- Email: [email protected]
Emission spectra of 1,2,3-butatriene cation H2CCCCH2+ by hollow-cathode glow discharge and extended negative glow discharge
Journal of Molecular Spectroscopy 297 (2014) 51–57
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Emission spectra of 1,2,3-butatriene cation H2 CCCCHþ 2 by hollow-cathode glow discharge and extended negative glow discharge Mitsunori Araki ⇑, Satoshi Uchida, Yuki Matsushita, Koichi Tsukiyama Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 16 December 2013 In revised form 13 January 2014 Available online 24 January 2014 Keywords: Diffuse interstellar bands Electronic transition Hollow cathode Extended negative glow Discharge Emission
a b s t r a c t The 1,2,3-butatriene cation H2 CCCCHþ 2 was observed with a monochromator in the optical emission from a hollow-cathode glow discharge and an extended negative glow discharge. This cation was effectively produced by the discharge of 2-butyne (H3CAC„CACH3), although no other organic species worked as its precursors. The observed emission bands were assigned to the 2B3u–X2B2g electronic transition on the basis of the chemical and physical investigations for the production and the reported photoelectron spectrum of H2CCCCH2. The frequency of the electronic transition’s v = 0–0 origin band was evaluated to be 20 381 cm1 from the emission band, which enables the comparison of the v = 0–0 absorption feature of H2 CCCCHþ 2 with diffuse interstellar bands. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Gaseous molecular ions have attracted considerable attention in the fields of gas-phase molecular spectroscopy and interstellar chemistry [1]. However, only a limited number of simple molecular ions have been measured with gas-phase spectroscopy, an essential task for the characterization of interstellar molecules. This is because the charge of molecular ions generally results in large collision cross sections with other molecules, making it difficult to produce a sufficiently large number of molecular ions required for laboratory gas-phase spectroscopy. As a result, complete characterization of even simple, commonly known molecular ions has rarely been addressed in spectroscopic studies. In the field of interstellar chemistry, diffuse interstellar bands (DIBs) were first discovered in the optical absorption spectra of stars [2]. DIBs are absorption bands of interstellar molecules in diffuse clouds between stars and the Earth. Although several hundred DIBs have been detected [3], the interstellar molecules causing these absorption bands are as yet unknown. To identify the carriers of the DIBs, candidate molecules must be generated in the laboratory to evaluate the frequencies of their electronic transitions, for their eventual comparison with astronomical DIB spectra [4]. Molecular ions, especially cations, in the gas phase are potential DIB molecules [5]. The electronic transitions of the cations of poly⇑ Corresponding author. Fax: +81 3 5261 4631. E-mail address: [email protected] (M. Araki). http://dx.doi.org/10.1016/j.jms.2014.01.009 0022-2852/Ó 2014 Elsevier Inc. All rights reserved.
aromatic hydrocarbons and unsaturated carbon chain molecules are known, or can be expected, to occur in the appropriate spectral region [6]. However, because cations are unstable, their electronic transitions are difficult to observe using a laboratory spectrometer system. To solve this difficulty, we developed a glow-discharge cell using a hollow cathode [7], in which cations can be effectively produced as high-density plasma by the hollow-cathode effect and an extended negative glow discharge cell using a coil [8,9]. A monochromator was utilized to detect cations in the discharge. We detected the 1,2,3-butatriene cation H2 CCCCHþ 2 in the discharge of 2-butyne. In this paper, we report the electronic transition of H2 CCCCHþ 2 and discuss the possibility that this cation is a potential DIB carrier. 2. Experiment description As a first step in this study, we developed a new dischargeemission spectrometer. A 20-mm-long hollow cathode having an internal diameter of 21 mm was installed 80 mm from the anode that was used to generate cations in a glow discharge, as shown in Fig. 1a. The discharge cell was made from 30-cm-long Pyrex glass of internal diameter 3.3 cm. The hollow-cathode glow discharge was produced by a pulsed voltage of 1300–1500 V with pulse width of 1 ms at 400 Hz. The pulsed voltage was produced by a combination of a fast high voltage transistor switch (BEHLKE HTS 31) and a custom-built high-voltage power supply. The typical
52
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Lens
Quartz Window
Pump
(a)
Sample gas
Cathode
Lens
Quartz Window
Anode (Ground)
Solenoid Coil
Pump
(b)
Sample gas
Anode (Ground)
Cathode
Solenoid Power Supply
(c)
(d)
Fig. 1. Discharge cells and electrodes. The monochromator was set to the left of the lens. (a) Hollow-cathode glow discharge cell. (b) Extended negative glow discharge cell. (c) Stainless-steel electrode used as the cathode and anode in the former cell and as the anode in the latter one. (d) Copper coil electrode used as the cathode in the extended negative glow discharge cell.
voltage and current between the electrodes were 500–800 V and 30–50 mA, respectively, using a ballast resistance of 20 kO. For the discharge molecules, we used a small hydrocarbon molecular gas at 0.2 torr without carrier gases. The monochromator (HORIBA Jobin Yvon iHR320) had a wide spectral range of 200–800 nm and used three gratings with groove densities of 1200–1800 g/mm. We applied a slit width of 0.1 mm, providing an FWHM resolution of 10 cm1. The discharge emission from the hollow cathode was focused on the inlet of the monochromator. The dispersed discharge emission was detected by a photomultiplier (Hamamatsu R928), to which a potential of 650 V was applied, and the signal was measured by a digital multimeter (Sanwa PC5000) via a lock-in amplifier (Femto LIA-MV-200-L). LabVIEW virtual instruments were used to manage the movement of the monochromator and acquire the signal from the lock-in amplifier. The observed spectra were calibrated to those for argon atomic lines. The production of cations was tested by comparing the electronic transitions of Ar+ and Cþ 2 with those of Ar and C2, respectively. Medium-strength lines were suitable for measuring the emission intensities of Ar and Ar+ and served to avoid intensity saturations. The 6 and 11 lines of Ar and Ar+ with upper-state energy levels of about 13 and 35–37 eV from the neutral ground state were located at wavelengths of 400–500 and 700–800 nm, respectively. These lines were selected as the sampling lines. The intensities were corrected by the reflectance ratios of the grating. In this experiment, a discharge having a pulse width of 1 ms at 10 Hz was utilized to avoid intensity canceling in the lock-in amplifier by the afterglow emission. On the basis of the Einstein coefficients
[10], the cation:neutral ratio of Ar in the hollow-cathode glow discharge was found to be Ar+/Ar = 0.1–0.01. Emission intensities of 4 the v = 0–0 origin bands for the B 4 R u X Rg electronic transition 3 3 of Cþ and the d P –a P electronic transition of C2 were meag u 2 sured at 507.2 and 516.5 nm, respectively. Using the reported transition dipole moments [11,12] of these molecules, the 4 3 cation:neutral ratio Cþ 2 ðB Ru Þ=C2 ðd Pg Þ of C2 in the discharge was estimated to be 1/2. The cation:neutral ratios of Ar and C2 suggest that cations can be produced effectively in the newly constructed hollow-cathode glow discharge. We also installed an extended negative glow discharge cell, with a Pyrex cell the same size as the hollow-cathode cell. Around the cell we wound an enameled-wire coil 10 cm in length with 400 turns per centimeter, as illustrated in Fig. 1b. A magnetic field of 0– 500 G was generated to confine the discharge. A coil cathode of copper wire was used to avoid shielding the magnetic field (Fig. 1d). The pulsed voltage was the same as for the hollow-cathode cell.
3. Results and discussion 3.1. Detection of emission bands The electronic transitions of many linear carbon chains measured by optical spectroscopy were compared with DIBs spectra. However no 1,2,3-butatriene cation H2 CCCCHþ 2 seems to have been investigated as a DIB molecule, and there are no reports for its
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
optical electronic transitions, even though it is one of the most simple linear carbon chains. Previously reported photoelectron spectroscopy observations of H2CCCCH2 [13] indicate that its 2B3u state is 2.55 ± 0.04 eV from the 2 B2g cation ground state. In earlier papers, these two states were labeled B2u and B3g, respectively. The difference in the labeling convention is because a different choice was made regarding the x- and y-axes. In this study, we use contemporary standard notation, where an carbon chain axis is an x-axis. The 2.55 ± 0.04 eV energy level indicates that the v = 0–0 origin band of the 2B3u–X2B2g electronic transition arises at 486 ± 8 nm. To produce this cation in a discharge we selected 2-butyne, which has a structure that is most similar to H2 CCCCHþ 2 out of all the commercially available chemical reagents. We observed two bands at 491 and 521 nm from the hollow-cathode glow discharge of 2-butyne (H3CAC„CACH3), as shown in Fig. 2a. The 491-nm wavelength of the observed band agrees well with the predicted one within the accuracy of the photoelectron spectrum. No other related bands were observed in the blue area of the 491-nm band, which is hence most likely to be the v = 0–0 origin band of the 2 B3u–X2B2g electronic transition of H2 CCCCHþ 2 , where the Dv = 0 vibronic band may partially overlap with the 491-nm band. 3.2. Chemical investigation The two bands were not observed from the discharges of benzene (see Fig. 2b), acetylene, allene, 2-butanol, cyclohexane, 3-hexyne, methylcyclohexane, o-xylene, 1-propanol, toluene or 1.3.5trimethylebenzene. 2-butyne was the only precursor to produce the two bands. This behavior suggests that the candidate producing these two bands is H2 CCCCHþ 2. Following the above logic, these two bands should be a product of fragmentation of the precursor and not the growth of the precursor. If a molecule is produced by growth in a discharge, it will be effectively produced by cooling of the cell [14,15]. We measured the 491-nm band with the hollow-cathode discharge cell cooled by dry ice. The resulting intensity of the band in the cooled spectrum was smaller than that at room temperature, as shown in Fig. 3, suggesting that the carrier of the 491-nm band is produced by fragmentation.
53
To verify that the molecule producing the two bands is not bearing nitrogen atoms, a sample mixture of 2-butyne (0.2 torr) and nitrogen (0.2 torr) was used in the hollow-cathode glow discharge. If the molecule included one or more nitrogen atoms, the intensities of the two bands may increase, which ultimately did not occur. A similar test was conducted for oxygen atoms with a sample mixture of 2-butyne (0.2 torr) and water (0.2 torr). No enhancement in intensity was observed with this mixture either. Therefore, the molecule producing these two bands does not involve N or O atoms. This is consistent with the present assignment of the two bands, although the discharge products are rarely made by using slight amounts of N2 and O2 air-leak gases in the cell. 3.3. Physical investigation To further support the assignment of these two bands, we verified whether or not the bands are produced by cations. Note that a cation is effectively produced inside the hollow cathode. In a normal setup, the cathode lies in front of the monochromator inlet, and the anode is at the far position, as shown in Fig. 1a. We inverted this setup by switching the positions of the two electrodes in order to judge the charge of the carrier. The intensities of the 516.5-nm band of neutral C2 and other peaks of interest were measured using both setups. The relative abundance of the two setups can be formulated as follows:
IiM =IiC2 InM =InC2
;
where Ii and In indicate peak intensities in the inverted and normal setups, respectively, and M is a potential molecule that can produce the peak. When a peak originates from a cation, the relative abundance can be <1. This outcome was confirmed by using the 507-nm band of HC4H+, as shown in Fig. 4. The relative abundance of HC4H+ was measured to be 0.8 ± 0.1. Accordingly, the relative abundance of the molecule producing the 491-nm band was measured to be 0.7 ± 0.1 (Fig. 4), which is consistent with assignment of the 491-nm band as a cation. The 521-nm band can be assigned to the vibronic band of the same electronic transition resulting from the unvarying relative intensities regardless of the discharge conditions in the hollow cathode cell, e.g. cell temperature, precursor type, and cell pressure. To be certain of this uniformity, the intensities of the 521- and 491-nm bands were measured in the extended negative glow discharge. Despite increasing the magnetic field from 0 to 500 G, the intensities of both bands were constant, whereas the bands of the three HC4H+ peaks and two C2 peaks consistently nearly doubled, as shown in Fig. 4. Thus the uniformity of the two bands was reconfirmed. 3.4. Vibrational analysis
Fig. 2. Observed spectra of the discharge emissions of (a) 2-butyne and (b) benzene. The symbol ‘‘H’’ indicates a saturated hydrogen atomic line and ‘‘*’’ lines by a fragment molecule with chemical behavior inconsistent with that of H2 CCCCHþ 2.
We measured the spectrum at dry-ice temperature to accurately determine the transition frequency of the v = 0–0 origin band component in the 491-nm band, as shown in Fig. 3. Callomon [16] reported that the intensity of the origin band increases in the cooled spectrum of HC4H+. The 491-nm band consists of two components. The intensity of the 491-nm band’s lower-frequency component decreased in the dry-ice-cooled spectrum. Thus, the v = 0–0 origin band may be the higher-frequency component, while the Dv = 0 vibronic band may be the lower-frequency component. The transition frequency of the v = 0–0 origin band was measured to be 20 381 cm1. The relative frequencies of the Dv = 0 vibronic band and the 521-nm band were determined to be 23 and 1207 cm1, respectively, in relation to the origin band.
54
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 3. Spectra of the 491-nm band at (a) room and (b) dry-ice temperatures. The width of the observed bands can be caused by the rotational structure which cannot be resolved by the monochromator used in this study.
Next, the vibrational modes were analyzed to ensure that the 20 381-cm1 band is in fact the origin band and that the bands located at 23 and 1207 cm1 are vibronic. To this end, we performed an ab initio calculation for the 2B3u and 2B2g states of H2 CCCCHþ 2 using CASSCF(5,4) and the cc-pVTZ basis set with Gaussian03 [17]. Due to structure optimization, the molecular structure of the 2B2g state was nonplanar and had D2 symmetry, which is consistent with reports by Yang et al. [18], Cattarius et al. [19], and Podkopaeva and Chizhov [20]. The 2B3u state was also nonplanar. The molecular structures of both H2 CCCCHþ 2 states are summarized in Table 1. The nonplanar structure is due to the double minimum potential for the torsional angle of the two CH2 surfaces around the planar structure of D2h. Although the bond lengths of both states are almost equivalent, the torsional angle of the 2B3u state (h) is remarkably larger than that of the 2B2g state. The difference between the torsional angles induces the vibronic bands in the 2 B3u–X2B2g electronic transition. The vibrational energy levels can be split into two levels owing to the tunneling in both electronic states. Tunneling causes splitting in the 2B2g state and is larger than that in the 2B3u state because the torsional angle in the 2B2g state is smaller than that in the 2B3u state, as shown in Table 1. The v = 1–1 band in the double minimum model is positioned at a lowerfrequency from the v = 0–0 origin band. Therefore, the band at 23 cm1 can be assigned to the v = 1–1 band of the torsional mode. The v = 2–2 and 2–0 bands could not be observed because of the insufficient population of the v = 2 level in the upper state. Our ab initio calculations suggest that the frequency of the torsional mode in the 2B2g state is 544 cm1 and that, except for this torsional mode, no allowed vibronic bands are located in the region within several hundred wavenumbers of the origin band. The vibrational band corresponding to v = 0–2, shown in the diagram in Fig. 5a, should appear at several hundred wavenumbers lower than the origin band, where it overlaps with the 507-nm band of HC4H+ (Fig. 2). To reduce interference from the intense 507-nm band, we subtracted the spectrum of the benzene discharge, presented in Fig. 2b, from that of the 2-butyne discharge, presented in Fig. 2a, because the spectrum of the benzene discharge does not
include peaks from H2 CCCCHþ 2 and does include the 507-nm band. From this analysis, a peak was observed at 708 cm1, shown in Fig. 5a. This peak can be assigned to the v = 0–2 band, and the band at 1207 cm1 can be assigned to the v = 1–3 band. The v = 0–2 band is at 499 cm1, far from the v = 1–3 band, although the v = 0–0 origin band is very close to the v = 1–1 band. This can be explained by the splitting between the v = 2 and 3 levels, which is larger than the splitting between the v = 0 and 1 levels. Therefore, the v = 2 vibrational level is already over the double-minimum potential barrier (see Fig. 5b). The vibrational assignments of the observed bands are summarized in Table 2. As an additional check for the v = 0–0 origin band assignment, we estimated the potential analysis of the torsional mode. The torsional mode was assumed to occur in an effective doubleminimum potential in the form
VðhÞ ¼ 2kh2 þ eðexpf4ah2 g 1Þ; proposed by Willitsch et al. [21], Pollard et al. [22], and Chau [23]. Thus, the torsional Hamiltonian becomes 2
HðhÞ ¼ Aþ
d
dh2
þ 2kh2 þ eðexpf4ah2 g 1Þ:
The corresponding Schrödinger equation was solved numerically using Meyer’s discrete variable representation method [24]. Calculations were performed on a 60 point equidistant grid within the intervals of 45° 6 h 6 45°, where h is defined as the half-torsional dihedral angle of the two CH2 planes. The rotational constant, A+ = 4.85 cm1, for the 2B2g ground state was obtained from our ab initio calculation. To estimate the observed vibrational term values in the 2B2g state, a tunneling splitting of 83.7 cm1 was assumed in the 2B3u upper state. Because a simultaneous determination of the tunneling splitting of both 2B3u and 2B2g states is not possible, it was necessary to assume the 83.7-cm1 splitting from the 2B3u state of C2 Hþ 4 as the only option [21]. We deemed that the assumed tunneling splitting was not far off from the true value, for the following two reasons. (1) We detected a temperature
55
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 4. Physical investigations of HC4H+ and H2 CCCCHþ 2 . (a) Normal positions of the two electrodes using the hollow-cathode glow discharge cell. (b) Inverted positions. (c) A magnetic field of 0 G applied in the extended negative glow discharge cell. (d) A magnetic field of 500 G applied.
Table 1 Molecular structures in the 2B3u and X2B2g states by ab initio calculation.ab
X2B2g 2
B3u
RCC1c (Å)
RCC2
c
1.351 1.342 1.397
1.250 1.230 1.250
(Å)
RCH (Å)
\HCC (°)
s d (°)
Barrier Height
1.085 1.075 1.072
119.04 120.24 119.33
31.01 33.34 41.73
624
e
(cm1) Ref. [25] This study This study
a
CASSCF(5,4)/cc-pVTZ. Electronic states are described by using the D2h point group according to the contemporary standard notation of this molecule, although the effective structure of this molecule is in the D2 point group (see text). c RCC1 and RCC2 are the bond lengths of the side and center C–C bonds, respectively. d Torsional angle of the two CH2 planes, s = 2h. e This barrier height is an energy of the transition state, having D2h symmetry located between the two equilibrium structures, based on the equilibrium structures as the reference energy level. b
dependence of the relative intensity of the two bands between the dry-ice and room temperatures, as shown in Fig. 3, although temperature dependence is negligible in the presence of very small tunneling splitting. (2) The large torsional angle calculated for the 2B3u state, as listed in Table 2, suggests that the tunneling split-
ting is smaller than the torsional vibration. The potential parameters of the 2B2g state, k and a, were fitted to the vibrational term values, while the potential parameter e was fixed to the C2 Hþ 4 value [21]. The potential parameters and potential curve thus obtained are described in Fig. 5b, and the calculated term values of the tor-
56
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 5. The vibrational structure and obtained potential of the torsional mode and its vibronic bands of the 2B3u–X2B2g electronic transition. The symbols # and * indicate the residual structures of HC4H+ and C2, respectively. The Ag and Au symbols in the energy level diagram of (a) indicate vibrational symmetries. (b) The potential parameters and the potential curve.
3.5. Astrochemical relevance
Table 2 a Observed frequencies in the 2B3u–X2B2g electronic transition of H2 CCCCHþ 2. Vibrational Assignment
0–0 1–1 0–2 1–3
c
Term value in 2B2g
Frequency
b
cm1
Relative cm1
Observed cm1
Calculated cm1
20 381 20 358 19 673 19 174
0 23 708 1207
0 107 708 1291
0 129 742 1267
d
a Electronic states are described by using the D2h point group, according to the traditional style of this molecule. b Vibrational term values of the torsional mode in the 2B2g ground state based on an assumed tunneling splitting of 83.7 cm1 in the 2B3u upper state. c Torsional mode. d Calculated term values with the potential function determined by least square fitting of the observed term values.
sional mode are listed in Table 2. Even though the calculated term values disagree with the observed term values by 20–30 cm1, the 660 cm1 barrier height of the calculated potential agrees with the 624 cm1 theoretical value [25]. These disagreements in the calculated term values may be caused by interactions from other vibrational modes. Our potential analysis of the torsional mode supports the conclusion that the 20 381-cm1 band is the v = 0–0 origin band of the 2B3u–X2B2g electronic transition in H2 CCCCHþ 2 and enables us to compare this band with DIBs. The 1-propenyl cation CCCHþ 3 can be produced by the discharge of 2-butyne, because CCCH3 radical has been suggested as one of the photodissociation products of 2-butyne [26]. The frequency of an electronic transition of CCCHþ was estimated to be 3 19 600 cm1 using ab initio calculation by Cameron et al. [27]. H3CCCCH+ and H2CCC+ are also possible as products of the discharge. Using TD-B3LYP and the cc-pVTZ basis set with Gaussian03 [17], we calculated the electronic transition of H3CCCCH+ to be 23 000 cm1 and the two transitions of H2CCC+ to be 18 500 and 22 000 cm1, while the 2B3u–X2B2g transition of H2 CCCCHþ 2 was estimated to be 19 300 cm1. Thus the three carbon chain cations also have the electronic transitions in the neighbor region of þ H2 CCCCHþ 2 . To reconfirm the assignment of H2 CCCCH2 , we hope that further studies will be conducted.
An absorption feature of H2 CCCCHþ 2 is expected by using the observed v = 0–0 emission band. The absorption band of the 2B3u– X2B2g electronic transition appears at 4905 Å (20 381 cm1). Although DIB absorbance was reported in this wavelength region at 4880 [28], 4882 [29], 4887 [28], 4947 [28], and 4951 [28] Å, no band agrees with the H2 CCCCHþ 2 absorption band. Therefore, the reported DIBs cannot be explained by this cation at this time, although we cannot rule out the possibility that new DIB is found to match this cation. 4. Conclusions In this study, an emission spectrometer with hollow-cathode glow discharge and extended negative glow discharge systems produced a spectrum including the 20 381-cm1 band from 2-butyne as the precursor. Photoelectron spectroscopy of H2CCCCH2 indicated that the 1,2,3-butatriene cation H2 CCCCHþ 2 produces a 2 B3u–X2B2g electronic transition in the 20 381-cm1 frequency region [13]. Using many precursors in the discharge, the chemical behavior of the band was tested. Physical investigations by electrode switching indicated that the carrier has a positive charge. Therefore the observed 20 381-cm1 band was assigned to the 1,2,3-butatriene cation H2 CCCCHþ 2 . The accompanying bands might be assigned to the torsional vibronic band on the basis of the double-minimum potential model. However, the 20381-cm1 band does not match the reported DIBs, and H2 CCCCHþ 2 does not correspond to the currently-known DIBs, even though this cation is one of the basic unsaturated carbon chain molecules. Acknowledgments We are grateful to Prof. John P. Maier at University of Basel for his spectroscopic comments. We thank Prof. Yasuki Endo at the University of Tokyo and Prof. Yoshihiro Sumiyoshi at Gunma University for their technical assistance in the development of the pulsed-discharge power supply. We are also thankful to Mr. Norihisa Kondo and Mr. Naoto Fujikawa for their assistances in the experiments. This study was funded by the Tokyo Ohka Foundation
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
for the Promotion of Science and Technology, the Research Foundation for Opto-Science and Technology, and a Grant-in-Aid for Scientific Research on Innovative Areas (No. 25108002). References [1] A.G.G.M. Tielens, Rev. Mod. Phys. 85 (2013) 1021–1081, http://dx.doi.org/ 10.1103/RevModPhys. 85.1021. [2] M.L. Heger, Lick Observatory Bull. 10 (1922) 141, http://dx.doi.org/10.5479/ ADS/bib/1922LicOB.10.141H. [3] L.M. Hobbs, D.G. York, J.A. Thorburn, T.P. Snow, M. Bishof, S.D. Friedman, B.J. McCall, T. Oka, B. Rachford, P. Sonnentrucker, D.E. Welty, Astrophys. J. 705 (2009) 32–45, http://dx.doi.org/10.1088/0004-637X/705/1/32. [4] T. Motylewski, H. Linnartz, O. Vaizert, J.P. Maier, G.A. Galazutdinov, F.A. Musaev, J. Krełowski, G.A.H. Walker, D.A. Bohlender, Astrophys. J. 531 (2000) 312–320, http://dx.doi.org/10.1086/308465. [5] A. Leger, L. D’Hendecourt, Astron. Astrophys. 146 (1985) 81–85. [6] M.K. Crawford, A.G.G.M. Tielens, L.J. Allamandola, Astrophys. J. 293 (1985) L45–L48, http://dx.doi.org/10.1086/184488. [7] M. Araki, H. Linnartz, P. Kolek, H. Ding, A. Boguslavskiy, A. Denisov, T.W. Schmidt, T. Motylewski, P. Cias, J.P. Maier, Astrophys. J. 616 (2004) 1301–1310, http://dx.doi.org/10.1086/424959. [8] M. Araki, H. Ozeki, S. Saito, Astrophys. J. 496 (1998) L53–L55, http://dx.doi.org/ 10.1086/311245. [9] F.C. De Lucia, E. Herbst, G.M. Plummer, G.A. Blake, J. Chem. Phys. 78 (1983) 2312–2316, http://dx.doi.org/10.1063/1.445004. [10] W.C. Martin, W.L. Wiese, Atomic spectroscopy, in: G.W.F. Drake (Ed.), National Institute of Standards and Technology, Gaithersburg, Maryland 20899. Chapter 10 in Atomic, Molecular, and Optical Physics Handbook, AIP Press, Woodbury, NY, 1996. [11] J.P. Maier, M. Rösslein, J. Chem. Phys. 88 (1988) 4614–4620, http://dx.doi.org/ 10.1063/1.453774.
57
[12] T.W. Schmidt, G.B. Bacskay, J. Chem. Phys. 127 (2007) 234310, http:// dx.doi.org/10.1063/1.2806988. [13] F. Brogli, E. Heilbronner, Else Kloster-Jensen, A. Schmelzer, A.S. Manocha, J.A. Pople, L. Radom, Chem. Phys. 4 (1974) 107–119. [14] T. Amano, Zdeneˇk Zelinger, T. Hirao, J. Takano, R. Toyoda, J. Mol. Spectrosc. 251 (2008) 252–255, http://dx.doi.org/10.1016/j.jms.2008.03.015. [15] S. Saito, K. Kawaguchi, E. Hirota, J. Chem. Phys. 82 (1985) 45–47, http:// dx.doi.org/10.1063/1.448767. [16] J.H. Callomon, Can. J. Phys. 34 (1956) 1046–1074. [17] M.J. Frisch et al., Gaussian 03, Revision A.1, Gaussian Inc, Pittsburgh PA, 2003. [18] Z.Z. Yang, L.S. Wang, Y.T. Lee, D.A. Shirley, S.Y. Huang, W.A. Lester Jr., Chem. Phys. Lett. 171 (1990) 9–13. [19] Chr. Cattarius, G.A. Worth, H.-D. Meyer, L.S. Cederbaum, J. Chem. Phys. 115 (2001) 2088–2100, http://dx.doi.org/10.1063/1.1384872. [20] O.Yu. Podkopaeva, Yu.V. Chizhov, J. Struct. Chem. 47 (2006) 420–426. [21] S. Willitsch, U. Hollenstein, F. Merkt, J. Chem. Phys. 120 (2004) 1761–1774, http://dx.doi.org/10.1063/1.1635815. [22] J.E. Pollard, D.J. Trevor, J.E. Reutt, Y.T. Lee, D.A. Shirley, J. Chem. Phys. 81 (1984) 5302–5309, http://dx.doi.org/10.1063/1.447672. [23] F.T. Chau, J. Mol. Struct. 131 (1985) 383–392, http://dx.doi.org/10.1016/00222860(85)87040-X. [24] R. Meyer, J. Chem. Phys. 52 (1970) 2053–2059, http://dx.doi.org/10.1063/ 1.1673259. [25] G.A. Worth, P. Hunt, M.A. Robb, J. Phys. Chem. A 107 (2003) 621–631, http:// dx.doi.org/10.1021/jp027117p. [26] A. Fahr, A.H. Laufer, J. Phys. Chem. A 109 (2005) 2534–2539, http://dx.doi.org/ 10.1021/jp0406058. [27] A. Cameron, J. Leszczynski, M.C. Zerner, B. Weiner, J. Phys. Chem. 93 (1989) 139–144, http://dx.doi.org/10.1021/j100338a031. [28] A. Bondar, Mon. Not. Roy. Astro. Soc. 423 (2012) 725–734, http://dx.doi.org/ 10.1111/j.1365-2966.2012.20910.x. [29] P. Jenniskens, F.-X. Desert, Astron. Astrophys. Suppl. 106 (1994) 39–78.
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Emission spectra of 1,2,3-butatriene cation H2 CCCCHþ 2 by hollow-cathode glow discharge and extended negative glow discharge Mitsunori Araki ⇑, Satoshi Uchida, Yuki Matsushita, Koichi Tsukiyama Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 16 December 2013 In revised form 13 January 2014 Available online 24 January 2014 Keywords: Diffuse interstellar bands Electronic transition Hollow cathode Extended negative glow Discharge Emission
a b s t r a c t The 1,2,3-butatriene cation H2 CCCCHþ 2 was observed with a monochromator in the optical emission from a hollow-cathode glow discharge and an extended negative glow discharge. This cation was effectively produced by the discharge of 2-butyne (H3CAC„CACH3), although no other organic species worked as its precursors. The observed emission bands were assigned to the 2B3u–X2B2g electronic transition on the basis of the chemical and physical investigations for the production and the reported photoelectron spectrum of H2CCCCH2. The frequency of the electronic transition’s v = 0–0 origin band was evaluated to be 20 381 cm1 from the emission band, which enables the comparison of the v = 0–0 absorption feature of H2 CCCCHþ 2 with diffuse interstellar bands. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Gaseous molecular ions have attracted considerable attention in the fields of gas-phase molecular spectroscopy and interstellar chemistry [1]. However, only a limited number of simple molecular ions have been measured with gas-phase spectroscopy, an essential task for the characterization of interstellar molecules. This is because the charge of molecular ions generally results in large collision cross sections with other molecules, making it difficult to produce a sufficiently large number of molecular ions required for laboratory gas-phase spectroscopy. As a result, complete characterization of even simple, commonly known molecular ions has rarely been addressed in spectroscopic studies. In the field of interstellar chemistry, diffuse interstellar bands (DIBs) were first discovered in the optical absorption spectra of stars [2]. DIBs are absorption bands of interstellar molecules in diffuse clouds between stars and the Earth. Although several hundred DIBs have been detected [3], the interstellar molecules causing these absorption bands are as yet unknown. To identify the carriers of the DIBs, candidate molecules must be generated in the laboratory to evaluate the frequencies of their electronic transitions, for their eventual comparison with astronomical DIB spectra [4]. Molecular ions, especially cations, in the gas phase are potential DIB molecules [5]. The electronic transitions of the cations of poly⇑ Corresponding author. Fax: +81 3 5261 4631. E-mail address: [email protected] (M. Araki). http://dx.doi.org/10.1016/j.jms.2014.01.009 0022-2852/Ó 2014 Elsevier Inc. All rights reserved.
aromatic hydrocarbons and unsaturated carbon chain molecules are known, or can be expected, to occur in the appropriate spectral region [6]. However, because cations are unstable, their electronic transitions are difficult to observe using a laboratory spectrometer system. To solve this difficulty, we developed a glow-discharge cell using a hollow cathode [7], in which cations can be effectively produced as high-density plasma by the hollow-cathode effect and an extended negative glow discharge cell using a coil [8,9]. A monochromator was utilized to detect cations in the discharge. We detected the 1,2,3-butatriene cation H2 CCCCHþ 2 in the discharge of 2-butyne. In this paper, we report the electronic transition of H2 CCCCHþ 2 and discuss the possibility that this cation is a potential DIB carrier. 2. Experiment description As a first step in this study, we developed a new dischargeemission spectrometer. A 20-mm-long hollow cathode having an internal diameter of 21 mm was installed 80 mm from the anode that was used to generate cations in a glow discharge, as shown in Fig. 1a. The discharge cell was made from 30-cm-long Pyrex glass of internal diameter 3.3 cm. The hollow-cathode glow discharge was produced by a pulsed voltage of 1300–1500 V with pulse width of 1 ms at 400 Hz. The pulsed voltage was produced by a combination of a fast high voltage transistor switch (BEHLKE HTS 31) and a custom-built high-voltage power supply. The typical
52
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Lens
Quartz Window
Pump
(a)
Sample gas
Cathode
Lens
Quartz Window
Anode (Ground)
Solenoid Coil
Pump
(b)
Sample gas
Anode (Ground)
Cathode
Solenoid Power Supply
(c)
(d)
Fig. 1. Discharge cells and electrodes. The monochromator was set to the left of the lens. (a) Hollow-cathode glow discharge cell. (b) Extended negative glow discharge cell. (c) Stainless-steel electrode used as the cathode and anode in the former cell and as the anode in the latter one. (d) Copper coil electrode used as the cathode in the extended negative glow discharge cell.
voltage and current between the electrodes were 500–800 V and 30–50 mA, respectively, using a ballast resistance of 20 kO. For the discharge molecules, we used a small hydrocarbon molecular gas at 0.2 torr without carrier gases. The monochromator (HORIBA Jobin Yvon iHR320) had a wide spectral range of 200–800 nm and used three gratings with groove densities of 1200–1800 g/mm. We applied a slit width of 0.1 mm, providing an FWHM resolution of 10 cm1. The discharge emission from the hollow cathode was focused on the inlet of the monochromator. The dispersed discharge emission was detected by a photomultiplier (Hamamatsu R928), to which a potential of 650 V was applied, and the signal was measured by a digital multimeter (Sanwa PC5000) via a lock-in amplifier (Femto LIA-MV-200-L). LabVIEW virtual instruments were used to manage the movement of the monochromator and acquire the signal from the lock-in amplifier. The observed spectra were calibrated to those for argon atomic lines. The production of cations was tested by comparing the electronic transitions of Ar+ and Cþ 2 with those of Ar and C2, respectively. Medium-strength lines were suitable for measuring the emission intensities of Ar and Ar+ and served to avoid intensity saturations. The 6 and 11 lines of Ar and Ar+ with upper-state energy levels of about 13 and 35–37 eV from the neutral ground state were located at wavelengths of 400–500 and 700–800 nm, respectively. These lines were selected as the sampling lines. The intensities were corrected by the reflectance ratios of the grating. In this experiment, a discharge having a pulse width of 1 ms at 10 Hz was utilized to avoid intensity canceling in the lock-in amplifier by the afterglow emission. On the basis of the Einstein coefficients
[10], the cation:neutral ratio of Ar in the hollow-cathode glow discharge was found to be Ar+/Ar = 0.1–0.01. Emission intensities of 4 the v = 0–0 origin bands for the B 4 R u X Rg electronic transition 3 3 of Cþ and the d P –a P electronic transition of C2 were meag u 2 sured at 507.2 and 516.5 nm, respectively. Using the reported transition dipole moments [11,12] of these molecules, the 4 3 cation:neutral ratio Cþ 2 ðB Ru Þ=C2 ðd Pg Þ of C2 in the discharge was estimated to be 1/2. The cation:neutral ratios of Ar and C2 suggest that cations can be produced effectively in the newly constructed hollow-cathode glow discharge. We also installed an extended negative glow discharge cell, with a Pyrex cell the same size as the hollow-cathode cell. Around the cell we wound an enameled-wire coil 10 cm in length with 400 turns per centimeter, as illustrated in Fig. 1b. A magnetic field of 0– 500 G was generated to confine the discharge. A coil cathode of copper wire was used to avoid shielding the magnetic field (Fig. 1d). The pulsed voltage was the same as for the hollow-cathode cell.
3. Results and discussion 3.1. Detection of emission bands The electronic transitions of many linear carbon chains measured by optical spectroscopy were compared with DIBs spectra. However no 1,2,3-butatriene cation H2 CCCCHþ 2 seems to have been investigated as a DIB molecule, and there are no reports for its
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
optical electronic transitions, even though it is one of the most simple linear carbon chains. Previously reported photoelectron spectroscopy observations of H2CCCCH2 [13] indicate that its 2B3u state is 2.55 ± 0.04 eV from the 2 B2g cation ground state. In earlier papers, these two states were labeled B2u and B3g, respectively. The difference in the labeling convention is because a different choice was made regarding the x- and y-axes. In this study, we use contemporary standard notation, where an carbon chain axis is an x-axis. The 2.55 ± 0.04 eV energy level indicates that the v = 0–0 origin band of the 2B3u–X2B2g electronic transition arises at 486 ± 8 nm. To produce this cation in a discharge we selected 2-butyne, which has a structure that is most similar to H2 CCCCHþ 2 out of all the commercially available chemical reagents. We observed two bands at 491 and 521 nm from the hollow-cathode glow discharge of 2-butyne (H3CAC„CACH3), as shown in Fig. 2a. The 491-nm wavelength of the observed band agrees well with the predicted one within the accuracy of the photoelectron spectrum. No other related bands were observed in the blue area of the 491-nm band, which is hence most likely to be the v = 0–0 origin band of the 2 B3u–X2B2g electronic transition of H2 CCCCHþ 2 , where the Dv = 0 vibronic band may partially overlap with the 491-nm band. 3.2. Chemical investigation The two bands were not observed from the discharges of benzene (see Fig. 2b), acetylene, allene, 2-butanol, cyclohexane, 3-hexyne, methylcyclohexane, o-xylene, 1-propanol, toluene or 1.3.5trimethylebenzene. 2-butyne was the only precursor to produce the two bands. This behavior suggests that the candidate producing these two bands is H2 CCCCHþ 2. Following the above logic, these two bands should be a product of fragmentation of the precursor and not the growth of the precursor. If a molecule is produced by growth in a discharge, it will be effectively produced by cooling of the cell [14,15]. We measured the 491-nm band with the hollow-cathode discharge cell cooled by dry ice. The resulting intensity of the band in the cooled spectrum was smaller than that at room temperature, as shown in Fig. 3, suggesting that the carrier of the 491-nm band is produced by fragmentation.
53
To verify that the molecule producing the two bands is not bearing nitrogen atoms, a sample mixture of 2-butyne (0.2 torr) and nitrogen (0.2 torr) was used in the hollow-cathode glow discharge. If the molecule included one or more nitrogen atoms, the intensities of the two bands may increase, which ultimately did not occur. A similar test was conducted for oxygen atoms with a sample mixture of 2-butyne (0.2 torr) and water (0.2 torr). No enhancement in intensity was observed with this mixture either. Therefore, the molecule producing these two bands does not involve N or O atoms. This is consistent with the present assignment of the two bands, although the discharge products are rarely made by using slight amounts of N2 and O2 air-leak gases in the cell. 3.3. Physical investigation To further support the assignment of these two bands, we verified whether or not the bands are produced by cations. Note that a cation is effectively produced inside the hollow cathode. In a normal setup, the cathode lies in front of the monochromator inlet, and the anode is at the far position, as shown in Fig. 1a. We inverted this setup by switching the positions of the two electrodes in order to judge the charge of the carrier. The intensities of the 516.5-nm band of neutral C2 and other peaks of interest were measured using both setups. The relative abundance of the two setups can be formulated as follows:
IiM =IiC2 InM =InC2
;
where Ii and In indicate peak intensities in the inverted and normal setups, respectively, and M is a potential molecule that can produce the peak. When a peak originates from a cation, the relative abundance can be <1. This outcome was confirmed by using the 507-nm band of HC4H+, as shown in Fig. 4. The relative abundance of HC4H+ was measured to be 0.8 ± 0.1. Accordingly, the relative abundance of the molecule producing the 491-nm band was measured to be 0.7 ± 0.1 (Fig. 4), which is consistent with assignment of the 491-nm band as a cation. The 521-nm band can be assigned to the vibronic band of the same electronic transition resulting from the unvarying relative intensities regardless of the discharge conditions in the hollow cathode cell, e.g. cell temperature, precursor type, and cell pressure. To be certain of this uniformity, the intensities of the 521- and 491-nm bands were measured in the extended negative glow discharge. Despite increasing the magnetic field from 0 to 500 G, the intensities of both bands were constant, whereas the bands of the three HC4H+ peaks and two C2 peaks consistently nearly doubled, as shown in Fig. 4. Thus the uniformity of the two bands was reconfirmed. 3.4. Vibrational analysis
Fig. 2. Observed spectra of the discharge emissions of (a) 2-butyne and (b) benzene. The symbol ‘‘H’’ indicates a saturated hydrogen atomic line and ‘‘*’’ lines by a fragment molecule with chemical behavior inconsistent with that of H2 CCCCHþ 2.
We measured the spectrum at dry-ice temperature to accurately determine the transition frequency of the v = 0–0 origin band component in the 491-nm band, as shown in Fig. 3. Callomon [16] reported that the intensity of the origin band increases in the cooled spectrum of HC4H+. The 491-nm band consists of two components. The intensity of the 491-nm band’s lower-frequency component decreased in the dry-ice-cooled spectrum. Thus, the v = 0–0 origin band may be the higher-frequency component, while the Dv = 0 vibronic band may be the lower-frequency component. The transition frequency of the v = 0–0 origin band was measured to be 20 381 cm1. The relative frequencies of the Dv = 0 vibronic band and the 521-nm band were determined to be 23 and 1207 cm1, respectively, in relation to the origin band.
54
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 3. Spectra of the 491-nm band at (a) room and (b) dry-ice temperatures. The width of the observed bands can be caused by the rotational structure which cannot be resolved by the monochromator used in this study.
Next, the vibrational modes were analyzed to ensure that the 20 381-cm1 band is in fact the origin band and that the bands located at 23 and 1207 cm1 are vibronic. To this end, we performed an ab initio calculation for the 2B3u and 2B2g states of H2 CCCCHþ 2 using CASSCF(5,4) and the cc-pVTZ basis set with Gaussian03 [17]. Due to structure optimization, the molecular structure of the 2B2g state was nonplanar and had D2 symmetry, which is consistent with reports by Yang et al. [18], Cattarius et al. [19], and Podkopaeva and Chizhov [20]. The 2B3u state was also nonplanar. The molecular structures of both H2 CCCCHþ 2 states are summarized in Table 1. The nonplanar structure is due to the double minimum potential for the torsional angle of the two CH2 surfaces around the planar structure of D2h. Although the bond lengths of both states are almost equivalent, the torsional angle of the 2B3u state (h) is remarkably larger than that of the 2B2g state. The difference between the torsional angles induces the vibronic bands in the 2 B3u–X2B2g electronic transition. The vibrational energy levels can be split into two levels owing to the tunneling in both electronic states. Tunneling causes splitting in the 2B2g state and is larger than that in the 2B3u state because the torsional angle in the 2B2g state is smaller than that in the 2B3u state, as shown in Table 1. The v = 1–1 band in the double minimum model is positioned at a lowerfrequency from the v = 0–0 origin band. Therefore, the band at 23 cm1 can be assigned to the v = 1–1 band of the torsional mode. The v = 2–2 and 2–0 bands could not be observed because of the insufficient population of the v = 2 level in the upper state. Our ab initio calculations suggest that the frequency of the torsional mode in the 2B2g state is 544 cm1 and that, except for this torsional mode, no allowed vibronic bands are located in the region within several hundred wavenumbers of the origin band. The vibrational band corresponding to v = 0–2, shown in the diagram in Fig. 5a, should appear at several hundred wavenumbers lower than the origin band, where it overlaps with the 507-nm band of HC4H+ (Fig. 2). To reduce interference from the intense 507-nm band, we subtracted the spectrum of the benzene discharge, presented in Fig. 2b, from that of the 2-butyne discharge, presented in Fig. 2a, because the spectrum of the benzene discharge does not
include peaks from H2 CCCCHþ 2 and does include the 507-nm band. From this analysis, a peak was observed at 708 cm1, shown in Fig. 5a. This peak can be assigned to the v = 0–2 band, and the band at 1207 cm1 can be assigned to the v = 1–3 band. The v = 0–2 band is at 499 cm1, far from the v = 1–3 band, although the v = 0–0 origin band is very close to the v = 1–1 band. This can be explained by the splitting between the v = 2 and 3 levels, which is larger than the splitting between the v = 0 and 1 levels. Therefore, the v = 2 vibrational level is already over the double-minimum potential barrier (see Fig. 5b). The vibrational assignments of the observed bands are summarized in Table 2. As an additional check for the v = 0–0 origin band assignment, we estimated the potential analysis of the torsional mode. The torsional mode was assumed to occur in an effective doubleminimum potential in the form
VðhÞ ¼ 2kh2 þ eðexpf4ah2 g 1Þ; proposed by Willitsch et al. [21], Pollard et al. [22], and Chau [23]. Thus, the torsional Hamiltonian becomes 2
HðhÞ ¼ Aþ
d
dh2
þ 2kh2 þ eðexpf4ah2 g 1Þ:
The corresponding Schrödinger equation was solved numerically using Meyer’s discrete variable representation method [24]. Calculations were performed on a 60 point equidistant grid within the intervals of 45° 6 h 6 45°, where h is defined as the half-torsional dihedral angle of the two CH2 planes. The rotational constant, A+ = 4.85 cm1, for the 2B2g ground state was obtained from our ab initio calculation. To estimate the observed vibrational term values in the 2B2g state, a tunneling splitting of 83.7 cm1 was assumed in the 2B3u upper state. Because a simultaneous determination of the tunneling splitting of both 2B3u and 2B2g states is not possible, it was necessary to assume the 83.7-cm1 splitting from the 2B3u state of C2 Hþ 4 as the only option [21]. We deemed that the assumed tunneling splitting was not far off from the true value, for the following two reasons. (1) We detected a temperature
55
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 4. Physical investigations of HC4H+ and H2 CCCCHþ 2 . (a) Normal positions of the two electrodes using the hollow-cathode glow discharge cell. (b) Inverted positions. (c) A magnetic field of 0 G applied in the extended negative glow discharge cell. (d) A magnetic field of 500 G applied.
Table 1 Molecular structures in the 2B3u and X2B2g states by ab initio calculation.ab
X2B2g 2
B3u
RCC1c (Å)
RCC2
c
1.351 1.342 1.397
1.250 1.230 1.250
(Å)
RCH (Å)
\HCC (°)
s d (°)
Barrier Height
1.085 1.075 1.072
119.04 120.24 119.33
31.01 33.34 41.73
624
e
(cm1) Ref. [25] This study This study
a
CASSCF(5,4)/cc-pVTZ. Electronic states are described by using the D2h point group according to the contemporary standard notation of this molecule, although the effective structure of this molecule is in the D2 point group (see text). c RCC1 and RCC2 are the bond lengths of the side and center C–C bonds, respectively. d Torsional angle of the two CH2 planes, s = 2h. e This barrier height is an energy of the transition state, having D2h symmetry located between the two equilibrium structures, based on the equilibrium structures as the reference energy level. b
dependence of the relative intensity of the two bands between the dry-ice and room temperatures, as shown in Fig. 3, although temperature dependence is negligible in the presence of very small tunneling splitting. (2) The large torsional angle calculated for the 2B3u state, as listed in Table 2, suggests that the tunneling split-
ting is smaller than the torsional vibration. The potential parameters of the 2B2g state, k and a, were fitted to the vibrational term values, while the potential parameter e was fixed to the C2 Hþ 4 value [21]. The potential parameters and potential curve thus obtained are described in Fig. 5b, and the calculated term values of the tor-
56
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
Fig. 5. The vibrational structure and obtained potential of the torsional mode and its vibronic bands of the 2B3u–X2B2g electronic transition. The symbols # and * indicate the residual structures of HC4H+ and C2, respectively. The Ag and Au symbols in the energy level diagram of (a) indicate vibrational symmetries. (b) The potential parameters and the potential curve.
3.5. Astrochemical relevance
Table 2 a Observed frequencies in the 2B3u–X2B2g electronic transition of H2 CCCCHþ 2. Vibrational Assignment
0–0 1–1 0–2 1–3
c
Term value in 2B2g
Frequency
b
cm1
Relative cm1
Observed cm1
Calculated cm1
20 381 20 358 19 673 19 174
0 23 708 1207
0 107 708 1291
0 129 742 1267
d
a Electronic states are described by using the D2h point group, according to the traditional style of this molecule. b Vibrational term values of the torsional mode in the 2B2g ground state based on an assumed tunneling splitting of 83.7 cm1 in the 2B3u upper state. c Torsional mode. d Calculated term values with the potential function determined by least square fitting of the observed term values.
sional mode are listed in Table 2. Even though the calculated term values disagree with the observed term values by 20–30 cm1, the 660 cm1 barrier height of the calculated potential agrees with the 624 cm1 theoretical value [25]. These disagreements in the calculated term values may be caused by interactions from other vibrational modes. Our potential analysis of the torsional mode supports the conclusion that the 20 381-cm1 band is the v = 0–0 origin band of the 2B3u–X2B2g electronic transition in H2 CCCCHþ 2 and enables us to compare this band with DIBs. The 1-propenyl cation CCCHþ 3 can be produced by the discharge of 2-butyne, because CCCH3 radical has been suggested as one of the photodissociation products of 2-butyne [26]. The frequency of an electronic transition of CCCHþ was estimated to be 3 19 600 cm1 using ab initio calculation by Cameron et al. [27]. H3CCCCH+ and H2CCC+ are also possible as products of the discharge. Using TD-B3LYP and the cc-pVTZ basis set with Gaussian03 [17], we calculated the electronic transition of H3CCCCH+ to be 23 000 cm1 and the two transitions of H2CCC+ to be 18 500 and 22 000 cm1, while the 2B3u–X2B2g transition of H2 CCCCHþ 2 was estimated to be 19 300 cm1. Thus the three carbon chain cations also have the electronic transitions in the neighbor region of þ H2 CCCCHþ 2 . To reconfirm the assignment of H2 CCCCH2 , we hope that further studies will be conducted.
An absorption feature of H2 CCCCHþ 2 is expected by using the observed v = 0–0 emission band. The absorption band of the 2B3u– X2B2g electronic transition appears at 4905 Å (20 381 cm1). Although DIB absorbance was reported in this wavelength region at 4880 [28], 4882 [29], 4887 [28], 4947 [28], and 4951 [28] Å, no band agrees with the H2 CCCCHþ 2 absorption band. Therefore, the reported DIBs cannot be explained by this cation at this time, although we cannot rule out the possibility that new DIB is found to match this cation. 4. Conclusions In this study, an emission spectrometer with hollow-cathode glow discharge and extended negative glow discharge systems produced a spectrum including the 20 381-cm1 band from 2-butyne as the precursor. Photoelectron spectroscopy of H2CCCCH2 indicated that the 1,2,3-butatriene cation H2 CCCCHþ 2 produces a 2 B3u–X2B2g electronic transition in the 20 381-cm1 frequency region [13]. Using many precursors in the discharge, the chemical behavior of the band was tested. Physical investigations by electrode switching indicated that the carrier has a positive charge. Therefore the observed 20 381-cm1 band was assigned to the 1,2,3-butatriene cation H2 CCCCHþ 2 . The accompanying bands might be assigned to the torsional vibronic band on the basis of the double-minimum potential model. However, the 20381-cm1 band does not match the reported DIBs, and H2 CCCCHþ 2 does not correspond to the currently-known DIBs, even though this cation is one of the basic unsaturated carbon chain molecules. Acknowledgments We are grateful to Prof. John P. Maier at University of Basel for his spectroscopic comments. We thank Prof. Yasuki Endo at the University of Tokyo and Prof. Yoshihiro Sumiyoshi at Gunma University for their technical assistance in the development of the pulsed-discharge power supply. We are also thankful to Mr. Norihisa Kondo and Mr. Naoto Fujikawa for their assistances in the experiments. This study was funded by the Tokyo Ohka Foundation
M. Araki et al. / Journal of Molecular Spectroscopy 297 (2014) 51–57
for the Promotion of Science and Technology, the Research Foundation for Opto-Science and Technology, and a Grant-in-Aid for Scientific Research on Innovative Areas (No. 25108002). References [1] A.G.G.M. Tielens, Rev. Mod. Phys. 85 (2013) 1021–1081, http://dx.doi.org/ 10.1103/RevModPhys. 85.1021. [2] M.L. Heger, Lick Observatory Bull. 10 (1922) 141, http://dx.doi.org/10.5479/ ADS/bib/1922LicOB.10.141H. [3] L.M. Hobbs, D.G. York, J.A. Thorburn, T.P. Snow, M. Bishof, S.D. Friedman, B.J. McCall, T. Oka, B. Rachford, P. Sonnentrucker, D.E. Welty, Astrophys. J. 705 (2009) 32–45, http://dx.doi.org/10.1088/0004-637X/705/1/32. [4] T. Motylewski, H. Linnartz, O. Vaizert, J.P. Maier, G.A. Galazutdinov, F.A. Musaev, J. Krełowski, G.A.H. Walker, D.A. Bohlender, Astrophys. J. 531 (2000) 312–320, http://dx.doi.org/10.1086/308465. [5] A. Leger, L. D’Hendecourt, Astron. Astrophys. 146 (1985) 81–85. [6] M.K. Crawford, A.G.G.M. Tielens, L.J. Allamandola, Astrophys. J. 293 (1985) L45–L48, http://dx.doi.org/10.1086/184488. [7] M. Araki, H. Linnartz, P. Kolek, H. Ding, A. Boguslavskiy, A. Denisov, T.W. Schmidt, T. Motylewski, P. Cias, J.P. Maier, Astrophys. J. 616 (2004) 1301–1310, http://dx.doi.org/10.1086/424959. [8] M. Araki, H. Ozeki, S. Saito, Astrophys. J. 496 (1998) L53–L55, http://dx.doi.org/ 10.1086/311245. [9] F.C. De Lucia, E. Herbst, G.M. Plummer, G.A. Blake, J. Chem. Phys. 78 (1983) 2312–2316, http://dx.doi.org/10.1063/1.445004. [10] W.C. Martin, W.L. Wiese, Atomic spectroscopy, in: G.W.F. Drake (Ed.), National Institute of Standards and Technology, Gaithersburg, Maryland 20899. Chapter 10 in Atomic, Molecular, and Optical Physics Handbook, AIP Press, Woodbury, NY, 1996. [11] J.P. Maier, M. Rösslein, J. Chem. Phys. 88 (1988) 4614–4620, http://dx.doi.org/ 10.1063/1.453774.
57
[12] T.W. Schmidt, G.B. Bacskay, J. Chem. Phys. 127 (2007) 234310, http:// dx.doi.org/10.1063/1.2806988. [13] F. Brogli, E. Heilbronner, Else Kloster-Jensen, A. Schmelzer, A.S. Manocha, J.A. Pople, L. Radom, Chem. Phys. 4 (1974) 107–119. [14] T. Amano, Zdeneˇk Zelinger, T. Hirao, J. Takano, R. Toyoda, J. Mol. Spectrosc. 251 (2008) 252–255, http://dx.doi.org/10.1016/j.jms.2008.03.015. [15] S. Saito, K. Kawaguchi, E. Hirota, J. Chem. Phys. 82 (1985) 45–47, http:// dx.doi.org/10.1063/1.448767. [16] J.H. Callomon, Can. J. Phys. 34 (1956) 1046–1074. [17] M.J. Frisch et al., Gaussian 03, Revision A.1, Gaussian Inc, Pittsburgh PA, 2003. [18] Z.Z. Yang, L.S. Wang, Y.T. Lee, D.A. Shirley, S.Y. Huang, W.A. Lester Jr., Chem. Phys. Lett. 171 (1990) 9–13. [19] Chr. Cattarius, G.A. Worth, H.-D. Meyer, L.S. Cederbaum, J. Chem. Phys. 115 (2001) 2088–2100, http://dx.doi.org/10.1063/1.1384872. [20] O.Yu. Podkopaeva, Yu.V. Chizhov, J. Struct. Chem. 47 (2006) 420–426. [21] S. Willitsch, U. Hollenstein, F. Merkt, J. Chem. Phys. 120 (2004) 1761–1774, http://dx.doi.org/10.1063/1.1635815. [22] J.E. Pollard, D.J. Trevor, J.E. Reutt, Y.T. Lee, D.A. Shirley, J. Chem. Phys. 81 (1984) 5302–5309, http://dx.doi.org/10.1063/1.447672. [23] F.T. Chau, J. Mol. Struct. 131 (1985) 383–392, http://dx.doi.org/10.1016/00222860(85)87040-X. [24] R. Meyer, J. Chem. Phys. 52 (1970) 2053–2059, http://dx.doi.org/10.1063/ 1.1673259. [25] G.A. Worth, P. Hunt, M.A. Robb, J. Phys. Chem. A 107 (2003) 621–631, http:// dx.doi.org/10.1021/jp027117p. [26] A. Fahr, A.H. Laufer, J. Phys. Chem. A 109 (2005) 2534–2539, http://dx.doi.org/ 10.1021/jp0406058. [27] A. Cameron, J. Leszczynski, M.C. Zerner, B. Weiner, J. Phys. Chem. 93 (1989) 139–144, http://dx.doi.org/10.1021/j100338a031. [28] A. Bondar, Mon. Not. Roy. Astro. Soc. 423 (2012) 725–734, http://dx.doi.org/ 10.1111/j.1365-2966.2012.20910.x. [29] P. Jenniskens, F.-X. Desert, Astron. Astrophys. Suppl. 106 (1994) 39–78.