The Unimolecular Dissociation of 1-Methylpyrene Cations: Why Are 1

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The Unimolecular Dissociation of 1-Methylpyrene Cations: Why Are 1Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion? Brandi West, Bethany Lowe, and Paul M. Mayer J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02667 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

The Unimolecular Dissociation of 1-Methylpyrene Cations: Why are 1-Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion? Brandi West, Bethany Lowe 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 1-methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modelling to determine a 0-K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K-1 mol-1 for this H-loss reaction. The ionization energy of 1methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring-strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the ∆fH0o (1-methylenepyrene cation) of 945 ± 31 kJ mol-1.


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Introduction The diffuse interstellar bands (DIBs) have been the subject of intense discussion for the better part of a century, but their chemical origins have remained elusive.1 Only C60+ has successfully been linked to some of the DIBs.2-4 One class of chemicals that have garnered significant attention are polycyclic aromatic hydrocarbons, PAHs.5-9 PAHs, as a group, have been postulated to be sources of DIBs due to the similarity between the IR emissions of the cations of large PAHs, and the DIBs and unidentified IR bands (UIRs).10-11 One molecule that has been investigated specifically for its potential role as a DIB source is 1-methylpyrene. The idea that substituted pyrenes might be likely candidates for some of the DIB bands was first suggested by Salama and Allamandola in 1992.12 They noted that the pyrene cation has an absorption at 439.5 nm when it is isolated in neon ice and the authors started a discussion as to whether this is a coincidence or if pyrene-related species are responsible for the 443-nm DIB feature.12

Léger,

d’Hendecourt and Défourneau conducted a matrix isolation experiment in which 1-methylpyrene was irradiated before and after introducing atomic hydrogen to the system.13 They concluded that the 1methylenepyrene radical was a likely candidate for two DIBs, the feature at 443.0nm previously hypothesized, and 756.5 nm.13 A concurrent computational study conducted by Parisel and Ellinger calculated the transition energies (up to 26,000 cm-1) and oscillator strengths for three isomers of methylpyrene cations (1-, 2-, and 4-methylpyrene), and concluded that the isomer most likely responsible for the DIBs is the 1-methylpyrene cation.14 The C17H11+ fragment ion from the dissociation of ionized 1-methylpyrene can have either a 1methylenepyrene (A) or a tropylium-containing (B) structure as shown in Figure 1. Kokkin et al. conducted photodissociation experiments on 1-methylpyrene cations and its fragmentation product 3 ACS Paragon Plus Environment


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ions.15 From a combination of experimental and computational results, they proposed a tentative mechanism with fast H-atom loss resulting in structure A immediately followed by isomerization into B.15 A more in-depth computational study of the C17H11+ structures was conducted by Rapacioli et al.16 Static and dynamic time-dependent density functional theory calculations (DFT/TD-DFT and dynamic DFTB/TD-DFTB) were performed to generate the IR and UV/VIS spectra for the two isomers, which were found to be quite distinct. The free-energy surfaces for the different isomerization channels were calculated yielding a total of seven isomerization steps in four different reaction pathways for the interconversion between the two structures, and concluded that isomerization between the two isomers would need photoactivation in the interstellar environment due to the size of the energy barriers along the reaction pathways.16 Recently, Pavol et al. published results of infrared multiplephoton dissociation (IRMPD) experiments for the 1-methylpyrene cation.

Precursor ions were

generated using electron impact and the resulting [M - H]+ ion trapped.

The results of these

experiments show strong support for the structure of [M - H]+ to be A as opposed to B.17 In the present work, the unimolecular dissociation of photoionized 1-methylpyrene is investigated using imaging photoelectron photoion coincidence spectroscopy (iPEPICO) and tandem mass spectrometry combined with Rice-Ramsperger-Kassel-Marcus (RRKM) theory modelling. Coupled cluster method (CCSD) calculations were also conducted on the reaction pathways and the resulting energetics were compared to the RRKM results to determine the nature of the C17H11+ ion obtained from internal-energyselected precursor ions.

Experimental 1-methylpyrene (> 97%) was purchased from Sigma-Aldrich (Sigma-Aldrich, Oakville, Canada) and used without further purification.


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iPEPICO iPEPICO experiments were conducted on the VUV beamline at the Swiss Light Source (Paul Scherrer Institut, Villigen, Switzerland) and have been previously described in detail.18-20 Briefly, monochromatic VUV synchrotron radiation (4−8 meV resolution depending on the photon energy) is used as a photoionization source to ionize gaseous 1-methylpyrene introduced via a heated oven inlet system.21 Electrostatic lenses direct the ions toward a time-of-flight (TOF) mass spectrometer, while the ejected electrons are velocity-mapped on an imaging multichannel plate (MCP) detector. The electrons are timeand position-stamped at the detector and the corresponding precursor or fragment photoions are detected in delayed coincidence. Threshold electrons account for the majority of the signal at the center of the MCP, whereas non-zero-kinetic energy, or “hot”, electrons are mostly detected according to their off-axis momentum. Some of the “hot” electrons will, coincidently, have the proper trajectory to hit the center spot, therefore the mass spectrum based on electrons detected in a ring around the center spot is used to account for this contamination. The ring signal can then be subtracted from the center signal to obtain the threshold photoionization mass spectrum. The photon energy range used was 13.65−15.85 eV, with data points taken every 0.05 eV, plus an additional point at 13 eV. Due to the low mass resolution of the TOF analyzer, it was necessary to deconvolve the M+· and (M − H)+ peaks using the multiple-peak fitting protocol included in the IGOR PRO 7 (WaveMetrics, Lake Oswego, OR) software package prior to correcting their abundances for 13C contributions. The mass-selected threshold photoelectron spectrum (TPES) of 1-methylpyrene was acquired from 7.24–7.84 eV to establish the ionization energy (Figure S1). The TPES exhibits a strong, narrow, primary ionization peak which means the vertical and adiabatic ionization energies (IE) are essentially the same at 7.27 ± 0.01 eV. This value was used to convert photon energies to ion internal energies for RRKM modeling. It is consistent with the IEs of other substituted PAHs.22, 23



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Tandem mass spectrometry Atmospheric pressure chemical ionization (APCI) experiments were performed on a Micromass Quattro LC (Waters Micromass, Manchester, U.K.) triple quadrupole mass spectrometer equipped with a Z-spray source. A 1 mg/mL solution of 1-methylpyrene in chlorobenzene24 was introduced into the APCI source with a syringe pump at a flow rate of 25 µL/min. The source block temperature was set to 90 °C while the probe temperature was kept at 400 °C. The corona and cone voltages were held at 4.00 kV and 25 V, respectively. The first and third quadrupole resolution was held constant at 15 (as set in the Masslynx software) to provide baseline separation of all mass spectral peaks. Collision-induced dissociation (CID) was carried out using argon as a collision gas (pressure = 1.0

10-3 mbar) over an energy range (Elab) of

1−38 eV.

Computational Methods Precursor neutrals, ions, transition states, intermediates and possible products were calculated using the Gaussian 09 suite of programs.25 All structures were calculated at the CCSD/6-31G(d)//B3-LYP/631G(d) level of theory (restricted for closed-shell species, unrestricted for open-shell species). The extracted vibrational frequencies and rotational constants were employed in the RRKM modeling of the data (see below). Transition states were confirmed with intrinsic reaction coordinate calculations.

RRKM modelling RRKM26 theory was used to determine the 0-K activation energy (E0) and entropy of activation (Δ‡S1000K) by modeling the experimental breakdown diagrams. The rate constant of each dissociation channel, k(E), is calculated using the formula: (1)


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where σ represents the reaction degeneracy, h is Planck’s constant, N‡ (E − E0) is the sum of the vibrational and rotational states for the transition state up to internal energy (E − E0) and ρ(E) is the density of states for the reactant ion at internal energy (E), as calculated via the Beyer and Swinehart direct count algorithm.27 Because no transition state was found when H loss produces A, the vibrational states of the transition state were approximated using the harmonic vibrational frequencies of the precursor ion, with a methyl group C-H bond stretch frequency removed as a substitute for the reaction coordinate. Of the remaining 3N − 7 (where N = number of atoms) modes for the transition state, the five modes with the lowest frequencies were scaled by a factor to adjust the entropy of activation (factors less than one increase ∆‡S while values greater than one decrease ∆‡S).28 The actual fitting of the experimental breakdown diagram was completed through the use of the minimal-PEPICO program.20 The program combines the physical parameters of the iPEPICO experiment at the Swiss Light Source with temperature (for the initial neutral molecule internal energy distribution) and the RRKM k(E) values for each channel (adjusted through the choice of E0 and ∆‡S) to calculate experimental branching ratios for the ion dissociation as a function of photon energy, which are then compared to the experimental breakdown curves. Photon energies were converted to ion internal energies using the ionization energy. The activation energies and entropies are then optimized to obtain the best fit to experiment. Error bars were established by finding the limits in E0 and ∆‡S that resulted in acceptable fits to the experimental data, which means a value of < 0.2% for equation 2 (2)

where Expt and Calc refer to the experimental and calculated relative ion abundances in the breakdown curves, respectively. For the APCI-CID experiments, the only difference was the inclusion in the model of an internal energy distribution changing with the centre-of-mass collision energy, Ecom. Here, the



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post-collision ions are assigned an effective temperature, Teff, depending on Ecom, and thus a “thermal” vibrational energy distribution, according to the relationship Teff = Ti + αEcom

(3)

where Ti represents the initial temperature and α describes the relationship between Ecom and the increase in the Teff.29 The model’s simplicity is also its weakness, but the results have been found to be in semi-quantitative agreement with those from iPEPICO.30 Since varying ∆‡S and α has a similar effect on the theoretical breakdown curve, they are treated as fitting parameters without quantitative meaning.

Results and Discussion Only one primary dissociation channel was observed in the unimolecular dissociation of ionized 1methylpyrene, the loss of a hydrogen atom: C17H12+● C17H11+ + H● Figure 2 shows the iPEPICO and CID breakdown diagrams with their respective theoretical fits. RRKM modeling of the iPEPICO breakdown curve resulted in E0 = 2.78 ± 0.25 eV and Δ‡S1000K = 6 ± 19 J K-1 mol-1, whereas modeling of the CID data yielded E0 ≈ 2.60 eV. No literature values are available for direct comparison, but the E0 values are similar for reactions from the related ions of toluene (E0 = 2.11 eV and Δ‡S1000K = -11.7 J K-1 mol-1) and methylnaphthalene (E0 = 2.41 ± 0.02 eV and Δ‡S1000K = -13.3 ± 1.5 J K-1 mol1 31

).

The current results are consistent with the size-independent reaction energies found for H loss

from sp3-carbons in PAHs ( = 2.34 eV).30 More significant is the difference in Δ‡S for H loss between ionized 1-methylpyrene and the ions of toluene32 and 1-methylnaphthalene33. For the last two ions, the Δ‡S values are negative, while for ionized 1-methylpyrene the value is possibly positive, suggesting


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isomerization reactions in the first two cases but a simple bond cleavage reaction in the last one, and thus loss of H from the methyl group. The mechanisms for the formation of product ions A and B from ionized 1-methylpyrene were explored at the CCSD/6-31G(d)//B3-LYP/6-31G(d) level of theory. In both cases the singlet ground state lies more than 1 eV below the triplet state, so only the singlet state products are discussed herein. In the case of B, two different reaction pathways were found, differing in how the methyl carbon inserts itself into the ring (toward the center of the molecule (α) or away from the center (β)). Figure 3 shows the reaction pathway energetics, with schematic structures shown for the α path (structures for the β path can be found in Figure S2 of supporting information). The structures calculated for the various intermediates and transition states are all in good agreement with the corresponding structures from ionized toluene.32 The transition state of highest energy in the pathway to B, TS4, lies at ≈ 3.20 eV, while the threshold for dissociation to A is only 2.54 eV, clearly favoring simple H loss from the methyl group. For ionized toluene, the highest energy TS for tropylium ion formation lies slightly below the dissociation threshold for H loss from the methyl group (2.116 eV compared to 2.177 eV).32 The reason for this reversal in relative energies for the two precursors is evident when the geometries for TS4 are compared (Figure 4). In the case of ionized toluene, TS4 has equivalent bond angles consistent with a planar seven-membered ring (128.7°). The presence of the three additional rings in 1-methylpyrene prevents the ring from expanding, with the most constrained angle being ∠123 with a value of 125.0° (Figure 4). The presence of geometric constraints prevents the ion from accessing routes of molecular relaxation which would lower the transition state energy, similar to what was observed previously in the case of the dissociation of ionized cyclopenta[d,e,f]phenanthrene.30 The barrier for any subsequent isomerization of A to B prior to further reaction is of the order of 3.5−4 eV,16 precluding this process from occurring in the present experiment.



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The iPEPICO results can be used to derive new thermochemistry. The standard enthalpy of formation, ∆fH0o, of 1-methylpyrene has not been measured, but can be estimated from Benson Group Additivity34 to be 194 ± 6 kJ mol-1 (corresponding to an effect on the ∆fH0o of pyrene due to the substitution of a H atom by a methyl group of -34 kJ mol-1, the same as that observed for naphthalene).35-36 Combined with the measured IE of 7.27 ± 0.01 eV (701 ± 1 kJ mol-1) this yields the ∆fH0o of 1-methylpyrene cations of 895 ± 7 kJ mol-1. The measured E0 of 2.78 ± 0.25 eV (268 ± 24 kJ mol-1) combined with the ∆fH0o of the H atom (218 kJ mol-1)36 and that of the 1-methylpyrene cation results in ∆fH0o (A) of 945 ± 31 kJ mol-1, which technically is an upper limit due to the possibility of a reverse energy barrier in the H loss reaction, but the possibly positive Δ‡S suggests this is unlikely to be significant.

Conclusion The dissociation of 1-methylpyrene cations was studied with iPEPICO spectroscopy and CID mass spectrometry. Both experiments were modeled with RRKM theory to derive the minimum reaction energies for the loss of an H atom. The energetics derived from this modelling (E0 = 2.78 ± 0.25 eV and Δ‡S = 6 ± 19 J K-1 mol-1from iPEPICO and E0 ≈ 2.60 eV from CID are the first experimental values reported for the dehydrogenation of ionized 1-methylpyrene, a molecule of interest in astrochemistry due to its strong spectral resemblance to some of the DIB bands. These energetics, combined with computationally-determined reaction coordinates allowed us to determine that the dehydrogenation product of 1-methylpyrene cations is the 1-methylenepyrene cation, A, in agreement with the literature. The tropylium-motif-containing fragment ion B requires significantly more energy to be formed due to steric hindrance in key transition states caused by the constraints imposed by the three benzene rings. The RRKM results allows us to derive a ∆fHo (A) of 945 ± 31 kJ mol-1.


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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 Dr. Andras Bödi.

Supporting Information Supporting Information Available. The complete citation for reference 25. Figure S1 is the TPES for 1methylpyrene. Gaussian 09 archive entries for all species calculated in this study.

Figure S2 is the

calculated reaction pathway for the formation of product B, showing the b structures. Figure S3 is the comparison of the reaction pathway for the formation of B between the B3LYP/6-31G(d) and the CCSD/6-31G(d) calculations. Table S1 is an excel spreadsheet containing the calculated energies, vibrational

frequencies

and

k(E)

values

for

the

processes

modelled.



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Figure Captions Figure 1: Structure of the 1-methylpyrene cation (C17H12+·) with those of the two most likely C17H11+ products A (1-methylene pyrene cation) and B (tropylium-containing cation).

Figure 2: iPEPICO (left) and CID (right) experimental breakdown diagrams of the 1-methylpyrene cation. The solid curves are the result of RRKM modelling.

Figure 3: Calculated reaction pathways for the dissociation of ionized 1-methylpyrene at the CCSD/631G(d)//B3-LYP/6-31G(d) level of theory. Solid lines correspond to the α intermediate (structures shown) and dashed lines correspond to the β intermediate.

Figure 4: B3-LYP/6-31G(d) calculated structures for transition state TS4 for formation of the tropyliummotif containing ion from ionized 1-methylpyrene (left) and ionized toluene (right, originally calculated at HF/6-31G(d,p) in reference 37 and re-optimized herein).


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Figure 1



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Figure 4


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The Unimolecular Dissociation of 1-Methylpyrene Cations: Why Are 1

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