Laboratory formation and photo-chemistry of fullerene/anthracene cluster cations
Junfeng Zhen, Weiwei Zhang, YuanYuan Yang, Qingfeng Zhu, Alexander G. G. M. Tielens
DDraft version October 28, 2019
Preprint typeset using L A TEX style emulateapj v. 12/16/11
LABORATORY FORMATION AND PHOTO-CHEMISTRY OF FULLERENE/ANTHRACENE CLUSTERCATIONS
Junfeng Zhen , , ∗ , Weiwei Zhang , ∗ , YuanYuan Yang , , , Qingfeng Zhu , Alexander G. G. M. Tielens CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology ofChina, Hefei 230026, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802, United States CAS Center for Excellence in Quantum Information and Quantum Physics, Hefei National Laboratory for Physical Sciences at theMicroscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China and Leiden Observatory, Leiden University, P. O. Box 9513, 2300 RA Leiden, The Netherlands
Draft version October 28, 2019
ABSTRACTBesides buckminsterfullerene (C ), other fullerenes and their derivatives may also reside in space.In this work, we study the formation and photo-dissociation processes of astronomically relevantfullerene/anthracene (C H ) cluster cations in the gas phase. Experiments are carried out usinga quadrupole ion trap (QIT) in combination with time-of-flight (TOF) mass spectrometry. The re-sults show that fullerene (C , and C )/anthracene (i.e., [(C H ) n C ] + and [(C H ) n C ] + ),fullerene (C and C )/anthracene (i.e., [(C H ) n C ] + and [(C H ) n C ] + ) and fullerene (C and C )/anthracene (i.e., [(C H ) n C ] + and [(C H ) n C ] + ) cluster cations, are formed in thegas phase through an ion-molecule reaction pathway. With irradiation, all the fullerene/anthracenecluster cations dissociate into mono − anthracene and fullerene species without dehydrogenation. Thestructure of newly formed fullerene/anthracene cluster cations and the bonding energy for these re-action pathways are investigated with quantum chemistry calculations.Our results provide a growth route towards large fullerene derivatives in a bottom-up process andinsight in their photo-evolution behavior in the ISM, and clearly, when conditions are favorable,fullerene/PAH clusters can form efficiently. In addition, these clusters (from 80 to 154 atoms or ∼ Subject headings: astrochemistry — methods: laboratory — ultraviolet: ISM — ISM: molecules —molecular processes INTRODUCTIONPolycyclic aromatic hydrocarbon (PAH) molecules andtheir derivatives are believed to be very ubiquitous inthe interstellar medium (ISM), where they are generallythought to be responsible for the strong mid-infrared(IR) features in the 3-17 µ m range that dominate thespectra of most galactic and extragalactic sources inspace (Allamandola et al. 1989; Puget & Leger 1989;Sellgren 1984; Genzel et al. 1998). The IR spectra of cir-cumstellar and interstellar sources have also revealed thepresence of buckminsterfullerene (C ) in space (Cami etal. 2010; Sellgren et al. 2010), which is thought to bechemically linked to PAHs (Bern´e & Tielens 2012; Zhenet al. 2014). In addition, several far-red diffuse inter-stellar bands (DIBs) are linked to the electronic transi-tions of C (Campbell et al. 2015; Walker et al.2015; Cordiner et al. 2017). Using the Hubble SpaceTelescope, the recent study by Cordiner et al. (2019)confirms the presence of all three expected C bandswith strength ratios in agreement with those extrapo-lated from [C − He] + laboratory measurements. In ad-dition, C cations undergo Jahn-Teller distortion whichremoves the icosahedral symmetry of C . This elimi-nates various spectral symmetry-selection rules, and alsoleads to three different rotational constants (Slanina et [email protected] & [email protected] al. 2003), these two aspects have important implicationsfor C /C spectral observations in space (Lykhin etal. 2019, and references therein). Hence, understand-ing the formation and destruction processes of PAHs andfullerene, and their derivatives (e.g., fullerene/PAH clus-ters) has attracted much attention in the field of molecu-lar astrophysics (Tielens 2013; Bern´e et al. 2015; Omont2016; Gatchell & Zettergren 2016; Candian et al. 2018).Singular value decomposition analysis for the IR spec-tra of photo-dissociation regions (PDRs) has revealed thepresence of a distinct emission component in the aro-matic infrared bands (AIBs) that has been attributed tothe presence of clusters of large molecules (Rapacioli etal. 2005; Bern´e et al. 2007). Likewise, the extended redemissions (EREs), which dominates the visual spectra ofreflection nebulae, has been attributed to luminescenceby charged PAH clusters (Rhee et al. 2007). In addition,Garc´ıa-Hern´andez & D´ıaz-Luis (2013a) proposed a pos-sible relation between specific diffuse interstellar bands,including the strongest at 4428 ˚A, and large fullerenesand buckyonions. Garc´ıa-Hern´andez et al. (2013b) sug-gested fullerene/PAH adducts as candidates to the car-riers of IR emission bands. And also, the formation anddestruction of PAH clusters in PDRs has been studiedby Rapacioli et al. (2006). So far, the experimentalevidence for the origin and evolution of clusters of largemolecules in the gas phase has been lacking. a r X i v : . [ phy s i c s . a t m - c l u s ] O c t Zhen et al.In our previous studies, we have reported laboratoryexperiments on van der Waals bonded, PAH clustersand their photochemical evolution towards large PAHmolecules in a bottom-up process (Zhen et al. 2018;Zhang et al. 2019). Given the presence of C in PDRssuch as NGC 7023 (Sellgren et al. 2010), studies ofclusters involving C have become of great interest aswell. The processing of fullerene clusters by energeticions (Gatchell & Zettergren 2016) is of relevance tointerstellar shocks. Hence, in order to understand theformation and photochemical evolution of such speciesin PDRs, we will simulate and focus on the ultraviolet(UV) processing of clusters of PAHs and fullerene in thelaboratory conditions.In addition, PAH/fullerene clusters offer a good ap-proach to cosmic dust (very small grains) in terms oftheir scale size and their photochemical behavior (Clay-ton et al 1999; Omont 2016). Indeed, the formationroute for fullerenes and their derivatives provide a cru-cial anchor point to test models for the formation andevolution of carbon-rich dust, and recent experimentaland quantum chemistry studies have started to elucidatethis (Dunk et al. 2013; Candian et al. 2019).It is known that fullerenes are electron-deficient poly-olefins that are able to form adducts with a number ofdifferent molecules (Komatsu et al. 1999). In particu-lar, buckminsterfullerene (C ) can react with catacon-densed PAHs (e.g., acenes such as anthracene and pen-tacene) to form fullerene/PAH adducts via Diels − Aldercycloaddition reactions (Komatsu et al. 1999; Briggs&Miller 2006; Garc´ıa-Hern´andez & D´ıaz-Luis 2013a;Garc´ıa-Hern´andez et al. 2013b; Sato et al. 2013). Dif-ferent yields of the neutral C /anthracene mono- andbis-adducts have been obtained in laboratory studies de-pending on the method employed in the production pro-cess, see e.g. Garc´ıa-Hern´andez & D´ıaz-Luis (2013a);Garc´ıa-Hern´andez et al. (2013b); Cataldo et al. (2014).But the majority of these studies focused on the solid orliquid phase. Laboratory formation and photochemistryof (cationic) fullerene/PAH clusters in the gas phase havebeen barely investigated. Selective ion flow tube (SIFT)experiments reveal no reaction between C and naph-thalene but do show adduct formation with corannulene(C H ) (Petrie & Bohme 2000). Dunk et al. (2013)also reported the formation of fullerene cluster cationsresulting from the gas-phase interaction of C and C with coronene (C H ) molecules under energetic con-ditions. In addition, laboratory studies of processing ofvan der Waals clusters of PAHs and of fullerenes in thegas phase by energetic ions (e.g., 24 keV O or 12 keVAr ) have revealed the formation of chemically bondedlarge species through direct knock-out of carbon atoms(Zettergren et al. 2010, 2013; Delaunay et al. 2015).In order to understand how (cationic) fullerenes aggre-gate with PAHs in the gas phase, we present an exper-imental and theoretical study on the photo-dissociationbehavior of fullerene/anthracene cluster cations in thegas phase. The interaction of the fullerene cations, C ,C , C , C , C and C with neutral an-thracene are investigated. We select anthracene (C H ,m/z=178) as an example of PAHs for this study, in viewof its relatively high vapor pressure at room temperature. EXPERIMENTAL METHODS Here, only a brief description of the experiment is pro-vided. More detailed information on the experimentalprocedures is available in Zhen et al. (2019). First ofall, a fullerene (C or C ) is evaporated by heating thepowder (J&K Scientific, with purity better than 99 %) inthe first oven at a temperature of ∼
613 K. Subsequently,evaporated C or C molecules are ionized using elec-tron impact ionization ( ∼
82 eV) and transported intothe ion trap via an ion gate and a quadrupole mass fil-ter. The high energy of the impacting electrons lead tofragmentation of the original fullerene through C lossesto form C and C or C and C , respectively.A second oven (neutral molecules source, ∼
300 K)is located under the trap to vaporize the molecules (an-thracene power, J&K Scientific, with a purity better than99 %), which can effuse continuously toward the center ofthe ion trap. In the ion trap, fullerene/anthracene clustercations are formed by reaction between fullerene cationsand neutral anthracene molecules. During this proce-dure, helium gas is introduced continuously into the trapvia a leak valve to thermalize the ion cloud through col-lisions ( ∼
300 K). Adduct formation presumably occursunder our experimental operating conditions. The thirdharmonic of an Nd:YAG laser (INDI, Spectra-Physics),355 nm, ∼ −
4) acts as a physical shield insidein the chamber and determines the interaction time ofthe light with the trapped ion clusters. The shutter isexternally triggered to guarantee that the ion cloud is ir-radiated only for a specified amount of time during eachcycle. A high precision delay generator (SRS DG535)controls the full timing sequence.Our setup operates with a typical frequency of 0.2 Hz,i.e., one full measuring cycle lasts 5.0 s. At the leadingedge of the master trigger, the ion gate is opened (0.0-4.0 s), allowing the ion trap to fill for a certain amountof ions. During this time, the trapped ion reacts withanthracene molecules to form new cluster cations. Oncethese clusters are formed, the stored waveform inverseFourier transform excitation (SWIFT) pulse is applied toisolate species within a given mass/charge (m/z) range(4.0-4.2 s) (Doroshenko & Cotter 1996). Afterwards ( ∼ EXPERIMENTAL RESULTS AND DISCUSSIONThe mass spectrum of the fullerene (C , C andC )/anthracene cluster cations, without SWIFT isola-tion and before laser irradiation, is shown in Figure 1(A).Clearly, a series of peaks of fullerene/anthracene clustercations are observed. As shown in Figure 1(B), exceptthe main fullerene (C ) mass peak (C , m/z=720),the mass spectra before irradiation reveal a small amountof residual fullerene (C ) fragments (C , m/z=672and C , m/z=696), due to the electron impact ioniza-tion and fragmentation (Zhen et al. 2014). We note thatthe peak intensity of m/z=721 ( C C ) is strongerullerene/anthracene cluster cations 3 Fig. 1.—
Upper panel (A): Mass spectrum of fullerene(C , C and C )/anthracene cluster cations, without SWIFT and be-fore laser irradiation; Panel (B-E): five zoom-in mass spectrum, revealing the presence of formed [C / / ] + , [(C H )C / / ] + ,[(C H ) C / / ] + , [(C H ) C / / ] + and [(C H ) C / ] + cluster cations, respectively. than m/z=720 ( C ) in here, as shown in Figure 1(B),which is off the natural carbon element abundance, i.e., C contained species have a stronger peak intensity thanpure C species in here, the possible reason may due tothe experimental setup conditions (quadrupole ion trap).We label the formed fullerene (C , C andC )/anthracene cluster cations in four zoom-inmass spectra in the lower panel of Figure 1(C-F). Thenew formed cluster cations are shown as follows: In Fig-ure 1(C), [(C H )C ] + (m/z=850), [(C H )C ] + (m/z=874) and [(C H )C ] + (m/z=898); In Figure1(D), [(C H ) C ] + (m/z=1028), [(C H ) C ] + (m/z=1052) and [(C H ) C ] + (m/z=1076); In Fig-ure 1(E), [(C H ) C ] + (m/z=1206), [(C H ) C ] + (m/z=1230) and [(C H ) C ] + (m/z=1254);In Figure 1(F), [(C H ) C ] + (m/z=1384) and[(C H ) C ] + (m/z=1408). We note that, as ob-served above in the mass spectrum of C , the masspeak intensity pattern of the [(C H )C ] + isotopo-logues does not reflect the natural abundance of carbonisotopes. Although the strongest peak is expected forions that comprise only C (m/z = 874), a system issueof the experimental setup gives this characteristic tothe peak caused by ions that contain a C atom. Weconsider that this issue does not affect the interpretationof our measurements because we do not observe anyclear difference in the behavior of the isotopologueswith regard to formation and dissociation. In addition,we also observe one “extra” peak (m/z=810). Whileno assignments can be provided, we suspect thatthis peak might be formed as a side-product, due tocontaminations in the ion trap chamber.Importantly, the adducts of C (e.g.,[(C H ) n C ] + ) always dominate the correspond-ing adducts of C (e.g., [(C H ) n C ] + ) and the latter is always at least as strong or dominate thecorresponding adducts of C (e.g., [(C H ) n C ] + ).C adducts only dominate over C adducts for n > , C and C )/anthracene cluster cations, we believe thatthe fullerene-derived cluster cations are formed by ion-molecule reaction pathways, i.e., C / / cations + neu-tral anthracene molecule. The reaction process betweenfullerene cations and neutral anthracene molecular oc-curs through sequential steps and add repeatedly an-thracene groups to the surface of fullerene cages. Basedon the obtained results, we propose the formation path-ways as shown below: [C / / ] + C H −→ [C H C / / ] + (1)[C H C / / ] + C H −→ [(C H ) C / / ] + (2)[(C H ) C / / ] + C H −→ [(C H ) C / / ] + (3)[(C H ) C / ] + C H −→ [(C H ) C / ] + (4) Figure 2 (A) shows the resulting mass spectrum oftrapped fullerene (C , C and C )/anthracene clustercations upon 355 nm irradiation at 1.3 mJ laser energies(irradiation times amounting to 0.5 s; i.e., typically ∼ is close to zero. The possible reason is that un-der laser irradiation, no C product formed, because Zhen et al. Fig. 2.—
Upper Panel (A): Mass spectrum of fullerene(C , C and C )/anthracene cluster cations trapped in QIT upon 355 nmirradiation at 1.3 mJ laser energy (irradiation times amountingto 0.5 s, from 4.4 − H )C / / ] + cluster cations, withoutirradiation (red) and irradiated at 355 nm (blue) in the range ofm/z=828-924; the ionization energy of C is greater than that of an-thracene. As such, the charge of cluster will localize onthe anthracene molecular group, rather than on the C molecular group and dissociation of the cluster will notlead to C . We will discuss this photo-dissociation be-havior further with theoretical calculations in the nextsection.We present a zoom in mass spectrum of[(C H )C / / ] + cluster cations in Figure 2(B),with and without laser irradiation. We can see thatonly mono-anthracene molecular group dissociationproducts are formed, and there is no evidence for otherfragmentation channels (e.g., dehydrogenation). Hence,we conclude that larger fullerene-PAH cluster cationsshrink to smaller clusters by sequentially sheddinganthracene molecules without dehydrogenation pathway.Accordingly, we propose the following photo-dissociationpathway for fullerene/anthracene cluster cations (thephoto-dissociation pathway for [(C H )C ] + will discuss later in the discussion section): [(C H ) C / ] + h ν → C H + [(C H ) C / ] + (5)[(C H ) C / / ] + h ν → C H + [(C H ) C / / ] + (6)[(C H ) C / / ] + h ν → C H + [C H C / / ] + (7)[C H C / ] + h ν → C H + [C / ] + (8) For the fullerene (C ) family, the typical massspectrum of the fullerene (C , C and C )/anthracenecluster cations are shown in Figure 3. Similar to Figure1, without laser irradiation (Figure 3, upper red spec-trum), besides C (m/z=792), C (m/z=816) andC (m/z=840), we detect newly formed cluster cationslabelled as: [(C H )C ] + (m/z=970), [(C H )C ] + (m/z=994) and [(C H )C ] + (m/z=1018);[(C H ) C ] + (m/z=1148), [(C H ) C ] + (m/z=1172) and [(C H ) C ] + (m/z=1196). Basedon the observed new species, we propose the formationreactions as: [C / / ] + C H −→ [C H C / / ] + (9)[C H C / / ] + C H −→ [(C H ) C / / ] + (10) The mass spectrum after irradiation is shown in Fig-ure 3 (middle blue spectrum, 0.9 mJ). Further detailson the photo-dissociation behavior of fullerene(C , C and C )/anthracene cluster cations are presented as adifferential spectrum (lower trace in Figure 3). Again,we do not observe the dehydrogenation process of thesefullerene (C , C and C )/anthracene cluster cations.Rather, larger clusters shrink to smaller clusters throughmono − anthracene loss: [(C H ) C / / ] + h ν −→ C H + [C H C / / ] + (11)[C H C / / ] + h ν −→ C H + [C / / ] + (12) In order to compare the reactivity of C andC relative to C in the cluster formation path-way, the intensity ratio of the fullerene/anthracenecluster cations to their parent fullerene cations areplotted in Figure 4: [(C H ) n C ] + /C , n=1, 2,3; [(C H ) n C ] + /C and [(C H ) n C ] + /C ,n=1, 2, 3, 4; respectively. From the intensity ratio com-parison, we conclude that C and C are more re-active towards adduct formation than C , and this istrue for n=1, 2, 3. In addition, we note that, C be-comes more reactive than C for n greater or equalto 4. Our conclusion is in line with previous studies ofreactions of the fullerene cation (C ) with cyclopen-tadiene (Becker et al. 1997). In these latter studies,C and C were shown to be more reactive to adductformation than C . Nevertheless, we stress that, de-spite this low reactivity of C , we do see cluster forma-tion of anthracene with C up to n=3. For the C family, the intensity ratio of [(C H ) n C ] + /C and[(C H ) n C ] + /C , n=1, 2, are plotted in Figure 4.The C family has a comparable adduct behavior tothe C family, and we conclude that C is more re-active towards adduct formation than C . We willdiscuss the adduct behavior with theoretical chemistrycalculations in the next section.ullerene/anthracene cluster cations 5 Fig. 3.—
Mass spectrum of fullerene(C , C and C )/anthracene cluster cations, without SWIFT and without irradiation (red),irradiated at 355 nm (blue) and the differential spectrum (black). In the no irradiation mass spectrum, revealing the presence of[(C H )C / / ] + and [(C H ) C / / ] + clusters, respectively. Fig. 4.—
The intensity ratio of formed fullerene/anthracene clus-ter cations to fullerene cations: [(C H ) n C ] + /C , n=1, 2,3; [(C H ) n C ] + /C and [(C H ) n C ] + /C , n=1, 2, 3,4; [(C H ) n C ] + /C and [(C H ) n C ] + /C , n=1, 2. THEORETICAL CHEMISTRY CALCULATIONRESULTSThe theoretical calculations are carried out at theB3LYP (Becke 1992; Lee et al. 1988) level with the6-31G(d, p) basis set, which is implemented in the Gaus-sian 16 program (Frisch et al. 2016). To accountfor the weak interaction (i.e. van der Waals force) be-tween fullerene and anthracene molecules, the dispersion-correction (D3) (Grimme et al. 2011) is included in thiswork. We mention here that the results reported heredo not include the basis set-superposition error (BSSE) correction, which usually results in slightly reduced bondenergies (Basiuk & Tahuilan-Anguiano 2019). In addi-tion, we only carried out theoretical calculations for thefullerene (C , C and C )/anthracene cluster cationssystem, due to the similarity in behavior for the fullerene(C ) family.For the fullerene (C and C ) cations, we assumethere is no carbon skeleton rearrangement (except forthe C loss at a local position) during the electron im-pact ionization and fragmentation process. After C loss,there are two main isomers of C , namely 7 C-ring and8 C-ring conformations, as shown in Figure 5. In agree-ment with earlier studies (Lee & Han 2004; Chen et al.2008; Candian et al. 2019), we found that the 7 C-ringisomer structure is more stable. Therefore, we only focuson this isomer in our following calculations. Based uponthe C study, after a further C loss, we identify twoisomers for C similarly (Figure 5). And the double 7C-rings conformation (the 7 C-rings are in opposite cageposition) is more stable, which is therefore selected in theC study.To understand the details of formation process of thefullerene/anthracene cluster cations, we take C + an-thracene, C (7 C-ring) + anthracene and C (6 C-ring) + anthracene as typical examples, to theoreticallystudy the adduct reaction process. We follow the mini-mum energy pathway from the van der Waals cluster tothe covalently bonded cluster, and at each step calculatedthe energy and the optimized structures. The energy andthe optimized structure for the reactant, transition states(TS1 and TS2), intermediary, product for the reactionpathway between C and anthracene, C (7 C-ring)and anthracene and C (6 C-ring) and anthracene areshown in Figure 6 and 7, and Table 1.As shown in Figure 6, with B3LYP+D3 functional Zhen et al.method, in the beginning, C and anthracene form avan der Waals molecular complex (Initial, the exother-mic energy is around -1.19 eV), and then it goes to anintermediary (Inter, -0.82 eV) through the first transi-tion states (TS1, -0.85 eV) that pass the first activatebarrier (0.37 eV). After that, the product (Product, theexothermic energy is around -1.31 eV) is formed throughthe second transition states (TS2, -0.73 eV) that passthe second activate barrier (0.08 eV). In the anthracene“landing” on the C process, the two carbon atomsfrom C are puckered out of the cage surface, the struc-ture of C H is modified to allow the 9, 10 C-atomsto bond to the C-atoms from the fullerene (Sato et al.2013).In order to check the accuracy of, e.g., the van derWaals interaction, M06-2X level are also calculatedand the results are presented in Figure 6. In M06-2X functional calculation, the adduct process is simi-lar to B3LYP+D3 functional method, and the calcula-tion result are (intial, -1.25 eV), (TS1, -0.87 eV), (in-ter, -0.91 eV), (TS2, -0.78 eV) and (Product, -1.55 eV).The relative energy values obtained with M06-2X func-tional method all slightly increase as compared to theB3LYP+D3 functional method, especially for the cova-lently bonded (-1.55 eV to -1.31 eV) species. In agree-ment with (Sato et al. 2013), B3LYP+D3 performsalmost as good as the M06-2X functional. For compu-tational reasons, the B3LYP+D3 functional method wasemployed in this work for other fullerene/PAH clustercations system.For the interaction of C with anthracene, due to thestructure of C , there are two reaction pathways: oneis “landing” on the “6 C-ring” and the other is “land-ing” on the “7 C-ring”. The result of the calculationare presented in Figure 7(A) and (B), respectively. Asshown in Figure 7(A), in the beginning, C (7 C-ring)and anthracene form a van der Waals molecular com-plex (Initial, the exothermic energy is around -1.12 eV),and then it goes to an intermediary (Inter, -1.39 eV)through the first transition states (TS1, -1.07 eV) thatpass the first activate barrier (0.05 eV). After that, theproduct (Product, the exothermic energy is around -1.31eV) is formed through the second transition states (TS2,-1.01 eV) that pass the second activate barrier (0.38 eV).For the other possible route in the interaction of C with anthracene, as shown in Figure 7(B), in the begin-ning, C (6 C-ring) and anthracene form a van derWaals molecular complex (Initial, the exothermic energyis around -0.83 eV), and then it goes to an intermediary(Inter, -0.34 eV) though the first transition states (TS1,-0.31 eV) that pass the first activate barrier (0.52 eV).After that, the product (Product, the exothermic energyis around -1.05 eV) is formed though the second transi-tion states (TS2, -0.14 eV) that pass the second activatebarrier (0.20 eV).For elucidate the difference in the chemical behavior ofthe three fullerene cations with anthracene, especially forthe newly formed multi-anthracene adducted clusters, wepresent in Figure 8 the optimized structures of the co-valently bonded clusters, [(C H ) (1 − C ] + (panel A),[(C H ) (1 − C ] + (panel B) and [(C H ) (1 − C ] + (panel C). To the covalently bonded clusters, mono-anthracene and fullerenes that are connected by two C-C single bonds in which the two (blue) C-atoms are fromC and two (red) C-atoms are from anthracene. All fourof these C-atoms are in sp hybridization where for thetwo C-atoms from anthracene one of the sp bonds is aC-H bond. As expected (Briggs &Miller 2006) and thediscussion presented above, the more reactive 9 & 10 C-atoms of anthracene are involved in the covalent bondformation.In Figure 8(A), [C H C ] + and the optimized struc-ture of [(C H ) C ] + and [(C H ) C ] + are ob-tained. For these structures, additional anthracene isalso added to (normal) 6 C-rings. Clearly, all the reac-tions are exothermic, with -1.3, -1.3 and -1.0 eV, respec-tively.Similar to C /anthracene, [C H C ] + and the op-timized structure of [(C H ) C ] + , [(C H ) C ] + and [(C H ) C ] + (shown in Figure 8(B)) are ob-tained. Base on the obtained result in Figure 7, for thesemulti anthracene adducted clusters − and analogouslyto C /anthracene and C /anthracene − additional an-thracene is added to (normal) 6 C-rings. Clearly, all thereactions are also exothermic, with -1.3, -1.2, -1.1 and-0.8 eV, respectively.As shown in Figure 8(C), similar to C /anthracenecluster cations, the structure of [C H C ] + consists ofone mono-anthracene molecule and one C +56 , connectedby two C-C single bonds. Again, two carbon atomsfrom the fullerene are in sp hybridization from one ofthe 7 C-rings, and two C-atoms from anthracene arein sp hybridization with an additional C-H bond. Ac-cordingly, the optimized structures of [(C H ) C ] + ,[(C H ) C ] + and [(C H ) C ] + are obtained andshown in Figure 8(C). For [(C H ) C ] + , the sec-ond anthracene molecule is added on another 7 C-ring.For [(C H ) C ] + and [(C H ) C ] + , anthracenemolecules are added to normal 6 C-rings. Clearly, allthe reactions are exothermic, with -1.7, -1.3, -0.9 and-0.8 eV, respectively.Comparison of the optimized structures of the cova-lently bonded clusters of the three fullerenes with an-thracene reveals clear differences. Specifically, with threeanthracene on C cage surface, the spherical shapeof C is almost unchanged. When we introduce fouranthracene on the C cage surface, the shape of theC cage shows significant modification changing froma spherical cage to a more tetrahedral cage. We surmisethat this difference reflects the very rigid structure ofC as compared to much more pliable C and C cages. DISCUSSIONThe experiments show that the fullerene cations, C and C , react much more readily with anthracene thanC . Likewise, C and C react more readily thanC . Following (Bohme 2016), we can attribute this tothe enhanced curvature of the surfaces of C & C and C & C with respect to C and C re-spectively. In the theoretical calculations, we can sepa-rate the formation process of fullerene/anthracene clus-ter cations into two stage: the first stage is from fullerenecation + anthracene to the van der Waals cluster (Ini-tial), the second stage is from the van der Waals clusterto covalent bonded cluster (Product). The formationullerene/anthracene cluster cations 7of both the van der Waals cluster and the covalentlybonded species are energetically downhill from the re-actants. The binding energies of the van der Waals clus-ter and the covalently bonded species are very similarfor both species, [C H C ] + and [C H C ] + . How-ever, there is a substantial energy barrier in the transi-tion from the van der Waals cluster involving C tothe covalently bonded cluster, while there is hardly abarrier involving C . While, in either process, thisbarrier is submerged, we surmise that it does play animportant role in the reaction process; That is, possi-bly the [C H C ] + cluster is quickly trapped in thevan der Waals, but with insufficient energy to over-come the energy barrier to the covalently bonded species.In contrast, after trapping in the van der Waals well,[C H C ] + can still react to form a covalently bondedspecies. We then further surmise that the [C H C ] + van der Waals cluster does not survive in the TOFmass spectrometer acceleration zone, while the covalentlybonded [C H C ] + species does. In this view, thesmall amount of [C H C ] + represents van der Waalsclusters that “survived” in the acceleration process.We do note that, in addition, at low temperatures(around 10 K), the substantial dipoles of C (1.1 De-bye) and C (1.26 Debye) can be expected to enhancethe reaction rate coefficient of these species by a fac-tor of 2 compared to C (0 Debye). However, at 300K in our experimental condition, the effect is negligible(Smith 2011). Finally, it should be emphasized thatreactions in the liquid and solid state can lead to cova-lently bonded structures (Garc´ıa-Hern´andez & D´ıaz-Luis2013a; Garc´ıa-Hern´andez et al. 2013b).As to the subsequent dissociation pathway initiated bythe laser irradiation in Figure 2, the clusters evolve to-wards breaking the bond between the fullerene and theanthracene groups. From our previous studies, we haveshown that the dissociation energy of H-loss is generally ∼ ∼ from 0.8 to1.7 eV as shown above). Hence, in agreement with theexperiments, loss of anthracene molecule should domi-nate over H-loss.In addition, as shown as below, the charge transfer canhappen between the C cation and anthracene (exother-mic reaction pathway, + 0.37 eV, equation 13) throughcluster cations. In contrast, the charge exchange reactionof C with anthracene is endothermic by 0.27 eV whichis thermodynamically unfavorable (equation 14). C H + [C ] + → [C H C ] + h ν → C H +10 + C + 0 . H + [C ] + → [C H C ] + h ν → C H +10 + C − . ASTRONOMICAL IMPLICATIONSWe experimentally and theoretically investigated theformation and photo-chemistry processes of a series oflarge fullerene derivatives (e.g., fullerene-PAH derivedclusters). Gas-phase reactions between fullerene (e.g.,C / , C / and C / cations) and PAHs (e.g., an-thracene) occur in our experimental setup, which pro-vide new insights into the evolution of fullerene (bottom-up growth) in the radiation fields in the ISM. Petrie Fig. 5.—
The optimized structure of C and C , blue carbonfor 7 C-ring or 8 C-ring.
Fig. 6.—
The reactant, transition states, intermediary, prod-uct, and the energy for the reaction pathway between C andanthracene with calculation B3LYP+D3 and M06-2X functionalmethod, respectively. & Bohme (2000) presented an experimental study ofC adduct reaction with one anthracene or corannu-lene (C H ) molecule. In these experiments, adductformation with anthracene did not occur but did occurwith corannulene. Our experiments also indicate veryinefficient adduct formation of anthracene with C (compared to C and C ). Dunk et al. (2013)demonstrated the cluster cations ([C H − C ] + and[C H − C ] + ) formation resulting from gas-phase in-teraction of C and C with coronene (C H ) un-der energetic conditions. In our study, we build uponthese studies by investigating the adduct formation be-havior of C /C /C and C /C /C with Zhen et al. TABLE 1The energy for the reactant, transition states (TS1 and TS2), intermediary, product for the reaction pathway betweenC and anthracene, C (7 C-ring) and anthracene and C (6 C-ring) and anthracene. C and anthracene C (7 C-ring) and anthracene C (6 C-ring) and anthraceneB3LYP+D3 level M06-2X level B3LYP+D3 level B3LYP+D3 levelHartree eV Hartree eV Hartree eV Hartree eVReactant -2825.828330 0.00 -2825.04131 0.00 -2749.492420 0.00 -2749.492420 0.00Initial -2825.872179 -1.19 -2825.08727 -1.25 -2749.533379 -1.12 -2749.522858 -0.83TS1 -2825.858382 -0.82 -2825.07328 -0.87 -2749.531803 -1.07 -2749.503947 -0.31Inter -2825.859637 -0.85 -2825.07484 -0.91 -2749.543600 -1.39 -2749.504750 -0.34TS2 -2825.85503 -0.73 -2825.07006 -0.78 -2749.529612 -1.01 -2749.497699 -0.14Product -2824.876509 -1.31 -2825.09826 -1.55 -2749.540684 -1.31 -2749.530882 -1.05 Fig. 7.—
The reactant, transition states, intermediary, product,and the energy for the reaction pathway between C (7 C-ring)and anthracene (panel A) and C (6 C-ring) and anthracene(panel B), respectively, with B3LYP+D3 functional method. anthracene, revealing the much greater reactivity of thesmaller fullerenes derived from C and C by succes-sive C losses. In addition, for the first time, we obtainedthe multi-PAHs adducting on the fullerene surface (e.g.,[(C H ) C ] + , four anthracene molecules on the C cage surface as one super large molecule clusters, with154 atoms and ∼ and C toadduct formation as compared to C is in line withthe study of Becker et al. (1997), which reported exper-imental evidence for the heightened chemical reactivity of C /C relative to C in the Diels-Alder reac-tion with cyclopentadiene. This difference in behaviorhas been related to the more pliable cage structure ofthe smaller fullerenes as suggested by Petrie & Bohme(2000); Becker et al. (1997) in their study of the Diels-Alder reaction of these fullerene cages with cyclopenta-diene.Our present study indicates that the smaller fullerenecations, C and C (C and C ), form adductswith PAHs much more readily than C (C ). Hence,if these smaller fullerene are present in space, formationof covalently bonding fullerene-based clusters could pro-duce an extended family of large molecules (together withthe van der Waals cluster of C with anthracenes).Likewise, these types of clusters may play a role in the IRspectral complexity of circumstellar environments whereC has been shown to be prominent (Cami et al. 2010;Sloan et al. 2014; Otsuka et al. 2014). (Bernard-Salaset al. 2012) could not explain satisfactorily the rela-tive intensities of IR emission bands attributed to C in PNes. They suggested that other substances, e.g.,C , could contribute to some of the bands, thus causingthe inconsistencies they observed. Because the spatialdistributions of fullerenes and PAHs do not overlap inPNes, we do not propose fullerene/PAH adducts as suchcontributors. Nevertheless, species formed by reactionbetween fullerene cations and molecules found in PNesmay be involved.In addition, it has been suggested that PAH clustersplay a role in the extended red emission prominent inmany interstellar and circumstellar environments (Rheeet al. 2007). These fullerene/PAHs adducts formedin our experiments may be relevant for this emission aswell. In addition, the covalent bond formation in theclusters considered here may be an important step in theformation of larger carbon grains (Dunk et al. 2013).In this paper, we study the subsequent photo-chemically driven evolution of such fullerene/PAHs clus-ter cations. The calculated binding energy is ∼ (and sp ) H-atoms in PAHs. In space, the weakest link is expected togo first and fragmentation after UV excitation will leadto loss of a PAH molecule from the cluster. Based uponthis low binding energy and using the density of states ofC and PAHs as a guide, we estimate that absorptionof a single 6 eV photon - which are readily available inPDRs - will be sufficient to lead to fragmentation (Tie-lens 2005). As this estimate scales with the internal en-ergy per atom, larger clusters will require concomitantlymore energetic photons. For much larger clusters, multi-ullerene/anthracene cluster cations 9 Fig. 8.—
The formation reaction pathway for [(C H ) n C ] + , n=1, 2, 3 in panel (A); the formation reaction pathway for[(C H ) n C ] + , n=1, 2, 3, 4 in panel (B); the formation reaction pathway for [(C H ) n C ] + , n=1, 2, 3, 4 in panel (C). (Bluecarbon is from fullerene group and red carbon is from C H group for C − C bond) photon events can then still lead to fragmentation in aPDRs environment (Bern´e et al. 2015). CONCLUSIONSThe first experimental results on the formation andphoto-chemical process of large fullerene/anthracenecluster cations in the gas phase are presented,which reveal a general cluster formation process forfullerene/PAHs cluster cations, i.e., constructed a seriesof fullerene-PAH derived cluster molecules. The clus-ter formation process is especially true for the smallerfullerene (C , C and C , C ) cations. In agreementwith earlier studies involving reactions with cyclopenta-diene (Becker et al. 1997; Bohme 2016), we concludethat C and C are much more reactive towardscluster formation than C . Quantum chemistry cal-culations demonstrate that these newly formed cluster species can be quite stable (the binding energy ∼ REFERENCESAllamandola, L. J., Tielens, A. G. G. M., Barker, J. R. 1989,ApJS, 71, 733Basiuk, V. A. & Tahuilan-Anguiano, D. E. 2019, CPL, 722,146Becke, A. D. 1992, JChPh, 96, 2155Becker, H., Scott, L. T., Bohme, D. K. 1997, Int. J.Mass,Spectrom. Ion Process., 167, 519Bernard-Salas, J., Cami, J., Peeters, E., Jones, A. P., Micelotta,E. R., and Groenewegen, M. A. T. 2012, ApJ, 757, 41Bern´e, O., Joblin, C., Deville, Y., et al. 2007, A & A, 469, 575Bern´e, O., Montillaud, J., & Joblin, C. 2015, A&A, 577, A133 Bern´e, O., Tielens, A. G. G. M. 2012, PNAS, 109, 401Briggs, J. B., and Miller, G. P. 2006, CRC, 9, 916Bohme, D. K. 1992, ChRe, 92, 1487Bohme D. K. 2009, Mass Spectrom. Rev. 28, 672Bohme, D. K. 2016, Philos Trans A Math Phys Eng Sci, 374,20150321Cami, J., Bernard-Salas, J., Peeters, E. & Malek, S. E. 2010, Sci,329, 1180Cami, J., Peeters, E., Bernard-Salas, J., Doppmann, G., DeBuizer, J. 2018, Galaxies, 6, 1010 Zhen et al.