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Structures of C2S4- molecular anion: Photoelectron spectroscopy and theoretical calculations
Chemical Physics Letters 457 (2008) 31–35
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Structures of C2 S 4 molecular anion: Photoelectron spectroscopy and theoretical calculations Yasushi Matsuyama, Takashi Nagata * Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan
a r t i c l e
i n f o
Article history: Received 8 February 2008 In final form 26 March 2008 Available online 31 March 2008
a b s t r a c t An intense beam of the dimer anion of CS2 was prepared by a pulsed-discharge nozzle, which enabled the measurement of the previously-reported 2.7-eV photoelectron band of C2 S 4 with higher spectral resolution at 355 nm. This measurement has revealed that the band consists of a vibrational progression with an average spacing of 0.13 ± 0.01 eV, indicating a bound electronic property of the residual C2S4 neutral. With the aid of molecular orbital calculations, the observed progression has been assigned to the transi2 1 tion from C2 S 4 ( B1) to the vibrationally-excited states of C2S4( A1), both possessing cyclic C2v structures. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Anions of dimeric molecules often form tightly bound species where the excess electron acts as a binding agent between the two constituent monomers. Among those species, the dimer anion of CS2 is of particular interest, as several different types of isomeric forms have been proposed to exist both experimentally and theoretically [1–8]. Hiraoka et al. determined the bond dissociation en1 ergy of ðCS2 Þ by high-pressure mass 2 to be 21.9 kcal mol spectrometric measurements, and proposed the formation of covalently-bonded C2 S 4 of C2v symmetry, which includes a dithiabutane ring structure [1]. Tsukuda et al. revealed by photoelectron spectroscopy that the C2 S 4 and CS2 CS2 forms of the dimer anion coexisted in ðCS2 Þ , and assigned a photoelectron band at the elec2 tron binding energy of 2.7 eV to the C2 S 4 anion [2]. This assignment was further confirmed in the recent photoelectron imaging experiment by Sanov and co-workers [3]. Maeyama et al. reported that SCCS was produced in the photodissociation of ðCS2 Þ2 , which provided a strong support for the cyclic C2v form of C2 S 4 [4]. More recently, Yu et al. measured the infrared vibrational spectrum of matrix-isolated C2 S 4 and identified the spectral carrier as a planar C–C bonded D2h form of C2 S 4 [5]. The electronic structures of ðCS2 Þ 2 were theoretically investigated by Sanov et al. [6]. Their MP2/6-31+G(d) calculations predicted the existence of four covalently-bonded forms along with one ion-molecule complex form; almost isoenergetic C2v(2B2) and C2v(2B1) forms of C2 S 4 were the most stable structures. The B3LYP/6-31+G(d) calculations by Zhang et al. [7] and B3LYP/6-311+G(d) by Zhou and Andrews [8], however, provided the D2h(2B3g) form as the global minimum structure. All the isomeric forms hitherto proposed and discussed are 1–6 (Scheme 1). The solid lines in Scheme 1 represent covalent bonds * Corresponding author. E-mail address: [email protected] (T. Nagata). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.068
and dotted lines are weak interactions; 1–5 are categorized as C2 S 4 molecular anions, while 6 as a CS2 CS2 ion-molecule complex. In isomeric form 2, the excess electron is accommodated in the antibonding molecular orbital between the S atoms, which weakens the S–S bond of the dithiabutane ring of isomer 3. As mentioned above, the arguments about existing isomeric forms are not yet all consistent. In the present study, we report on the vibrational structures appearing in the 2.7-eV photoelectron band of C2 S 4 , which give us a clue as to the isomeric form of the dimeric anion responsible for the photodetachment process. The experimental results are discussed in conjunction with molecular orbital calculations, aiming to settle the controversy regarding the existing isomeric forms of C2 S 4. 2. Experimental and computational methods The experimental apparatus consists of a cluster-anion source, a time-of-flight (TOF) mass discriminator and a photoelectron spec trometer [9]. The C2 S 4 anion was produced as ðCS2 Þ2 species in a supersonic jet by using a pulsed-discharge nozzle (PDN) [10,11]. A sample gas of Ar containing 1% of CS2 was expanded into the source chamber through the PDN operated at 1500 V (20 ls duration). The stagnation pressure was typically 0.3 MPa. The product anions were extracted at 20 cm downstream from the nozzle, perpendicularly to the initial beam direction by applying a pulsed electric field, and admitted into the TOF mass discriminator. The mass-selected ðCS2 Þ 2 anions were then intersected with unfocused either third (355 nm) or fourth (266 nm) harmonic of a Q-switched Nd:YAG laser at the photodetachment region. The kinetic energies of the photoelectrons were measured by a magnetic-bottle type electron spectrometer. In the high-resolution measurements, the ion packets were decelerated to reduce Doppler broadening prior to photodetachment. The typical resolution of our setup was
32
Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35
Scheme 1.
DE/E 7% for electrons having a kinetic energy of 1 eV. Each spectrum presented in this Letter represents an accumulation of 30 000 laser shots with background subtraction. Molecular orbital calculations are carried out to obtain the optimized geometries, the total energies and the vertical detachment energies (VDEs) of C2 S 4 by using the GAUSSIAN 03 program package [12]. The geometry optimizations are performed at the B3LYP level of theory, and the single-point energy calculations at the CCSD(T) level. The basis sets employed are aug-cc-pVDZ and 6-311+G(2d). Frequency analyses are also made to check whether the optimized geometries are located at potential minima.
Fig. 1. Photoelectron spectra of ðCS2 Þ 2 measured at (a) 266 nm (4.66 eV) and (b) 355 nm (3.49 eV). In panel (a) the dotted lines are the best-fit Gaussian profiles obtained by the deconvolution processing. The arrows indicate the positions of the 1 þ 2 C2 S CS 4 bands. The band assignments for the CS2 ðX Rg Þ CS2 2 ðX A1 Þ CS2 and 2 CS2 ða3 B2 Þ CS2 CS 2 ðX A1 Þ CS2 transitions are also given. In panel (b) the combs are shown for the vibrational progression of C2S4.
3. Results and discussion 3.1. Photoelectron spectra Fig. 1a shows the photoelectron spectra of ðCS2 Þ 2 measured at 266 nm (4.66 eV). The photoelectron counts are plotted against the electron binding energy defined as eBE = hm – eKE, where hm and eKE represent the photon energy and the kinetic energy of photoelectrons, respectively. The spectral profile is intrinsically identical to that reported in Ref. [2]; the spectral assignments confirmed so far are also indicated in Fig. 1 [2,3,13]. In the present measurement, we focus our attention on the tiny features in the 2.4–3.4 eV range. After an appropriate deconvolution processing, two band components are recognized at 2.68(2) and 3.19(2) eV as indicated by arrows in the inset of Fig. 1a. This implies that the 2.4–3.4 eV features contain contributions from the electronic transitions assignable to photodetachment processes of more than one C2 S 4 isomeric forms, as pointed out by Sanov and co-workers [3]. Although the assignment of the C2 S 4 band is unquestionable, there remains a question about the chemical identity of the spectral carrier: i.e., which of the isomeric forms 1–5 are responsible for the 2.68- and 3.19-eV bands. In order to uncover more detailed features of these photoelectron bands, 355 nm (3.49 eV) radiation was employed as the detachment laser because a better spectral resolution can be achieved for the photoelectrons with smaller eKE in the TOF measurement. Fig. 1b depicts the ðCS2 Þ 2 spectrum measured at 355 nm. In the 2.68-eV band, a vibrational progression with an average spacing of 0.13 ± 0.01 eV (1048 ± 80 cm1) is well recognized due to the improved spectral resolution. The progression is assignable to the Franck–Condon leap from the ground state
of C2 S 4 to the vibrationally-excited states of neutral C2S4. This implies that the C2S4 neutral is prepared in a bound electronic state via the photodetachment, and that the equilibrium geometry of C2S4 differs significantly from that of C2 S 4 . The geometrical change associated with the electronic transition from C2 S 4 to C2S4 occurs along the normal mode which composes the observed vibrational progression, as is generally the case according to the Franck–Condon principle. This statement is a key to the spectral assignment for the 2.68-eV band, as will be discussed in Section 3.3. It is also obvious in Fig. 1b that the photoelectron intensity starts to increase drastically around eBE > 3 eV in the 355 nm spectrum, whereas the tail of the high-energy band arises at 3.4 eV in the 266 nm spectrum. This observation is consistent with the result given by Sanov et al., who observed a low-eKE feature specific to the autodetachment process in the ðCS2 Þ 2 photoelectron spectra measured at 400 and 530 nm [3]. The appearance of the low-eKE feature prevents us from observing the detailed profile of the 3.19-eV band. 3.2. Theoretical calculations Geometry optimizations were performed first at the B3LYP/augcc-pVDZ level, starting with the geometries proposed by Sanov et al. at MP2/6-31+G(d) [6] and by Zhang et al. at B3LYP/631+G(d) [7]. The reliabilities of the optimizations were further checked by the B3LYP/6-311+G(2d) calculations. Fig. 2 shows the optimized structures for five C2 S configurations and one 4 CS 2 CS2 obtained at B3LYP/6-311+G(2d). The isomeric forms are individually assigned numbers 1–6 after Scheme 1. Although the
Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35
33
Fig. 2. Optimized geometries of C2 S 4 (1–5) and CS2 CS2 (6) obtained at the B3LYP/6-311+G(2d) level. Filled circles correspond to C atoms and open circles to S atoms. Bond lengths and bond angles are given in Å and deg. Note that 1 is optimized as a transition structure with one imaginary vibrational frequency.
Table 1 Calculated stabilization energies (EST) and vertical detachment energies (VDE) of stable isomeric forms 2–6 of C2 S 4 Isomers
2 3 4 5 6 c CS 2
6-311+G(2d)a
Aug-cc-pVDZb
EST (eV)
VDE (eV)
EST (eV)
VDE (eV)
0.84 0.88 0.32 0.37 0.31 –
2.94 2.41 3.67 1.61 0.95 1.02
1.00 0.92 0.45 0.39 0.33 –
3.07 2.54 3.83 1.70 1.16 1.23
a EST and VDE values are obtained by single-point calculations at CCSD(T)/6311+G(2d) with employing the geometries optimized at B3LYP/6-311+G(2d). b Same as above except that aug-cc-pVDZ basis set is used. c VDE of CS 2 monomer anion is calculated for reference.
optimized structures 1–6 are essentially identical to those reported in Refs. [5–8], the present B3LYP calculations mark a sharp contrast with the previous ones, which predicted 1 as the global minimum structure [5,7,8]. In the present study, B3LYP/aug-cc-pVDZ provides isomer 2 as the global minimum structure while 3 is the most stable geometry at B3LYP/6-311+G(2d). Both of the present B3LYP calculations predict 1 as an unstable geometry in a transition state. The stabilization energies with reference to the 2 1 CS 2 ðX A1 Þ þ CS2 ðX Rg Þ limit, and VDEs for the stable isomers obtained by the CCSD(T) calculations are listed in Table 1. As for the energy ordering of isomers 1–3, the discrepancy arises from the difference in the levels of theory: CCSD(T) with a larger basis set, employed in the present study, gives a more accurate estimate of the total energy. A more serious issue is the question whether 1 is the global minimum structure as predicted by the previous B3LYP calculations [5,7,8], or a transition structure as obtained in the present study. In regard to this issue, it should be pointed out that (1) isomers 1 and 2 are close in energy (Table 1) and, more importantly, that (2) their electronic states belong to the same symmetry type of point groups: the 2B3g species of D2h point group resolves into 2B2 of C2v by symmetry lowering. This suggests that isomers 1(2B3g, D2h) and 2(2B2, C2v) are located on the same adiabatic potential energy surface either as a transition state or as a local minimum. In order to examine the possibility of the interconversion between 1 and 2, we calculated the energy profile for the conformational conversion process. In the calculations, the
Fig. 3. Energy profiles for the interconversion 2 ? 1 ? 20 obtained by B3LYP calculations with various basis sets: filled circles: aug-cc-pVDZ, open circles: 6-311+G(2d), filled triangles: 6-311+G(2df), open triangles: 6-311+G(3df). The reaction coordinate is the angle h = \SCC.
angle SCC was chosen as the reaction coordinate and varied from 115° to 122° at an interval of 0.1°. At a fixed value of \SCC, the transient structure was obtained by optimizing all the parameters other than \SCC under the constraint that C2 S 4 retains C2v symmetry. Fig. 3 depicts the energy profiles calculated by the B3LYP method with employing various basis sets, such as aug-cc-pVDZ, 6-311+G(2d), 6-311+G(2df) and 6-311+G(3df). As readily seen in Fig. 3, the calculated energy profiles are well represented by
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Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35
Fig. 4. Geometry of C2S4(1A1) neutral obtained at the B3LYP/6-311+G(2d) optimization, starting with the configuration of isomer 3 as the initial geometry. Also shown are the actual forms of the a1 vibrational modes in C2S4(1A1).
double-minimum potential curves. The maximum of each curve around 119° corresponds to the transition state, where C2 S 4 takes on the D2h configuration of isomer 1. Two minima are located at 116–117.5° and 120–121.5°, depending on the basis sets used in the calculations. These minima correspond to the local minimum configuration of isomer 2. Although the calculated energy profiles show a distinct double-minimum feature, the barrier height is estimated to be in the order of several thousandths of eV, which is obviously a tiny quantity beyond the accuracy of B3LYP calculations. All that we can say from the present results in Fig. 3 are that (i) isomers 1 and 2 are adiabatically connected to each other, as predicted on the basis of the group-theoretical consideration, and that (ii) they are even structurally-indistinguishable. Within the framework of the present B3LYP calculations, C2 S 4 is fluttering along the energy profile as 2 ? 1 ? 20 ? 1 ? 2 (Fig. 3). The electronic property of the ‘fluttering’ C2 S 4 is characterized primarily as 2(2B2, C2v), while the average structure is 1(2B3g, D2h). 3.3. Spectral carrier of the C2 S 4 photoelectron bands Based on the results in the preceding sections, we are now in the position to make spectral assignments for the 2.68- and 3.19eV bands observed in the ðCS2 Þ 2 photoelectron spectrum. By comparing the experimentally determined VDE values with calculated ones, we assign the 2.68-eV band to isomer 3, and 3.19-eV band to isomer 2. As these assignments are based substantially on the results of the VDE calculations, it is necessary to assess the reliability of the present theoretical calculations, where VDEs are estimated by single-point energy calculations at the CCSD(T) level with the optimized geometries given by the B3LYP method. For the assessment, we calculated the VDEs by a different theoretical method. The VDE values for isomers 2 and 3 are estimated again by single-point CCSD(T) calculations, but this time with the optimized geometries obtained at MP2/6-311+G(2d) or MP2/aug-cc-pVDZ instead. The VDE calculations with the 6-311+G(2d) basis set provide 2.85 and 2.35 eV (3.04 and 2.48 eV with aug-cc-pVDZ) for isomers 2 and 3, respectively. These values are in reasonable agreement with those listed in Table 1. It is also taken into account in the assignments that CCSD(T) calculations tend to underestimate VDEs: for example, the VDE of CS 2 is determined to be 1.46 eV by photoelectron spectroscopy [2] whereas the present calculations give 1.02 eV (CCSD(T)/6-311+G(2d)) and 1.23 eV (CCSD(T)/ aug-cc-pVDZ). The assignment for the 2.68-eV band is further confirmed by examining the normal mode of neutral C2S4 responsible for the vibrational progression observed in the 355 nm spectrum. Sanov et al. revealed that removal of the excess electron from 3 led to the formation of a cyclic C2S4 neutral with 1A1 symmetry [6]. Fig. 4 shows the structure of the 1A1 neutral species (30 ) optimized at B3LYP/6-311+G(2d) along with several actual forms of normal vibrations selected for discussion. The main difference in geometry between 3(2B1) and 30 (1A1) is the C–C internuclear distance (see Figs. 2 and 4). The C–C bond length is calculated to be 1.42 Å for
3 and 1.51 Å for 30 . Therefore, it is expected that photodetachment process 3(2B1) + hm ? 30 (1A1) + e promotes considerably the C–C stretching vibration of the residual neutral. As both 3(2B1) and 30 (1A1) are of the same C2v symmetry, the promoted vibrational motion should belong to a1 symmetry. Among the normal motions displayed in Fig. 4, the 1210-cm1 mode meets the requirement. Hence, we assign the vibrational progression of the 2.68-eV band to the electronic excitation from the 2B1 anion state to the vibrationally-excited states of the 1A1 neutral with several quanta in the 1210-cm1 mode. The average spacing of the progression is determined to be 1048 cm1, which is smaller by 13% than the predicted value (1210 cm1). This arises partly from the neglect of the scaling factor for the calculated frequencies, and primarily from the neglect of anharmonicity of the vibrations, which have not been taken into consideration in the present calculations. As for the 3.18-eV band, less information is available for the spectral assignment. It should be noted that photodetachment pro2 2 0 1 cess of C2 S 4 ( B2) also proceeds as 2( B2) + hm ? 3 ( A1) + e [6]. On the analogy of the above discussion, the electronic transition occurs with excitation of the S–S vibrational motion, because of the large difference in the S–S internuclear distance between 2(2B2) and 30 (1A1). If we could reduce the intensity of the interfering low-eKE feature due to autodetachment by choosing an adequate photodetachment wavelength other than 355 nm, a vibrational progression with 400 or 500 cm1 spacing (Fig. 4) could be observed in the 3.18-eV band. In the present study, we have aimed to identify the isomeric forms of C2 S 4 responsible for the 2.4–3.4-eV bands in the ðCS2 Þ2 photoelectron spectrum, and identified 2(2B2, C2v) and 3(2B1, C2v) as the existing forms. Another outstanding issue regarding the C2 S 4 anions concerns their photodissociation processes: whether or not the C2 S 4 photodissociation proceeds in an ‘isomer-specific’ manner. Acknowledgments The authors are grateful to Professor K. Takatsuka for the loan of high-performance computers, which enabled us to carry out the present calculations. Professors Y. Endo and Y. Sumiyoshi are also acknowledged for generously providing us with the pulsed-discharge nozzle technique. This work was supported by Grants-inAid for Scientific Research from JSPS (No. 18550007), and from MEXT (No. 19029011). References [1] [2] [3] [4] [5] [6] [7]
K. Hiraoka, S. Fujimaki, K. Aruga, J. Phys. Chem. 98 (1994) 1802. T. Tsukuda, T. Hirose, T. Nagata, Chem. Phys. Lett. 279 (1997) 179. R. Mabbs, E. Surber, A. Sanov, Analyst 128 (2003) 765. T. Maeyama, T. Oikawa, T. Tsumura, N. Mikami, J. Chem. Phys. 108 (1998) 1368. L. Yu, A. Zeng, Q. Xu, M. Zhou, J. Phys. Chem. A108 (2004) 8264. A. Sanov, W.C. Lineberger, K.D. Jordan, J. Phys. Chem. A 102 (1998) 2509. S.W. Zhang, C.G. Zhang, Y.T. Yu, B.Z. Mao, F.C. He, Chem. Phys. Lett. 304 (1999) 265. [8] M. Zhou, L. Andrews, J. Chem. Phys. 112 (2000) 6576.
Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35 [9] T. Tsukuda, M. Saeki, S. Iwata, T. Nagata, J. Phys. Chem. A 101 (1997) 5103. [10] Y. Endo, H. Kohguchi, Y. Ohshima, Faraday Discuss. 97 (1994) 341. [11] M. Nakajima, Y. Yoneda, Y. Sumiyoshi, T. Nagata, Y. Endo, J. Chem. Phys. 119 (2003) 7805.
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[12] M.J. Frisch et al., GAUSSIAN 03, Revision C.02, Gaussian, Inc., Wallingford CT, 2004. [13] K.H. Bowen, J.G. Eaton, in: R. Naaman, Z. Vager (Eds.), The Structure of Small Molecules and Ions, Plenum Press, New York, 1987, p. 147.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Structures of C2 S 4 molecular anion: Photoelectron spectroscopy and theoretical calculations Yasushi Matsuyama, Takashi Nagata * Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan
a r t i c l e
i n f o
Article history: Received 8 February 2008 In final form 26 March 2008 Available online 31 March 2008
a b s t r a c t An intense beam of the dimer anion of CS2 was prepared by a pulsed-discharge nozzle, which enabled the measurement of the previously-reported 2.7-eV photoelectron band of C2 S 4 with higher spectral resolution at 355 nm. This measurement has revealed that the band consists of a vibrational progression with an average spacing of 0.13 ± 0.01 eV, indicating a bound electronic property of the residual C2S4 neutral. With the aid of molecular orbital calculations, the observed progression has been assigned to the transi2 1 tion from C2 S 4 ( B1) to the vibrationally-excited states of C2S4( A1), both possessing cyclic C2v structures. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Anions of dimeric molecules often form tightly bound species where the excess electron acts as a binding agent between the two constituent monomers. Among those species, the dimer anion of CS2 is of particular interest, as several different types of isomeric forms have been proposed to exist both experimentally and theoretically [1–8]. Hiraoka et al. determined the bond dissociation en1 ergy of ðCS2 Þ by high-pressure mass 2 to be 21.9 kcal mol spectrometric measurements, and proposed the formation of covalently-bonded C2 S 4 of C2v symmetry, which includes a dithiabutane ring structure [1]. Tsukuda et al. revealed by photoelectron spectroscopy that the C2 S 4 and CS2 CS2 forms of the dimer anion coexisted in ðCS2 Þ , and assigned a photoelectron band at the elec2 tron binding energy of 2.7 eV to the C2 S 4 anion [2]. This assignment was further confirmed in the recent photoelectron imaging experiment by Sanov and co-workers [3]. Maeyama et al. reported that SCCS was produced in the photodissociation of ðCS2 Þ2 , which provided a strong support for the cyclic C2v form of C2 S 4 [4]. More recently, Yu et al. measured the infrared vibrational spectrum of matrix-isolated C2 S 4 and identified the spectral carrier as a planar C–C bonded D2h form of C2 S 4 [5]. The electronic structures of ðCS2 Þ 2 were theoretically investigated by Sanov et al. [6]. Their MP2/6-31+G(d) calculations predicted the existence of four covalently-bonded forms along with one ion-molecule complex form; almost isoenergetic C2v(2B2) and C2v(2B1) forms of C2 S 4 were the most stable structures. The B3LYP/6-31+G(d) calculations by Zhang et al. [7] and B3LYP/6-311+G(d) by Zhou and Andrews [8], however, provided the D2h(2B3g) form as the global minimum structure. All the isomeric forms hitherto proposed and discussed are 1–6 (Scheme 1). The solid lines in Scheme 1 represent covalent bonds * Corresponding author. E-mail address: [email protected] (T. Nagata). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.068
and dotted lines are weak interactions; 1–5 are categorized as C2 S 4 molecular anions, while 6 as a CS2 CS2 ion-molecule complex. In isomeric form 2, the excess electron is accommodated in the antibonding molecular orbital between the S atoms, which weakens the S–S bond of the dithiabutane ring of isomer 3. As mentioned above, the arguments about existing isomeric forms are not yet all consistent. In the present study, we report on the vibrational structures appearing in the 2.7-eV photoelectron band of C2 S 4 , which give us a clue as to the isomeric form of the dimeric anion responsible for the photodetachment process. The experimental results are discussed in conjunction with molecular orbital calculations, aiming to settle the controversy regarding the existing isomeric forms of C2 S 4. 2. Experimental and computational methods The experimental apparatus consists of a cluster-anion source, a time-of-flight (TOF) mass discriminator and a photoelectron spec trometer [9]. The C2 S 4 anion was produced as ðCS2 Þ2 species in a supersonic jet by using a pulsed-discharge nozzle (PDN) [10,11]. A sample gas of Ar containing 1% of CS2 was expanded into the source chamber through the PDN operated at 1500 V (20 ls duration). The stagnation pressure was typically 0.3 MPa. The product anions were extracted at 20 cm downstream from the nozzle, perpendicularly to the initial beam direction by applying a pulsed electric field, and admitted into the TOF mass discriminator. The mass-selected ðCS2 Þ 2 anions were then intersected with unfocused either third (355 nm) or fourth (266 nm) harmonic of a Q-switched Nd:YAG laser at the photodetachment region. The kinetic energies of the photoelectrons were measured by a magnetic-bottle type electron spectrometer. In the high-resolution measurements, the ion packets were decelerated to reduce Doppler broadening prior to photodetachment. The typical resolution of our setup was
32
Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35
Scheme 1.
DE/E 7% for electrons having a kinetic energy of 1 eV. Each spectrum presented in this Letter represents an accumulation of 30 000 laser shots with background subtraction. Molecular orbital calculations are carried out to obtain the optimized geometries, the total energies and the vertical detachment energies (VDEs) of C2 S 4 by using the GAUSSIAN 03 program package [12]. The geometry optimizations are performed at the B3LYP level of theory, and the single-point energy calculations at the CCSD(T) level. The basis sets employed are aug-cc-pVDZ and 6-311+G(2d). Frequency analyses are also made to check whether the optimized geometries are located at potential minima.
Fig. 1. Photoelectron spectra of ðCS2 Þ 2 measured at (a) 266 nm (4.66 eV) and (b) 355 nm (3.49 eV). In panel (a) the dotted lines are the best-fit Gaussian profiles obtained by the deconvolution processing. The arrows indicate the positions of the 1 þ 2 C2 S CS 4 bands. The band assignments for the CS2 ðX Rg Þ CS2 2 ðX A1 Þ CS2 and 2 CS2 ða3 B2 Þ CS2 CS 2 ðX A1 Þ CS2 transitions are also given. In panel (b) the combs are shown for the vibrational progression of C2S4.
3. Results and discussion 3.1. Photoelectron spectra Fig. 1a shows the photoelectron spectra of ðCS2 Þ 2 measured at 266 nm (4.66 eV). The photoelectron counts are plotted against the electron binding energy defined as eBE = hm – eKE, where hm and eKE represent the photon energy and the kinetic energy of photoelectrons, respectively. The spectral profile is intrinsically identical to that reported in Ref. [2]; the spectral assignments confirmed so far are also indicated in Fig. 1 [2,3,13]. In the present measurement, we focus our attention on the tiny features in the 2.4–3.4 eV range. After an appropriate deconvolution processing, two band components are recognized at 2.68(2) and 3.19(2) eV as indicated by arrows in the inset of Fig. 1a. This implies that the 2.4–3.4 eV features contain contributions from the electronic transitions assignable to photodetachment processes of more than one C2 S 4 isomeric forms, as pointed out by Sanov and co-workers [3]. Although the assignment of the C2 S 4 band is unquestionable, there remains a question about the chemical identity of the spectral carrier: i.e., which of the isomeric forms 1–5 are responsible for the 2.68- and 3.19-eV bands. In order to uncover more detailed features of these photoelectron bands, 355 nm (3.49 eV) radiation was employed as the detachment laser because a better spectral resolution can be achieved for the photoelectrons with smaller eKE in the TOF measurement. Fig. 1b depicts the ðCS2 Þ 2 spectrum measured at 355 nm. In the 2.68-eV band, a vibrational progression with an average spacing of 0.13 ± 0.01 eV (1048 ± 80 cm1) is well recognized due to the improved spectral resolution. The progression is assignable to the Franck–Condon leap from the ground state
of C2 S 4 to the vibrationally-excited states of neutral C2S4. This implies that the C2S4 neutral is prepared in a bound electronic state via the photodetachment, and that the equilibrium geometry of C2S4 differs significantly from that of C2 S 4 . The geometrical change associated with the electronic transition from C2 S 4 to C2S4 occurs along the normal mode which composes the observed vibrational progression, as is generally the case according to the Franck–Condon principle. This statement is a key to the spectral assignment for the 2.68-eV band, as will be discussed in Section 3.3. It is also obvious in Fig. 1b that the photoelectron intensity starts to increase drastically around eBE > 3 eV in the 355 nm spectrum, whereas the tail of the high-energy band arises at 3.4 eV in the 266 nm spectrum. This observation is consistent with the result given by Sanov et al., who observed a low-eKE feature specific to the autodetachment process in the ðCS2 Þ 2 photoelectron spectra measured at 400 and 530 nm [3]. The appearance of the low-eKE feature prevents us from observing the detailed profile of the 3.19-eV band. 3.2. Theoretical calculations Geometry optimizations were performed first at the B3LYP/augcc-pVDZ level, starting with the geometries proposed by Sanov et al. at MP2/6-31+G(d) [6] and by Zhang et al. at B3LYP/631+G(d) [7]. The reliabilities of the optimizations were further checked by the B3LYP/6-311+G(2d) calculations. Fig. 2 shows the optimized structures for five C2 S configurations and one 4 CS 2 CS2 obtained at B3LYP/6-311+G(2d). The isomeric forms are individually assigned numbers 1–6 after Scheme 1. Although the
Y. Matsuyama, T. Nagata / Chemical Physics Letters 457 (2008) 31–35
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Fig. 2. Optimized geometries of C2 S 4 (1–5) and CS2 CS2 (6) obtained at the B3LYP/6-311+G(2d) level. Filled circles correspond to C atoms and open circles to S atoms. Bond lengths and bond angles are given in Å and deg. Note that 1 is optimized as a transition structure with one imaginary vibrational frequency.
Table 1 Calculated stabilization energies (EST) and vertical detachment energies (VDE) of stable isomeric forms 2–6 of C2 S 4 Isomers
2 3 4 5 6 c CS 2
6-311+G(2d)a
Aug-cc-pVDZb
EST (eV)
VDE (eV)
EST (eV)
VDE (eV)
0.84 0.88 0.32 0.37 0.31 –
2.94 2.41 3.67 1.61 0.95 1.02
1.00 0.92 0.45 0.39 0.33 –
3.07 2.54 3.83 1.70 1.16 1.23
a EST and VDE values are obtained by single-point calculations at CCSD(T)/6311+G(2d) with employing the geometries optimized at B3LYP/6-311+G(2d). b Same as above except that aug-cc-pVDZ basis set is used. c VDE of CS 2 monomer anion is calculated for reference.
optimized structures 1–6 are essentially identical to those reported in Refs. [5–8], the present B3LYP calculations mark a sharp contrast with the previous ones, which predicted 1 as the global minimum structure [5,7,8]. In the present study, B3LYP/aug-cc-pVDZ provides isomer 2 as the global minimum structure while 3 is the most stable geometry at B3LYP/6-311+G(2d). Both of the present B3LYP calculations predict 1 as an unstable geometry in a transition state. The stabilization energies with reference to the 2 1 CS 2 ðX A1 Þ þ CS2 ðX Rg Þ limit, and VDEs for the stable isomers obtained by the CCSD(T) calculations are listed in Table 1. As for the energy ordering of isomers 1–3, the discrepancy arises from the difference in the levels of theory: CCSD(T) with a larger basis set, employed in the present study, gives a more accurate estimate of the total energy. A more serious issue is the question whether 1 is the global minimum structure as predicted by the previous B3LYP calculations [5,7,8], or a transition structure as obtained in the present study. In regard to this issue, it should be pointed out that (1) isomers 1 and 2 are close in energy (Table 1) and, more importantly, that (2) their electronic states belong to the same symmetry type of point groups: the 2B3g species of D2h point group resolves into 2B2 of C2v by symmetry lowering. This suggests that isomers 1(2B3g, D2h) and 2(2B2, C2v) are located on the same adiabatic potential energy surface either as a transition state or as a local minimum. In order to examine the possibility of the interconversion between 1 and 2, we calculated the energy profile for the conformational conversion process. In the calculations, the
Fig. 3. Energy profiles for the interconversion 2 ? 1 ? 20 obtained by B3LYP calculations with various basis sets: filled circles: aug-cc-pVDZ, open circles: 6-311+G(2d), filled triangles: 6-311+G(2df), open triangles: 6-311+G(3df). The reaction coordinate is the angle h = \SCC.
angle SCC was chosen as the reaction coordinate and varied from 115° to 122° at an interval of 0.1°. At a fixed value of \SCC, the transient structure was obtained by optimizing all the parameters other than \SCC under the constraint that C2 S 4 retains C2v symmetry. Fig. 3 depicts the energy profiles calculated by the B3LYP method with employing various basis sets, such as aug-cc-pVDZ, 6-311+G(2d), 6-311+G(2df) and 6-311+G(3df). As readily seen in Fig. 3, the calculated energy profiles are well represented by
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Fig. 4. Geometry of C2S4(1A1) neutral obtained at the B3LYP/6-311+G(2d) optimization, starting with the configuration of isomer 3 as the initial geometry. Also shown are the actual forms of the a1 vibrational modes in C2S4(1A1).
double-minimum potential curves. The maximum of each curve around 119° corresponds to the transition state, where C2 S 4 takes on the D2h configuration of isomer 1. Two minima are located at 116–117.5° and 120–121.5°, depending on the basis sets used in the calculations. These minima correspond to the local minimum configuration of isomer 2. Although the calculated energy profiles show a distinct double-minimum feature, the barrier height is estimated to be in the order of several thousandths of eV, which is obviously a tiny quantity beyond the accuracy of B3LYP calculations. All that we can say from the present results in Fig. 3 are that (i) isomers 1 and 2 are adiabatically connected to each other, as predicted on the basis of the group-theoretical consideration, and that (ii) they are even structurally-indistinguishable. Within the framework of the present B3LYP calculations, C2 S 4 is fluttering along the energy profile as 2 ? 1 ? 20 ? 1 ? 2 (Fig. 3). The electronic property of the ‘fluttering’ C2 S 4 is characterized primarily as 2(2B2, C2v), while the average structure is 1(2B3g, D2h). 3.3. Spectral carrier of the C2 S 4 photoelectron bands Based on the results in the preceding sections, we are now in the position to make spectral assignments for the 2.68- and 3.19eV bands observed in the ðCS2 Þ 2 photoelectron spectrum. By comparing the experimentally determined VDE values with calculated ones, we assign the 2.68-eV band to isomer 3, and 3.19-eV band to isomer 2. As these assignments are based substantially on the results of the VDE calculations, it is necessary to assess the reliability of the present theoretical calculations, where VDEs are estimated by single-point energy calculations at the CCSD(T) level with the optimized geometries given by the B3LYP method. For the assessment, we calculated the VDEs by a different theoretical method. The VDE values for isomers 2 and 3 are estimated again by single-point CCSD(T) calculations, but this time with the optimized geometries obtained at MP2/6-311+G(2d) or MP2/aug-cc-pVDZ instead. The VDE calculations with the 6-311+G(2d) basis set provide 2.85 and 2.35 eV (3.04 and 2.48 eV with aug-cc-pVDZ) for isomers 2 and 3, respectively. These values are in reasonable agreement with those listed in Table 1. It is also taken into account in the assignments that CCSD(T) calculations tend to underestimate VDEs: for example, the VDE of CS 2 is determined to be 1.46 eV by photoelectron spectroscopy [2] whereas the present calculations give 1.02 eV (CCSD(T)/6-311+G(2d)) and 1.23 eV (CCSD(T)/ aug-cc-pVDZ). The assignment for the 2.68-eV band is further confirmed by examining the normal mode of neutral C2S4 responsible for the vibrational progression observed in the 355 nm spectrum. Sanov et al. revealed that removal of the excess electron from 3 led to the formation of a cyclic C2S4 neutral with 1A1 symmetry [6]. Fig. 4 shows the structure of the 1A1 neutral species (30 ) optimized at B3LYP/6-311+G(2d) along with several actual forms of normal vibrations selected for discussion. The main difference in geometry between 3(2B1) and 30 (1A1) is the C–C internuclear distance (see Figs. 2 and 4). The C–C bond length is calculated to be 1.42 Å for
3 and 1.51 Å for 30 . Therefore, it is expected that photodetachment process 3(2B1) + hm ? 30 (1A1) + e promotes considerably the C–C stretching vibration of the residual neutral. As both 3(2B1) and 30 (1A1) are of the same C2v symmetry, the promoted vibrational motion should belong to a1 symmetry. Among the normal motions displayed in Fig. 4, the 1210-cm1 mode meets the requirement. Hence, we assign the vibrational progression of the 2.68-eV band to the electronic excitation from the 2B1 anion state to the vibrationally-excited states of the 1A1 neutral with several quanta in the 1210-cm1 mode. The average spacing of the progression is determined to be 1048 cm1, which is smaller by 13% than the predicted value (1210 cm1). This arises partly from the neglect of the scaling factor for the calculated frequencies, and primarily from the neglect of anharmonicity of the vibrations, which have not been taken into consideration in the present calculations. As for the 3.18-eV band, less information is available for the spectral assignment. It should be noted that photodetachment pro2 2 0 1 cess of C2 S 4 ( B2) also proceeds as 2( B2) + hm ? 3 ( A1) + e [6]. On the analogy of the above discussion, the electronic transition occurs with excitation of the S–S vibrational motion, because of the large difference in the S–S internuclear distance between 2(2B2) and 30 (1A1). If we could reduce the intensity of the interfering low-eKE feature due to autodetachment by choosing an adequate photodetachment wavelength other than 355 nm, a vibrational progression with 400 or 500 cm1 spacing (Fig. 4) could be observed in the 3.18-eV band. In the present study, we have aimed to identify the isomeric forms of C2 S 4 responsible for the 2.4–3.4-eV bands in the ðCS2 Þ2 photoelectron spectrum, and identified 2(2B2, C2v) and 3(2B1, C2v) as the existing forms. Another outstanding issue regarding the C2 S 4 anions concerns their photodissociation processes: whether or not the C2 S 4 photodissociation proceeds in an ‘isomer-specific’ manner. Acknowledgments The authors are grateful to Professor K. Takatsuka for the loan of high-performance computers, which enabled us to carry out the present calculations. Professors Y. Endo and Y. Sumiyoshi are also acknowledged for generously providing us with the pulsed-discharge nozzle technique. This work was supported by Grants-inAid for Scientific Research from JSPS (No. 18550007), and from MEXT (No. 19029011). References [1] [2] [3] [4] [5] [6] [7]
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