IR, NIR, and UV Absorption Spectroscopy of C602+ and C603+ in

Letter pubs.acs.org/JPCL

IR, NIR, and UV Absorption Spectroscopy of C602+ and C603+ in Neon Matrixes Bastian Kern, Dmitry Strelnikov,* Patrick Weis, Artur Böttcher, and Manfred M. Kappes Institute of Physical Chemistry II, KIT, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: C602+ and C603+ were produced by electron-impact ionization of sublimed C60 and charge-state-selectively codeposited onto a gold mirror substrate held at 5 K together with neon matrix gas containing a few percent of the electron scavengers CO2 or CCl4. This procedure limits charge-changing of the incident fullerene projectiles during matrix isolation. IR, NIR, and UV−vis spectra were then measured. Ten IR absorptions of C602+ were identified. C603+ was observed to absorb in the NIR region close to the known vibronic bands of C60+. UV spectra of C60, C60+, and C602+ were almost indistinguishable, consistent with a plasmon-like nature of their UV absorptions. The measurements were supported by DFT and TDDFT calculations, revealing that C602+ has a singlet D5d ground state whereas C603+ forms a doublet of Ci symmetry. The new results may be of interest regarding the presence of C602+ and C603+ in space. SECTION: Spectroscopy, Photochemistry, and Excited States

S

ince the unequivocal IR detection of fullerenes in space in 20101−4 (and references therein), there has been renewed interest in laboratory spectroscopic measurements of neutral and charged fullerenes. In particular, there have so far been no spectroscopic studies of the higher cationic charge states C602+ or C603+. Accounts of such reactive species in condensed phase are limited to the recent preparation of a fullerenium (2+) salt.5 While the reactivity of these fullerene multications has been studied in the gas phase using mass-spectrometric methods,6 no vibrational or electronic spectroscopy has yet been reported for C602+/3+. Recently, we have built a new UHV setup for the study of mass-selected molecular ions in cryogenic matrixes and have used it to detect several previously unknown C60+ and C60− IR absorptions.7 In the present study, we continue our work on fullerene ions and now report measurements for C602+ and C603+. C602+ Vibrational Spectrum. Figure 1 shows representative 1270−1590 cm−1 sections of IR spectra recorded after depositing C602+ into Ne + 0.15% CCl4 (Figure 1 B) and Ne +1% CO2 (Figure 1 C) matrixes, respectively. Among a series of initially unidentified features, the spectra show the known characteristic absorptions of matrix isolated C60+7 (Figure 1D). C60+ originates from electron capture by C602+ during deposition. Such charge-changing also leads to the formation of small amounts of neutralized C60. Comparing IR spectra of the matrixes obtained by C602+ deposition with those made by depositing C60+ in Ne + 0.15% CCl4 and Ne + 1% CO2, most of the unidentified IR absorptions can be assigned to C602+ as indicated in the Figure. Full IR spectra recorded over the 380− 1590 cm−1 range can be found in Supporting Information Figures 1−3. For an easier comparison to the calculated spectrum (see Figure 2), we next substitute those regions of the experimental © 2014 American Chemical Society

Figure 1. (A) C603+ deposited in Ne + 0.15% CCl4 (green). (B) C602+ deposited in Ne + 0.15% CCl4 (red). (C) C602+ deposited in Ne + 1% CO2 (black) matrixes. (D) Extracted C60+ IR spectrum in neon for comparison (blue).7 C602+ absorbances were scaled to have similar values at 1396 cm−1. Absorption features assigned to C602+, C60+, and C60 are labeled by 2+, +, and 0, respectively. In spectrum A, the symbol * indicates an unassigned complex containing a Cl atom that is not seen in the Ne + 1% CO2 matrix.

Received: December 4, 2013 Accepted: January 15, 2014 Published: January 15, 2014 457

dx.doi.org/10.1021/jz402630z | J. Phys. Chem. Lett. 2014, 5, 457−460

The Journal of Physical Chemistry Letters

Letter

fits the “extracted” spectrum very well. Note that the spectral range of the FTIR spectrometer was limited by the KBr beam splitter cutoff. Absorptions below 380 cm−1 could therefore not be measured. Furthermore, the region between 600 and 800 cm−1 is dominated by strong CO2 or CCl4 features, making it difficult to observe any C602+ absorptions possibly present. Experimental and theoretical frequencies and intensities are summarized in Table 1. Two approaches were used to estimate experimental integrated molar absorptivities. In the first approach, we estimated the total amount of C602+ in a sample from the integrated incident ion current corrected for charge state. (Such an estimation has been shown to work quite well for deposited C60+.) The other approach was to use the absorption intensities of counterions, CCl3−Cl−,8 to estimate the total number of cations present in the matrix (the total matrix should be roughly neutral). Integrated molar absorptivities of CCl3−Cl− were estimated from samples of C60+ in Ne + 0.15% CCl4, for which the absorptivities of C60+ had been previously determined.7 Both approaches gave roughly similar values of integrated molar absorptivities. The overall accuracy of molar absorptivities tabulated for C602+ in Table 1 is estimated to be ∼40%. The agreement between the RI-DFT spectrum calculated for singlet C602+ (D5d) and the measurement is very good. Some of the IR absorption features recorded show fine structure most likely due to matrix influence (multiple site effects), similar to the splitting of the C60 (T1u(4)) absorption feature observed in neon.7 Spectral predictions for the D5d and C1 triplet states of C602+ do not reproduce the observed experimental spectrum

Figure 2. Extracted experimental IR spectrum of C602+ in neon (top) versus theoretical RI-DFT prediction for singlet C602+ (D5d) (bottom). Calculated frequencies are unscaled. Dipole allowed vibrational normal modes, not seen in the experimental data, are labeled in italics.

IR spectra that do not contain IR spectral features assigned to C602+ by straight lines. In doing so we construct an “extracted” IR spectrum of C602+. As is apparent, the calculated spectrum

Table 1. Assignment of Experimentally Identified Singlet C602+ (D5d) Absorption Features by Comparison with DFT Calculations: Frequency (ν), Full Width at Half-Maximum (fwhm), and Integrated Molar Absorptivity (ANe in Neon Matrix at 5 K; A Calculated) experiment

theory

mode

ν (cm )

fwhm (cm )

ANe (km·mol )

ν (cm )

A (km·mol−1)

E1u(17), A2u(9)c

1565.5 1562.1 1559.9 1556.7 1552.5 1396.4 1323.1 1322.6 1320.6 1307.3 1304.8 1302.7 1278.3 1275.2 1232.3 1230.6 1228.5 1168.9 974.1 972.7 952.9 951.7 525.7

1.2

45

1565.5 1559.3

160 25.5

3.8 1.2

50 8

1424.1 1323.2

188 30.7

1.1

12

1307.2

56

1.4

11

1291.0

47.7

1.2

7

1232.0

22.4

1.9 1.4

1 5

1178.2 976.2

11.3 18.3

1.2

8

955.7

43.8

0.6

2

518.1 514.2

12.2 10.3

E1u(15) E1u(14)

E1u(13)

A2u(7) E1u(12)

E1u(11) A2u(5) E1u(10) A2u(2), E1u(3)c

−1 a

−1 b

−1

−1

a

Multiple absorption lines are due to multiple site effects or other distortions in Ne matrix, the most intense line is in italics. bfwhm of the most intense absorption line. cIt is not possible to assign a specific normal vibrational mode due to multiple site effects. 458



Letter

the well-known C60+ absorptions were seen. In contrast, after deposition of C603+, new absorptions appeared at 899 and 857 nm. Note that NIR electronic absorptions are about a factor of 1000 stronger than typical IR vibrational transitions. This explains why we can observe C603+ NIR electronic absorption features whereas IR features are below the detection limit of our experiment. Electronic spectra were calculated at the TDDFT level for C60+ (D5d), C602+ (D5d - in both triplet singlet states), as well as for doublet C603+ (Ci). These calculations predict significant absorption strength for triplet C602+ and doublet C603+ in the vicinity of C60+ NIR absorption; see Figure 3 E−H. For singlet C602+, no strong NIR absorptions are predicted. For samples containing mostly C602+, no strong absorptions were experimentally observed in this spectral range. We therefore conclude that C602+ has a singlet electronic ground state. For samples containing C603+, we observe two new NIR absorptions at 899 and 857 nm. We assign the stronger absorption at 899 nm to the band origin of the 2Ag ← 12Au transition. The energy difference between these two absorptions is 545 cm−1, which roughly corresponds to a radial cage vibration frequency. Therefore, we assign the 857 nm absorption feature to a vibronic component of the 2Ag ← 12Au transition. UV absorption spectra (200−400 nm) of samples containing various amounts of C60, C60−, C60+, C602+, and C603+ were found to be almost identical. This can be rationalized by taking into account TDDFT calculations (see Supporting Information, Figures 5−6), which predict close-lying UV electronic transitions for all species mentioned. Further differentiation is complicated by the broad UV absorption lines observed with halfwidths on the order of 2000 cm−1. In conclusion, we have used soft-landing of mass-to-charge selected ion beams into (codeposited) cryogenic neon matrixes doped with electron scavengers to accumulate sufficient sample densities to enable absorption spectroscopy of matrix isolated C602+ and C603+. Over the spectral range 6000 − 380 cm−1, C602+ shows 10 vibrational transitions which are in good agreement with DFT predictions for the singlet D5d ground state. This assignment is consistent with measurements in the NIR range, which show no strong absorption features attributable to C602+, as predicted by TDDFT calculations for singlet C602+ (D5d). C603+ containing matrixes were similarly prepared and studied. Because of dominant electron transfer upon deposition, the sample densities accumulated were insufficient for IR spectroscopy. However, two NIR absorptions were observed at 899 and 857 nm. Comparison with TDDFT calculations allows us to assign them to an electronic transition from a doublet C603+ (Ci) ground state. The UV spectra of matrix-isolated C60, C60+, and C602+ all show broad almost indistinguishable bands, consistent with a plasmon-like nature of the corresponding absorptions. Our results may be of interest toward establishing the presence of C602+ and C603+ in space.

(see Supporting Information, Figure 4). In addition, the observations in the NIR (see later) also support a singlet electronic ground state for C602+. Therefore, we can rule out a triplet C602+ electronic ground state as proposed in a previous theoretical study.9 Note that our DFT calculations also predict the triplet state to be 0.05 eV lower in energy than the singlet. Apparently, such a minimal energy difference is within the error of the calculation. Comparing IR line-widths of C602+ and C60+, one observes that C602+ spectral features are generally narrower than those of C60+ at comparable frequencies. As a corollary, C602+ IR bands often manifest partially resolved fine structure (multiple distinguishable matrix sites); C60+ bands do not. We have no simple explanation for this at present. Deposition of C603+ resulted in IR spectra containing primarily C602+ and very weak C60+ IR absorption features (Figure 1 A). We could not clearly identify any new features belonging to C603+. A possible reason is the high electron affinity of C603+ leading to dominant electron transfer from CO2 or CCl4 to C603+ (experimental ionization energies: C60: 7.6 eV (first), 11.4 eV (second), 16.6 eV (third);10 Ne: 21.56 eV; CO2: 13.8 eV; CCl4: 11.5 eV11). It would be necessary to significantly modify the apparatus to obtain higher incident ion currents or to deposit for considerably longer times to record an IR spectrum of C603+. C602+ and C603+ Electronic Spectra. NIR absorption spectra were recorded after deposition of C60+, C602+, or C603+ into Ne + 0.15% CCl4 matrixes as well as after deposition of C603+ into a Ne + 1% CO2 matrix (Figure 3A−D). Upon C602+ deposition, no new features were observed in the NIR (Figure 3 B). Only

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METHODS Experimental Methods. A description of the experimental setup and measurement procedure has recently been published.7 In brief, C602+/3+ were produced in an electron impact ionization source, mass-selected by a RF quadrupole filter, and codeposited with neon mixtures (Ne + 1% CO2, Ne + 0.15% CCl4) onto a gold mirror substrate held at 5 K. CO2 and CCl4 were used as electron scavengers, capturing secondary electrons

Figure 3. (A) C+60 deposited in Ne + 0.15% CCl4 (blue). (B) C602+ deposited in Ne + 0.15% CCl4 - leading to a distribution of matrix isolated C602+ and C60+ (red). (C) C603+ deposited in Ne + 1% CO2 (green). (D) C603+ deposited in Ne + 0.15% CCl4 (green). (E) TDDFT of C60+ D5d (blue). (F) TDDFT of triplet C602+ (D5d) (red). (G) TDDFT of singlet C602+ (D5d) (red). (H) TDDFT of doublet C603+ (Ci) (green). Experimental absorbances are scaled to yield similar C60+ absorption intensity at 966 nm. Calculated frequencies are unscaled. 459



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emitted from metal surfaces bombarded by the mass-selected cations along the incident ion beam path (and forming counterions as a result). C602+ and C603+ were deposited over 12−24 h at ion currents of 40 and 5 nA, respectively. The ratio of matrix gas (Ne 5.0, Air Liquide) to guest fullerene was about 5000:1 in all experiments. IR spectra were measured using a Bruker IFS66v/S FTIR spectrometer equipped with both a CuGe detector (6000−380 cm−1) and an MCT detector (6000−500 cm−1). For measurements with the CuGe and MCT detector, KRS-5 windows and a KBr beam splitter were used. For all measurements the resolution was set to 0.25 cm−1, and 2400 scans were averaged. The near-infrared region 800 − 1025 nm was monitored using a RXN1 Kaiser Optics process-Raman spectrometer (with the excitation laser turned off) and a halogen light source. UV−vis measurements (200−800 nm) were performed using an ARC SpectraPro-500 spectrograph equipped with an EG&G PARC 1456A detector and halogen or deuterium lamps. Computational Details. DFT quantum chemical calculations were carried out using the Turbomole software package.12 Geometry optimization and vibrational analysis were done at the RI-DFT BP86/def2-SV(P) level of theory. First, the geometries of C602+/3+ were optimized without symmetry restrictions in C1. The optimized geometry was analyzed to have some symmetry and optimized again within the corresponding symmetry restriction. The symmetries of the lowest energy structures of C602+ and C603+ were determined as D5d and Ci, respectively, consistent with previous ab initio calculations.13 The symmetry lowering from neutral C60 (Ih) is due to static Jahn−Teller effect. Harmonic frequencies were calculated for singlet and triplet C602+ (D5d) and C602+ (C1) as well as for doublet C603+ (Ci) and C603+ (C1). TDDFT calculations were performed to simulate electronic absorption spectra (RPA singlet excitations for singlet C602+ and UHF RPA excitations for doublet C603+ and triplet C602+).12

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Letter

REFERENCES

(1) Cami, J.; Bernard-Salas, J.; Peeters, E.; Malek, S. Detection of C60 and C70 in a Young Planetary Nebula. Science 2010, 329, 1180−1182. (2) Berné, O.; Mulas, G.; Joblin, C. Interstellar C60+. Astron. Astrophys. 2013, 550, L4. (3) Otsuka, M.; Kemper, F.; Hyung, S.; Sargent, B. A.; Meixner, M.; Tajitsu, A.; Yanagisawa, K. The Detection of C60 in the Wellcharacterized Planetary Nebula M1−11. Astrophys. J. 2013, 764, 77. (4) Iglesias-Groth, S.; Esposito, M. A Search for near Infrared Bands of the Fullerene Cation C60+ in the Protoplanetary Nebula IRAS 01005 + 7910. Astrophys. J., Lett. 2013, 776, L2. (5) Riccò, M.; Pontiroli, D.; Mazzani, M.; Gianferrari, F.; Pagliari, M.; Goffredi, A.; Brunelli, M.; Zandomeneghi, G.; Meier, B. H.; Shiroka, T. Fullerenium Salts: A New Class of C60-Based Compounds. J. Am. Chem. Soc. 2010, 132, 2064−2068. (6) Bohme, D. K. Buckminsterfullerene Cations: New Dimensions in Gas-Phase Ion Chemistry. Mass Spectrom. Rev. 2009, 28, 672−693. (7) Kern, B.; Strelnikov, D.; Weis, P.; Böttcher, A.; Kappes, M. M. IR Absorptions of C60+ and C60 in Neon Matrixes. J. Phys. Chem. A 2013, 117, 8251−8255. (8) Lugez, C. L.; Jacox, M. E.; Johnson, R. D. Matrix Isolation Study of the Interaction of Excited Neon Atoms with CCl4: Infrared Spectra of the Ion Products and of Cl2CCl··Cl. J. Chem. Phys. 1998, 109, 7147−7156. (9) Petrie, S. Getting a Theoretical Handle on Fullerene Ions: Quantum Chemical Calculations on the Reactions of C60+, C602+ and C603+ with Ammonia. Int. J. Mass Spectrom. 2006, 255−256, 213−224. (10) Wörgötter, R.; Dünser, B.; Scheier, P.; Märk, T. D. Appearance and Ionization Energies of C60−2mz+ and C70−2mz+ Ions (with z and m up to 4) Produced by Electron Impact Ionization of C60 and C70, Respectively. J. Chem. Phys. 1994, 101, 8674−8679. (11) Lias, S. Ionization Energy Evaluation. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard,W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD. http://webbook.nist.gov (accessed December 2, 2013). (12) TURBOMOLE V6.4 2012, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007, TURBOMOLE GmbH, since 2007; available from http://www. turbomole.com. (13) Cioslowski, J.; Patchkovskii, S.; Thiel, W. Electronic Structures, Geometries, and Energetics of Highly Charged Cations of the C60 Fullerene. Chem. Phys. Lett. 1996, 248, 116−120.

ASSOCIATED CONTENT

S Supporting Information *

Full IR spectra of C602+ in Ne, Ne + 0.15% CCl4, and Ne + 1% CO2 matrixes. Comparison of IR spectra with predictions from DFT calculations for C602+ in both D5d and symmetries C1 for singlet and triplet ground states. Comparison of electronic absorption spectra predicted by TDDFT calculations for C60, C60−, C60+, C602+, and C603+ in 200−500 nm range. UV spectra of various amounts of C60, C+60, and C602+ in Ne + 1% CO2 matrixes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49-72160843310. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the DFG-funded Excellence Cluster “Center for Functional Nanostructures (CFN)”, Project 4.6. We also acknowledge support by KIT and Land Baden-Württemberg. 460


IR, NIR, and UV Absorption Spectroscopy of C602+ and C603+ in

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