Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose

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Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose CH? Brandi J. West, Lukas Lesniak, and Paul M. Mayer J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01879 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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The Journal of Physical Chemistry

Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose C2H2? Brandi J. West, Lukas Lesniak and Paul M. Mayer* Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Canada K1N 6N5

*Corresponding author Paul M Mayer, [email protected] 1-613-562-5800 ext 6038

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Abstract The reaction mechanisms for the loss of C2H2 from the ions of anthracene, phenanthrene, tetracene and pyrene were calculated at the B3-LYP/6-311++G(2d,p) level of theory and compared to that previously published for ionized naphthalene. A common pathway emerged involving the isomerization of the molecular ions to azulene-containing analogues, followed by the contraction of the seven-member ring into a five- and four-member fused ring system, leading to the cleavage of C2H2. The key transition state was found to be for this last process, and its relative energy was consistent going from naphthalene to tetracene. That for pyrene, though, was significantly higher due to the inability of the azulene moiety to achieve a stable conformation because of the presence of the three fused rings. Thus, C2H2 loss is discriminated against in pericondensed PAHs. For catacondensed PAHs, C2H2 loss also drops in relative abundance as the PAH gets larger due to the increase in the number of available hydrogen atoms, increasing the rate constant for H atom loss over that for C2H2 loss as PAH size increases. The unimolecular reactions of four cyano-substituted polycyclic aromatic hydrocarbon (PAH) ions were also probed as a function of collision energy by collision-induced dissociation tandem mass spectrometry. As the size of the ring-system increases HCN loss decrease in importance relative to other processes (H and C2H2 loss). 9-cyanophenanthrene ions were chosen for further exploration by theory and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The calculated reaction pathway and energetics for C2H2 loss were consistent with those found above. The calculations suggest that larger PAHs of interest in the interstellar environment will behave independently of a CN substituent.


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Introduction Since the development of the PAH hypothesis in 1986, it has generally been accepted that polycyclic aromatic hydrocarbons (PAHs), both neutral and their cations, are important players in the identities of the unidentified IR bands (UIRs) and the diffuse interstellar bands (DIBs).1-5 Photo-processing of these ions includes their dissociation, and thus the unimolecular reactions of ionized PAHs have been extensively explored by mass spectrometry, both by collisional activation6-22 and photo-initiated processes.18,20-41 A common unimolecular reaction for smaller PAH ions like naphthalene and anthracene is the loss of C2H2 and other hydrocarbon fragments. However, an observed trend in all of these experiments is the decreasing amount of hydrocarbon loss relative to hydrogen (atomic and molecular) loss as the size of the PAH ring system increases.18 To understand this trend, we have computationally explored the unimolecular pathway to C2H2 loss in a series of PAH ions ranging from naphthalene42 to anthracene, phenanthrene, tetracene and pyrene. Included also are cyano-substituted PAHs in which the CN group does not play a dominate role in the ion chemistry. Johnsson et al43 have previously published a calculated pathway for the loss of C2H2 from anthracene that suggests the ion first isomerizes to ionized phenanthrene prior to the formation of the biphenylene product ion. Based on our previous work on the dissociation of ionized naphthalene,42 we found an alternative route that appears also to be common to anthracene, phenanthrene, tetracene, pyrene and cyanophenanthrene that can rationalize the observed trend in reactivity. The loss of C2H2 from ionized naphthalene was shown to yield predominantly the pentalene ion containing two fused 5member rings.42 IRMPD studies by Oomens and co-workers44,45 suggest this is a fairly general reaction based on the facile nature of forming such species in the dissociation of substituted PAH ions, and so we extended the calculated pathway of ionized naphthalene to the other systems listed above to determine if this route explained the observed trends in reactivity.



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Computational Methods Precursor neutrals, ions, transition states, intermediates and possible products were calculated using the Gaussian 16 suite of programs.46 All structures were calculated at the B3-LYP/6-311++G(2d,p) level of theory (restricted for closed-shell species, unrestricted for open-shell species). The extracted vibrational frequencies and rotational constants were employed in the kinetic modeling of the imaging photoelectron photoion coincidence (iPEPICO) data for cyanophenanthrene (see supporting information). Transition states were confirmed with intrinsic reaction coordinate calculations.

Results and Discussion In earlier work at the B3-LYP/6-311+G(3df,2p)//B3-LYP/6-31G(d) level of theory, it was shown that the kinetically most favorable route to the loss of C2H2 from ionized naphthalene was via the 7- and 5member-ring-containing azulene ion, which then undergoes a contraction of the 7-member ring to form a 4- and 5-member ring pair, ultimately leading to the production of pentalene ions,42 which was subsequently confirmed by Oomans by IRMPD spectroscopy.44 The reaction pathway is reproduced in Figure 1. The highest energy transition state (4.73 eV) in this pathway is the final one, which involves the cleavage of the cyclobutadiene ring to make the final product. This pathway was generalized to anthracene, phenanthrene, tetracene and pyrene ions at the B3-LYP/6-311++G(2d,p) level of theory, and these pathways are superimposed on the above naphthalene ion path in Figure 1 (structures for all ions and transition states are shown in Figure 2). All of the mechanistic aspects are conserved across this range of PAH ions, including the participation of an azulene-analogue. For phenanthrene and anthracene, the mechanism converges at structure c, the azulene-analogue. Structure c can either return to phenanthrene (but not anthracene due to the high energy of TS(bc)) or continue on to dissociate. The highest energy 4 ACS Paragon Plus Environment

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transition state in each case is that for the final step, cleavage of the C2H2 group from the cyclobutadiene ring. The energies calculated for these transition states in the cases of naphthalene, anthracene, tetracene and phenanthrene ions are consistent with the experimentally-derived activation energies for C2H2 loss derived from iPEPICO data18,20,21,47 and time-resolved photodissociation experiments,37,38,40 approximately 4.5 eV. However, in the case of tetracene, it is evident that the structures containing a four-member ring after intermediate c are starting to require higher energies. For pyrene, the reaction pathway is shifted even higher in relative energy starting at c, pushing the final transition state to almost 6 eV. This effectively prevents C2H2 loss from competing with hydrogen loss in this ion. This increase in relative energy is the result of the difficulty in forming the four-member ring as the remaining fused-rings of the pyrene ion prevent it from adopting the most favorable set of bond angles (as was observed in the dissociation of methylpyrene ions),48 and the collapse of this ring to a 5- and 4-member fused ring pair yields a strained system.

This is in contrast to the pathway calculated by Johnsson et al.,43 which lies significantly higher in energy, but which could come into play at higher internal energies. They calculated a path for ionized anthracene to form ethynylnaphthalene via a ring-opening reaction of a terminal benzene ring (maximum energy transition state of 4.99 eV) and one for the formation of biphenylene ions via isomerization to a phenanthrene skeleton (maximum transition state of 5.64 eV). Our mechanism also includes the possibility of anthracene isomerizing to phenanthrene (see above). Also calculated was a pathway to form ionized acenaphthalene from phenanthrene, with a maximum barrier of 4.95 eV (relative to ionized phenanthrene).

Catacondensed PAH ions also exhibit a decrease in hydrocarbon loss with increasing ring size.18,47 In this case it is not a structural feature that discriminates against C2H2 loss, but rather a kinetic one. The 5 ACS Paragon Plus Environment


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increasing availability of near equivalent hydrogen atoms in larger PAH ions increase  in the RRKM rate equation (see equation s1, supporting information), leading to higher rate constants relative to C2H2 loss. Figure 3 illustrates this comparing the k(E) vs E curves for reactions with the same vibrational frequencies and E0 values, but differing  values.

When it comes to substituted PAH ions, the substituent often drives the unimolecular chemistry (as was observed previously for 1-methylpyrene and 1-nitropyrene).48,49 However, cyano-substituted PAH ions were found to largely undergo the same unimolecular reactions as unsubstituted PAH ions. Figure 4 contains the collision-induced dissociation (CID) breakdown curves for the four molecular ions under study (1- and 2-cyanonaphthalene, 9-cyanoanthracene and 9-cyanophenanthrene, see supporting information for details of the CID methodology). As the carbon skeleton gets larger, there is a shift in the dominant fragmentation pathway away from 27 Da (HCN) loss towards H and C2H2 loss, the main processes observed for PAH ions. Ionized benzonitrile only loses HCN,50 so clearly there is a trend towards the ions behaving more like PAH ions as the ring-system expands. The dissociation onset centre-of-mass collision energy (Ecom) values for the three fragmentation channels were extracted from the breakdown curves to serve as a qualitative comparison for the reaction energies. The measured onsets for C2H2 loss are approximately 4 eV in all four cases, while those for HCN loss increase from 2.5-3 eV for the cyanonaphthalenes to 3.5-4 eV for cyanoanthracene and cyanophenanthrene ions. The C2H2 loss values are consistent with previous work that showed the E0’s for this channel to be independent of PAH ring size.18 The iPEPICO breakdown graph (see supporting information for details of the iPEPICO experiment and modeling methodologies) for ionized 9-cyanophenanthrene is shown in Figure 5. Careful calibration revealed loss of H, C2H2 and HCN. Acetylene loss was confirmed by the modelled energetics; the RRKM E0 for this channel, 4.2 ± 0.1 eV, lies some 1 eV below the threshold calculated (5.2 eV, CCSD/6-31G(d)//B3LYP/6-31G(d)) for CN loss forming the lower energy triplet state [phenanthrene-H]+ ion, and within 0.1 eV 6 ACS Paragon Plus Environment

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of the calculated maximum transition state energy of 4.3 eV, Figure 1. The C2H2 loss pathway was explored in the case of 9-cyanophenanthrene (also shown in Figure 1) and was found to follow the same reaction coordinate as the other ions, meaning the CN group does not exert an influence on the reaction mechanism. Given their identification in interstellar environments,51 the results suggest that large CNsubstituted PAHs will not be significant sources of CN-containing neutrals. The results may also suggest that the C2H2 loss pathway for PAH ions with other, non-reactive, substituents may also follow suit.

Conclusion The mechanism for C2H2 loss from ionized PAH ions was explored across the series naphthalene, anthracene, phenanthrene, tetracene and pyrene. A pathway involving an azulene-containing analogue and the formation of a double five-member-ring product ion was found for all of these ions. The three fused-rings in pyrene raised the energy of the latter portion of the mechanisms in which the 7-member ring collapses to a 5- and 4-member fused ring system prior to cleavage of a C2H2 moiety, effectively discriminating against this channel relative to hydrogen atom loss. Cyano-substitution, which produces HCN loss in aniline, was found to have a waning influence in the ion chemistry in larger PAHs, to the point where HCN loss is barely observed in 9-cyanophenanthrene ions.

Acknowledgements P.M.M. thanks the Natural Sciences and Engineering Research Council of Canada for continued financial support. The iPEPICO experiments were carried out at the VUV beamline of the Swiss Light Source at the Paul Scherrer Institute with the support of Drs. Andras Bödi and Patrick Hemberger.



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Supporting Information Supporting Information Available. Experimental and RRKM modeling procedures are presented. The complete citation for reference 46. Figure S1 is the TPES for 9-cyanophenanthrene.


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Figure Captions Fig. 1. Calculated reaction pathways for C2H2 loss from naphthalene,42 anthracene, phenanthrene, tetracene, pyrene and 9-cyanophenanthrene ions, obtained at the B3-LYP/6-311++G(2d,p) level of theory. Representative structures shown for the phenanthrene ion pathway, plus the key intermediate in the pyrene ion pathway (see text). Fig. 2. Optimized structures for the species calculated in Fig 1. Fig. 3. Illustrative RRKM k(E) vs E curves for reactions with the same vibrational frequencies and E0 values, but differing  values. Fig. 4. Collision-induced dissociation breakdown curves for ionized a) 1-cyanonaphthalene, b) 2cyanonaphthalene, c) 9-cyanoanthracene and d) 9-cyanophenanthrene. See supporting information for experimental procedures. Fig. 5. iPEPICO breakdown curves for 9-cyanophenanthrene ions. Solid lines are the result of RRKM modeling (see supporting information for details).


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Fig 1.



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Fig 2


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Fig 3



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Fig 4.


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100

[M]

+.

[M -H]

+

[M -C2H2]

80

Relative Abundance (%)

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[M -HCN]

+.

+.

60

40

20

0 16.5

17.0

17.5

18.0

Photon Energy (eV)

Fig.5



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Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose

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