Infrared Spectrum of Protonated Corannulene H+C20H10 in Solid

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Infrared Spectrum of Protonated Corannulene HC H in Solid para-Hydrogen and its Potential Contribution to Interstellar Unidentified Infrared Bands Pavithraa Sundararajan, Masashi Tsuge, Masaaki Baba, and Yuan-Pern Lee ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00089 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Earth and Space Chemistry

Infrared Spectrum of Protonated Corannulene H+C20H10 in Solid para-Hydrogen and its Potential Contribution to Interstellar Unidentified Infrared Bands Pavithraa Sundararajan,1 Masashi Tsuge,1,2,* Masaaki Baba,3 and Yuan-Pern Lee1,4,5,* 1

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan. 2

3

4

Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan.

Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan 5

Institute of Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.

Keywords: polycyclic aromatic hydrocarbon, corannulene, protonated species, infrared, para-hydrogen matrix

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Abstract Polycyclic aromatic hydrocarbons (PAH) and their derivatives, including protonated and cationic species, are suspected to be carriers of the unidentified infrared (UIR) emission bands observed from the galactic and extra-galactic sources. We extended our investigations of infrared (IR) spectra of protonated planar PAH to a non-planar PAH, corannulene (C20H10), which is regarded as a fragment of a fullerene, C60. The protonated corannulene H+C20H10 was produced on bombarding a mixture of corannulene and para-hydrogen (p-H2) with electrons during deposition at 3.2 K. During maintenance of the electron-bombarded matrix in darkness the intensities of IR lines of protonated corannulene decreased because of neutralization by electrons that was slowly released from the trapped sites. The observed lines were classified into two groups according to their responses to secondary irradiation at 365 nm. Eighteen lines in one group are assigned to the lowest-energy species among five possible isomers, hub-H+C20H10, and seventeen in another group to rim-H+C20H10, the species of second lowest energy. Spectral assignments were derived based on a comparison of the observed spectra with those predicted with the B3PW91/6-311++G(2d,2p) method. The observed IR spectrum of hub-H+C20H10 resembles several bands of the Class-A UIR bands.


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INTRODUCTION The unidentified infrared (UIR) emission bands are discrete infrared emissions from interstellar media (ISM), circumstellar regions, star-forming regions, and extra-galactic objects.1,2,3 The UIR emission bands have features inherent to aromatic C–H and C=C chemical bonds, leading to a hypothesis that polycyclic aromatic hydrocarbons (PAH) and their derivatives, including ions and protonated species, might be these emitters,2,3,4,5 but the identification of the exact forms of the PAH responsible for the UIR bands remains a major challenge.6 Because the proton affinities of PAH are large, proton transfer to a PAH to yield a protonated PAH (H+PAH) is a feasible process. Another process that might produce H+PAH is a hydrogen addition to PAH cations; i.e., H + PAH+ → H+PAH. Thus, H+PAH are consequently postulated to be present in the ISM.7 Le Page et al.8 and Snow et al.9 discussed H+PAH as potential carriers of UIR bands. The production of H+PAH in the laboratory and measurements of their infrared spectra are hence critical for the identification of the UIR emission bands. In this regard, we have performed experiments to produce protonated planar PAH in p-H2 matrices by electron bombardment during deposition of a mixture of PAH and p-H2; 10,11,12 some resemblance between UIR bands and laboratory infrared spectra of H+PAH is attained for large H+PAH such as protonated coronene (C24H13+)11 and ovalene (C32H15+).12 From the spectral point of view, this resemblance might indicate the existence of H+PAH in the astronomical environments. Although the formation and survival of H+PAH have yet been demonstrated,13,14 it is of astrochemical importance to accumulate laboratory spectral data on H+PAH and other classes of PAH for the future identification of carriers of the UIR emission bands. Álvaro Galué and Díaz Leines suggested that inclusion of nonplanar structural components in aromatic core-structures (π domains) induces spectral patterns similar to the UIR bands. 15 Corannulene (C20H10) is a highly symmetric (symmetry C5v) nonplanar PAH with five benzene rings that are fused with a pentagonal ring at the center, as shown in Figure 1(a). It is the smallest stable 3 ACS Paragon Plus Environment


symmetric PAH that adopts a bowl-shaped structure, and can be regarded as a fragment of fullerene (C60). Corannulene has been suggested to play a role in the formation of C60 in interstellar environments.16 Rouillé et al. reported the infrared absorption spectrum of corannulene in CsI pellets, the Raman spectrum of solid corannulene, and the ultraviolet (UV), visible, and infrared (IR) absorption spectra of corannulene in an argon matrix at 12 K.16 Laboratory microwave spectra of corannulene revealed a dipole moment of 2.07 D, indicating the feasibility of its astronomical detection.17 Although Pilleri et al. failed to observe transitions attributable to corannulene in the Red Rectangle region with the IRAM 30-m telescope,18 they indicated that corannulene could exist in the Red Rectangle nebula according to the estimated rates of photodissociation and production. The existence of corannulene in the ISM was also inferred from a spectral analysis of the diffuse interstellar bands at 6614 and 6196 Å.19 Figure 1 shows possible sites for protonation of corannulene. Some quantum-chemical computations of the relative energies of isomers of protonated corannulene are reported.20,21,22 Frash et al. calculated these energies and concluded that the protonation at the hub-site has the least energy and those at the rim- and spoke-sites have energies higher by 8 and 57 kJ mol−1, respectively.22 Rice et al. produced protonated corannulene by chemical ionization via C2H5OH2+ and reported the electronic absorption spectra attributable to corannulene protonated at the rim-site, rim-H+C20H10 shown in Figure 1(c), both in the gaseous phase and in a Ne matrix.20 Alvaro Galué et al. produced gaseous protonated corannulene with electrospray and reported the infrared multiphoton dissociation (IRMPD) spectrum of corannulene protonated at the rim-site.23 These authors considered only rim-H+C20H10 according to the analogy with protonated coronene, in which protonation on the carbon atom of the outer ring was observed. The reason why the most stable hub-H+C20H10 remains unobserved requires further investigation. The advantages of the IRMPD method, especially its mass-selectivity and a variety of ionization methods, for spectral studies of PAH+ and H+PAH and comparison with UIR bands have been 4 ACS Paragon Plus Environment

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demonstrated.5,24 However, because the IRMPD spectrum typically has broad spectral features that are red-shifted from infrared absorption bands due to anharmonicity, and because the IRMPD spectrum is an action spectrum that might not reflect the true IR intensities, it is challenging to distinguish unambiguously various isomers of protonated species with IRMPD spectra. In contrast, our method of electron bombardment of PAH in p-H2 during deposition allows us to distinguish various isomers of the protonated species because the proton-transfer process induces negligible fragmentation, and the species in solid p-H2 shows IR spectra with narrow lines, an excellent ratio of signal to noise, wide spectral range, and true relative intensities.25,26 Our preliminary results on protonated corannulene in a p-H2 matrix indicated the existence of the most stable isomer, hub-H+C20H10,26 It is hence imperative to conduct further experiments to confirm the assignments and to identify the sites favorable for protonation of corannulene. With improved experiments, we have observed IR lines of both hub- and rim-H+C20H10 in solid p-H2. The spectral assignments and their astronomical implications are discussed in the following sections.

METHODS A detailed description of our experimental setup and p-H2 converter used to prepare p-H2 has been reported.27 In brief, the experiments were performed with a closed-cycle helium-refrigerator system capable of cooling a gold-coated copper plate to 3.2 K. This copper plate served as a matrix sample substrate and a mirror to reflect the incident IR beam to the detector. A Fourier-transform infrared (FTIR) spectrometer, equipped with a KBr beam splitter and a Hg-Cd-Te detector to cover the spectral range 400–5000 cm−1, was used to record the spectrum of the matrix. In this work, 500 scans with resolution 0.25 cm−1 were used to record each spectrum. Gaseous mixtures of C20H10/p-H2 were prepared in situ during deposition. Corannulene (C20H10, 94 % purity, Kanto Chemical) was heated to 408 K in a stainless steel tube (outer diameter 6 mm) to increase its vapor pressure 28 and p-H2 was passed through this tube. The gaseous mixture of 5 ACS Paragon Plus Environment


corannulene/p-H2 was deposited onto the substrate at 3.2 K over a period of 10 h at a flow rate ∼12 mmol h−1. Because the vapor pressure of C20H10 is low, we were unable to determine accurately the mixing ratio of C20H10/p-H2. Using the method reported by Ruzi et al.29 and the quantum-chemically predicted IR intensities of some intense lines of corannulene, we estimated the mixing ratio of C20H10/p-H2 in our experiments to be (3.0 ± 0.3) ppm; the error represents only the fitting error from various lines. In the experiments to produce protonated corannulene, the matrix was bombarded during deposition with an electron beam of current 30 µA and kinetic energy 270 eV from an electron gun. To distinguish the protonated species from various species produced after electron bombardment, we maintained the matrix in darkness for 30 h and then subjected it to secondary photolysis at wavelengths 365±10, 445±20, and 520±20 nm from light-emitting diodes. Para–hydrogen was prepared in another closed-cycle refrigerator system capable of cooling to 10 K. Normal-H2 (99.9999 %) was passed through a copper tube filled with Fe(OH)3; in these experiments, the conversion temperature was set to 13 K. Quantum-chemical calculations were performed with Gaussian 09 (Revisions C.01 and D.01).30 Geometrical optimization and harmonic vibrational analysis were performed with the B3PW91/6-311++G(2d,2p) method; this method has been applied to several PAH and H+PAH investigated in our laboratory. The harmonic vibrational wavenumbers were scaled with factors of 0.958 for >2500 cm−1 and 0.978 for 60 km mol−1 (Table 2); lines with more than 90 % of the maximal intensity are listed in bold face. For rim-H+C20H10, eight intense lines are predicted near 1612, 1578, 1513, 1429, 1402, 1386, 1319, and 857 cm−1 with IR intensities >60 km mol−1 (Table 3) For spoke-H+C20H10, seven intense lines are predicted near 2725, 1604, 1568, 1500, 1442, 1370, and 856 cm−1 with IR intensities >60 km mol−1 (Table S-3). Five intense lines in group A+ with wavenumbers 1607.0, 1348.7, 1324.3, 878.3, and 856.7 cm−1, with the two most intense lines in bold face, agree with those predicted for hub-H+C20H10. Six intense lines in group B+ at 1611.5, 1581.2, 1509.2, 1381.0, 1326.3, and 854.1 cm−1, with the line at 1611.5 cm−1 being most intense, agree with those predicted for rim-H+C20H10; two additional lines at 1419.2 and 1400.9 cm−1 with slightly less intensity also correlate well with the predicted pattern. The predicted spectral pattern of spoke-H+C20H10, with the most intense line near 1370 cm−1, agrees poorly with observed spectral patterns of groups A+ and B+. 11 ACS Paragon Plus Environment


For hub-H+C20H10, in addition to these five intense lines, six lines in group A+ with medium intensities at 2771.9, 1473.8, 1456.9, 1424.7, 1118.1, and 559.4 cm−1, and seven additional weaker ones also agree satisfactorily with the predicted spectrum, as compared in Table 2. The observed 18 lines in group A+ are hence assigned to hub-H+C20H10. An average absolute deviation of 8.5 ± 4.4 cm−1 for hub-H+C20H10 indicates satisfactory agreement between experimental and predicted vibrational wavenumbers. For rim-H+C20H10, in addition to these nine intense lines, five lines in group B+ with medium intensities at 2868.9, 1467.5, 1431.5, 1059.3, and 950.2 cm−1, and four additional weaker ones also agree satisfactorily with predicted spectrum, as compared in Table 3. The observed 17 lines in group B+ are hence assigned to rim-H+C20H10. An average absolute deviation of 5.1 ± 3.6 cm−1 for rim-H+C20H10 indicates satisfactory agreement between experimental and predicted vibrational wavenumbers. According to our time-dependent density functional theory (TD-B3PW91/6-311++G(2d,2p)) calculations, hub-, rim-, and spoke-H+C20H10 have low lying excited states in region 600–250 nm, as shown in Figure S-3. In experiment, lines in groups A+ and B+, assigned to hub- and rim-H+C20H10, respectively, decreased in intensity upon irradiation with 365 nm light, in agreement with theoretical predictions. However, because of the large uncertainties in prediction, to distinguish the magnitudes of depletion, 10% decrease of hub-H+C20H10 and 20% decrease of rim-H+C20H10, according to the predicted spectra is challenging. Rice et al. reported that rim-H+C20H10 is dissociated to H + C20H10+ upon excitation to the 41A state (397–354 nm) in the gaseous phase,25 consistent with our observation. The negligible decrease upon irradiation at 254 nm might be due to a small absorption coefficient and the similar behavior at 405, 445, and 520 might be attributed to insufficient photon energy to dissociate H+C20H10. Figure S-4 compares lines in groups A+ and B+ with the predicted spectra of concave-hub and concave-spoke-H+C20H10, but the agreement is poor. Figure S-5 compares other unassigned cationic 12 ACS Paragon Plus Environment

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lines (marked with * in Figure 3) with the predicted spectra of spoke-, concave-hub, and concave-spoke-H+C20H10. The poor agreement indicates that the carrier of these lines is unlikely to be a protonated corannulene. Another possible carrier is the corannulene radical cation C20H10+; observed unassigned cationic lines are compared with the IRPMD spectrum of C20H10+ (Ref. 23) in Figure S-6. The IRMPD spectrum of C20H10+ consists of five broad features near 1332, 1222, 1154, 1092, and 813 cm−1, whereas intense members of these unassigned cationic lines were observed at 1576.1, 1318.0, 1106.2, 1082.9, 1033.6, 954.6, and 902.1 cm−1. The agreement is unsatisfactory, especially in the spectral region below 1000 cm−1. In a Ne matrix, electronic absorption features of C20H10+ in the visible region were found to be depleted upon irradiation with λ > 260 nm,20 in contrast to the stability of these cationic lines in solid p-H2 upon secondary photolysis at 365, 405, 445, and 520 nm. The spectral carrier of these unassigned cationic lines is hence unlikely to be C20H10+. We are so far unable to provide a definitive assignment of these cationic lines; further experimental and theoretical investigations are required. Figure S-7 compares the stick IR absorption spectra of hub-H+C20H10 and rim-H+C20H10 isomers in solid p-H2 with the IRPMD spectrum of H+C20H10; the stick spectra were also convoluted with a Gaussian function with a full width at half maximum (FWHM) of 30 cm−1 for comparison. Alvaro Galué et al. assigned the observed IRMPD spectrum to rim-H+C20H10 following the analogy with protonated coronene, in which protonation on the carbon atom of the outer ring was observed.22 Although the broad spectral bands and expected red shifts of the IRMPD spectrum prevent a direct comparison with the IR absorption spectra in solid p-H2, it appears that, in addition to rim-H+C20H10, hub-H+C20H10 might contribute to the observed IRMPD spectrum because two unique bands in 1100−1200 cm−1, observed only for hub-H+C20H10 in p-H2, were observed in the IRMPD spectrum and the overall pattern of the IRMPD spectrum fits slightly better with our observed spectrum of hub-H+C20H10. We note that the distribution of various isomers might depend on the method of



production of H+C20H10 and that isomerization during the heating process of IRMPD cannot be excluded.

C. Mechanism of formation The bombardment of a p-H2 matrix with electrons during deposition produces H3+ and H atoms;31 these species are expected to react with guest molecules to produce protonated species via proton transfer and mono-hydrogenated species via neutralization of the protonated species or reaction of the guest molecules with hydrogen, as demonstrated for planar PAH.25,26 The production of the hub-H+C20H10 and rim-H+C20H10 isomers and the absence of other isomers of H+C20H10 in an electron-bombarded C20H10/p-H2 matrix is explicable based on the potential-energy diagram presented in Figure 2. As the exothermicity of the protonation reaction H3+ + C20H10 is greater than isomerization barriers for the proton to move to the neighboring carbon, all carbon sites are accessible before the internal energy of H+C20H10 is relaxed. Once a protonated isomer is formed and stabilized in solid p-H2 at 3.2 K, isomerization is unlikely except through tunneling from concave-hub and concave-spoke isomers, which have barriers < 14 kJ mol−1. Because the energy of hub- and rim-H+C20H10 is much less than that of the other three isomers, one expects that the densities of states of hub- and rim-H+C20H10 are much greater than others near the energy of formation from a proton transfer. The chance of forming hub- and rim-H+C20H10 before significant energy relaxation is hence much enhanced. Another consideration is the partial charge on each carbon atom of C20H10. According to a Mulliken population analysis, the average charge on the carbon atoms of hub- and rim-sites is negative (−0.12 e and −0.09 e, respectively), whereas that of the spoke-site is positive (+0.07 e). Taking into account the above considerations and the number of carbon atoms (five hub and ten rim carbons), a proton attack at the hub- and rim-sites might have similar probabilities, whereas proton attack at other sites is unfavorable. The abundance of hub- and rim-H+C20H10 in a p-H2 matrix was estimated to be 0.14 ± 0.04 ppm and 0.10 ± 0.04 ppm using the method suggested by Ruzi et al.29 In electron-bombarded naphthalene 14 ACS Paragon Plus Environment

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(C10H8)/p-H2 matrix experiments, even though only 1-C10H9+ was observed after deposition, 2-C10H9+ was observed immediately after irradiation at 365 nm.32 Because 2-C10H9+ was unstable, it was converted to 1-C10H9+ within 30 min after irradiation; hence, it is possible that 2-C10H9+ was also produced upon electron bombardment, but rapidly decomposed afterward. According to calculations, 2-C10H9+ is higher in energy by 12−15 kJ mol−1,32,33 and is separated from 1-C10H9+ by a barrier of only ~30 kJ mol−1.33 In other protonated PAH (pyrene, coronene, and ovalene) studied in our laboratory,10,11,12 only the most stable isomers were observed. For these cations the second-lowest energy isomers have energies at least 40 kJ mol−1 higher than the most stable ones, consistent with our postulated mechanism of formation. Apparently the barrier for isomerization and the relative energy, which also affects the density of state for formation, play critical roles on the branching for formation of various isomers upon protonation. More experimental and theoretical results are necessary to confirm this model.

D. Relation to the UIR emission bands In the course of our investigations of the IR spectra of protonated planar PAH, we have demonstrated that protonated PAH larger than protonated pyrene (1-C16H11+) have the chromophore required to account for prominent features of the UIR emission bands (Figure 2 of ref 26). As the size of a PAH increases, the observed bands shift toward the positions of the UIR emission bands. The laboratory spectra of protonated coronene (1-C24H13+)11 and protonated ovalene (7-C32H15+)12 were found to reproduce well some features of the UIR bands. The experimental IR absorption spectra of corannulene (C20H10), protonated corannulene isomers (hub-H+C20H10 and rim-H+C20H10), protonated coronene (1-C24H13+),11 and protonated ovalene (7-C32H15+)12 recorded in solid p-H2 are compared with the interstellar Class A UIR features34 in Figure 6; we present the UIR emission spectrum from NGC 7023 because it is representative of Class A UIR features. Experimental spectra are presented with sticks according to observed wavenumbers and integrated intensities; solid red lines represent spectra convoluted with 15 ACS Paragon Plus Environment


Gaussian profiles with full width 30 cm−1 at half maximum. When comparing experimental results with the astronomical UIR bands, one should bear in mind the following aspects: (i) the relative spectral shifts from the gas- phase values induced by solid p-H2 are typically smaller than 1 %; more importantly, (ii) experimental spectra obtained in this work are absorption spectra of matrix-isolated species, which might differ slightly from the UIR bands originating from UV induced emission; typically one might expect a small (10–30 cm−1) red shift for the emission bands because of anharmonicity.35,36,37,38,39 The relative intensities might also differ between emission and absorption spectra, but the information on vibrational relaxation is generally lacking. The most intense feature of the infrared spectrum of C20H10, Figure 6(a), is at 11.9 µm, which is associated with the out-of-plane CH-bending mode. The observed wavelength is similar to those known for duo hydrogen of neutral planar PAH,40 which are thought to contribute to the 12.0-µm feature of the UIR emission. For C20H10, the intensities of the in-plane CH-stretching, ring deformation, and CC-stretching bands at 8.8, 7.6, and 7.0 µm, respectively, are, however, much smaller than for the out-of-plane CH-bending mode. This character has also been recognized for neutral planar PAH, and led to a proposal that ions of PAH might contribute to the UIR emission.10 As presented in Figures 6(b) and 6(c), protonation alters the spectral pattern significantly. The wavelength of the out-of-plane CH-bending band shifts to the blue by 0.24–0.56 µm; the convoluted features of other modes become as intense as that of the out-of-plane CH-bending mode. As a result, the IR spectrum of hub-H+C20H10, Figure 6(c), resembles closely the UIR Class A spectrum, Figure 6(f). Particularly of interest is the observation of the CC-stretching mode at 6.2 µm that coincides with the UIR band. Because the smaller protonated planar PAH show absorption bands near 6.3 µm, the nitrogen-substituted PAH or their cations were postulated to be the possible carriers of the UIR band near 6.2 µm.41,42 In Figure 6, we also compare the convoluted spectra of protonated coronene (1-C24H13+, trace d) and protonated ovalene (7-C32H15+, trace e) with the UIR band in trace (f). A similarity of the 16 ACS Paragon Plus Environment

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spectrum of hub-H+C20H10 to that of protonated coronene (1-C24H13+) is clearly illustrated. However, the CC-stretching band of protonated coronene was observed in region 6.2−6.6 µm, inconsistent with the UIR band near 6.2 µm, whereas the CC-stretching band of protonated corannulene at 6.2 µm fits well with the UIR band. On the other hand, in region near 8.6 µm, the bands of protonated coronene fits better than those of protonated corannulene. For protonated ovalene, the enhanced combination and overtone transitions induce a blue shift of the CC-stretching band toward 6.3 µm, but the intensity of the out-of-plane CH-bending mode near 11.0 and 11.8 µm is diminished. Although satisfactory agreement is observed between laboratory IR spectrum of protonated corannulene (hub-H+C20H10) and the UIR Class A spectrum, we are unable to confirm definitely that protonated nonplanar PAH are the carrier of the UIR band because only one such species has been investigated. Further experiments on more protonated non-planar PAH are desirable.

CONCLUSION The infrared spectra of two isomers of protonated corannulene, hub-H+C20H10 and rim-H+C20H10, were recorded in an electron-bombarded corannulene/p-H2 matrix. The formation of the two most stable isomers of protonated corannulene in an electron-bombarded p-H2 matrix is consistent with a theoretical prediction that their energies are within 5 kJ mol−1. Upon protonation at the hub-site, a line of the out-of-plane CH-bending mode of neutral corannulene at 837.1 cm−1 is blue-shifted to 856.7 and 878.3 cm−1; intensities of lines of the in-plane CH-stretching, ring deformation, and CC-stretching modes are significantly enhanced. These alterations of the spectral pattern upon protonation are similar to those observed for large planar PAH. The experimental IR spectrum of protonated corannulene (hub-H+C20H10) in solid p-H2 resembles several bands of the interstellar Class A UIR features, implying that protonated non-planar PAH might be another important class of PAH inventory in the ISM.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at the ACS Publication website at DOI:xxxx Partial IR spectra of C20H10/p-H2 mixture in region 550–1625 cm−1 (Figure S-1) and in the CH-stretching region (Figure S-2); predicted UV-Vis absorption spectra of H+C20H10 isomers (Figure S-3);comparison of observed lines in groups A+ and B+ with the predicted spectra of concave-huband concave-spoke-H+C20H10 isomers (Figure S-4); comparison of unassigned cationic lines with the predicted spectra of possible H+C20H10 isomers (Figure S-5) and with the IRMPD spectrum of C20H10+ (Figure S-6); comparison of the infrared spectra of rim- and hub-H+C20H10 in solid p-H2 with the IRMPD spectrum of H+C20H10 (Figure S-7); comparison of vibrational wavenumbers and IR intensities of C20H10 (Table S-1) and hub- and rim-H+C20H10 (Table S-2); predicted vibrational wavenumbers and IR intensities of spoke-, concave-hub-, and concave-spoke-H+C20H10 (Table S-3).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Ministry of Science and Technology, Taiwan (grant No. MOST106-2745-M009-001-ASP and MOST107-3017-F009-003) and the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Japan Society for the Promotion of Science (JSPS KAKENHI grant No. JP18K03717) partially supported this work. The National Center for High-Performance Computation of Taiwan provided computer time.


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Table 1. Relative energies of protonated corannulene isomers and transition states (TS) for isomerization calculated with the B3PW91/6-311G++(2d,2p) method. Isomera

Relative energy/ kJ mol−1

∆Eb/ kJ mol−1

hub

0.0

−440.1

rim

5.6

−434.5

spoke

55.5

−384.6

concave-hub

106.0

−334.1

concave-spoke

106.7

−333.4

TS1 (hub ↔ spoke)

89.4

–

TS2 (spoke ↔ rim)

99.9

–

TS3 (rim ↔ concave-spoke)

117.8

–

TS4 (concave-spoke ↔ concave-hub)

119.8

–

a

See Figure 1 for protonation sites.

b

Energy relative to C20H10 + H3+ → H+C20H10 + H2.



Table 2. Comparison of experimental vibrational wavenumbers (cm−1) and relative IR intensities of hub-H+C20H10 (group A+) with corresponding values predicted with the B3PW91/6-311++G(2d,2p) method.a Calculatedb

Mode

IRMPDc

p-H2

ν11

2808 (22)d

2771.9 (31)e

ν13

1611 (79)

1607.0 (86)

ν16

1513 (7)

1520.9 (6)

ν17

1510 (14)

1517.4 (17)

ν18

1472 (37)

1473.8 (32)

ν20

1442 (26)

1456.9 (23)

ν21

1438 (7)

1451.0 (12)

ν22

1430 (27)

1424.7 (21)

ν27

1354 (68)

1348.7 (66)

1394

ν28

1332 (100)

1324.3 (100)

1320

ν35

1196 (9)

1192.5 (9)

1198

ν40

1122 (27)

1118.1 (27)

ν43

1035 (9)

1028.1 (9)

ν52

881 (98)

878.3 (96)

ν53

858 (19)

856.7 (15)

ν60

769 (9)

752.1 (9)

ν64

644 (10)

642.5 (10)

ν71

545 (24)

559.4 (25)

1594

850

a

Table S-2 presents a list of all vibrational modes.

b

Harmonic vibrational wavenumbers are scaled with factors 0.958 for the region above 2500 cm−1 and 0.978 below 2500

cm−1. c

From Ref. 23. Although the spectrum was assigned to rim-H+C20H10, contributions from hub-H+C20H10 are

possible; see text. d e

The predicted IR intensities shown in parentheses are normalized to the intensity of the most intense line,107 km mol−1.

Integrated IR intensities as a percentage of the most intense line at 1324.3 cm−1 are shown in parentheses.


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Table 3. Comparison of experimental vibrational wavenumbers (cm−1) and relative IR intensities of rim-H+C20H10 (group B+) with corresponding values predicted with the B3PW91/6-311++G(2d,2p) method.a Mode

Calculatedb

IRMPDc

p-H2

ν11

2848 (16)d

2868.9 (15)e

ν15

1612 (100)

1611.5 (100)

ν16

1578 (44)

1581.2 (53)

ν17

1513 (35)

1509.2 (34)

ν20

1461 (12)

1467.5 (9)

ν21

1443 (17)

1431.5 (16)

ν22

1429 (31)

1419.2 (27)

ν24

1402 (34)

1400.9 (28)

ν26

1386 (35)

1381.0 (35)

1394

ν28

1338 (8)

1338.7 (10)

1320

ν29

1319 (37)

1326.3 (35)

ν43

1060 (19)

1059.3 (14)

ν49

959 (13)

950.2 (16)

ν50

944 (4)

941.9 (3)

ν52

857 (43)

854.1 (45)

ν57

803 (6)

792.0 (7)

ν63

663 (6)

665.9 (6)

1594

1198

850

a

Table S-2 presents a list of all vibrational modes.

b

Harmonic vibrational wavenumbers are scaled with factors 0.958 for the region above 2500 cm−1 and 0.978 below 2500

cm−1. c

From Ref. 23. Although the spectrum was assigned to rim-H+C20H10, contributions from hub-H+C20H10 are

possible; see text. d e

The predicted IR intensities shown in parentheses are normalized to the intensity of the most intense line, 203 km mol−1.

Integrated IR intensities as a percentage of the most intense line at 1611.5 cm−1 are shown in parentheses.



Figure 1. Geometries of (a) C20H10, (b) hub-H+C20H10, (c) rim-H+C20H10, (d) spoke-H+C20H10, (e) concave-hub-H+C20H10, and (f) concave-spoke-H+C20H10 optimized with the B3PW91/6-311++G(2d,2p) method. ZPVE-corrected relative energies (in kJ mol−1) are shown in parentheses. Bond lengths and angles are in Å and degree, respectively.


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Figure 2. Calculated relative energies of protonated corannulene isomers (hub-, rim-, spoke-, concave-spoke-, and concave-hub-H+C20H10) and transition states connecting them. Calculations were performed with the B3PW91/6-311++G(2d,2p) method; zero-point vibrational energies were corrected with unscaled harmonic vibrational energies.



Figure 3. Comparison of experimental and predicted partial IR spectra of C20H10. (a) Experimental absorption spectrum in solid p-H2, (b) simulated stick spectrum according to scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method, and (c) experimental absorption stick spectrum of C20H10 in an Ar matrix at 12 K. The Ar matrix spectrum is reproduced according to the reported wavenumbers and spectral heights.16 In (a), blue arrows indicate lines assigned to fundamental modes of C20H10 and red dots indicate lines tentatively assigned to overtone or combination modes.


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Figure 4. Partial infrared spectra of the electron-bombardment experiments on C20H10/p-H2 in regions 840– 960 and 1300–1630 cm−1. (a) The spectrum of the matrix upon electron bombardment during deposition at 3.2 K for 10 h with lines of C20H10 stripped; (b) difference spectrum of the matrix sample in darkness at 3.2 K for 30 h, (c) difference spectrum after further secondary photolysis at 365 nm for 20 min. Traces (b) and (c) are shifted by 0.012 and 0.018 for clarity. The lines in groups A+ and B+ are indicated with red and blue arrows, respectively. The lines due to unassigned cationic species are marked with *.



Figure 5. Comparison of experimental infrared spectrum with simulated spectra in regions 840–960 and 1300–1630 cm−1. (a) Inverted spectrum of Figure 4(b), the difference spectrum of the electron-bombarded C20H10/p-H2 matrix in darkness at 3.2 K for 30 h. Infrared stick spectra of (b) hub-H+C20H10, (c) rim-H+C20H10, and (d) spoke-H+C20H10 simulated according to scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method.


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Figure 6. Comparison of UIR bands with experimental infrared spectra of H+PAH in solid p-H2. (a) C20H10, (b) rim-H+C20H10, (c) hub-H+C20H10, (d) protonated coronene (1-C24H13+),11 (e) protonated ovalene (7-C32H15+),12

and (f) UIR Class A spectrum observed from the Iris nebula, NGC 7023.34 Experimental

spectra are presented with sticks, of which the heights represent integrated intensities. Solid red lines represent spectra convoluted with Gaussian profiles of FWHM 30 cm−1.



TOC Figure


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Figure 1 79x136mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2 78x53mm (300 x 300 DPI)


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Figure 3 149x80mm (300 x 300 DPI)



Figure 4 149x66mm (300 x 300 DPI)


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Figure 5 149x88mm (300 x 300 DPI)



Figure 6 74x114mm (300 x 300 DPI)


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Infrared Spectrum of Protonated Corannulene H+C20H10 in Solid

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