Identification of the fragment of the 1-methylpyrene cation by mid-IR spectroscopy
Pavol Jusko Aude Simon Gabi Wenzel Sandra Brünken Stephan Schlemmer Christine Joblin
physics.chem-ph






airap,auzk]Pavol Jusko
alcpq]Aude Simon
airap]Gabi Wenzel
anj,auzk]Sandra Brünken
auzk]Stephan Schlemmer
airap]Christine Joblincor1

[cor1]christine.joblin@irap.omp.eu

[airap]Institut de Recherche en Astrophysique et Planétologie (IRAP), Université
de Toulouse (UPS), CNRS, CNES,
9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
[auzk]I. Physikalisches Institut, Universität zu Köln,
Zülpicher Str. 77, 50937 Köln, Germany
[alcpq]Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC,
Université de Toulouse (UPS) and CNRS, 118 Route de Narbonne, 31062 Toulouse, France
[anj]Radboud University, Institute for Molecules and Materials, FELIX Laboratory,
Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands



The fragment of the 1-methylpyrene cation, , is expected to exist in two isomeric forms,
1-pyrenemethylium  and the tropylium containing species . We measured 
the infrared (IR) action spectrum of cold  tagged with Ne using
a cryogenic ion trap instrument coupled to the FELIX laser. Comparison of the
experimental data with density functional theory calculations allows us to identify the
 isomer in our experiments. The IR Multi-Photon Dissociation spectrum was also recorded
following the  loss channel.
Its analysis suggests combined 
effects of anharmonicity and isomerisation while heating the trapped ions, as shown by 
molecular dynamics simulations.












PAH 22 pole cryogenic ion trap Ne tagging spectroscopy molecular 
dynamics simulations
















§ INTRODUCTION

The photoproduct of the 1-methylpyrene cation, , has been pointed
out in matrix-isolation studies as a candidate of interest to account for some of the diffuse
interstellar bands including the strongest one at 4428 Å <cit.>. This has
motivated experimental studies using the ion trap setup PIRENEA in which 
CH_3-C_16H_9^+ ions were trapped, irradiated by UV-VIS photons and its 
photofragment,  ,  could be isolated. First multi-photon dissociation (MPD) spectra 
were obtained in order to reveal the structure of these ions <cit.>.
Rapacioli et al. performed extensive calculations on the possible isomers 
and isomerisation pathways <cit.>. These studies have shown that both the
1-pyrenemethylium isomer, , and the isomer containing a tropylium cycle,  (see
Figure <ref>), could be present under the experimental conditions used in
<cit.>. Both isomers were found to be quasi degenerate (cf. Table 3 in <cit.>).
In addition, theoretical calculations showed several isomerisation pathways 
with barrier heights in the range of 3.5-4.0 eV <cit.>. This, together with 
the fact that both isomers coexist at formation, could explain why the recorded bands in
MPD measurements were so broad.  

Infrared (IR) spectroscopy is an obvious technique to assign
absorption bands to the  and  species and thus to disentangle the
nature of the 1-methylpyrene cation photoproduct.
Similarly to the case of the electronic transitions described
above, absorption of multiple photons at IR wavelengths can be used to probe IR transitions
of stored ions.
This technique is well-known as IRMPD spectroscopy and has been successfully applied to 
different types of molecules and molecular complexes including polycyclic aromatic 
hydrocarbons (PAHs), which are of interest for this study <cit.>. 
A drawback of the technique is that it implies the heating of the trapped ions by successive
IR photon absorption. As a result, anharmonic effects, which lead to modifications of the IR
spectrum with internal temperature, can therefore induce strong deviations of the recorded 
spectra compared to linear IR absorption spectra <cit.>.
In a case like the  and  species, an additional
complication can arise from isomerisation processes induced by the heating.

The recent development of a cryogenic trap instrument <cit.> at the 
FELIX[Free Electron Lasers for Infrared eXperiments, <www.ru.nl/felix>] 

Laboratory <cit.> opens new perspectives for this study.  Indeed, it allows to use
a rare gas tagging often referred to as messenger technique which provides information on 
the IR spectrum of the cold ion–rare gas complex upon the absorption of a single photon.
This type of spectra was earlier reported in molecular beam experiments, in particular on
jet-cooled complexes of cationic naphthalene-Ar, phenanthrene-Ar and
phenanthrene-Ne <cit.>. The possibility to build and store such
complexes in a cryogenic ion trap opens new perspectives <cit.>, 
especially to isolate and study fragments of the parent species.

In this article, we present the first IR spectrum of the Ne tagged  species
together with the IRMPD spectrum of the bare cation. The analysis of the experimental 
spectra involves both static Density Functional Theory (DFT) calculations to interpret 
the cold IR spectrum and molecular dynamics simulations to gain insights into the hot 
IRMPD spectrum.





§ METHODS



 §.§ Experimental


The experimental spectrum has been recorded on the FELion beamline at the
FELIX <cit.> Laboratory at the Radboud University in The Netherlands.
The FELion instrument at the beamline <cit.> consists of a cryogenic 22-pole 
rf ion trap equipped with two quadrupole mass filters allowing to mass select and analyse
the ions to be studied, as well as the products created in the trap. 

The ions of interest were created in a storage ion source <cit.> via
electron bombardment of the 1-methylpyrene neutral gas evaporated directly
into the UHV chamber from a solid 1-methylpyrene sample (Sigma-Aldrich).
The sample was heated to a temperature of 40^∘ C in order to achieve
a sufficient vapour pressure. Several thousands ions of mass-selected  cations
(m/z=215, cf. Figure S1 in the Supplementary Information (SI) for the different experimental 
conditions used in this work) were stored in the trap for 
each experimental cycle, while
one of the following two spectroscopic techniques was applied:


  
IR Pre-Dissociation (IR-PD) of the weakly bound complex with Ne The trap was
  	operated at a nominal temperature
    
    of 8-9 K. Upon ion injection
    high quantities of  a Ne:He 1:2 gas mixture were introduced in order to promote ternary
    attachment of Ne atoms to  cations with a yield of only ≲ 1 %.
    The depletion (single photon process) of the ·Ne complex was then 
    recorded as a function of wavelength.


  
IR Multi-Photon Dissociation (IRMPD) of the primary ion  These experiments 
    were performed at a trap temperature of about 200 K. He gas was introduced 
    only to aid ion thermalisation and was removed prior to irradiation. The FELIX laser 
    beam was focussed into the trap region to facilitate multi-photon absorption processes. 
    The typical irradiation time to record the spectrum is 3 s and the major 
    fragment corresponds 
    to the loss  of acetylene,  (cf. Table S1 of SI). The number of produced fragment 
    ions at m/z=189 is therefore recorded as a function of wavelength.

The FELIX laser used for the experiment delivers up to 30 mJ into the 22 
pole trap in a single macropulse (about 5 μs long), at a 10 Hz 
repetition rate. The macropulse is composed of micropulses with energy in the 
10 μJ range and with a typical length in the picosecond range. 
A spectral bandwidth of approximately σ=0.5% was used.







 §.§ Computational

Two computational approaches were used in order to i) predict the IR harmonic spectra and ii)
gain insight into the dissociation dynamics during the IRMPD process.


  
Static DFT calculations These were carried out in order to obtain the theoretical IR 
harmonic spectra of  the two isomers of . In a recent computational study of the
isomerisation reactions of  isomers, Kharnaior et al. <cit.> have
concluded that the B3LYP exchange-correlation functional <cit.>
provides good results for molecules of similar size and structure.
We therefore optimised the geometries of the two isomers of  using the B3LYP hybrid 
functional in conjunction with a 6-31G(d,p) basis set <cit.>. 
These were found to be quasi degenerate at this level, with  being more stable than 
 by 0.5 kJ.mol^-1 when including zero-point energy corrections.
The harmonic spectra were obtained by full 
diagonalization of the weighted Hessian matrix.
The calculations were achieved with the Gaussian09 suite of programs <cit.>. 


  
Dissociation dynamics PAHs with high enough internal energy may undergo many possible 
isomerisation reactions <cit.>, among them H migrations <cit.>, external 
ring openings and formation of 5-membered rings <cit.>. This leads to many different 
possible dissociation paths, and an exhaustive study of reactions mechanisms with DFT becomes 
very fastidious. A first insight into reaction kinetics, branching ratios and mechanisms can 
be obtained from molecular dynamics (MD) simulations in which the electronic structure is 
described on-the-fly at the Self-Consistent-Charge Density Functional based Tight Binding 
(SCC-DFTB) level of theory <cit.> (MD/SCC-DFTB), as shown in our previous work 
<cit.>.  To get insight  into isomerisation and dissociation mechanisms of the 
two isomers of , we ran extensive simulations following the procedure detailed in 
Section 2 of <cit.>. We also performed calculations on the regular pyrene 
cation as a reference system.
For each molecular ion of interest, 720 simulations of 500 ps 
(δt=0.1 fs)  were achieved with random initial velocities in the 
(NVE) ensemble, at 19 eV (lowest energy 
necessary to observe a dissociation event) and 24  eV (to obtain significant branching ratios).



The drawback of such simulations is their time limitation (less than 1 ns for a system of 
∼30 atoms), making the use of high internal energy mandatory to observe a sufficient
number of events. The calculations were achieved with the deMonNano package <cit.>.







§ RESULTS AND DISCUSSION




 §.§ Infrared spectra


Figure <ref> shows the two experimental spectra that were recorded, the IR-PD spectrum
in the top panel and the IRMPD spectrum in the bottom panel. The two experimental spectra
are rather different. The IR-PD spectrum exhibits several weak bands, whereas the IRMPD
spectrum depicts 3 strong bands, which fall close to some of the bands in the IR-PD spectrum. 
Further insight into the band assignment comes from the comparison with the computed DFT harmonic
IR absorption spectra  of the two isomers,   and ,
which are reported in the middle panel. 






Both isomers were
found to have an ^1A' ground-state.  All band frequencies were multiplied by a 
unique scaling factor of 0.975 obtained by adjusting the position of the  most intense band
in the calculated spectrum of  at 1656  to the  band located at 1615  in 
the IR-PD spectrum. This factor is in accordance with those used previously for similar 
systems <cit.>. 


All IR bands in the 1100-1670 region involve C-C stretching and in-plane C-H bending modes 
(a' symmetry; Figures S2 and S3  in the SI for more information on the assignment of 
vibrational bands). They differ significantly between the two isomers.



Although the obtained IR-PD spectrum is rather noisy due to the difficulty of efficiently tagging
the  cation with Ne,

the comparison with the DFT spectra allows us to identify  in
these experiments. 

This assignment is based on at least 5 bands as listed in Table <ref>. There is no evidence
for characteristic signatures of , but the low signal-to-noise ratio of the spectrum combined
with the expected lower band intensities of  compared to  (cf. Table <ref>) 
are not in favour of detecting this ion.
In the case of the IRMPD spectrum, we also observe three bands that could be attributed to .
This conclusion is however challenged by the fact that the band measured at 1615  in the PD 
experiments, which is the strongest in the theoretical spectrum of , has no counterpart in
the IRMPD spectrum. The band at about 1530  is also missing.
On the other hand, none of the predicted bands of  are observed in our IRMPD spectrum, 
which raises the question of the species that are traced by this latter spectrum.





It is interesting to compare our results with previous IRMPD spectra recorded on substituted 
benzylium (Bz^+) and tropylium (Tr^+) cations. Zins et al. <cit.> compared the IRMPD 
spectrum of the tert-butyl derivative
of with B3LYP calculations using the same scaling factor of 0.975 as in our work.
They concluded that only Bz^+ can be observed. The characteristic band of the Tr^+ 
isomer at 1506  was not observed but the authors argued that this is a sensitivity matter, 
considering that the computed absorption intensity of Tr^+ is a factor of ∼10 weaker 
than that of the strongest band of Bz^+. The IRMPD spectra of methyl-substituted species were
recorded by Chiavarino et al. <cit.>, showing that the major band of Bz^+ 
and Tr^+ species falls at 1600 and 1480 , respectively. 
Finally, Morsa et al. <cit.> were able to produce preferentially the derivatives of 
Bz^+ and Tr^+ isomers of the C_11H_15^+ 
ion by tuning the activation regime of the precursor ion. The major 
bands of Bz^+ and Tr^+ derivatives were recorded in the IRMPD spectra at 1615 and 1425,
respectively. All the above studies show that Bz^+ has a characteristic IR band at 
∼ 1600, whereas a band is expected at ∼ 1500 for Tr^+ species. Although our
species are significantly different from these previously studied systems, their calculated IR 
spectra also carry these characteristic features, which are neither present in the spectrum of  the 
regular neutral pyrene nor cationic pyrene (cf. Figure S5 in the SI). This can be 
understood if one considers
the change in charge distribution from C_16H_10^+ to 
  and   (see Mulliken 
charge distribution on Figure S4 in the SI) that leads to a dipole moment of 1.29 D for both 
isomers, in the XY plane following the orientation reported in Figure <ref> (more specifically
along the Y axis for ). This  is expected to lead to an enhancement of the IR intensity 
for the modes whose projection in the direction of the dipole moment is significant,  which is
the case for some δ _CH + ν _CC modes (in plane C-H bend and C-C stretch) such as
those  reported for instance  on Figure S2 in the SI for  and on Figure S3 (e)-(f) in 
the SI for   . 










 §.§ Dissociation dynamics


In order to shed more light on the measured IRMPD spectrum, we report in the following further 
insights into the dissociation dynamics of  ions. Dissociation rates, branching ratios and 

structures of the products and reaction intermediates were obtained from 
MD/SCC-DFTB simulations following the procedure detailed in Section 2.2 and starting from the 
two isomers  and . The branching ratios and dissociation rates are determined at 
24 and 19 eV (cf. Figure <ref>). The major fragment for both isomers corresponds to 
the loss of , H loss being second, but much lower. At both calculated energies, the 
dissociation of  is found to be slightly more efficient than that of .
We compared these results with calculations on the regular pyrene cation (cf. Figure S6 in 
the SI). For this ion, we know from comparison with photo-absorption experiments 
<cit.> that our calculations overestimate the contribution of the  channel and 
this challenging issue is currently under investigation. Still the /H fragmentation 
branching ratio is found to be larger for both isomers of  than for cationic pyrene. 
This indicates that  is a major fragmentation channel for  ions, which is also 
shown by our experimental results (cf. Table S1 in the SI) and a previous study <cit.>.



From our simulations at 19 eV, we could get insights into the product structures and 
possible dissociation mechanisms. Some snapshots of simulations that we estimated to be 
representative are reported in Figure <ref>  and in Figure S7 in the SI for 
both  and  . We found that a common structure for the dissociation product 
is the presence of a five-membered ring as earlier suggested <cit.>.
The other noticeable product has undergone ring opening to form an acetylene terminal group.
This terminal structure was also observed as the final product in several simulations 
for both isomers. It is presented in the case of  in Figure S7 (a) in the SI.

All paths were found to involve H migrations, leading to the formation of quite long time 
scale intermediates. H migration also leads to the formation of sp^3 carbon atoms and 
weakening of the CC bond. In the case of , one can notice the formation of a 
terminal -CH-CH_2 vinylidene function, which often appears as a long time intermediate 
in the simulations (cf. Figure <ref> (a)). On the other example reported in 
Figure S7 (a) in the SI, we see that isomerisation into the tropylium-like isomer occurs 
following a mechanism invoked in Rapacioli et al. <cit.> (out of plane 
deformation of the PAH, CH_2 insertion and H migration). In the case of , one can 
observe the direct formation of a 5-membered ring (through a [5+4] intermediate, see 
first structure at the bottom of Figure <ref> (b)) from the 7-membered cycle, 
followed by  loss. In the other selected path in Figure S7 (b) in the SI, one can 
see the variety of possible isomerisation reactions involving H migration, external ring 
opening and formation of acetylene terminal groups and five-membered rings. Interestingly, 
only the tropylium cycle in and the  C_6-CH_2  part of are altered, 
showing their enhanced fragility with respect to the rest of the carbon skeleton.




Overall, the results of these simulations can help us to rationalise the IRMPD spectrum. 
The simulations suggest a lower but comparable internal energy to dissociate  and 
 as well as the pyrene cation. The dissociation threshold for the pyrene cation has
been observed to be around 9 eV in other ion trap experiments using synchrotron 
radiation <cit.>, corresponding to a micro-canonical temperature of 2200 K. We 
can therefore use this value as a first estimate of the internal energy required to 
dissociate . With this amount of internal energy, the system will be able to overcome 
the  to  isomerisation barrier of 3.5-4.0 eV <cit.>. Our 
simulations also suggest that the heated molecular ion can experience many isomeric forms 
on its way to dissociation involving not only the two lowest-energy isomers but also 
higher-energy ones. 
For IRMPD to occur, the irradiated ion should absorb continuously the IR laser irradiation 
to reach high enough internal energy/ temperature to dissociate. Isomerisation will affect 
the IR absorption spectrum. Figure S8 of the SI aims at illustrating this effect by mixing 
the harmonic spectrum of  with those of the 3 isomers involved in the calculated 
dissociation path presented in Figure <ref>, panel (a). Although this spectrum 
cannot be compared directly to the IRMPD spectrum, it shows the spectral ranges with higher 
global absorption cross-section for the mixture of isomers. These spectral ranges are likely 
to have higher heating efficiency upon laser irradiation and therefore a higher IRMPD signal. 
Although this constructed global spectrum shows 3 comparably intense bands, 
it still does not explain the missing 1615  band in the experimental IRMPD spectrum.
Anharmonicity is another effect that can affect the IR spectrum.
Rapacioli et al. <cit.> calculated the IR spectra of  
 and  as a function of temperature up to 2000 K. The spectra exhibit significant 
band shifts and broadening with temperature. Hence, the recorded IRMPD spectrum is expected 
to reflect the net efficiency of heating the ion while the absorption IR spectrum is 
modified by both isomerisation and anharmonic effects, making its interpretation very tricky.
The impact of isomerisation during the IRMPD process has been previously discussed in the 
case of C_7H_9^+ <cit.>. We note that this issue has not been reported in the 
IRMPD studies on substituted benzylium (Bz^+) and tropylium (Tr^+) cations that are 
described in the previous section. The reason is that, for these systems, the involved 
dissociation energies were much lower making complex isomerisation pathways less competitive.



















§ CONCLUSION


In summary, this work demonstrates the benefit of the combination of a widely tunable
IR laser source (FELIX) with cold ions prepared in a 22-pole ion trap in probing the 
role of isomerisation in the photophysics of PAHs. 
In the specific case of , we showed that the IRMPD spectrum is difficult to analyse 
and can only bring limited information.
This spectrum lacks the characteristic band around 1600, which 
has been observed in previous IRMPD spectra of benzylium-type species. Possible reasons 
include anharmonic effects and isomerization while heating the trapped ions, as suggested 
by molecular dynamics simulations. Further experimental work either on the anharmonic 
spectra or on the dissociation dynamics of  would help to solve this issue.
On the opposite, a clear relation between the one photon absorption 
bands of the IR-PD experiment and DFT predictions unravels the presence of the  
 isomer in our experimental conditions. The  isomer was not observed 
due to the limited signal-to-noise ratio. Thus, the presence of this isomer at 
formation cannot be excluded.















§ ACKNOWLEDGMENTS


We greatly appreciate the experimental support provided by the FELIX team.
The research leading to this result is supported by the European Research 
Council under the European Union's Seventh Framework Programme ERC-2013-SyG, 
Grant Agreement n. 610256 NANOCOSMOS. We acknowledge support from the project
CALIPSOplus under the Grant Agreement 730872 from the EU Framework
Programme for Research and Innovation HORIZON 2020. S. B. and St. S. also 
acknowledge support from DFG SPP 1573 grant BR 4287/1-2.
P. J. and the operation of the 22-pole ion trap were partially funded by the 
DFG via the Gerätezentrum “Cologne Center for Terahertz Spectroscopy” 
(DFG SCHL 341/15-1). G. W. is supported by the H2020-MSCA-ITN-2016 Program 
(EUROPAH project, G. A. 722346).
A. S. would like to thank the computing facility CALMIP for generous 
allocation of computer resources. Finally, we thank the anonymous referees 
for helping improving this manuscript, in particular the analysis of the 
IRMPD spectrum.



§ SUPPLEMENTARY MATERIAL


Supplementray material can be found online at: <https://doi.org/10.1016/j.cplett.2018.03.028>.



§ REFERENCES




