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Photo-fragmentation spectroscopy of benzylium and 1-phenylethyl cations

The electronic spectra of cold benzylium (C6H5-CH2+) and 1-phenylethyl (C6H5-CH-CH3+)cations have been recorded via photofragment spectroscopy. Benzylium and 1-phenylethyl cations produced from electrosprayed benzylamine and phenylethylamine solutions, respectively, were stored in a cryogenically cooled quadrupole ion trap and photodissociated by an OPO laser, scanned in parts of the UV and visible regions (600-225 nm). The electronic states and active vibrational modes of the benzylium and 1-phenylethyl cations as well as those of their tropylium or methyl tropylium isomers have been calculated with ab initio methods for comparison with the spectra observed. Sharp vibrational progressions are observed in the visible region while the absorption features are much broader in the UV. The visible spectrum of the benzylium cation is similar to that obtained in an argon tagging experiment [V. Dryza, N. Chalyavi, J.A. Sanelli, and E.J. Bieske, J. Chem. Phys. 137, 204304 (2012)], with an additional splitting assigned to Fermi resonances. The visible spectrum of the 1-phenylethyl cation also shows vibrational progressions. For both cations, the second electronic transition is observed in the UV, around 33 000 cm-1 (4.1 eV), and shows a broadened vibrational progression. In both cases the S2 optimized geometry is non planar. The third electronic transition observed around 40 000 cm-1 (5.0 eV) is even broader with no apparent vibrational structures, which is indicative of either a fast non-radiative process or a very large change in geometry between the excited and the ground states. The oscillator strengths calculated for tropylium and methyl tropylium are weak. Therefore, these isomeric structures are most likely not responsible for these absorption features. Finally, the fragmentation pattern changes in the second and third electronic states: C2H2 loss becomes predominant at higher excitation energies, for both cations.

The electronic spectra of cold benzylium (C 6 H 5 -CH 2 + ) and 1-phenylethyl (C 6  1-phenylethyl cation also shows vibrational progressions. For both cations, the second electronic transition is observed in the UV, around 33 000 cm -1 (4.1 eV), and shows a broadened vibrational progression. In both cases the S 2 optimized geometry is non planar. The third electronic transition observed around 40 000 cm -1 (5.0 eV) is even broader with no apparent vibrational structures, which is indicative of either a fast non-radiative process or a very large change in geometry between the excited and the ground states. The oscillator strengths calculated for tropylium and methyl tropylium are weak. Therefore, these isomeric structures are most likely not responsible for these absorption features. Finally, the fragmentation pattern changes in the second and third electronic states: C 2 H 2 loss becomes predominant at higher excitation energies, for both cations.

I. Introduction
Carbocations are ions with a positively charged carbon atom. 1 In particular, aromatic ions appear frequently as short-lived reactive intermediates in organic reaction mechanisms. 2 There has been considerable interest in the study of carbocations because of their importance in chemical reactions. [3][4][5] The knowledge of their structures and properties is central to the understanding of their chemical reactivity, and IR spectroscopy methods have contributed much to a better understanding of the ground state of these species. 6,7 However, very little information is available on their electronic spectroscopy in the literature.
While electronic transitions of neutral aromatic molecules are more energetic (UV range), the lowest-lying electronic states of aromatic cations are in the visible region. [8][9][10] Because of their large energy range of electronic transitions and their associated width, aromatic ionic species are regarded as potential carriers of the Diffuse Interstellar bands (DIBs). [11][12][13][14] Some closed-shell aromatic cations have been characterized in rare gas matrices 15,16 and in the gas phase. 7,[17][18][19][20][21] Protonated species can be produced by several techniques such as chemical ionization, 17 pulsed-electrical discharge 20 or in an electrospray ionization source (ESI), 18,19 and investigated through IR multiple photon dissociation spectroscopy (IRMPD) 7,17,19 or cold ion UV photofragment spectroscopy. 22,23 Cold traps that can be cooled down to ~10 K have become the ideal tool for studying optical properties of isolated gas phase bio-molecules, like protonated amino acids and peptides. 18,24 The ESI source is a very convenient ion source: not only it allows to produce protonated molecules, but also radical cations and collision induced fragments of protonated molecules, [25][26][27][28][29][30][31] which can further be trapped and cooled down. Therefore ESI is an easy technique to produce gas phase radical or closed-shell cations, and the coupling with the low temperatures attainable in a cold ion trap greatly simplifies the study of the electronic spectroscopy of such large isolated molecular species.
The C 7 H 7 + ion has long been a challenge to mass spectroscopists with its two possible cyclic isomeric forms: the benzylium cation (C 6 H 5 CH 2 + ), a six membered ring with an attached methylidene group, stabilized through resonance and the tropylium cation, a seven membered ring, which is extremely stable because of its six π electrons aromatic structure. It is known that these two isomers can undergo interconversion. 15,[32][33][34][35][36][37] The benzylium (or tropylium) ion C 7 H 7 + (m/z 91) and the 1-phenylethyl cation C 8 H 9 + (m/z 105) are among the simplest aromatic closed-shell cations and specifically prominent in the mass spectra of alkylbenzenes and related compounds. [33][34][35][37][38][39][40] For the C 8 H 9 + ion, rearrangement pathways between the 1-phenylethyl cation and the methyl tropylium cation have also been discussed. 32,41,42 Until recently, only broad electronic absorptions have been observed for C 7 H 7 + cations in the solution phase, in Argon matrices, and in ion traps with significant differences in band positions and assignments. 15,36,[43][44][45][46] More recently, vibrationally resolved electronic spectra of benzylium and tropylium cations have been recorded in a Neon matrices, 16 but the influence of the matrix is still non negligible, i.e. band broadening and hindered rotational motions. The Argon-tagged benzylium cation has also been studied through laser-induced dissociation 47 and two electronic states have been evidenced, one in the visible and one in the UV region.
The calculated electronic structure and simulated vibronic spectra are in very good agreement with the experiment.
From an experimental as well as theoretical point of view, the 1-phenylethyl cation has received less attention compared to the benzylium and tropylium cations. Recently, IRMPD spectroscopy has been used to differentiate between the 1-phenylethyl form and the methyltropylium structure 42 as well as to characterize hydroxy-substituted benzylium and tropylium ions. 48 We present here the photo-fragmentation spectra of cold and isolated gas phase benzylium and 1-phenylethyl cations, on a very wide spectral range in the visible and UV regions. The assignment of electronic states and vibrational modes is achieved by comparison with ab initio calculations.

A. Experiment
The experimental setup, similar to the one developed by Wang and Wang, 18,38-39 has already been described. 23 Ions are produced in an electrospray ionization source (from Aarhus

B. Calculations
Ab initio calculations have been performed with the TURBOMOLE program package, 51 making use of the resolution-of-the-identity (RI) approximation for the evaluation of the electron-repulsion integrals. 52 The equilibrium geometry of ground electronic state (S 0 ) has been determined at the MP2 53 level. Excitation energies and equilibrium geometry of the lowest excited singlet state (S 1 ) have been determined at the RI-CC2 level. [43][44] Calculations were performed with two basis sets: the correlation-consistent polarized valence triple-zeta cc-pVTZ basis set for the benzylium cation and the double-zeta cc-pVDZ basis set for the 1-phenylethyl cation. 56 The vibrations in the ground and excited states have been calculated and the vibronic spectra simulated using PGOPHER software 57 for the Frank-Condon analysis.

Benzylium cation (m/z 91)
In this part, we present the photo-fragment spectroscopy of the benzylium cation in the visible (18 800 cm -1 -24 400 cm -1 ) and the UV (30 000 cm -1 -44 500 cm -1 ). The main ionic fragments issued from the photo-dissociation of benzylium ions (m/z 91) are (i) C 5 H 5 + ion (m/z 65) issued from acetylene loss (-C 2 H 2 ) and (ii) C 3 H 3 + ion (m/z 39) a fragment frequently observed in the dissociation of aromatic ions corresponding to C 4 H 4 loss (or two C 2 H 2 loss) (see Figure SI1 in the Supplementary Material).
a. Visible spectrum Figure 1 shows the photo-fragment spectrum of cold benzylium (Bz + ) cation in the visible, recorded on the m/z 65 fragment. It is a well-structured S 1 ← S 0 transition, with the band origin observed at 523.9 nm (2.36 eV, 19 089 cm -1 ). This value is very close to the one extrapolated from the Bz + -Ar and Bz + -Ar 2 transition measurements by Dryza et al.
(19 082 cm -1 ). 47 The spectrum has been recorded further to the red, i.e. up to 800 cm -1 from the band origin (see Figure SI2 in Supplementary Material), and no other bands have been detected to the red of the origin. Above 24 400 cm -1 , the first excited state photofragmentation spectrum decreases rapidly and no other absorption bands appear between 25 000 and 30 400 cm -1 .
The spectrum exhibits a long vibrational progression in  13 = 504 cm -1 (Mulliken notation). This progression has already been discussed elsewhere. 14,16,47 Apart from the 0-0 transition which is a single band, all other vibrational bands are split by c.a. ~10 cm -1 . This splitting, not observed in other experiments, 16,47 increases with the number of quanta (14 cm -1 for one quantum in  13 to 28 cm -1 for 3 quanta in  13 ). Our calculations, in agreement with previous ones, 47 indicate that there is only one vibrational mode around 500 cm -1 and the splitting could be due to Fermi resonances. A combination band of two out-of-plane vibrations can be responsible for this splitting (vide infra).
In addition to the main progression, three other progressions starting at 1176 cm -1 , 1236 cm -1 and 1350 cm -1 above the band origin are observed (Fig. 1). The first frequency corresponds to   mode (observed at 1181 cm -1 in the experiment of Dryza et al.). 47 The other modes will be tentatively assigned on the basis of calculated frequencies in the discussion section. Above 24 400 cm -1 , the first excited state photo-fragmentation spectrum decreases rapidly and further absorption bands appear higher in energy at around 30 000 cm -1 (see next section).

b. UV spectrum
At higher energy, in the UV, two states have been identified: S 2 starting at about 32 000 cm -1 (3.95 eV) and S 3 around 40 000 cm -1 (5 eV) (Fig. 2). A broadened vibrational progression on the  13 mode (c.a. 500 cm -1 ) is observed for the S 2 state, and no apparent vibrational structure is observed for the S 3 state. The absence of well-structured vibronic bands is a clear indication that some fast dynamics occurs in these excited states, which smears out the vibrational structure. The branching ratio between m/z 39 and m/z 65 photofragments changes when the energy increases: at the onset of the S 2 state at 32 000 cm -1 , m/z 65 is stronger in intensity than m/z 39 but decreases within 4000 cm -1 further in the blue, while in the S 3 state, at higher energy, m/z 65 (C 2 H 2 loss) is predominant.

b. UV spectrum
At higher energy (between 28 500 and 44 500 cm -1 ), in the UV, the observed signal is quite complex and broad, as for the benzylium ion (Fig. 4). The first band starting at 29 500 cm -1 (3.65 eV) exhibits a double bump structure on which is superimposed a broad progression similar to the one observed in the visible region (inset of Fig. 4). In the latter region, the fragmentation pattern does not change with energy while in the UV, the branching ratio changes with increasing energy: the production of the lightest fragment (m/z 77), which corresponds to C 2 H 4 (or CH-CH 3 ) loss, decreases while the C 2 H 2 loss (leading to m/z 79) is favored with increasing energy. The H 2 loss channel (m/z 103) is also present and has the same spectral pattern as m/z 79.
Further in the blue, a broad band with no apparent vibrational structure is observed, centered at 39 000 cm -1 (4.8 eV), leading predominantly to C 2 H 2 loss, as was the case for the benzylium ion.

Benzylium cation (m/z 91)
The benzylium cation has been calculated extensively in two recent papers 14,47 using different methods (TD-DFT mostly). It has a C 2v structure in the ground state and the vertical energies and the oscillator strengths calculated at the RI-CC2/cc-pVTZ level are presented in and a fourth mode observed at 1350 cm -1 , which is tentatively assigned to 2 quanta in  30 (calculated at 1346 cm -1 ).
The S 2 state can be optimized when the C 2v symmetry is maintained and the adiabatic transition energy is then calculated at 4.43 eV. When the symmetry constraint is released, the CH 2 group starts to rotate and the full optimization leads to a geometry in which the CH 2 plane is perpendicular to the aromatic ring, as calculated in TD-DFT. 47 In the optimization process, the S 2 state crosses S 1 at around 3 eV. It is noteworthy to mention that the excited states calculated in the 5 eV region, where an intense band is observed (Fig. 2), have very low (or zero) oscillator strength.

Tropylium cation (m/z 91)
One could argue that some of the UV bands could be assigned to the tropylium ion, as in the neon matrix study. 16 Tropylium has a D 7h symmetry. Since TURBOMOLE 51 does not handle CC2 calculations with D 7h symmetry, this cation has been calculated using the TD-DFT (B3LYP/cc-pVDZ) method with the D 7h symmetry and using the RI-CC2/cc-pVDZ method with a lower C 2v symmetry. The lowest excited states are found at similar energies with the two methods. The results of excited state calculations in TD-DFT (B3LYP/cc-pVDZ) for the tropylium ion are presented in Table II.  2 12.65 0 1 A' 1 12.95 0 a The S 1 geometry optimization using RI-CC2/cc-pVDZ method leads to a first excited state energy of 4.77 eV.
From our calculations, it does not seem that a tropylium excited state with reasonable oscillator strength lies in the 5 eV energy region. The observed transition in the 5 eV region is broad and without any apparent vibrational structure (Fig. 2), which is indicative either of an excited state undergoing a fast non-radiative decay or of a very large change in geometry as compared to the ground state leading to spectral congestion. The first excited state optimization leads to a geometry close to that of the ground state, with an energy of 4.77 eV (RI-CC2/cc-pVDZ). To test the possibility of a conical intersection with another excited state at some distance from the equilibrium geometry, the first excited state has been optimized starting from a symmetry-broken geometry. The D 7h symmetry structure is recovered with the optimization process, which indicates the presence of a local minimum on the excited state potential energy surface. Therefore, this state should exhibit sharp vibrational bands, as observed for the first excited state of the benzylium ion. Thus, it seems that the tropylium ion is not observed in the present experiment and may be difficult to observe in this UV spectral range, because of the low oscillator strengths of the transitions. Note that there is one state ( 1 E' 1 state in table II) that has a large oscillator strength. However, the geometry relaxation of this state has been performed and leads to a transition energy of 6.09 eV after optimization and thus is probably not responsible for the 5 eV absorption.

1-phenylethyl cation (m/z 105)
The ground state optimized structure of the 1-phenylethyl cation has a phenyl-CH-CH 3 structure of C s symmetry (Fig. 5a). The phenyl-CH 2 -CH 2 form is not a minimum in the ground state potential energy surface. Geometry optimization starting from the phenyl-CH 2 -CH 2 form leads without barrier to the phenyl-CH-CH 3 form located at -0.77 eV in energy. The vertical transition energies given in Table III are calculated with a ground state optimized phenyl-CH-CH 3 geometry. The adiabatic S 1 (1 1 A') ← S 0 (X 1 A') transition energy, including the ZPE correction, has been calculated at 2.62 eV, which is in very good agreement with the observed band origin at 2.61 eV. It is interesting to note that in the S 1 state, the methyl group has rotated by 60° as compared to the ground state geometry. The stabilization energy due to this rotation can be calculated in freezing the methyl rotation in C s symmetry when optimizing S 1 , and is 0.033 eV.
In the S 2 state (2 1 A' in C s symmetry) optimization process, the methyl group rotates out of the benzene plane along the C 1 -C 7 axis, as seen in Figure 5. For an angle of 64°, the S 1 and S 2 states become degenerate and at 90° the S 2 state is the lowest state at 2.94 eV.

FIG. 5. (a), (b) and (c)
Optimized geometries for the first electronic states (S 0 , S 1 and S 2 ) of the 1-phenylethyl cation. In the S 1 state (b), the methyl group has rotated by 60° around the C 7 -C 8 axis as compared to S 0 (a), but the molecule still keeps the benzene plane as plane of symmetry. In the S 2 state (c), the CH 3 group rotates around the C 1 -C 7 axis and comes out of the benzene plane.
The first A" state calculated vertically at 5.19 eV can be optimized in C s symmetry, which results in an adiabatic transition energy at 4.62 eV. The change in geometry with respect to the ground state is mostly due to an increase of the C 1 -C 4 distance in the aromatic ring from 2.80 Å to 3.026 Å as in S 1 . Thus we would expect to see a progression of 400/500 cm -1 as in the S 1 (1 1 A') ← S 0 (X 1 A') transition.

The methyl-tropylium (m/z 105)
As for the tropylium ion, the methyl-tropylium has been calculated at the CC2/cc-pVDZ level (Table IV). The first excited state does not exhibit a large change in geometry as compared to the ground state and has non-negligible oscillator strength (0.019).
If present in the experiment, it should be characterized by well-defined vibrational structures with sharp peaks around 37 000 cm -1 .

Benzylium ion (m/z 91)
The spectrum recorded in the visible region for cold, bare benzylium ions is very similar to the one obtained for the argon complex. 47 This justifies that indeed the messenger technique does not perturb much the spectroscopic measurements and that the shift of the band origin can be correctly predicted: 59 the estimated value obtained from the argon tagging experiment (19 082 ± 20 cm -1 ) is in remarkable agreement with our measurement of the S 1 band origin at 19 089 ± 10 cm -1 . The visible spectrum of the argon complex shows a main vibrational progression built on  13 , a ring deformation mode that reflects the change in geometry between the ground and excited states. 47 The same progression is observed here and in addition, three other progressions involving  13 are observed starting on vibrational modes at 1176, 1236 and 1350 cm -1 . These modes can be tentatively assigned on the basis of calculated frequencies and simulated spectrum (see the simulated spectrum in Figure  All the progressions show the same band splittings: all the bands containing  13 are split, by 14 cm -1 for the band involving one quantum in  13 , 21 cm -1 for the band involving two quanta in  13 , and 37 cm -1 for the band involving three  13 quanta. As a consequence, the value of the  13 vibration is difficult to evaluate, and is estimated to be 504 ± 8 cm -1 , a value which is slightly smaller than that obtained in previous experiments (Ar tagging or matrix).
The band splittings are not due to hot bands: no hot bands have been observed to the red of the band origin, and the simulated spectrum shows only one weak hot band ( in the simulated spectrum at 100 K) that does not seem to be present in the experimental spectrum (see Figure SI4 in the Supplementary Material). The splittings observed in the experimental spectra are most probably due to Fermi resonances. In our calculations as well as in previous ones, 47 there is no vibrational mode close to the  13 mode, but the combination of two low out-of-plane modes could be responsible for Fermi resonances. Indeed, a combination of the lowest mode calculated at 185 cm -1 with another out-of-plane mode calculated at 313 cm -1 leads to 498 cm -1 , not far from the value of the  13 mode.
The bands that do not involve  13 quanta in the spectrum of cold benzylium are narrower (FWHM = 12 ± 2 cm -1 at the S 1 origin and on the and bands) than the bands of the Bz + -Ar complex (FWHM~60 cm -1 at the S 1 origin) indicating probably a participation of van der Waals low frequency modes in the profiles recorded in the Ar tagged experiment.

1-phenylethyl cation (m/z 105)
The spectrum of 1-phenylethyl cation in its first excited state is quite similar to the benzylium one, except that low frequency vibrations (c.a. 50 cm -1 ) are superimposed on the

Benzylium ion (m/z 91)
The second electronic state of the benzylium ion starting at around 3.95 eV may be assigned to the second excited state (1 1 A 1 ), calculated at 4.43 eV in the C 2v symmetry (without ZPE correction). But when the C 2v symmetry is released, the CH 2 group rotates without barrier and gets perpendicular to the aromatic ring, with its energy decreasing to 2.44 eV. With such a geometry change, the 0-0 transition is not expected to be observed due to vanishing Franck-Condon factors. Broad bands are observed in the 4 eV region (Fig. 2), which means that the potential energy surface is rather flat in the Franck-Condon region or has a kind of local minimum in the planar configuration. The first part of the spectrum shows broadened vibronic bands, as the spectrum recorded in neon matrices 16 but the width is larger and the spectrum seems to be blueshifted as compared to the matrix spectrum. The CH 2 rotation may be blocked in the condensed phase, which would lead to a sharper spectrum.
Around 5 eV, an intense very broad band is observed. It is not easily assigned to the C 2v benzylium isomer. Indeed, the S 3 and S 4 states calculated at 5 and 5.4 eV have very small oscillator strengths. This band is large with no apparent vibrational progressions, which indicates either a fast non-radiative decay and/or a very large change in geometry. However one should keep in mind that the intensity in a photo-fragmentation spectrum depends on several factors that are not controlled, such as the fragmentation efficiency, so that the contribution of S 3 and S 4 states cannot be completely ruled out. It is also possible to assign this band to higher excited states of benzylium not calculated at the ground state geometry, which would undergo a strong change in geometry that could open Franck-Condon activity in this 5 eV region.
Based on the calculated electronic transitions in the 5 eV spectral range, it does not seem that the tropylium ion would contribute to the experimental spectrum (no allowed calculated electronic transitions in this spectral range). The first tropylium excited state that has a large oscillator strength is the 1 E' 1 state, but the geometry relaxation of this state has been performed (TD-DFT in C s symmetry) and leads to an energy of 6.09 eV after optimization and thus is probably not responsible for the 5 eV absorption.
The presence of other linear isomers that could absorb in the 5 eV region in the trap cannot be excluded.

1-phenylethyl cation (m/z 105)
As for benzylium ions, two very broad transitions are observed in the UV region, centered around 4 eV (33 000 cm -1 ) and 4.8 eV (39 000 cm -1 ). The second electronic transition starts at 3.65 eV with some vibrational progressions, which is not the case for the third transition at 4.8 eV.
In the 4 eV band, the fragmentation pathways are changing as a function of the However, once again, the intensity in a photo-fragmentation spectrum depends on the fragmentation efficiency that is not known.
Alternatively, this band could be assigned to the first excited state calculated for the methyl tropylium cation. But the calculated geometry change for this state with respect to the ground state is rather small, and therefore we would have expected a well-resolved vibronic spectrum.
The question remains open, as in the case of the benzylium ion where the 5 eV band is not clearly assigned.

V. Conclusion
The benzylium and 1-phenylethyl cations have been produced by collision-induced fragmentation of protonated benzylamine and protonated phenylethylamine from an electrospray ion source. The electronic spectra of benzylium and 1-phenylethyl cations were obtained via photo-fragment spectroscopy in an energy range from 2 eV to 5. mode by a few tens of wavenumbers, probably due to a combination band of two out-of-plane vibrations close to  13 mode. The visible spectrum of 1-phenylethyl cation shows a weak band origin and a vibrational progression on a mode of c.a. 400-500 cm -1 that looks similar to the progression of the benzylium cation, on which is superimposed a low frequency progression reminiscent of a methyl hindered rotation.
The second electronic state, around 4 eV, shows a broadened vibrational progression in the two ions. This corresponds to a large geometry change as indicated by the RI-CC2 optimizations that give a 90° rotation of the methylidene group for the S 2 state of benzylium ion and an out-of-plane torsion around the C 1 -C 7 axis for the S 2 state of the 1-phenylethyl cation.
The third electronic state (around 5 eV) is even broader, without any vibrational progression. The absence of vibronic structure can be interpreted as resulting from a fast nonradiative decay or from a very large change in geometry as compared to the ground state.
However, the calculations performed in this study do not allow a clear assignment of the third band either to the benzylium and 1-phenylethyl cation or to their isomeric forms tropylium or methyl-tropylium ions. More accurate methods may provide insight to these excited states and their fragmentation pathways.

Supplementary Material
"See Supplementary Material Document No. for: -Photo-fragmentation mass spectrum of the benzylium cation at 20 623 cm -1 (2.56 eV) -Comparison between the experimental and simulated spectra for benzylium cation in