From planes to bowls: photodissociation of the bisanthenequinone cation
Tao Chen, Junfeng Zhen, Ying Wang, Harold Linnartz, Alexander G. G. M. Tielens
FFrom planes to bowls: photodissociation of the bisanthenequinone cation
Tao Chen , , Junfeng Zhen , , Ying Wang , Harold Linnartz , Alexander G. G. M. Tielens Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, 10691, Stockholm, Sweden Leiden Observatory, Leiden University, PO Box 9513, NL 2300 RA Leiden, the Netherlands Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands
Abstract
We present a combined experimental and theoretical study of the photodissociation of the bisanthenequinone (C H O ) cation,Bq + . The experiments show that, upon photolysis, the Bq + cation does not dehydrogenate, but instead fragments through thesequential loss of the two neutral carbonyl groups, causing the formation of five-membered carbon cycles. Quantum chemicalcalculations confirm this Bq + → [Bq - CO] + → [Bq - 2CO] + sequence as the energetically most favorable reaction pathway. Forthe first CO loss, a transition state with a barrier of ∼ ∼ ∼ ∼ Keywords:
Dissociation, Hydrocarbon molecules, Bisanthenquinone
1. Introduction
Bisanthenequinone (Bq) belongs to the family of the poly-cyclic aromatic hydrocarbon (PAH) quinones. These moleculesare known products in the photo-oxidation of environmentallyrelevant PAHs (Alam et al., 2014; Garc´ıa, 1994). During in-complete combustion processes quinones are released into theatmosphere (Iinuma et al., 2007; Layshock et al., 2010; Vala-vanidis et al., 2006). They are also used in (in)organic synthe-sis, as oxidizing agent (March, 2005) or because of their phar-macological relevance (Patai, 1974; Liu, 2010). They may alsobe of astronomical relevance (Tielens, 2008), the motivation forthe present study.The vibrational signatures of PAHs dominate the mid-infrared spectra of many objects in space and are key contrib-utors to the energy and ionization balance of the gas (Tielens,2008). Interstellar PAHs are assumed to form in processes akinto soot formation in the cooling ejecta of carbon-rich Red Gi-ant stars as they flow from the stellar photosphere into the in-terstellar medium (Frenklach and Feigelson, 1989; Cherchne ff et al., 1992). Subsequently, they are further processed for mil-lions of years by photons of the interstellar radiation field (Tie-lens, 2013). Driven by this astrophysical interest, experimentsfocusing on the photoexcitation of PAHs have attracted muchinterest in recent years (Zhen et al., 2014a, 2016) (and refer-ences therein). A number of di ff erent processes can take place,varying from sequential fragmentation (Ekern et al., 1998; Westet al., 2012; Zhen et al., 2014a), isomerization (Dyakov et al.,2006; Johansson et al., 2011; Solano and Mayer, 2015; Si-mon et al., 2017; Trinquier et al., 2017) and ongoing ionization Corresponding author: Tao Chen ([email protected]) (Holm et al., 2011; Zhen et al., 2015). Dedicated studies ofthe involved dissociation channels provide information on themolecular dynamics at play and this is interesting, both from anastronomical and physical chemical point of view (Holm et al.,2011; Chen et al., 2015). Particularly processes changing thenature of the carbon skeleton have been the topic of recent stud-ies. In photodissociation regions (PDRs) in space, (large) PAHs(with more than 50 C-atoms) are considered starting pointsin the formation of other species, including fullerenes, carboncages and smaller hydrocarbon chains (Pety et al., 2005; Bern´eand Tielens, 2012; Zhen et al., 2014b; West et al., 2012). AlsoPAHs with functional side groups may be important. Insidemolecular clouds, PAHs are expected to be trapped in low tem-peratures ( ∼
10 K) ice mantles (Guennoun et al., 2011b,a), con-sisting mainly of H O with traces of CH OH, CO , CO, andNH . Photolysis of these complex ice mixtures is known tofunctionalize PAHs with alcohol (-OH), ketone ( > C = O), amino(-NH ), methyl (-CH ), methoxy (-OCH ), cyano / isocyano (-CN, -NC), and carboxyl (-COOH) groups (Bernstein et al.,2002; Cook et al., 2015). When molecular cloud ices are ex-posed to the strong radiation field of a newly formed massivestar in a PDR, ice molecules can be returned to the gas phasethrough various processes (Tielens, 2013). Photolysis of thesenewly formed PAHs with functionalized side-group additions,like methyl, methoxy, hydroxyl or carbonyl groups may play acomparable important role (Bernstein et al., 2002).Quinones mainly dissociate through consecutive losses ofcarbonyl units (Beynon et al., 1959; Proctor et al., 1981; Panet al., 2008). Their fragmentation mechanisms are well estab-lished for small species, e.g. substituted 1,4-naphthoquinone(Becher et al., 1966; Mari et al., 1966; Stensen and Jensen, Preprint submitted to Chemical Physics Letters September 27, 2018 a r X i v : . [ phy s i c s . a t m - c l u s ] S e p ff ect of acetone site-substitution on photostability and photoreactivity. The exper-iments are conducted using quadrupole ion trap time-of-flight(QIT-TOF) mass spectrometry. Quantum chemical calculationsare performed to explore the dissociation pathways and result-ing geometry changes.The article is organised as follows: Section 2 and 3 describethe experimental setup and computational methods used for thisstudy. Section 4 shows the experimental and theoretical results,and discusses the astrophysical relevance. The conclusions fol-low at the end.
2. Experimental setup
The photofragmentation experiments on Bq cations areconducted with i-PoP, our instrument for photodissociationof PAHs (Zhen et al., 2014b) that comprises a commercialquadrupole ion trap time-of-flight system. A typical experimentworks in the following way: commercially available Bq powder(purity higher than 99.0 % from Kentax) is heated to ∼
500 K ina small oven. Subsequently, the evaporated molecules are ion-ized by an electron gun and the ions are guided into the ion trap.Once the ions are trapped, a stored waveform inverse Fouriertransform (SWIFT) excitation technique (Doroshenko and Cot-ter, 1996) is used to isolate a specific range of mass / charge(m / z) species. After a short time delay (typically ∼ ∼
298 K) throughcollisions with He bu ff er gas that is continuously added to thetrap. Subsequently, the ion cloud is irradiated with light pulsesgenerated by a tunable dye laser (LIOP-TEC, Quasar2-VN)pumped by a Nd:YAG laser (DCR-3, Spectra-Physics), oper-ated at 10 Hz. A solution of DCM dye is used to produce626-nm-light as well as 312 and 208 nm radiation through dou-bling and tripling of the original light. The fragments and intactmolecules after irradiation with several photons are acceleratedby a negative square pulse to transfer them from the trap to thefield-free TOF region, where the corresponding mass signalsare detected using a microchannel plate (MCP) detector. Thefull system operates at pressures of the order of 10 − mbar orbetter.
3. Computational methods
Our theoretical calculations are carried out using densityfunctional theory (DFT). The dissociation energies, transitionstate energies and dipole moments presented in this work arecalculated using the hybrid density functional B3LYP (Becke,1992; Lee et al., 1988) as implemented in the Gaussian 16 pro-gram (Frisch et al., 2016). All structures are optimized usingthe 6-311 ++ G(2d,p) basis set. The vibrational frequencies are
Figure 1: The electron impact induced mass spectrum of Bq cation, before(black curve) and after (blue (A), red (B) and green (C) curve) SWIFT signalisolation. The blue curve at mass ∼
380 amu corresponds to the Bq + . The redcurve at mass ∼
352 amu corresponds to one carbonyl loss from Bq + ([Bq -CO] + ). The green curve at mass ∼
324 amu corresponds to the two carbonyllosses from Bq + ([Bq - 2CO] + ). The small peaks on the right side of the mainpeak in graphs A and B are isotopic contributions from C or O, but there isnot enough oxygen incorporated to generate detectable amounts of isotopomersgiven the apparent signal-to-noise ratio: only C enriched species are actuallyvisible in the mass spectra. SWIFT isolation e ff ectively reduces isotopes in themass spectrum of graph C, i.e. [Bq - 2CO] + . calculated for the optimized geometries to verify that these cor-respond to minima or first-order saddle points (transition states)on the potential energy surface (PES). We have taken the zeropoint vibrational energy (ZPVE) into account. The ZPVE val-ues are scaled by the empirical factor 0.965 to correct for anhar-monic e ff ects (Andersson and Uvdal, 2005). It should be notedthat these corrections will not influence our conclusions regard-ing the dissociation energies and transition states calculations.The calculated energies for the small [6-31G(d)] and the largerbasis set [6-311 ++ G(2d,p)] are very similar, and thus, only theresults from the larger basis set calculations are presented here.The PES of all possible dissociation pathways is scanned fortransition state calculations. Intrinsic reaction coordinate (IRC)calculations (Fukui, 1981; Dykstra, 2005) are performed to con-firm that the transition state structures are connected to theircorresponding local PES minima. In all cases we have onlyconsidered the ground state PES, as the non-radiative decay forsuch a large molecule is very rapid: ∼ − s (Vierheilig et al.,1999; Zewail, 2000). Only the lowest spin state is considered,which seems to be a well-justified approach to predict reason-able energies in comparison to the experimental results (Holmet al., 2011).
4. Results and discussion
Figure 1 shows the mass spectrum of the trapped Bq + afterelectron impact ionization of the Bq precursor species. The topmass spectrum is obtained without laser irradiation and without2 igure 2: Mass spectrum of Bq + (left panels) and [Bq - CO] + (right panels) before (lower graph in each panel) and after irradiation (upper graph in each panel)by 626 nm (top panels), 312 nm (middle panels) and 208 nm (bottom panels). The blue (red) curves represent SWIFT selected Bq + ([Bq - CO] + ) without laserirradiation. The black curves show the e ff ect upon irradiation. No proof for dehydrogenation is found, meaning that CO is the dominant dissociation channel forBq + and [Bq - CO] + (see text). SWIFT signal isolation. It illustrates that the parent cations ex-perience substantial fragmentation due to the electron impactionization. The improvement in mass purity becomes clearfrom the lower three curves, where mass peaks at m / z ∼ + ), at ∼
352 for the [Bq - CO] + andat ∼
324 for the [Bq - 2CO] + are shown using the SWIFT pulseisolation. The mass spectra of Bq + and [Bq - CO] + show thatthe relatively small isotopic contributions from C or O, atm / z ∼
381 & 382 and 353 & 354 are further suppressed by theSWIFT pulse. Isolation of the parent mass peak, excludes Ccontributions in the mass spectrum of the [Bq - 2CO] + .Figure 2 shows the resulting mass-spectrum for Bq + irra-diation (left panels) and [Bq - CO] + irradiation (right panels)for three di ff erent wavelengths: 626 nm (upper panels), 312 nm(middle panels), and 208 nm (bottom panels). In all panels theSWIFT reference mass spectrum (without laser) is included aswell, in blue for Bq and in red for [Bq - CO] + . Clearly, due tomulti-photon absorption in experiments for the three di ff erentwavelengths, only minor changes have been observed in the re-sulting mass spectra. For this reason, in the remaining of thisarticle, we only will present and discuss the 626 nm data. Thefragmentation through the loss of two CO-units is clearly thedominant dissociation pathway. Once this process has ended,the [Bq-2CO] + starts behaving like a regular PAH (Chen et al.,2015). In Figure 2 (left bottom panel) it can be seen that aC -loss channel results in m / z signal at 300, and corresponding C H -loss in m / z signal at 298.As shown in Figure 1, the precursor molecule is located atm / z ∼ / z ∼
352 and 324, very similar to the electron impact inducedfragmentation. The separation between precursor and first frag-ment peak as well as between first and second fragment peakamounts to 28 Da mass di ff erences. This corresponds to oneoxygen plus one carbon (m / z = + and [Bq - CO] + except those due to isotopes.Hydrogen loss channels are not found. Our interpretation ofthese observations is that Bq photofragmentation is governed byloss of the two CO units only, transforming Bq into [Bq - CO] + and [Bq - 2CO] + . It is interesting to note that this CO-loss chan-nel is clearly preferred above dehydrogenation, i.e., fragmenta-tion through H-atom loss. For many regular large PAHs, i.e.,PAHs without side groups, H loss and 2H / H loss are dominantdissociation channels (Chen et al., 2015). This observation isremarkable although not fully unexpected as the same observa-tion was reported for smaller species like 1,2-naphthoquinoneand others (Pan et al., 2008). Also in the case of PAHs withother side group additions, like methoxy and methyl groups,dissociation pathways other than H-losses were reported (Zhenet al., 2016).In order to further understand the details of the dissociationprocess, quantum chemical calculations are performed. Figure3 .2 (TS)4.9 0.0H-loss CO-loss 1.03.8 (TS2) +CO0.0 2.04.7 (TS1)4.7 (Bq+)([Bq-CO]+) Figure 3: Calculated dissociation energies and reaction barriers for H losses(three di ff erent H losses in Bq + and six in [Bq - CO] + , here only the energeti-cally most favorable channel is shown) and CO losses from Bq and [Bq - CO] + cation. The barrier for CO loss from Bq + is significantly lower than the lowestdissociation energy of H-loss. Two reasonable barriers are found for the secondCO loss from [Bq - CO] + , one is comparable with H-loss, the other one is 0.9eV. All values are given in eV. + and [Bq - CO] + . Givenits molecular geometry, Bq + has three possible ways to losethe first hydrogen, These have been considered and our calcu-lations show that the lowest dissociation energy for H loss fromBq + is about 4.9 eV (see Figure 3 for the position of such Hatom in Bq + ), which is very similar to values found for regularPAHs (Chen et al., 2015). The barrier for one CO loss fromBq + is calculated to be ∼ ∼ − . Following the first barrier several shallowintermediate states are found before CO completely dissociatesfrom the [Bq - CO] + plane (final state). However these interme-diate states do not a ff ect the general trend for the dissociationprocess found here and therefore only the first transition stateand final state are shown in Figure 3. This explains why no Hloss is seen around peak m / z ∼ + . In Figure 3 it is shownthat [Bq - CO] + has six di ff erent dissociation channels for Hlosses. The lowest dissociation energy for H loss from [Bq -CO] + is about 4.7 eV (see Figure 3 for the position of such Hatom in Bq + ). Two possible dissociation pathways are found for the second CO loss from [Bq - CO] + , with barriers of ∼ ∼ − and -301 cm − . During this process, the planar structure of themolecule distorts into a bowl structure and these two transitionstates correspond to a CO molecule above and below the bowl(see Figure 3 for a structural representation). The energy forTS1 (4.7 eV) is comparable to the value calculated for an Hloss in [Bq - CO] + . As no H loss is observed in the mass spec-trum of [Bq - CO] + , it is unlikely that this provides an activefragmentation channel. The other channel is lower in energyand prefers CO loss above dehydrogenation, explaining why noH loss around the peak at m / z ∼
352 is found. Interestingly,whereas the [Bq - CO] + barely changes its molecular structureupon CO loss, we find that after removing the second CO group,Bq + loses its planar geometry and starts bending to form a bowlshaped PAH. This is discussed below.Figure 4 shows the calculated structure changes of Bq + ,[Bq - CO] + , and [Bq - 2CO] + and their corresponding dipolemoments. The bending of the aromatic structure presently ob-served results from the conjunction of (i) the particular arrange-ment of aromatic cycles in the structure of Bq + and (ii) the spe-cific position of each CO group, not only with respect to thestructure but also relative to each other. For [Bq-CO] + the for-mation of a pentagon at the periphery of this species after theloss of the first CO group does not lead to a large scale restruc-turing. However, after formation of the second pentagon, themolecular structure has to turn into a bowl-like geometry. Wenote that for this species the two pentagons are only separatedby one C-C bond which will make it easier to initiate bowl for-mation, compared to larger PAHs. The original Bq + is fullysymmetric and has no dipole. The highly asymmetric interme-diate ([Bq - CO] + ) has a dipole moment of 3.89 D, situated inthe plane, and the final product ([Bq - 2CO] + ) ends up with a2.54 D dipole moment pointing inwards. It is clear that disso-ciation of CO and the curling of a PAH plane induces a dipolemoment which would make such a molecule, in principle, de-tectable by radio astronomy (Lovas et al., 2005).The work described here shows a rather specific pathwaytowards pentagon formation in PAHs. When exposed to UVphotons, experiments show that large PAHs can quickly looseall of their peripheral hydrogens and subsequently isomerize tocages and fullerenes, typically after losing C -units from thecarbon skeleton. It was shown that C can form from C H (Zhen et al., 2014a) in line with the suggestion that the in-creased abundance of C in photodissociation regions can beascribed to complete H-loss followed by C-loss creating pen-tagons and initiating fullerene formation (Chuvilin et al., 2010;Bern´e and Tielens, 2012). The study reported here shows thatfor the studied Bq, pentagon formation and curling of the pla-nar PAH structure can commence before H-stripping starts, trig-gered by the dissociation of both side groups. The size of the in-volved molecule and the position of the side groups, obviously,are relevant parameters in this process. Our result also has somesimilarities with recent work by de Haas et al (de Haas et al.,2017) showing that the loss of a HCN-fragment in nitrogen con-taining PAHs o ff ers a facile pathway towards pentagon forma-tion. We also mention the formation of fulvene-type isomers4 igure 4: Calculated dipole moments for optimized structures of Bq + , [Bq - CO] + , and [Bq - 2CO] + cations. Due to the change of structure, permanent dipolemoments are induced. Arrows on each structure indicate the direction of the total dipole moment, and the corresponding values are given beneath each molecule inunits of Debye. leading to bowl-shaped structures as theoretically predicted by(Trinquier et al., 2017). As a consequence, the relevance ofpentagon formation before H-stripping - as discussed in otherrecent work - may be a more general process taking place inspace, but this will remain to be sorted out in future studies.
5. Conclusion
Experiments and quantum chemical calculations are per-formed for understanding the photodissociation processes ofBq cations. The mass spectrum of Bq cation upon laser irra-diation with three di ff erent wavelengths (626 nm, 312 nm, and208 nm) presents clear evidence for pure neutral CO-losses, i.e.,no other fragments are detected as a first or second dissocia-tion product. The CO-losses are highly favorable for all testedwavelengths. Our quantum chemical calculations reveal that forBq the transition barrier for CO-loss is only ∼
6. Acknowledgments
This work is supported by Swedish Research Council (Con-tract No. 2015-06501). Facility is supported by the SwedishNational Infrastructure for Computing (SNIC). We acknowl-edge the European Union (EU) and Horizon 2020 fundingawarded under the Marie Skłodowska Curie action to the EU-ROPAH consortium, grant number 722346. Studies of inter-stellar PAHs at Leiden Observatory are supported through aSpinoza award.
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