of S(3P) and S(1D) atoms with CS2.5 They found local minimum energy structures corresponding to the C2v and D3h forms of CS3, and cis and trans forms of SCSS on the lowest singlet and triplet surfaces. Ma et al. recorded the infrared spectrum of CS3 in an argon matrix prepared by the cocondensation of carbon disulfide with high-frequency discharged argon.7 On the basis of isotopic substitution measurements and theoretical frequency calculations, they have assigned the observed infrared absorptions to the stretching and bending vibrations of CS3 with C2v symmetry. They also found that CS3 dissociated to form an SCS···S complex upon visible light irradiation. On the whole, those previous studies indicate that at least two possible isomeric forms exist for the © XXXX American Chemical Society

carbon sulfide composed of one C atom and three S atoms in the neutral, cationic and anionic states: the carbon trisulfide, CS3, C-centered configuration and the carbon disulfide Ssulfide, SCSS, chain configuration. To the best of our knowledge, however, no attempt has been made to confirm the exisitence of these two isomeric forms in the same charge state with compelling spectroscopic evidence. In this article, we focus our attention on the preparation and spectroscopic identification of the two isomeric forms of CS3− in the gas phase. One advantage of choosing the anionic system is that it enables us to use an optimal combination of mass spectrometry and electronic spectroscopy. The target anions are generated by two different processes: (i) the dissociative electron attachment to vinylene trithiocarbonate (1,3-dithiole2-thione) and (ii) ion−molecule reactions in the gas discharge of CS2/Ar. Process (i), shown in Scheme 1, is the same procedure as that employed by Sülzle et al.4 except that in the present study the reaction takes place in an electron-impact Scheme 1

Received: July 10, 2016 Revised: August 17, 2016

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DOI: 10.1021/acs.jpca.6b06900 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

processes effectively. Photoelectrons were extracted perpendicularly to both the ion and the laser beams by a static electric field and projected onto a 40-mm -diameter microchannel plate (MCP) coupled to a phosphor screen. The MCP was gated with a 200 ns time window coincident with the photoelectron arrival in order to reduce background noise. Photoelectron images on the phosphor screen were recorded using a charge coupled device camera. The images were acquired typically for 20 000−30 000 laser shots. Photoelectron spectra were reconstructed from the obtained images using the BASEX program.12 The measured electron kinetic energy was calibrated against the known photoelectron bands of S− and I−.13,14 Ab initio calculations for exploring the stable structures of CS3− and SCSS− were performed by using the Gaussian 03 program package.15 The MP2/aug-cc-pVDZ level of theory was employed in the initial search for the minimum energy structures. The minimum energy structures obtained by the MP2 calculations were further optimized at a higher level of theory. All of the geometry optimizations and frequency calculations were eventually made at the CCSD(T)/aug-ccpVDZ level. An inspection of the harmonic frequencies ensured that the optimized geometries were located at potential local minima.

ionized free jet so that product anions are further stabilized by collisional relaxation. Process (ii) is undertaken by using a pulsed discharge nozzle.9 Subsequent reactions of the discharge product anions with CS2 in the nozzle beam provide many kinds of anionic species, including anions with the composition formula CS3−. The electronic properties of these target anions are then probed by anion photoelectron spectroscopy, from which the vertical detachment energies (VDEs) of the anions are experimentally determined. In addition to these measurements, ab initio calculations are performed to explore the minimum energy structures for the anions of carbon trisulfide and carbon disulfide S-sulfide. The calculations also provide the stabilization energies and VDEs for the optimized structures. By combining these experimental and theoretical results, we identify the two distinctive isomeric forms of CS3− and discuss their formation mechanism.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental apparatus used in the present study has been described in detail previously.10 Briefly, the apparatus consists of a negative-ion source, a time-of-flight (TOF) mass spectrometer, and a photoelectron spectrometer. The target molecular anions were prepared by exploiting two different kinds of processes: dissociative electron attachment to vinylene trithiocarbonate (1) and ion−molecule reactions in a discharged CS2/Ar mixture. The dissociative electron attachment was conducted in an electron-impact ionized free jet. Argon gas was expanded supersonically through a pulsed valve (0.8 mm nozzle diameter) with a stagnation pressure of 0.3 MPa. The free jet was crossed with a 150 eV electron beam (current ≈ 100 μA) in the expansion region, which generated secondary slow electrons within the jet. A glass tube containing a powder of 1 was located ∼20 mm from the pulsed valve and was heated to ∼50 °C; this provided the sample gas with ambient pressure of ∼4.0 × 10−3 Pa. The sample was entrained spontaneously into the ionized jet and reacted with the slow electrons. The product anions were stabilized by collisional relaxation under the free jet condition. The anions were extracted ∼15 cm downstream from the nozzle, perpendicular to the initial beam direction by applying a pulsed electric field. They were further accelerated up to 1.25 keV and mass analyzed with a 1.9 m TOF mass spectrometer. The major product anions were m/z = 90 and 108 species, which corresponded to composition formulas C2H2S2− and CS3− (Figure S1(a) in Supporting Information). The discharge of a CS2/Ar gas mixture was conducted by using a pulsed discharge nozzle.9 Argon gas (0.3 MPa) containing a trace amount of CS2 (volume percentage ≈ 1%) was expanded through a pair of electrodes attached to the tip of the nozzle. A pulsed high voltage of 1.5 kV (duration ≈ 20 μs) was applied to the electrodes synchronously with the gas expansion. The discharged free jet contained various kinds of anionic species formed via fragmentation and/or ion−molecule reactions; the major product anions were observed at m/z = 76 (CS2− by composition formula) and 152 (C2S4−) along with several minor products at m/z = 32 (S−), 64 (S2−), 108 (CS3−), and 120 (C2S3−) (Figure S1(b)). Photoelectron measurements were performed by using a velocity-map imaging spectrometer11 at the end of the TOF tube. The m/z = 108 anions were crossed with either the third (355 nm) or fourth (266 nm) harmonic of a pulsed Nd:YAG laser in the center of the spectrometer. The laser fluence was kept below 8 mJ pulse−1 cm−2 to suppress multiphoton

3. RESULTS AND DISCUSSION 3.1. Carbon Trisulfide Anion, CS3−. Figure 1 shows the photoelectron spectra of m/z = 108 species prepared via the

Figure 1. Photoelectron spectra of the m/z = 108 species formed via the dissociative electron attachment to vinylene trithiocarbonate. (a) Spectrum recorded at 355 nm. The comb indicates a possible vibrational progression. (b) Spectrum recorded at 266 nm. (c) Spectrum of the S− atomic anion obtained at 355 nm. (d) Spectrum obtained by subtracting trace (c) from trace (a). In traces (a), (b), and (c), no photoelectron signals are discernible in the electron binding energy below 1.2 eV and hence are not shown.

dissociative electron attachment to vinylene trithiocarbonate (1). The spectrum measured at 355 nm consists of a single band at around 3.2 eV that possesses a vibrational progression at intervals of 565 ± 10 cm−1 (Figure 1a). In the spectrum measured at 266 nm (Figure 1b), another band with a higher intensity appears at around 3.8 eV. The vibrational features of the 3.2 eV band are severely blurred in the 266 nm spectrum because of the degradation of spectral resolution owing to the increased kinetic energy of photoelectrons. The VDE values determined from the band maxima are 3.21 ± 0.02 and 3.82 ± B


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The Journal of Physical Chemistry A 0.02 eV for those observed bands, respectively. A small peak at around 2.1 eV in the 355 nm spectrum, marked with an asterisk in Figure 1a, disappears in the measurement at 266 nm. This indicates that the 2.1 eV band arises from the photoproduct from the m/z = 108 anion at 355 nm. As was argued by Sülzle et al., dissociative electron attachment to 1 leads to the formation of the carbon trisulfide anion, CS3−, having either C2v or D3h symmetry.4 With this in mind, we performed ab initio calculations for the C2v and D3h configurations of CS3− at the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. Although both configurations, C2v and D3h, were fully optimized at the MP2 level of theory, the C2v structure is no longer a stationary point at the CCSD(T) level; geometry optimization starting from the C2v configuration converged to the D3h form in the CCSD(T)/aug-cc-pVDZ calculations. It is concluded from the CCSD(T) result that the D3h form is the global minimum energy structure of CS3−. In regard to this point, we refer particularly to symmetry-breaking effects, which have been rigorously investigated by Eisfeld and Morokuma on the NO3 structure (isovalent to CS3−).16 Although both C2v and D3h equilibrium structures were predicted for the NO3 radical in the earlier studies, they have demonstrated by CASSCF and MR-SDCI calculations that only the D3h structure is the true local minimum whereas the C2v structure is a spurious local minimum due to symmetrybreaking effects caused by an inadequate treatment of electron correlations.16 They also showed that the spurious C2v structure was calculated to be lower in energy in the HF approximation because of the lack of electron correlation whereas the D3h structure was energetically favored in the MP2 method as a result of the overestimation of resonance effects. The C2v and D3h structures were calculated to be almost isoenergetic in the CCSD and CCSD(T) methods, where the electron correlation was treated more accurately. Recently, Cappa and Elrod found the same symmetry-breaking effects in the CO3 and CO3− system (isovalent to CS3 and CS3−, respectively).17 They examined the energy ordering of the C2v and D3h local minima of CO3 and CO3− as a function of the theoretical method employed: HF, MP2, CCSD, or CCSD(T). They found precisely the same behavior of the energy ordering as observed in the NO3 study. Consequently, they proposed that the D3h structure was the true minimum energy configuration for each species. In the present study of CS3−, we checked the energy ordering of the C2v and D3h local minima at each level of theory and found the same behavior upon increasing the level of theory from HF to MP2 to CCSD except that only the D3h configuration was obtained as the minimum energy structure at CCSD(T). This finding is indicative of the influence of symmetry-breaking effects on the CS3− geometry optimization and consequently reinforces our conclusion on the D3h equilibrium structure. The global minimum energy structure of CS3− optimized by the CCSD(T) calculations is shown in Figure 2. The VDEs were evaluated by subtracting the CCSD(T) energy of the anion from those of the neutral species retaining the anion equilibrium geometry in the lowest singlet or triplet manifold. The calculated VDEs along with the CCSD(T) energies are summarized in Table 1. By extending the CCSD(T) calculations to the CS3 neutral, both D3h and C2v forms were obtained as local minima on the singlet potential energy surface; CS3(D3h, 1A1′) was calculated to be 0.40 eV higher in energy than CS3(C2v, 1A1). By taking into consideration the symmetry-breaking effects pointed out by Cappa and Elrod,17

Figure 2. Optimized geometries and energetics for CS3− and CS3. Bond lengths and bond angles are given in units of angstroms and degrees, and adiabatic energy differences are given in units of electronvolts. *The C2v isomeric form assigned to a pseudolocal minimum structure due to the symmetry breaking effect is also shown for reference. (See the text for details.)

Table 1. CCSD(T) Energies and VDEs for CS3− and CS3 species CS3− CS3

symmetry D3h C2vc D3h D3h

term

E (ha)a

ΔE (eV)b

VDE (eV)

A2′ A1c 1 A1′ 3 E″

−1231.120253 −1231.021193 −1231.006434 −1230.983258

0.00 2.70 3.10 3.73

3.13d 3.73e

2

1

a

Total energy calculated at the CCSD(T)/aug-cc-pVDZ level. Relative energy with reference to the global-minimum anion state, CS3−(D3h, 2A2′). cPseudolocal minimum structure with C2v symmetry due to the symmetry-breaking effect. (See the text for details.) d Vertical detachment energy for the transition from CS3−(D3h, 2A2′) to the singlet neutral state. eVertical detachment energy for the transition from CS3−(D3h, 2A2′) to the triplet neutral state. b

we assign D3h(1A1′) as the equilibrium geometry of the singlet neutral, although the CCSD(T) energy of D3h(1A1′) is higher than that of C2v(1A1). This choice of geometry, however, does not affect our spectral assignments discussed below as long as the discussion remains within the confines of vertical transitions from CS3− (D3h, 2A2′). The CCSD(T) calculations provided a D3h form of CS3 as only one optimized structure on the triplet surface. The energetics obtained by the calculations for all of the relevant species is illustrated schematically in Figure 2. On the basis of the fact that the calculated VDEs reasonably reproduce the experimentally determined values (Table 1), we identify the D3h form of CS3− as the spectral carrier of the photoelectron bands shown in Figure 1. The 3.2 eV band is assignable to the transition from the ground-state anion (D3h, 2 A2′) to the singlet neutral (D3h, 1A1′), and the 3.8 eV band to that to the triplet neutral (D3h, 3E″). Within the context of these assignments, the vibrational progression appearing in the 3.2 eV band is ascribable to the vibrational motion of neutral CS3 in the singlet manifold. Thus, we assign the observed frequency of the progression (νobs = 565 cm−1) to the totally symmetric C−S stretching vibration of CS3 (D3h, 1A1′) (νcalc = 541 cm−1). This seems most consistent with the finding that C


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The possible existence of isomeric form(s) other than CS3− (D3h, 2A2′) has already been pointed out in the literature.4,18 A prime candidate is the anion of carbon disulfide S-sulfide, SCSS−, with cis and trans conformations, whereas only the trans form was predicted as a stable structure of SCSS− in HF/ 6-31G* calculations.4 In the present study, we performed geometry optimization at the CCSD(T)/aug-cc-pVDZ level to explore the possible isomeric forms of SCSS− in the electronic ground state and those of SCSS neutrals in the lowest singlet and triplet states. The optimized geometries and their CCSD(T) energies are summarized in Figure 4 and Table 2.

the CCSD(T) calculations predict a substantial change in the C−S equilibrium nuclear distances between the anion and the neutral states (Figure 2). This inference is also supported by the Franck−Condon simulation of the photoelectron band envelope (section S2 in the SI). The small peak at around 2.1 eV in the 355 nm spectrum is eventually ascribable to the photoemission from S− generated via the 355 nm photodissociation of CS3−. We found the intensity of the 2.1 eV peak being quadratically dependent on the detachment laser power, indicating the relevance of a twophoton process such as photodissociation, CS3− + hν → S−(2P) + CS2, followed by photodetachment, S−(2P) + hν → S(3P2) + e−, at 355 nm. We actually confirmed the production of S− from CS3− as a dominant photoproduct at 355 nm by photofragment mass spectrometry (more details in the SI). In Figure 1c, the photoelectron spectrum of S− measured at 355 nm is shown for comparison. By subtracting trace 1c from trace 1a, we have obtained the photoelectron spectrum of CS3− (D3h, 2A2′) at 355 nm (trace 1d in Figure 1). 3.2. Carbon Disulfide S-Sulfide Anion, SCSS−. Figures 3a shows the photoelectron spectrum of the m/z = 108 species

Figure 3. Photoelectron spectrum of the m/z = 108 species formed in the discharged supersonic jet of CS2/Ar. (a) Spectrum measured at 355 nm. The dotted line is the m/z = 108 spectrum obtained by the dissociative electron attachment (reproduced from Figure 1a). (b) Spectrum obtained by subtracting the contributions of CS3− and S− bands from trace (a). Decomposed band shapes obtained in the band envelope analysis are shown with dotted blue lines. Figure 4. Optimized geometries and energetics for SCSS− and SCSS. Bond lengths and bond angles are given in units of angstroms and degrees, and adiabatic energy differences are given in units of electronvolts. All of the species displayed have Cs symmetry.

formed in the discharged supersonic jet of a gas mixture of CS2/Ar. Although the observed spectral features around 3 eV are somehow congested, the vibrational structures characteristic of the 3.2 eV band of CS3− (D3h, 2A2′) and the 2.1 eV band of S− are obviously discernible. The subtraction of these CS3− and S− contributions from trace 3a consequently reveals the existence of new photoelectron bands as shown in Figure 3b, which are attributable to the photoemission from the m/z = 108 species having isomeric forms other than CS3− (D3h, 2A2′). The newly obtained spectrum consists of a dominant peak at 3.0 eV and a relatively broad shoulder at around 2.8 eV. The spectral profile was decomposed into two Gaussian envelopes by curve fitting (Figure 3b), from which the peak positions were determined to be 3.02 ± 0.03 eV for the sharp band and 2.80 ± 0.03 eV for the broad shoulder. There is a noticeable difference in bandwidth between these two bands; the 2.8 eV band is much broader (fwhm = 0.50 eV) than the 3.0 eV band (fwhm = 0.16 eV). The photoelectron measurement at 266 nm has scarcely provided additional information on these bands; hence, the 266 nm spectrum is not presented here.

As seen in Figure 4, the CCSD(T) calculations for SCSS− provide two local minima corresponding to the cis and trans forms of Cs symmetry. The lowest-energy structure of SCSS− takes on a cis conformation; the trans form is higher in energy by 0.12 eV. The two forms are separated by a potential barrier of 0.27 eV with reference to the cis form on the Cs energy surface. The barrier height ensures the existence of two conformers as distinctive isomeric forms of SCSS−. As for neutral SCSS in the singlet manifold, geometry optimization starting from either the cis or trans conformation converged to an identical local-minimum structure having a nearly linear S− C−S configuration (Figure 4). In contrast to the singlet manifold, both cis and trans conformers are located at local minima in the lowest triplet state. The neutral structures obtained in the present CCSD(T) calculations are essentially D


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CS2 are vulnerable to attack by S−. Because of this result, we can infer that, in the present study using the pulsed-discharge free jet, those collision complexes formed by the S− attack are subject to collisional relaxation with high efficiency in the free jet environment, leading eventually to the production of CS3− and SCSS−. Recently, Sanov et al. observed the production of CS3− in the photodissociation of (CS2)n− (n = 2−4), though CS3− is not a structural formula but a composition formula in this case.19 They concluded that CS3− was formed by the association reaction between nascent photoproduct S− with CS2 solvent within the CS2−-based clusters: CS2−·CS2 + hν → CS + S−·CS2 → CS + CS3− for the case of n = 2. Because (CS2)2− was observed to be a major product anion in our pulsed-discharge free jet, we cannot exclude the possibility that CS3− and SCSS− are formed by the same mechanism via [(CS2)2−]* energized in the pulsed discharge. Consequently, however, this formation mechanism is intrinsically equivalent to the S− + CS2 reaction except that it takes place in cluster environments. As for energetics, the stabilization energies are calculated to be 1.74 eV for CS3−, 0.62 eV for cis-SCSS−, and 0.50 eV for trans-SCSS− with reference to S− + CS2. Thus, all of the association reactions between S− and CS2 occur with some amount of excess energy, which is dissipated inevitably by collisional relaxation for CS3−/SCSS− production. An orbital correlation study shows that the singly occupied molecular orbital (SOMO) of CS3− is constructed by the overlap between the 3p orbital of S− and the HOMO (πg) of CS2 and that the SOMO of SCSS− occurs mainly as a result of the overlap between S− (3p) and the LUMO (πu) of CS2 (more details in the SI). The electronic ground states of CS3− and cis-/transSCSS− correlate directly to the S−(2P) + CS2(1Σg+) limit. In line with the orbital correlation study, we also calculated the potential energy profile for the isomerization between cis- and trans-SCSS−. The calculations were performed at the MP2/augcc-pVDZ level to save computational time. The S−C−S angle, denoted as θ, was chosen as the parameter representing the isomerization coordinate by considering the fact that the frequency analysis in the CCSD(T) geometry optimization showed that the SCS bending mode possessed the lowest vibrational frequency among the possible vibrational modes relevant to the isomerization. In the calculations, θ was varied in the range of 134−232° over an interval of 4°. The transient structure at each fixed θ value was obtained by optimizing all parameters other than θ. The calculations were made in a predictor-corrector way in that structure parameters obtained at a given θ value were used as the initial parameters for the subsequent optimization at θ + Δθ. Figure 5 depicts the calculated potential profile plotted against θ. As shown in Figure 5, SCSS− has a Cs structure in the transition state and retains planar symmetry during the isomerization. Also note that the 3p orbital of S− interacts with the πu orbital of CS2 to form the SOMO of SCSS− in a head-on manner in the transition state but in a side-by-side manner in the local minimum structures. Accordingly, the 3p orbital apparently rotates its orientation with respect to the S−S bond direction with the progression of isomerization (Figure 5). Although the potential profile in Figure 5 is indicated on the scale of MP2 energy, the transition-state geometry was reoptimized at the CCSD(T)/aug-cc-pVDZ level, which provides the CCSD(T) barrier height for isomerization: 0.27 eV with reference to the cis form and 0.15 eV with reference to the trans form (Figure 4). We conclude from these arguments that the cis and trans

Table 2. CCSD(T) Energies and VDEs for SCSS− and SCSS species SCSS

−

conformera cis trans

SCSS cis trans

E (ha)b

ΔE (eV)c

A′ 2 A′ 1 A′

−1231.078807 −1231.074491 −1230.995986

1.13 1.25 3.38

3

−1230.976677 −1230.973906

3.91 3.98

term 2

3

A″ A″

VDE (eV)

2.83d 2.87e 2.85f 2.87g

a All of the species listed here have Cs symmetry. bTotal energy calculated at the CCSD(T)/aug-cc-pVDZ level. cRelative energy with reference to the global-minimum anion state, CS3−(D3h, 2A2′). d Vertical detachment energy for the transition from cis-SCSS−(Cs, 2 A′) to the singlet neutral state. eVertical detachment energy for the transition from trans-SCSS−(Cs, 2A′) to the singlet neutral state. f Vertical detachment energy for the transition from cis-SCSS−(Cs, 2A′) to the triplet neutral state. gVertical detachment energy for the transition from trans-SCSS−(Cs, 2A′) to the triplet neutral state.

identical to those calculated previously at the MP2 and B3LYP levels of theory.5,7,8 The VDE values calculated for the cis and trans forms of SCSS− are listed in Table 2. We infer from these calculated values that the cis and trans forms are indistinguishable from each other by means of the VDE measurement. It is also found that, both in the cis and trans forms, the calculated VDE for the singlet manifold differs little from that for the triplet although the two manifolds differ in CCSD(T) stabilization energy by 0.53 eV in the cis form and by 0.60 eV in the trans form (Figure 4). By comparing the experimental data with the calculation results, the observed photoelectron bands are ascribable to the photodetachment from one or both of the two SCSS− conformers. The 2.8 eV band is assigned to the transition to the singlet manifold because its broad bandwidth implies a large difference in the equilibrium geometry between the initial anion and the final neutral states. The 3.0 eV band is eventually assigned to the transition to the triplet. These assignments are further confirmed by the Franck−Condon simulation of the band profiles; the simulation qualitatively reproduces a much broader bandwidth for the transition to the singlet manifold (section S2 in the SI). The possible coexistence of the cis and trans conformers will be discussed below in terms of the formation mechanism of SCSS− under the present experimental conditions. 3.3. Formation Mechanism of CS3− and SCSS−. As discussed in the previous section, both the carbon trisulfide anion (CS3−) and carbon disulfide S-sulfide anion (SCSS−) are formed in the pulsed-discharge free jet. A simple but plausible candidate for the formation mechanism is the reaction between S− and CS2, although neither CS3− nor SCSS− was observed as a product of the S− + CS2 reaction in the drift-tube experiment performed by Bierbaum et al.18 Their measurements using 34S− as the reagent anion revealed that the reaction of 34S− with CS2 occurred by either isotope exchange, 34S− + CS2 → S− + 34SCS, or sulfur abstraction, 34S− + CS2 → 34SS− + CS. From the observed kinetic energy dependence of the rate coefficients, they infer that the isotope exchange proceeds via the CS3− collision complex formed by 34S− nucleophilic attack on the carbon atom of CS2. The absence of the 32S32S− product anion in the abstraction reaction is interpreted as evidence for the mechanism that the abstraction occurs through direct 34S− attack on the sulfur atom of CS2 at kinetic energy above 0.3 eV.18 Those results indicate that both the carbon and sulfur of E


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possible coexistence of cis- and trans-SCSS− in the discharged free jet. The present investigation experimentally and theoretically demonstrates the existence of distinctive isomeric forms for the small carbon sulfide, C1S3, in its anionic and neutral states. This not only shows clear evidence of specific bond formation in the C1S3 system but also provides a basic understanding of the capability inherent in CmSn species to form a variety of structure morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. Energy profile for the cis−trans isomerization of SCSS− in the electronic ground state. The relative energy with reference to the local-minimum cis form is plotted against ∠S−C−S. The singly occupied molecular orbital of SCSS− changes its shape with the progression of the isomerization, as depicted above.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b06900. Mass spectra of the product anions prepared by the experimental setup in this study, Franck−Condon simulations of photodetachment transitions of CS3− and SCSS−, photofragment mass spectrum of CS3− at 355 nm, and singly occupied molecular orbitals of CS3− and SCSS− (PDF)

forms of SCSS− exist as distinctive conformational isomers energetically separated by an insurmountable potential barrier under the free-jet conditions. The two forms possibly coexist in our pulsed discharge beam because it is unlikely that SCSS− can be selectively produced into either of the two conformers via the S− + CS2 reaction.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

4. CONCLUSIONS Two distinctive isomeric forms of CS3−, the carbon trisulfide anion with a C-centered CS3 configuration and the carbon disulfide S-sulfide anion with a chain SCSS configuration, have been prepared and identified by a combination of mass spectrometry and anion photoelectron spectroscopy. The carbon trisulfide anion, CS3−, is formed as a major product of the dissociative electron attachment to vinylene trithiocarbonate (1,3-dithiole-2-thione). The observed photoelectron spectrum of CS3− consists of two bands with the band maxima at 3.21 ± 0.02 and 3.82 ± 0.02 eV, respectively, which can be compared directly with the theoretically calculated vertical detachment energies (VDEs). In the present study, we employ the CCSD(T)/aug-cc-pVDZ level of theory for the geometry optimizations and energy calculations in an attempt to prevent the effects of symmetry breaking, which can provide spurious local minima having lower symmetry as demonstrated in isovalent systems such as CO3− and NO3.16,17 The CCSD(T) calculations eventually provide only one optimized geometry with D3h symmetry as the global minimum structure for CS3−. On the basis of this result along with the calculated VDE values, the observed photoelectron bands are assigned to the photodetachment transitions from the ground state (2A2′) of the CS3− anion to the singlet (1A1′) and triplet (3E″) states of the CS3 neutral. We then explore the discharge products of the CS2/Ar mixture for the other isomeric form, SCSS−, in anticipation of the S− + CS2 reaction leading to the formation of SCSS−. The photoelectron spectroscopic measurements reveal the existence of new bands other than the 3.21 and 3.82 eV bands of CS3−: a broad band centered at 2.80 ± 0.03 eV and a relatively sharp band at 3.02 ± 0.03 eV. With the aid of the CCSD(T) calculations, these bands are assigned to the photodetachment transitions from the ground state (2A′) of either cis- or transSCSS−, neither of which has been spectroscopically identified to date, to the singlet (1A1′) and triplet (3A″) states of the corresponding SCSS neutrals. The calculations also indicate the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Grants-in-Aid for Scientific Research (grants 15K17804 and 26410004) from the Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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Photoelectron Spectroscopy and Ab Initio Calculations of CS3

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