IR photofragmentation of the Phenyl Cation: Spectroscopy and Fragmentation Pathways
S. D. Wiersma, A. Candian, J. M. Bakker, G. Berden, J. R. Eyler, J. Oomens, A. G. G. M. Tielens, A. Petrignani
IIR photofragmentation of the Phenyl Cation: Spec-troscopy and Fragmentation Pathways † Sandra D. Wiersma, a , b , e ‡ Alessandra Candian, a ‡ Joost M. Bakker, b Giel Berden, b John R.Eyler, d Jos Oomens, b , Alexander G. G. M. Tielens, c Annemieke Petrignani ∗ a , c We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio (C H +5 )and perdeutero (C D +5 ) forms, in the 260–1925 cm − (5.2–38 µ m) spectral range, and investigatethe observed photofragmentation. The spectral and fragmentation data were obtained using InfraredMultiple Photon Dissociation (IRMPD) spectroscopy within a Fourier Transform Ion Cyclotron Res-onance Mass Spectrometer (FTICR MS) located inside the cavity of the free electron laser FELICE(Free Electron Laser for Intra-Cavity Experiments). The A singlet nature of the phenylium ion isascertained by comparison of the observed IR spectrum with DFT calculations, using both harmonicand anharmonic frequency calculations. To investigate the observed loss of predominantly [2C,nH](n = − ) fragments, we explored the potential energy surface (PES) to unravel possible isomeriza-tion and fragmentation reaction pathways. The lowest energy pathways toward fragmentation includedirect H elimination, and a combination of facile ring-opening mechanisms ( ≤ . eV), followed byelimination of H or CCH . Energetically, all H-loss channels found are more easily accessible thanCCH -loss. Calculations of the vibrational density of states for the various intermediates show thatat high internal energies, ring opening is the thermodynamically the most advantageaous, eliminat-ing direct H-loss as a competing process. The observed loss of primarily [2C,2H] can be explainedthrough entropy calculations that show favored loss of [2C,2H] at higher internal energies. The phenyl cation, C H + , also known as phenylium, is a bench-mark system for several organic species such as the aryl andarene groups. As an abundant component in hydrocarbonplasmas, it plays a key role in flames and combustion chem-istry where ring growth according to the HACA mechanism (H-abstraction/acetylene-addition, C H ) takes place. Phenyl andother benzene derivatives are thereby considered to be a corner-stone for aromatic growth, forming Polycyclic Aromatic Hydro-carbons (PAHs). The release of arylic compounds such as C H + a Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, PO Box 94157,1090 GD, Amsterdam, The Netherlands E-mail: [email protected] b Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toer-nooiveld 7, 6525 ED Nijmegen, The Netherlands c Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Nether-lands d Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL32611-7200, U.S.A. e Institut de Recherche en Astrophysique et Planétologie, 9 avenue du Colonel Roche, BP44346, 31028 Toulouse Cedex 4, France † Electronic Supplementary Information (ESI) available: optimised structures of in-termediates and transition states, density of state calculations for phenylium, detailsabout anharmonic calculations.‡ These authors contributed equally to this work into Earth’s atmosphere has sparked interest in the reaction mech-anisms in which they could be involved. Larger PAHs containing more than 40 C atoms are known tobe present in space, and to play a key role in interstellar chem-istry.
They are observed via infrared emission in character-istic bands, called Aromatic Infrared Bands (AIBs). Although in-dividual PAH species are still to be identified, benzene , andbenzonitrile have been firmly identified in space. Benzene hasalso been detected in the atmospheres of Titan and on giant plan-ets. Phenylium is currently one of the proposed precursors inthe interstellar synthesis of benzene.
Phenylium’s high reactivity stems from its electrophilicity,which is induced in the singlet state by its vacant, non-bonding σ orbital . The empty σ orbital induces sp hybridization ofthe C-atom, causing a substantial deformation of the hexago-nal frame as is depicted in the left structure in Fig. 1. In itstriplet state (right structure), the σ orbital is singly occupied byan electron that has been removed from the π -system. This re-stores the sp hybridization of the carbon center and hence thehexagonal shape of the C H +5 cation, but sacrifices the aromatic-ity of the system. Whereas there exists a general agreementthat the A state is the ground state, the energy difference be-tween the triplet and singlet states is somewhat uncertain, with Journal Name, [year], [vol.] , a r X i v : . [ phy s i c s . c h e m - ph ] F e b ig. 1 The geometries of the two different possible electronic states,showing how the singlet structure leads to a distorted hexagonal struc-ture. calculations covering a range from 77 to 137 kJ mol − .Photoelectron spectroscopy of the phenyl radical did not shedany light on the singlet-triplet (S-T) gap. Some experimentalsupport of a singlet ground state was offered in 2000, when amatrix-isolated IR spectroscopy study of phenylium reported vi-brational frequencies at 713 and 3110 cm − . These two bandswere found to be consistent with a scaled harmonic frequencyprediction calculated with B3LYP/cc-pVDZ for the A state. Adecade later, a gas-phase, IR photofragmentation spectrum ofargon-tagged phenylium was reported, presenting five bands inthe 3 µ m range characteristic for C − H stretch vibrations. Thecomplementary calculations at the MP2/aug-cc-pVTZ and scaledMP2/6-311++G(2df,2pd) levels of theory showed that theseC − H stretch vibrations shift, depending on the position of theAr tag relative to the phenylium cation. In the A state, theAr atom donates electron density into the empty σ orbital onthe carbocation center and forms a σ bond in the plane of themolecule. In the B state, the Ar atom shares electron densitywith the π system of the cation and localizes above the face of thering. In 2012, an extensive experimental and theoretical investi-gation of several singly dehydrogenated PAH cations reported alinear correlation between the size of the PAH molecule and thesize of the S-T gap, predicting triplet electronic ground states forall singly dehydrogenated PAHs and a singlet configuration forphenylium. Although experimental verification was presentedfor the larger aryl cations by applying IR Multiple Photon Dis-sociation (IRMPD) spectroscopy using the Free Electron Laserfor Infrared Experiments (FELIX), no experimental spectrum ofphenylium was reported. Phenylium proved too resistant againstIRMPD, preventing the recording of the fingerprint IR spectrumof bare ( i.e., untagged), gas-phase phenylium.Experimental evidence pertaining to isomerization and frag-mentation is likewise limited. Such information is of importanceas a structural rearrangement of phenylium would affect reac-tion and dissociation pathways that are of importance to the re-action networks in flames and the interstellar medium. Recently,Shi et al. theoretically explored the phenylium potential en-ergy surface (PES), identifying 60 possible isomers in which theywere able to connect 28 through transition states. A laboratorystudy on the photodissociation of the (neutral) phenyl radicalfound H-loss to occur upon irradiation at 248 nm and both H and [2C,2H]-loss at 193 nm. The fragmentation of phenyliumupon collisions with He and N O has also been reported to leadto [2C,2H]-loss.
Furthermore, many studies on the largerPAH species have shown a trend for smaller and irregular PAHsto be more likely to lose [2C,2H] than H or H upon photofrag-mentation. It would thus be of great interest to find out ifphenylium follows this trend.In this paper, we present the IRMPD spectrum of bare, isolated,gas-phase phenylium, in its perprotio (C H +5 ) and perdeutero(C D +5 ) forms in the 5–40 µ m region. We use the FT-ICRmass spectrometer in the optical cavity of the IR Free ElectronLaser for Intra-Cavity Experiments (FELICE), demonstratingthat the higher fluences available there are able to efficiently ex-cite IRMPD-resistant species. † The observed fragmentationproducts are interpreted by investigating several isomerizationand fragmentation pathways using DFT calculations.
The IR spectrum of phenylium was recorded in the Fourier Trans-form Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS)located inside the optical cavity of FELICE, one of the free-electron lasers of the FELIX Laboratory. A schematic of the ap-paratus can be found in Petrignani et al. ; only details relevantto this study are given here.Phenylium is generated in an electron impact ionizer. To formthe dehydrogenated cation of benzene, bromobenzene (C H Bror C D Br Merck/Sigma-Aldrich) is used as precursor and placedin a glass vial connected via a leak valve to the instrument, oper-ated at source chamber pressures of several − mbar. The bro-mobenzene molecules are subsequently ionized with 40-eV elec-trons generating phenylium (C H + m / z = , or C D +5 , m / z = ).All ions are directed to a quadrupole mass selector, used in ra-dio frequency (rf) only mode, to guide all ions. The continuouslyproduced ions are then accumulated and collisionally cooled withroom-temperature Ar gas at an ambient pressure of − mbar ina linear quadrupole ion trap with rectilinear rods segmented inthree parts. The accumulated ions are extracted into a largeelectrostatic deflection quadrupole located inside the laser cavity.The ions are deflected by 90 ◦ to be parallel to the laser beam intoa 1-m-long, rf-guiding quadrupole, which leads to four ion trap-ping and detection cells positioned in the bore of the 7-T FT-ICRMS. The center of cell 1 is located at the focus of the FELICElaser beam, and cells 2, 3, and 4 are positioned progressively 100mm away from the previous cell. The magnet can be moved alongits axis so that one of the four ICR cells can be selected to be in thesweet spot of the magnet. Considering the Rayleigh range of 82mm, the laser fluence is reduced by a factor of 2.3 for each nextcell. The high photon densities provided in the laser focus werenot required to achieve fragmentation. To limit power broaden-ing effects, the experiments were therefore conducted in cell 4.For C H +5 the ions were irradiated for 4.85 s at a repetition rateof 10 Hz amounting to 48 macropulses. After these experiments,development on the alignment optics made it possible to achieve Journal Name, [year], [vol.][vol.]
The IR spectrum of phenylium was recorded in the Fourier Trans-form Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS)located inside the optical cavity of FELICE, one of the free-electron lasers of the FELIX Laboratory. A schematic of the ap-paratus can be found in Petrignani et al. ; only details relevantto this study are given here.Phenylium is generated in an electron impact ionizer. To formthe dehydrogenated cation of benzene, bromobenzene (C H Bror C D Br Merck/Sigma-Aldrich) is used as precursor and placedin a glass vial connected via a leak valve to the instrument, oper-ated at source chamber pressures of several − mbar. The bro-mobenzene molecules are subsequently ionized with 40-eV elec-trons generating phenylium (C H + m / z = , or C D +5 , m / z = ).All ions are directed to a quadrupole mass selector, used in ra-dio frequency (rf) only mode, to guide all ions. The continuouslyproduced ions are then accumulated and collisionally cooled withroom-temperature Ar gas at an ambient pressure of − mbar ina linear quadrupole ion trap with rectilinear rods segmented inthree parts. The accumulated ions are extracted into a largeelectrostatic deflection quadrupole located inside the laser cavity.The ions are deflected by 90 ◦ to be parallel to the laser beam intoa 1-m-long, rf-guiding quadrupole, which leads to four ion trap-ping and detection cells positioned in the bore of the 7-T FT-ICRMS. The center of cell 1 is located at the focus of the FELICElaser beam, and cells 2, 3, and 4 are positioned progressively 100mm away from the previous cell. The magnet can be moved alongits axis so that one of the four ICR cells can be selected to be in thesweet spot of the magnet. Considering the Rayleigh range of 82mm, the laser fluence is reduced by a factor of 2.3 for each nextcell. The high photon densities provided in the laser focus werenot required to achieve fragmentation. To limit power broaden-ing effects, the experiments were therefore conducted in cell 4.For C H +5 the ions were irradiated for 4.85 s at a repetition rateof 10 Hz amounting to 48 macropulses. After these experiments,development on the alignment optics made it possible to achieve Journal Name, [year], [vol.][vol.] , he same results for C D +5 with only two macropulses in 0.21 sat a repetition rate of 5 Hz.During storage in cell 4, the desired mass, i.e., m / z = forC H +5 or m / z = for C D +5 , is isolated by ejecting all othermasses present via a Stored Waveform Inverse Fourier Transform(SWIFT) pulse. After irradiation of the ion cloud, a mass spec-trum is recorded at each frequency step. The intensity for eachfragment ion is recorded as function of FELICE frequency. TheIRMPD spectrum is obtained by taking the ratio of the total frag-ment ion count and the total ion count, i.e., the sum of the parentand fragment ions, giving the yield Y ( ν ) Y ( ν ) = ∑ I f rag ( ν ) I par ( ν ) + ∑ I f rag ( ν ) This yield is then normalized to the macropulse energy that is ob-tained by monitoring the FELICE power. For this a fraction of thelaser beam is coupled out through a 0.5 mm radius hole in thecavity end mirror and is directed onto a power meter (CoherentEPM1000). Wavelength calibration was performed by directingthe IR beam to a grating spectrometer (Princeton InstrumentsSpectraPro). The spectral bandwidth is near transform-limitedand is at ∼ Density Functional Theory (DFT) calculations presented in thiswork were performed using the Gaussian16 package. Thestructures of perprotio and perdeutero phenylium were opti-mised at the B3LYP/N07D level, assuming singlet and tripletground states and the double harmonic spectra were then cal-culated. The anharmonic quartic force field vibrational spectrawere also calculated using generalised second-order vibrationalperturbation theory at the B3LYP/N07D level. This combina-tion was shown to yield very good results for anharmonic fre-quency studies of PAHs. We used the keywords Opt=VeryTightand Int(Grid=200974) and default values for resonance thresh-olds. Further steps we took to improve the quartic force field an-harmonic spectrum are described in detail in the ESI † . In short,some low-energy normal modes do not exhibit the expected redshift when including anharmonic behaviour. Instead, these nor-mal modes exhibited unusual blue shifts and were treated har-monically instead. This procedure resulted in a better agreementwith experiments. All theoretical stick spectra were convolutedwith a Gaussian line shape with a FWHM of 100 cm − to matchthe experimental bandwidth.The PES describing the photodissociation of C H +5 was in-vestigated at the M06-2X/6-311++G(3df,2pd) level, consider-ing both singlet closed-shell and triplet open-shell systems. Forthe latter, the HOMO and LUMO were mixed (Guess=Mix) todestroy α - β orbitals and spatial symmetries, and thus reproduc-ing a correct UHF wave function. This specific level of theorywas chosen because of the proven performances of the M06-2Xfunctional coupled to a triple zeta quality basis set in predictingenergy barriers and differences in energies for isomers in manysystems, including hydrocarbon rings and chains. For exam-ple, the difference in energy between acetylene (C H ) and vinyli- dene (CCH ) is calculated to be 1.85 eV while the best theoret-ical prediction using coupled cluster methods is 1.86 eV. In-termediates and transition states were found with the Berny al-gorithm and harmonic frequencies were calculated to verify thetrue nature of the stationary points (one imaginary frequency fortransition states and zero for minima). Finally, Intrinsic Reac-tion Coordinate calculations were performed to check if transi-tion states were connecting the correct minima. S-T gaps wereevaluated for both C H +5 and C D +5 with electronic energies es-timated at the M06-2X/6-311++G(3df,2pd) level and zero-pointvibrational energies estimated at the anharmonic B3LYP/N07Dlevel. The S-T gap for C H +5 − Ar was estimated using M06-2X/6-311++G(3df,2pd) for both the electronic and zero-point vibra-tional energy, with the inclusion of empirical dispersion (GD3)which calculates the long-range potentials needed for the simula-tion of noble gas tagging. In the following, IRMPD spectra of both perprotio C H +5 andperdeutero C D +5 phenylium are presented, and compared toboth harmonic and anharmonic DFT spectra to ascertain whether A or B is the phenylium ground state. IR fragmentation massspectra have been recorded at different wavelengths, and are an-alyzed for both isotopologs to gain insight in the underlying lossmechanisms. Using high-level DFT calculations, the PES behindthese loss mechanisms is explored starting from the experimen-tally confirmed ground state. The experimental IRMPD spectrum of perprotio phenylium isshown in Fig. 2a as the black solid curve (top). The spectrumis constructed using the four fragment masses shown in Fig. 3. Itfeatures five broad resonances between 300 and 1800 cm − , witha FWHM of roughly 80–100 cm − . These experimental featuresare listed in Table 1. Depicted below the experimental curve arethe predictions based on different types of calculations, all em-ploying the B3LYP functional and N07D basis set. The red curve(second from the top) shows the harmonic spectrum for the sin-glet configuration, with the frequencies scaled by a factor of 0.96.The experimental spectrum agrees best with this singlet one, bothin terms of overall shape and peak positions. A comparison isgiven in Table 1, along with the vibrational mode descriptions.The equivalent triplet state (Fig. 2 blue trace, third from thetop) does not show any resemblance in overall shape. An indi-cation of some contribution to the experimental spectrum mightbe inferred from the predicted 1363 cm − resonance, which isvery close to the observed 1382 cm − feature. This feature is theleast well reproduced for the singlet state. However, this couldalso be indicative of some poorly predicted modes. The limitedexperimental resolution, predictive power, and complex relation-ship between calculated linear absorption strengths and observedfragmentation, impede reliable quantification. A dominant sin-glet character can, however, clearly be inferred.The anharmonic spectrum was also calculated using the samefunctional and basis set. The green, bottom curve in Fig. 2a Journal Name, [year], [vol.] , .00.20.40.60.81.0 (a) C H B3LYP/N07Danharmonic A no r m a li z ed I R M P D y i e l d ( a . u . )
380 696 1052 1382 1710 I R i n t en s i t y ( k m m o l - )
400 600 800 1000 1200 1400 1600 1800050100150 B B3LYP/N07Dharmonic, x0.96 A experiment wavenumber (cm -1 ) (b) C B3LYP/N07Danharmonic A
738 1015 1184 1337 1648735 1061 1234 1350 169013631177770
B3LYP/N07Dharmonic, x0.96 B B3LYP/N07Dharmonic, x0.96 A experiment wavenumber (cm -1 )
754 1078 1259 1369 1727
Fig. 2
Comparison between the IR experimental spectrum of (a) phenylium-H and (b) phenylium-D (black curve, top), and the calculated singlet(red, second from top) and triplet (blue, third from top) spectra along with the anharmonic singlet (green, bottom) spectrum, for the respectiveisotopologs. The peak positions of the experimental spectrum were determined by applying Gaussian fits to the individual features, and using theirmaxima to describe the experiment. The harmonic curves were scaled with a factor of 0.96, while the anharmonic curve is unscaled. All theoreticalspectra are convoluted with a 100 cm − FWHM Gaussian waveform.
Table 1
Observed band frequencies (cm − ) and relative intensities together with the calculated singlet frequencies (cm − ) and IR intensities (km · mol − ),and the descriptions of the corresponding vibrational modes for preprotio (left panel) and perdeutero (right panel) phenylium. The listed harmonicfrequencies are scaled by a factor of 0.96. The abbreviations oop and ip stand for out-of-plane and in-plane. C H +5 C D +5 Harmonic calc. Experiment Harmonic calc. ExperimentDescription Freq. IR int. Freq. Norm. Y Description Freq. IR int. Freq. Norm. Y CCC oop empty C 375 9 380 0.10 CCC oop empty C 343 14ring compression 399 21 ring compression 383 19ring twisting 505 3ring compression 630 5 quintet CD oop
520 63quintet CH oop
699 153 696 1.00 ring compression 588 7ring breathing 939 6 CD ip scissoring 731 6 738 0.74ring breathing 1037 15 ring compression 735 3CH ip scissoring 1065 46 1052 0.71CH ip scissoring 1139 8 ring breathing 962 5CH ip rocking 1238 12 CC stretch 1061 41 1015 1.00CC stretch 1302 39 1382 0.40 CC stretch 1234 15 1184 0.30CC stretch 1430 12 CC stretch 1353 10 1337 0.46CC stretch 1724 33 1710 0.14 CC stretch 1690 27 1648 0.15 Journal Name, [year], [vol.][vol.]
699 153 696 1.00 ring compression 588 7ring breathing 939 6 CD ip scissoring 731 6 738 0.74ring breathing 1037 15 ring compression 735 3CH ip scissoring 1065 46 1052 0.71CH ip scissoring 1139 8 ring breathing 962 5CH ip rocking 1238 12 CC stretch 1061 41 1015 1.00CC stretch 1302 39 1382 0.40 CC stretch 1234 15 1184 0.30CC stretch 1430 12 CC stretch 1353 10 1337 0.46CC stretch 1724 33 1710 0.14 CC stretch 1690 27 1648 0.15 Journal Name, [year], [vol.][vol.] , hows the unscaled, anharmonic, spectrum calculated for the sin-glet state. It is quite similar to the scaled harmonic spectrumand provides a similarly reasonable match with experiment asthe scaled harmonic spectrum. In comparison to the latter, itshows an improved match for the experimental 696 and 1382cm − features, while the deviation from the experimental 380and 1710 cm − features is increased. Overall, taking anharmonic-ity into account does not seem to lead to significant improve-ments compared to the harmonic spectrum. Most modes in theanharmonic spectrum appear blueshifted compared to the exper-imental spectrum. We attribute this to an experimental redshiftcaused by the high-intensity multiple-photon absorption. Theanharmonic calculation represents a linear absorption spectrumand will therefore not line up exactly with the high-intensity IR-MPD spectra reported here. Deviations between experiment andprediction can be masked more easily with the freedom to choosea scaling factor for the harmonic calculations.We also present the IRMPD spectrum of perdeutero phenylium,C D +5 , in Fig. 2b. The experimental spectrum (black, top) dis-plays four broad bands between 600 and 1900 cm − . The 1015cm − band shows a large, high-frequency shoulder. A deconvo-lution into two Gaussian curves yields band maxima at 1015 and1184 cm − . Best agreement is again found between the experi-mental spectrum and the harmonic prediction for the singlet con-figuration (red, second from the top). The dominant feature inthe experimental spectrum – at 1015 cm − – is mirrored by theintense band at 1061 cm − , whereas no activity is predicted atthese frequencies for the triplet state (blue, third from the top).The 738 cm − and 1648 cm − experimental features are quiteaccurately predicted by the singlet spectrum, where the tripletspectrum shows only little or no intensity. Overall, the shapeand band positions agree best with the singlet ground state pre-diction. Although again a triplet contribution to the spectrumcannot be excluded, the phenylium in its A state is clearly thedominant species observed, confirming our interpretation of theperprotio spectrum. Just as for the perprotio phenylium, the un-scaled, anharmonic singlet spectrum (green, bottom curve) showsgreat similarity with the scaled harmonic prediction, but does notyield a much improved fit for any of the observed features.The experimental features and the scaled, harmonic resonancesare presented in Table 1. Comparison between the perdeuteroand perprotio phenylium reveals an expected red-shift of the vi-brational modes. Where the perprotio phenylium exhibits modeswith mixed CH ip bending and CC stretching character in the1000–1600 cm − range, in perdeuterated phenylium, the CD ip counterpart is less prominent than the CC stretching component.We compared the nature of these modes between C H +5 andC D +5 by inspecting their animated vibrations. Many of thesemixed modes in the C H +5 spectrum have no one-on-one equiva-lent counterpart with significant intensity in the C D +5 spectrum.Of the modes presented in the Table, six show direct counterpartswith significant intensity. They are indicated with matching col-ors.Our calculations of the S-T gap for perprotio phenylium withDFT calculations (using M06-2X for the electronic energies andB3LYP for the zero-point energies) yield a value of 121 kJ mol − H - 1 - 1 Fig. 3
IRMPD mass spectra of perprotio phenylium (C H +5 , m / z = − (top) and 380 cm − (bottom). D - 1 - 1 Fig. 4
IRMPD mass spectrum of perdeutero phenylium (C D +5 , m / z = − (top) and 1648 cm − (bottom). (120 kJ mol − for C D +5 ), close to the highest values reportedin literature ( ≤ kJ mol − ). The S-T gap of argon-taggedphenylium Ar − C H +5 , calculated including empirical dispersionto account for the long-range potentials needed to describe argon-tagging, is 99 kJ mol − , whereas a dispersion-corrected calcula-tion for bare phenylium yielded 91 kJ mol − . This indicates asmall stabilization of 8 kJ mol − for the singlet state by the σ -coordinated — or in-plane — argon atom, as was previously sug-gested as well. We conclude that the spectra for both perprotio and perdeuterosystems clearly indicate that the observed phenylium is in its A singlet ground state. DFT calculations confirm these findings. Upon resonant IR irradiation, several fragment masses appear.Figure 3 shows mass spectra recorded at two different resonancesin the IR spectrum of perprotio phenylium. The top panel showsthe fragmentation that occurred close to the maximum of the696 cm − band at 680 cm − . Four fragment ions are observedat m / z =
51, 50, 49 and 37 in addition to the parent ion at
Journal Name, [year], [vol.] , / z = . The mass spectrum recorded at the weakest IR reso-nance at 380 cm − (bottom panel) displays only one fragmentat m / z = . Fragmentation mainly occurs through the loss of[2C,nH]-moieties with n = − ( m / z = − ) plus a small con-tribution of [3C,4H]-loss ( m / z = ). The m / z = and 49 chan-nels appear mostly along this dominant m / z = mass, while the m / z = channel only appears at the most intense resonances.Notably, no significant H or 2H loss ( m / z = and ) is observedfor any of the resonances. This was also observed in an ear-lier IRMPD measurements of naphthyl cation, showing [2C,2H]as the dominant loss channel although the relatively low massresolution in that experiment could have obscured observation ofH-loss.For perdeutero phenylium, a mass spectrum recorded at 1275cm − — on the flank of the IR resonance at 1337 cm − – - is givenin the top panel of Fig. 4, displaying two main fragment massesat m / z = and 52 next to the parent mass at m / z = . Contri-butions from [nD]-loss at m / z = , 78 and 76 are small and areonly present in the flanks of the 1337 cm − and 1015 cm − reso-nances. No m / z = ion signal is detected, which would have in-dicated [3C,4D]-loss. The mass spectrum recorded at the weakestIR resonance (1648 cm − ) is displayed in the bottom panel andonly shows one fragment mass, at m / z = . Similar to its per-protio counterpart, perdeutero phenylium mainly loses [2C,2D]( m / z = ), with [2C,3D] ( m / z = ) appearing along the mainfragment in all resonances other than the weakest 1648 cm − feature.Both the perprotio and perdeutero phenylium thus show theloss of [2C,2H/D] as their dominant loss channel, with the lossof [2C,3H/D] as the second strongest. The most salient differ-ences are that the perprotio phenylium is the only one to showthe loss of [3C,4H/D], while the perdeutero phenylium is the onlyone to show the loss of [nH/D]. In spite of these differences, itis clear that these smaller channels are secondary to the loss of[2C,2H/D]. We attribute the observation of lower-mass fragmentsfor C H +5 to a higher-energy loss channel, that is made accessi-ble by a combined effect of both the IR absorption strength of theexcited mode and laser intensity.To rationalize the observed mass spectra obtained upon IR ir-radiation, we investigated the PES describing the photofragmen-tation of phenylium at the M06-2X/6-311++G(3df, 2pd) level.The results are summarized in Fig. 5 where the colors and romannumerals highlight energy regimes dominated by a given mecha-nism. The lowest energy regime is that of ring opening and subse-quent isomerization processes. Ring opening (I, orange) is clearlyenergetically more favorable than H-atom loss (II, red) processes.Direct H-loss from phenylium involves a simple C − H bond break-ing requiring 5.09 eV. This value is slightly higher (0.2 eV) thanobtained at the CCSD(T) level, which is likely due to the multi-configuration character of the m -benzyne C H +4 cation. Ringopening — leading to the chain-like intermediate int2 — can oc-cur along two different reaction paths. The lowest energy path in-volves a single transition state ts3 at 2.41 eV, where the breakingof the alpha C − C bond is followed by the migration of one H-atomto form a CH -moiety. The higher energy path goes through ts1 at2.88 eV involving the migration of an H atom to a nearby carbon creating int1 . This is followed by the subsequent cleavage of theC − C bond with a concerted migration of one H to the final carbonof the chain, involving ts2 at 4.9 eV. In both paths the transitionstates and the intermediate have a singlet open-shell character.The 2.41 eV energy of ts3 compares very well with the value of2.44 eV (56.2 kcal/mol) obtained with CCSD(T) while it is lowcompared to the 3.33 eV value obtained with QCISD(T). De-spite the uncertainties of the different methods, direct H-loss fromphenylium is clearly energetically less favorable than isomeriza-tion.Several chain-like C H +5 isomers exist, lying a couple of eVsabove ground-state phenylium. The structures investigated inFig. 5 are amongst the lowest in energy. Similar reaction pathswere found to be available for the all isomers investigated here,namely int2 , int3 , int4 , and int5 , with comparable energies. Inthe following, we describe the different reaction pathways avail-able for isomer int3 as an example, the energies are given withrespect to int3 . For this isomer, the lowest energy pathways areisomerization reactions (I, orange): a rotation along the C H central group ( ts5 , 2.03 eV) producing the cis form of int3 — int2 , and H-migration ( ts7 , 2.09 eV) leading to int4 . An ad-ditional H-transfer leads to the formation of a methyl group in int5 .Energetically, the next class of reactions is H-loss (II, red), re-quiring energies ranging from 3.65 eV to 4.8 eV depending onwhich H-atom is eliminated. * The CCH -loss reaction (III, blue)is found to be even more endoergic, requiring energies above5.74 eV ( ts6 ) for the direct dissociation and above 8.70 eV forthe sequential H+CCH loss (IV, green). Direct CCH n losses (with n = , ) also require energies higher than 6 eV.Based on the PES, the following scenario can be put forward.When phenylium is excited up to an energy of around 4 eV, iso-merization reactions (such as ring opening/closing, H-migration,and rotation) can occur. Because the isomerization barriers areenergetically very close and lower than the dissociation barriersthe process is expected to yield a mixed population of C H +5 iso-mers, including phenylium.This situation can be described quantitatively within the pre-equilibrium approximation. In this approximation, the relativeabundance of each isomer in an equilibrated mixture of isomerscan be determined via the vibrational density of states (VDOS) atinternal energy E. The abundance of isomer X can be calculatedas [ X ] = ρ X ( E − E , X ) / ∑ i ρ i ( E − E , i ) where ρ i ( E − E , i ) is the density of states of isomer i evalu-ated at energy E, with E , i , the relative energies of isomer i, allgiven with respect to the ground state of phenylium as a refer-ence. For each isomer, the VDOS was calculated using the directcount method on unscaled harmonic frequencies from M06-2X/6-311+G(3df,2pd) calculations. The resulting abundances are plot-ted in Fig. 6. Phenylium constitutes 90% of this mixture up to3 eV of internal energy, but already around 4 eV its contribution * In Fig.5 we reported only the lowest energy H-loss channel for each intermediate. Journal Name, [year], [vol.][vol.]
Journal Name, [year], [vol.] , / z = . The mass spectrum recorded at the weakest IR reso-nance at 380 cm − (bottom panel) displays only one fragmentat m / z = . Fragmentation mainly occurs through the loss of[2C,nH]-moieties with n = − ( m / z = − ) plus a small con-tribution of [3C,4H]-loss ( m / z = ). The m / z = and 49 chan-nels appear mostly along this dominant m / z = mass, while the m / z = channel only appears at the most intense resonances.Notably, no significant H or 2H loss ( m / z = and ) is observedfor any of the resonances. This was also observed in an ear-lier IRMPD measurements of naphthyl cation, showing [2C,2H]as the dominant loss channel although the relatively low massresolution in that experiment could have obscured observation ofH-loss.For perdeutero phenylium, a mass spectrum recorded at 1275cm − — on the flank of the IR resonance at 1337 cm − – - is givenin the top panel of Fig. 4, displaying two main fragment massesat m / z = and 52 next to the parent mass at m / z = . Contri-butions from [nD]-loss at m / z = , 78 and 76 are small and areonly present in the flanks of the 1337 cm − and 1015 cm − reso-nances. No m / z = ion signal is detected, which would have in-dicated [3C,4D]-loss. The mass spectrum recorded at the weakestIR resonance (1648 cm − ) is displayed in the bottom panel andonly shows one fragment mass, at m / z = . Similar to its per-protio counterpart, perdeutero phenylium mainly loses [2C,2D]( m / z = ), with [2C,3D] ( m / z = ) appearing along the mainfragment in all resonances other than the weakest 1648 cm − feature.Both the perprotio and perdeutero phenylium thus show theloss of [2C,2H/D] as their dominant loss channel, with the lossof [2C,3H/D] as the second strongest. The most salient differ-ences are that the perprotio phenylium is the only one to showthe loss of [3C,4H/D], while the perdeutero phenylium is the onlyone to show the loss of [nH/D]. In spite of these differences, itis clear that these smaller channels are secondary to the loss of[2C,2H/D]. We attribute the observation of lower-mass fragmentsfor C H +5 to a higher-energy loss channel, that is made accessi-ble by a combined effect of both the IR absorption strength of theexcited mode and laser intensity.To rationalize the observed mass spectra obtained upon IR ir-radiation, we investigated the PES describing the photofragmen-tation of phenylium at the M06-2X/6-311++G(3df, 2pd) level.The results are summarized in Fig. 5 where the colors and romannumerals highlight energy regimes dominated by a given mecha-nism. The lowest energy regime is that of ring opening and subse-quent isomerization processes. Ring opening (I, orange) is clearlyenergetically more favorable than H-atom loss (II, red) processes.Direct H-loss from phenylium involves a simple C − H bond break-ing requiring 5.09 eV. This value is slightly higher (0.2 eV) thanobtained at the CCSD(T) level, which is likely due to the multi-configuration character of the m -benzyne C H +4 cation. Ringopening — leading to the chain-like intermediate int2 — can oc-cur along two different reaction paths. The lowest energy path in-volves a single transition state ts3 at 2.41 eV, where the breakingof the alpha C − C bond is followed by the migration of one H-atomto form a CH -moiety. The higher energy path goes through ts1 at2.88 eV involving the migration of an H atom to a nearby carbon creating int1 . This is followed by the subsequent cleavage of theC − C bond with a concerted migration of one H to the final carbonof the chain, involving ts2 at 4.9 eV. In both paths the transitionstates and the intermediate have a singlet open-shell character.The 2.41 eV energy of ts3 compares very well with the value of2.44 eV (56.2 kcal/mol) obtained with CCSD(T) while it is lowcompared to the 3.33 eV value obtained with QCISD(T). De-spite the uncertainties of the different methods, direct H-loss fromphenylium is clearly energetically less favorable than isomeriza-tion.Several chain-like C H +5 isomers exist, lying a couple of eVsabove ground-state phenylium. The structures investigated inFig. 5 are amongst the lowest in energy. Similar reaction pathswere found to be available for the all isomers investigated here,namely int2 , int3 , int4 , and int5 , with comparable energies. Inthe following, we describe the different reaction pathways avail-able for isomer int3 as an example, the energies are given withrespect to int3 . For this isomer, the lowest energy pathways areisomerization reactions (I, orange): a rotation along the C H central group ( ts5 , 2.03 eV) producing the cis form of int3 — int2 , and H-migration ( ts7 , 2.09 eV) leading to int4 . An ad-ditional H-transfer leads to the formation of a methyl group in int5 .Energetically, the next class of reactions is H-loss (II, red), re-quiring energies ranging from 3.65 eV to 4.8 eV depending onwhich H-atom is eliminated. * The CCH -loss reaction (III, blue)is found to be even more endoergic, requiring energies above5.74 eV ( ts6 ) for the direct dissociation and above 8.70 eV forthe sequential H+CCH loss (IV, green). Direct CCH n losses (with n = , ) also require energies higher than 6 eV.Based on the PES, the following scenario can be put forward.When phenylium is excited up to an energy of around 4 eV, iso-merization reactions (such as ring opening/closing, H-migration,and rotation) can occur. Because the isomerization barriers areenergetically very close and lower than the dissociation barriersthe process is expected to yield a mixed population of C H +5 iso-mers, including phenylium.This situation can be described quantitatively within the pre-equilibrium approximation. In this approximation, the relativeabundance of each isomer in an equilibrated mixture of isomerscan be determined via the vibrational density of states (VDOS) atinternal energy E. The abundance of isomer X can be calculatedas [ X ] = ρ X ( E − E , X ) / ∑ i ρ i ( E − E , i ) where ρ i ( E − E , i ) is the density of states of isomer i evalu-ated at energy E, with E , i , the relative energies of isomer i, allgiven with respect to the ground state of phenylium as a refer-ence. For each isomer, the VDOS was calculated using the directcount method on unscaled harmonic frequencies from M06-2X/6-311+G(3df,2pd) calculations. The resulting abundances are plot-ted in Fig. 6. Phenylium constitutes 90% of this mixture up to3 eV of internal energy, but already around 4 eV its contribution * In Fig.5 we reported only the lowest energy H-loss channel for each intermediate. Journal Name, [year], [vol.][vol.] , ig. 5 Potential Energy Surface (PES) connecting phenylium and several low-lying isomers with potential fragmentation pathways (M06-2X/6-311++G(3df,2pd) level of theory). Energies are in eV and calculated with respect to phenylium, loss channels are labeled between parentheses.Molecular structures depicted in the PES are cations. The colors (background and labels) and roman numerals identify energy regimes associated withdifferent processes going from low to higher energy: isomerization (0 to ≈ ≈ ≈ D +5 will be similar with minor differences in the energetics due to zero point vibrational energy differencesbetween perprotio and perdeuterated phenylium. Journal Name, [year], [vol.] , s down to 50%. From this point, the sum of int 2 and int 3dominates the mixture of isomers up to 14 eV, where their contri-bution decreases below 50% and isomers of int 4 and int 5 domi-nates. Thus it is a mixture of C H +5 isomers that will determinethe fragmentation observed experimentally, assuming that at in-termediate internal energies of several eVs all species are in thequasi-continuum † and readily absorb the extra photons neededfor fragmentation. It appears that the ring opening in phenyliumbrings a thermodynamic advantage: int 2 and int 3 possess morelow-energy vibrational modes that increase their density of stateswith respect to phenylium. A similar behavior was also observedfor the naphthalene cation. Starting at an energy of around 4.8 eV, H-loss reactions froma mixed population of C H +5 isomers become accessible. DirectH-loss from the ring might also happen, but it is not expectedto be competitive with H-loss from the population of open iso-mers. Finally CCH /C H -loss reactions open up for energiesabove 5.8 eV.This theoretical picture appears to be at odds with what wasobserved in the experiment, which reveals only C-loss channels.Two studies on the reactivity of phenylium showed only the lossof [2C,2H] as a collisionally induced dissociation (CID) byprod-uct at room temperature. In these guided ion beam stud-ies, the collisions are expected to lead to a less gentle excitationthan in our IR-MPD experiment, but the lack of hydrogen loss ap-pears consistent with the current observations. In contrast, forthe neutral phenyl radical, photofragmentation studies using 248nm light (5.0 eV per photon) showed H-loss only, whereas addi-tional [2C,2H]-loss was observed when using 193 nm light (6.4eV). Although these findings cannot be directly compared dueto the difference in charge state, it is interesting to note that theshift from H-loss to [2C,2H]-loss occurs in the range between 5.0and 6.4 eV, the same range calculated here for the phenyl ion.The lack of detection of the H-loss channel may be explainedif the [2C,nH]-peaks in the mass spectra are the result of sequen-tial loss of H followed by the loss of [2C, ( n − ) H]-units. In thisscenario, isomerization takes place first producing a collection ofisomers, within the microsecond timescale of the experiment. Inany of these isomers, fast loss of [2C, ( n − ) H] follows the initialloss of H-atoms, thus obscuring this channel.The high barriers calculated for the [2C,H]-loss reactions fromC H +4 isomers plus the sole detection of [2C,2H] in the CID ex-periments might suggest that another mechanism is at play. Itcould also be speculated that a kinetic shift might be responsiblefor the lack of detection of H-loss on C H +5 , i.e. , that a moleculeneeds to be excited to higher internal energies to observe anyfragmentation during the experimental timescale ( ∼ µ s). Thishas been observed both in large and small aromatic systems. At these higher internal energies ( ≥ eV) the dominant fragmen-tation channel might be different from that at lower internal en-ergies.To evaluate if there is a change in the dominant fragmentationchannel at higher energies we calculated entropies of activation atT=1000 K ( ∆ S ) for H-loss and CCH -loss from int2 , using theunscaled harmonic frequencies for int2 and ts4 . Since H-lossis a barrierless reaction, we started from an optimised structure
Fig. 6
Relative concentrations of phenylium, int 2 + int 3 and int 4 + int5 in the equilibrated mixture M as function of the internal energy of thephenylium molecules in the pre-equilibrium approximation. where the eliminated H is located 4 Å away from the remain-ing C H +4 fragment. For the H-loss reaction, ∆ S is 23.02J mol − K − , while for CCH -loss ∆ S is 51.58 J mol − K − .This means that, even if the loss of an H atom requires a lowerenergy barrier (3.63 eV) than the loss of CCH (4.52 eV), the re-action rate of CCH -loss will grow faster than the rate of H-loss asfunction of internal energies due to the larger change in entropy.Eventually the CCH -loss channel becomes dominant at high in-ternal energy. A similar situation was observed for the compe-tition between H- and H -loss channels in the pyrene cation. This is true also for C D +5 , since the calculated ∆ S for D-lossand CCD -loss are 27.21 and 55.65 J mol − K − , respectively.The lack of detection of the H-loss channel in C H +5 couldthus be either due to sequential loss or to a kinetic shift. Exper-iments on a nanosecond timescale could disentangle the directand/or sequential origins of the C-loss channels. A kinetic mod-eling study of the fragmentation pathways of phenylium and itsisomers, which is beyond the scope of this paper, could help shedlight on this matter. The first IR spectra of isolated, gas-phase perprotio phenyliumand perdeutero phenylium covering the 5.2–38 µ m range are pre-sented. Our experiments confirm the assignment of the singletground-state configuration and reveal only limited anharmonicbehavior. The C H +5 ions fragment primarily through loss of[2C,nH] ( n = − ), with [2C,2H]-loss being the dominant losschannel, in agreement with other works. The exploration of thePES of phenylium through DFT calculations and pre-equilibriumcalculations of the VDOS for various intermediates revealed facilering-opening pathways, suggesting a mixture of open isomers tobe available upon photolysis. H-loss was not detected in exper-iment. This could either be explained by fast sequential loss orby kinetic effects; the entropy ( ∆ S ) calculations show that athigh internal energies, CCH -loss proceeds at a faster rate than H- Journal Name, [year], [vol.][vol.]
Relative concentrations of phenylium, int 2 + int 3 and int 4 + int5 in the equilibrated mixture M as function of the internal energy of thephenylium molecules in the pre-equilibrium approximation. where the eliminated H is located 4 Å away from the remain-ing C H +4 fragment. For the H-loss reaction, ∆ S is 23.02J mol − K − , while for CCH -loss ∆ S is 51.58 J mol − K − .This means that, even if the loss of an H atom requires a lowerenergy barrier (3.63 eV) than the loss of CCH (4.52 eV), the re-action rate of CCH -loss will grow faster than the rate of H-loss asfunction of internal energies due to the larger change in entropy.Eventually the CCH -loss channel becomes dominant at high in-ternal energy. A similar situation was observed for the compe-tition between H- and H -loss channels in the pyrene cation. This is true also for C D +5 , since the calculated ∆ S for D-lossand CCD -loss are 27.21 and 55.65 J mol − K − , respectively.The lack of detection of the H-loss channel in C H +5 couldthus be either due to sequential loss or to a kinetic shift. Exper-iments on a nanosecond timescale could disentangle the directand/or sequential origins of the C-loss channels. A kinetic mod-eling study of the fragmentation pathways of phenylium and itsisomers, which is beyond the scope of this paper, could help shedlight on this matter. The first IR spectra of isolated, gas-phase perprotio phenyliumand perdeutero phenylium covering the 5.2–38 µ m range are pre-sented. Our experiments confirm the assignment of the singletground-state configuration and reveal only limited anharmonicbehavior. The C H +5 ions fragment primarily through loss of[2C,nH] ( n = − ), with [2C,2H]-loss being the dominant losschannel, in agreement with other works. The exploration of thePES of phenylium through DFT calculations and pre-equilibriumcalculations of the VDOS for various intermediates revealed facilering-opening pathways, suggesting a mixture of open isomers tobe available upon photolysis. H-loss was not detected in exper-iment. This could either be explained by fast sequential loss orby kinetic effects; the entropy ( ∆ S ) calculations show that athigh internal energies, CCH -loss proceeds at a faster rate than H- Journal Name, [year], [vol.][vol.] , oss. The presented results constitute the successful IRMPD spec-troscopy of an IRMPD-resistant ion using the FELICE FT-ICR MS.Developments are currently in progress that improve ion produc-tion, storage, and alignment, providing access to higher ion andphoton densities. The current results demonstrate the unique ca-pabilities of FELICE in spectroscopic characterization of stronglybound species which would not be possible with other instru-ments, thus paving the way to the IRMPD spectroscopic charac-terization of highly photostable, astronomically relevant speciesdown to the far-infrared. Author contributions
A.P. and J.O. conceptualized the study. A.P. and A.G.G.M.T. ac-quired the funding for the work. S.D.W, J.R.E., A.P., J.M.B. andG.B performed the experiments. A.C. performed the theoreticalcalculations. S.D.W, A.P. and A.C. conducted the data analysis.S.D.W., A.C. and A.P. wrote the manuscript. All authors con-tributed to the scientific discussion and gave critical reviews ofthe manuscript.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the
Nederlandse Organisatie voorWetenschappelijk Onderzoek (NWO) for the support of the FELIXLaboratory. In particular, we would like to thank A. F. G. vander Meer and B. Redlich of the FELIX Laboratory for their sup-port during the development of the FELICE FT-ICR apparatus.We would also like to thank Prof. W. J. Buma and two anony-mous reviewers for their constructive comments. This work issupported by the VIDI grant (723.014.007) of A.P. from NWO.Furthermore, A.C. gratefully acknowledges NWO for a VENI grant(639.041.543). Calculations were carried out on the Dutch na-tional e-infrastructure (Cartesius and LISA) with the support ofSurfsara, under projects NWO Rekentijd 16260 and 17603.
Notes and references
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