Threshold Photoionization of Fluorenyl, Benzhydryl ... - ACS Publications

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Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers Melanie Lang,† Fabian Holzmeier,† Ingo Fischer,*,† and Patrick Hemberger*,¶ †

Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany Molecular Dynamics Group, Paul Scherrer Institut (PSI), CH-5232 Villigen PSI, Switzerland

¶

S Supporting Information *

ABSTRACT: Two π-conjugated radicals, fluorenyl (C13H9) and benzhydryl (C13H11), as well as the carbene diphenylmethylene (C13H10) were studied by imaging photoelectron− photoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and α-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IEad) for all three species were extracted. Values of 7.01 ± 0.02 eV for fluorenyl and 6.7 ± 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IEad of 6.8 ± 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE0K(C13H9+/C13H9Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IEad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck−Condon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 ± 0.04) and (8.11 ± 0.04) eV were determined for C26H18 and C26H22, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames.

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INTRODUCTION In this paper, we discuss the threshold photoelectron spectra of the radicals fluorenyl and benzhydryl, applying synchrotron radiation in the vacuum ultraviolet (VUV) energy range. Radicals are important intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) and soot in flames.1 In particular, aromatic and resonantly stabilized species are expected to accumulate in a combustion environment and contribute to the growths of larger particles. Because such particles are carcinogenic, their formation is unwanted, and considerable effort is dedicated toward understanding the details of the process. Photoionization utilizing synchrotron radiation can be employed to monitor the concentration of key intermediates in flames online.2−4 However, ionization and appearance energies of radicals and their precursors have to be known beforehand to distinguish between structural isomers in flames. Moreover, PAHs as neutral molecules or hydrogenated and even dehydrogenated ions are discussed as possible carriers of diffuse interstellar absorption bands (DIBs) and unidentified infrared emission (UIR) bands.5,6 In a series of experiments, we have shown the potential of synchrotron radiation studies in connection with coincidence spectroscopy for investigating the photoionization and dissociative photoionization of reactive intermediates. Among the previously investigated species are allyl,7 ethyl, and propargyl8 as well as C9H7 isomers9 and small carbenes.10,11 © XXXX American Chemical Society

Ionization energies and the vibronic structure of the ions were determined, and in some cases, the threshold for dissociative photoionization was identified. It was shown in the past that photoionization2,4 and threshold photoelectron spectra9,12 provide a molecular fingerprint and are a versatile tool to distinguish between isomers. Here, we extend this work to the PAH radicals fluorenyl, C13H9, and diphenylmethyl, C13H11, commonly called benzhydryl, as well as the carbene diphenylmethylene, C13H10. Scheme 1 represents all species investigated in the present work, as well as their precursors. Few spectroscopic data are available for all of the investigated species. The adiabatic ionization energy (IEad) of fluorenyl was first measured by Pottie and Lossing.13 They obtained IEad = 7.03 eV by electron impact ionization. Vala et al. examined the optical absorption recording a strong visible band at 494.6 nm, proposing fluorenyl to be responsible for the DIB at 488.2 nm.14 Another absorption band was detected in the UV range at 283.1 nm.15 Furthermore, the infrared spectrum14 and the preresonance Raman transitions were determined.15 All of these experiments were performed in an Ar matrix at 12 K using either photolysis or electron impact dissociation of fluorene as a precursor. Harrison and Lossing16 also performed Received: March 30, 2013 Revised: May 31, 2013

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dx.doi.org/10.1021/jp403158z | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Scheme 1. Structure and Formation of the Species Investigated in the Present Work

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EXPERIMENTAL SECTION

The experiments were performed at the VUV beamline of the Swiss Light Source at the Paul Scherrer Institute in Villigen (Switzerland). A detailed description of the beamline is found in the literature.48,49 Thus, only a brief overview is given in the following section. VUV synchrotron radiation is generated in a X04DB bending magnet and collimated onto a platinum-coated copper mirror that reflects it to a plane incidence monochromator using a 600 grooves mm−1 grating. The grating is optimized for the 5−15 eV energy range and yields a resolution of 5 meV at 15.764 eV, measured at the 11s′ transition of Ar. The same Rydberg series was used to calibrate the grating at both the first and second order. Higher harmonic radiation in the 7−14 eV range was suppressed in a gas mixture of argon (30 mol %), neon (60 mol %), and krypton (10 mol %) in a differentially pumped gas filter operating at a pressure of 10−13 mbar. Suppression of higher harmonics below 7 eV was realized by a MgF2 window. The iPEPICO apparatus consists of a velocity map imaging (VMI) electron spectrometer with a position-sensitive Roentdek DLD40 detector50 and a Wiley−McLaren time-of-flight (TOF) mass spectrometer.51 Both electrons and ions were accelerated within a constant field of 120 V·cm−1 onto microchannel plates. Moderate extraction fields accelerate the ions over a time of a few microseconds, permitting us to measure rate constants in the 103−107 s−1 range. The photoelectrons and photoions are correlated by a multiple start/multiple stop data acquisition setup52 as implemented in the i2PEPICO software.48,53 Threshold photoelectrons were selected with a resolution of 5−7 meV by analyzing the central part and taking into account the hot electron contamination of the image as described by Sztáray and Baer.54,55 The data were corrected for false coincidences by subtracting the background of the masscorrelated TPE spectra. The data were recorded with a 10 meV step size and an integration time of 90 s per data point for fluorenyl and with a step size of 5 meV and an acquisition time of 120 s per point for benzhydryl and normalized by the photon flux as measured by an AXUV photodiode.49 A flange providing the molecular source was attached to a differentially pumped vacuum chamber. 9-Bromofluorene (fluoBr) and α-aminodiphenylmethane (adpm) were purchased from Sigma Aldrich and used without further purification. The precursors were heated in a sample container to 175 °C, seeded in argon (0.01− 0.2 bar), and expanded through a 100 μm nozzle7,8,11,56,57 into a resistively heated silicon carbide (SiC) tube, where the radicals are generated. The heating power for the SiC tube was optimized to achieve the best efficiency for precursor to radical conversion. Quantum chemical calculations were carried out using the Gaussian09 suite of programs.58 For comparison, a variety of methods was employed. Density functional theory (DFT) using the B3LYP functional and the 6-311++G(d,p) basis set was utilized to compute vibrational frequencies and the groundstate structures.59 Gaussian-4 theory (G4)60 and the complete basis set quadratic Becke3 (CBS-QB3) model61 were utilized to obtain reliable thermochemical parameters of neutrals and cations.62 The difference between the zero-point corrected electronic energies at the equilibrium geometries of the cationic and neutral species and the products of dissociation was taken as the adiabatic ionization energy (IEad) and the appearance energy (AE0K). Excited states of the cations were calculated with time-dependent DFT (TD-DFT), applying the B3P86

electron impact ionization experiments of the benzhydryl radical and found an IEad of 7.32 eV. Several photochemical studies were performed in a glassy matrix as well as in the gas phase. The absorption spectrum of benzhydryl, showing a maximum at 325 nm, was reported in several experiments.17,18 Also, the transient absorption and emission spectra were examined for several electronic states of benzhydryl. In these experiments, it was studied as an adsorbate on zeolites,17 in solution,19−21 or in silica gel at room temperature.22 Moreover, it was generated from chlorinated diphenylmethane by pulse radiolysis at 77 K.23 Diphenylmethylene (C13H10) has gained attention similar to that of the radical. Several electron paramagnetic resonance (EPR) experiments in different rigid matrixes24−27 as well as an electron nuclear double resonance experiment28 revealed that the ground state of the carbene is the triplet. Excitation, absorption and emission spectra were recorded as well.29 Moreover, the first excited triplet state of diphenylmethylene was examined by time-resolved EPR,30 absorption,31 and fluorescence spectroscopy.32 Mass spectra recorded in a phenylacetylene flame show contributions of m/z = 165 and 167, which might be assigned to fluorenyl and benzhydryl.2 Interestingly, recent studies propose that o-benzyne (o-C6H4) plays a key role in PAH formation in combustion of aromatic fuels.33−40 Further computational studies on the ring growth mechanism of obenzyne with the benzyl radical (C7H7) indicate that the benzhydryl radical might be the most stable C13H11 isomer on the product potential energy surface for this reaction.41 The knowledge of the properties of both dimers is scarce. For both 9,9′-bi-9H-fluorene (C26H18) and 1,1′,1″,1‴-(1,2ethanediylidene)tetrakis-benzene (C26H22, sym-tetraphenylethane), analytical 1H-nuclear magnetic resonance (NMR) experiments42,43 and X-ray crystallography were performed.44 Several 13C NMR experiments of C26H22 were carried out,45,46 and the absorption and emission of 9,9′-bi-9H-fluorene in solution and its photochemical reactivity were studied as well.47 B



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functional and a 6-311++G(d,p) basis set.63,64 To calculate the IEad for the dimers of both radicals by DFT, the ωB97XD65 functional and a 6-311++G(d,p) basis set were applied, which include corrections for dispersion and long-range interactions. DFT structures, force constant matrices, and frequencies were used unscaled to calculate Franck−Condon (FC) factors with the program FCFit 2.8.8.66 The stick spectra were subsequently convoluted with a Gaussian function in order to compare them with the experimental spectra. The breakdown diagram and the center-of-gravity (CoG) plot of fluoBr were modeled using the MinimalPEPICO program.67

All threshold electrons were collected, that is, no mass selection was applied. A value of 8.10 eV was determined for IEad of the precursor 9-bromofluorene (fluoBr). Excited electronic states of the fluoBr cation are visible at 9.10 and above 10.0 eV. Note that TD-DFT computations yielded energies of 8.58, 8.65, 8.80, 9.20, and 9.90 eV for the excited electronic states of the cation. However, spin−orbit splitting was ignored in these computations. The peaks at 9.36 and 9.98 eV in the TPE spectrum are caused by iodine (I2) present in the sample.68 Upon scanning beyond 9 eV, the precursor starts to fragment and loses a Br atom. The resulting fluorenyl daughter ions appear in the same mass channel as the radical of interest and can thus be incorrectly assigned to the desired reactive intermediate. In order to elucidate the dissociative ionization of the precursor, a breakdown curve was recorded as shown in Figure 2 (right) following the reaction in eq 1.

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RESULTS AND DISCUSSION 1. Mass Spectra and Dissociative Photoionization of the Precursors. Mass spectra of the precursors as depicted in Figure 1 with and without pyrolysis were recorded to check the efficiency of the pyrolysis. As visible from Figure 1b, fluoBr could not be completely converted to the desired radical.

fluoBr + hν → fluo+ + e− + Br

(1)

In a breakdown diagram, the fractional abundances of the parent and daughter ions are plotted as a function of the photon energy taken from mass-selected threshold photoelectron spectra. As visible in Figure 2 (right), there is a “peak” in the daughter ion signal and a corresponding “dip” in the parent ion fractional abundance between 8.55 and 9.10 eV. This feature is caused by fluorene (C13H10), which is present as an impurity in the commercial fluoBr precursor sample. The mass spectrum in Figure 1a (inset) reveals that the mass signal of the fluorenyl ion (m/z = 165) with its asymmetric TOF peak shape due to the kinetic shift encloses the mass signal of the fluorene ion (m/z = 166). As a consequence, the threshold photoelectron signal of the fluorenyl fragment contains threshold photoelectrons of fluorene. An excited electronic state of the fluorene cation becomes accessible in the range between 8.55 and 9.20 eV. Such a state was observed before at 8.92 eV.69 The corresponding feature in the breakdown diagram is therefore not related to the dissociative photoionization of fluoBr. For this reason, the data points between 8.55 and 9.10 eV were disregarded in the analysis. The asymmetric peak shape of the fluorenyl fragment ion (fluo+) reveals the first dissociation channel to be slow, which leads to a kinetic shift of the observed fragmentation thresholds.70,71 Close to the threshold, the ion is metastable and does not dissociate completely during the time that it spends in the acceleration region of the TOF mass spectrometer.72 To include the kinetic shift in the analysis of AE0K of the fluorenyl fragment ion, the breakdown diagram and the TOF distributions have to be modeled simultaneously. Due to the small signal-to-noise ratio, TOF peak shape fitting was impractical for the rate calculations. Instead, the CoG of the fragment ion peak as a function of the photon energy was modeled to get the experimental rates k(E) because our mass spectrometer allows to measure rates in the range of 103 < k < 107 s−1. The fitted CoG curve is shown in Figure 3. Deviations of the fit at later flight times from the experimental data can be explained by the contribution of fluorene to the signal. For the rate calculations, the Simplified Statistic Adiabatic Channel Model (SSACM)67,73 was applied. This approach has been demonstrated to work well for several dissociation reactions before and is thus not further described.74−77 DFT vibrational frequencies of the precursor ion and the fragment ion were used as input data to calculate the density and number of states of the parent and the fragment ions. To achieve a good fit to the experimental data, we had to adjust the

Figure 1. Mass spectra of fluoBr without pyrolysis (a) and with active pyrolysis (b) at 8.70 eV. The inset in (a) indicates the presence of fluorene (m/z = 166) at 8.70 eV. Traces (c) and (d) show mass spectra of adpm at 9.15 eV, (c) without and (d) with active pyrolysis.

Because pyrolysis was incomplete, the photoionization and dissociative photoionization of the precursors were examined too. On the left-hand side of Figure 2, the threshold photoelectron spectrum (TPES) without pyrolysis is depicted.

Figure 2. Threshold photoelectron spectrum without pyrolysis (left) and breakdown diagram of fluoBr (right). The peaks at 9.36 and 9.98 eV are due to iodine (I2), present as a sample impurity. The appearance energy AE0K (fluoBr, fluo+) was determined from a fit of the fractional abundances and the CoG distribution of the fragment ion peak to a SSACM model, as described in the text. C



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Figure 3. CoG fit curve of fluorenyl fragment ion. Due to overlapping mass signals of the fluorenyl fragment ion (m/z=165/166) and fluorene ion (m/z=166), the fit deviates at long flight times from the experimental data (blue).

Figure 4. Dissociative photoionization of adpm. The loss of phenyl and benzene corresponding to m/z = 106 and 105 are the most important channels.

analysis difficult. However, neither fragment interferes with the observation of benzhydryl and diphenylmethylene. At about 10.4 eV, adpm starts to lose a hydrogen atom. Again, the mass channels at m/z = 182 and 183 cannot be separated because the peaks are broadened by the kinetic shift. Therefore, the m/z = 182 H-loss channel is not shown in Figure 4. It is indicated by the slight signal increase of the combined m/z = 182/183 signal (black open circles) at about 10.45 eV. The most important dissociative photoionization channels for the experiments on reactive species described below are loss of NH2 and NH3, respectively. However, neither C13H11+, corresponding to the benzhydryl cation, nor C13H10+, the cation of diphenylmethylene, is formed with significant intensity (green diamonds). Only a small signal is visible at around 9.8 eV. Therefore, the dissociative photoionization of the precursor to C13H11+ and C13H10+ is a parallel pathway of minor importance that can be ignored in the analysis of the radical data below. 2. Photoionization of Fluorenyl and Its Dimer. After the dissociative photoionization of fluoBr and adpm has been explored, the photoionization of the radical can be investigated. Figure 5 shows the TPE signal of fluorenyl as a function of the

density of states by deleting several low-frequency modes.76,78 The experimental rate curve was extrapolated to the 0 K appearance energy, where the rates vanish. A value of 9.4 eV for AE0K was obtained. Due to our procedure, a comparably large error might be associated with AE0K; therefore, the value is only given to the first decimal place. The simplified empirical adjustment chosen by us is justified by (a) the perturbation of the signal from fluorene impurities, (b) the unknown temperature in the beam, and (c) our interest in the radical rather than the precursor. The temperature of 465 K obtained from the fit correlates with the sample temperature of around 450 K and indicates limited cooling in the molecular beam experiment. A more accurate determination of the appearance energy requires an extensive study of the precursor dissociative photoionization that is beyond the scope of this work. In addition, we computed the AE0K by quantum chemistry. Equation 2 was used to calculate AE0K based on computed zero-point corrected electronic energies E0 of the fragment cation (fluo+), the neutral fragment bromine (Br), and the neutral parent (fluoBr). AE0K = E0(fluo+) + E(Br) − E0(fluoBr)

(2)

The electronic energies were taken from the size-consistent G4 and CBS-QB3 computations. We obtained values of 9.69 and 9.37 eV for AE0K, in good agreement with the value of 9.4 eV derived from the experiment. Taking into account the IEad and the AE0K of the fluoBr, the C−Br bond dissociation energy of the ion can be calculated to be 1.3 eV (125 kJ·mol−1). From the breakdown diagram on the right-hand side of Figure 2, it follows that in experiments with active pyrolysis, the precursor fragmentation does not contribute to the radical signal below 9 eV. Benzhydryl radical is generated by pyrolysis from adpm, as depicted in Figure 1d. The mass spectra showed even at high pyrolysis power no full conversion to the radical. Besides benzhydryl, two peaks at m/z = 105 and 106 appear with activated pyrolysis, which might be assigned to the aminobenzyl radical and benzylimine generated by phenyl and benzene abstraction of the precursor. Thus, the dissociative photoionization of the precursor was again studied to elucidate a possible contribution to the radical and carbene signal. In Figure 4, the most prominent channels are the parallel loss of phenyl radical, C6H5, and benzene (C6H6), leading to the fragment ions with m/z = 106 (red triangles) and 105 (blue full circles). As both fragments are created in a slow dissociation process, the mass peaks overlap at threshold, rendering an

Figure 5. TPES of the fluorenyl radical (blue). The high-energy part is underestimated by the FC simulation based on the computed geometries (red line).

photon energy. Because the mass spectrum depicted in Figure A in the Supporting Information shows only one peak at m/z = 165 in the relevant energy range, all threshold photoelectrons were taken into account, and no mass selection was performed. The signal rises at a photon energy of about 6.60 eV and reaches a maximum at 7.01 eV. This value is assigned to be the adiabatic ionization energy IEad. Electron impact ionization D



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brominated fluorene isomers might appear as contaminations in the precursor synthesis, for example, 1-, 2-, 3-, or 4bromofluorene. For that reason, the corresponding fluorenyl radicals of these precursors and three further possible C13H9 radical rearrangement products, 1H-benz[e]indenyl (a), 1Hbenz[f]indenyl (b), and 1H-phenalenyl (c) have been investigated theoretically. An energy diagram showing the structures of the isomers and all calculated ionization energies is given in the Supporting Information (Figure C). All calculated IEad values are higher, or the isomers less stable than fluorenyl. Isomer (c) is the only one with its absolute energy lying below that of fluorenyl, but the computed IEad of 6.41 eV is too low to assume a contribution to the spectrum depicted in Figure 5. We therefore conclude that the computations underestimate the geometry change upon ionization. With the information determined here, a BDE of the C−Br bond in neutral fluoBr can be determined through a thermochemical cycle. With the radical IEad of 7.01 eV and AE0K(fluoBr, fluo+) = 9.4 eV, the BDE of the C−Br bond of the neutral is determined to be (9.4−7.01 eV) at around 2.4 eV, corresponding to 230 kJ·mol−1. Compared to the cation, the bond dissociation energy of the neutral is almost twice as large, whereas the C−Br bond length decreases by about 2 pm only. At first glance, this seems to be counterintuitive. However, dissociation on the potential energy surface of the neutral results in two radicals (bromine and fluorenyl), whereas dissociation on the ionic surface generates a closed-shell ion and a radical, a process that needs less energy. The mass spectra with active pyrolysis (Figure 1b) reveal an interesting feature at m/z = 330 that can be assigned to a fluorenyl dimer. Previous work by IR/UV double resonance spectroscopy indicated that such dimerization reactions show a remarkable selectivity.80 Figure 6 shows the photoion yield

measurements with selection of the kinetic energy of the electrons yielded a similar value of 7.03 eV.13 The IEad’s computed with CBS-QB3 and G4 composite methods are 7.07 and 7.17 eV, respectively (Table 1). Above 7.01 eV, the TPE Table 1. Experimental and calculated ionization energies of the molecules examined in the present work IE(exp) /eV fluoBr, C13H9Br fluorenyl, C13H9 adpm, C13H11NH2 benzhydryl, C13H11 diphenylmethylene, C13H10 fluorenyl dimer, C26H18 benzhydryl dimer, C26H22 a

8.10 7.01 8.00 6.7 6.8 7.69 8.11

IE(calc) /eV a

8.20 7.17a/7.07b 8.44a 6.60a/6.65b 6.77(T)a 7.79c 7.99c

IE(lit) /eV − 7.0713 − 7.3216 − −

CBS-QB3. bG4. cDFT ωB97XD/6-311++G**

signal decreases slowly, but no individual vibrational features can be resolved. The red lines display a FC simulation66 that is based on the geometries, force constants, and vibrational frequencies (unscaled) computed at the B3LYP/6-311++g(d,f) level of theory. The FC factors (red sticks) were subsequently convoluted with a Gaussian function (fwhm = 100 meV). Both the fluorenyl radical as well as the cation possess C2v symmetry. Upon ionization from the X 2B1 neutral ground state to the closed-shell X+ 1A1 state of the cation, mainly the two totally symmetric in-plane bending modes ν3+ and ν14+ are supposed to be active. They are depicted in Figure B of the Supporting Information. Furthermore, the appearance of the first overtone of ν3+ and of a combination band ν3+ν14+ can be expected. As is evident from the dashed lines, the transitions can tentatively be associated with some of the features visible in the spectrum. Although the overall agreement between experiment and simulation is reasonable, there are some deviations visible on the low- and high-energy sides. The former is most likely due to the fact that hot and sequence band transitions are not simulated. Because the temperature of pyrolytically generated radicals in a continuous beam is known to be around 500 K,79 a considerable fraction of radicals is produced in rotationally and vibrationally excited states. In particular, the presence of sequence bands can be expected. The deviations on the highenergy side might be explained by a geometry change upon ionization that is more pronounced than expected from theory. A table containing all calculated geometry parameters of the fluorenyl radical and its ion is given in the Supporting Information (Table A). The most pronounced change in geometry is associated with the C7−C7 bond in the fivemembered ring; see Figure D (Supporting Information) for labeling of atoms. A difference of around 2 pm was computed by B3LYP/6-311(2d,d,f) and B3LYP/GTbas3. The C2C1C2 bond angle ϕ2_1_2 is predicted to change by 1.4°. Because the overall computed geometry change is small, we investigated alternative explanations to account for the broad shoulder on the high-energy side. Dissociative photoionization of the precursor can be excluded below 9.4 eV, as discussed above. Dissociative photoionization of the fluorenyl dimer, which is detected at higher energies, can be excluded as well because the IEad of the dimer is determined to be at 7.69 eV, above the IEad of the monomer (vide infra). A further possibility is the presence of a second C13H9 isomer, originating from a rearrangement in the pyrolysis. Moreover, other

Figure 6. The photoion yield curve and the TPES (inset) of the m/z = 330 signal show an onset at around 7.69 eV that can be assigned to the IEad of the fluorenyl dimer.

curve and the TPES of this molecule. From the experimental data, we assign the IEad to be at 7.69 ± 0.04 eV. We calculated the structures and the ionization energies of nine stable dimer structures (C26H18) by DFT, using the ωB97XD functional and a 6-311++g(d.f) basis set (see Table C in the Supporting Information). The best match is obtained for the direct dimer 9,9′-bi-9H-fluorene (Scheme 1), where both fluorenyl moieties are connected by the carbon atom C1 in the five-membered ring. Note that the resonance stabilization of the radical would permit dimerization to occur also at several other sites. The calculated value of 7.79 eV is in excellent agreement with the E



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energy of 6.6 eV. Note that contributions from the carbene 13C isotopologue to the radical signal cannot be avoided. DFT calculations using the B3LYP functional and either a 6311(2d,d,f) or a GTbas3 basis set reveal only a small geometry change upon photoionization. The structures of the radical, the carbene, and their corresponding cations were all found to have C2 symmetry. The bond lengths and angles of all three species are tabulated in the Supporting Information (Table B). Thus, the ν″ = 0 to ν+ = 0 transition is expected to dominate the spectrum. To simulate the spectrum, we used the computed geometries and varied the ionization energy of the radical and carbene until the best fit to the experimental spectrum was achieved. We assumed relative intensities of 3:2 based on the mass spectrum at 6.80 eV (Figure 7). The relative intensity accounts for the difference in both the number density and the ionization cross section. From the best fit to the experimental spectrum (red line in Figure 8, upper trace), we extract an IEad = 6.7 eV for the radical and IEad = 6.8 eV for the triplet carbene. For comparison, the calculations predict the IEad of benzhydryl to be at 6.60 (CBS-QB3) and 6.65 eV (G4). The IEad of triplet diphenylmethylene is calculated to be at 6.77 eV (CBS-QB3). The TPE band is much broader than expected for two diagonal transitions. However, the torsional motions of the two benzene moieties act as internal rotors in the neutral as well as cationic species. Thus, several excited states will be populated in the neutral, and transitions from all of them contribute to the TPE spectrum. Such contributions are not accounted for by a FC simulation; thus, the bands in the simulated spectrum are actually found to be narrower than the experimental ones. In principle, contribution from the singlet diphenylmethylene should be taken into account as well. The total energy (CBSQB3) of the singlet is calculated to be 0.10 eV above the triplet carbene, with an IEad of 6.67 eV. Nevertheless, in the simulation, we neglected its contribution to the spectrum. The presence of two independent fit parameters (two ionization energies) leads to error bars of around 100 meV associated with the IEad. However, the IEad of benzhydryl of 6.7 eV derived from our data is significantly lower than the value reported in the literature. The earlier value, 7.32 eV, was based on electron impact ionization mass spectrometry16 and is clearly outside of the error bars of the data presented in Figure 8. Accordingly, our conclusion is that a significant downward revision of the ionization energy of benzhydryl is warranted. At higher photon energies, we also detect a dimer of benzhydryl (m/z = 334). The photoion yield curve and the TPES (Figure 9) deliver an ionization energy of 8.11 eV. We

experimental one of 7.69 eV. The calculations thus revealed that 9,9′-bi-9H-fluorene is the most stable neutral fluorenyl dimer structure. 3. Photoionization of Benzhydryl, Diphenylmethylene, and the Benzhydryl Dimer. When the benzhydryl precursor adpm is pyrolyzed, the mass spectrum at 6.80 eV, depicted in Figure 7, shows three overlapping mass peaks at

Figure 7. Mass spectrum of adpm with active pyrolysis at 6.80 eV. The three overlapping mass signals at m/z = 166−168 correspond to the diphenylmethylene benzhydryl, dpm radical, and 13C isotopologue of the radical. Carbene and the radical are present in a ratio of around 2:3.

m/z = 166−168, corresponding to the mass signals of benzhydryl (m/z = 167), its 13C isotopologue (m/z = 168), and a signal at m/z = 166 that can be assigned to diphenylmethylene. While benzhydryl formation is associated with NH2, the carbene is generated in the pyrolysis tube together with NH3 as the second product. Because all three mass peaks overlap and the 13C satellite of the carbene (m/z = 166) will also contribute to the signal at m/ z = 167, full mass selectivity is difficult to achieve. Figure 8

Figure 8. TPES of diphenylmethylene and benzhydryl (upper trace). The lower trace shows the mass-selected TPE signals of the carbene and the radical within a narrow mass range. This permits identification of the later onset of the carbene signal, starting at around 6.6 eV.

(upper trace) shows the threshold photoelectron spectrum of the adpm pyrolysis products, recording all threshold electrons. Thus, the signal contains contributions from masses 166−168. In the lower trace of Figure 8, the signal within a narrow mass gate is depicted. The carbene signal was integrated from m/z = 165.3 to 166.3, and the radical signal was integrated from m/z = 166.7 to 167.7. Narrowing the integration intervals leads to a quickly deteriorating signal-to-noise ratio. Despite this, different onsets for both species can be identified. One can recognize that the TPE signal of the radical starts to rise at around 6.3 eV, whereas the carbene signal increases at a slightly higher photon

Figure 9. TPES of the benzhydryl dimer. Both the inserted photoion yield curve and the TPES give a value of 8.11 eV for the adiabatic ionization energy. F



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estimate an error of ±0.04 eV to be associated with this value, based on the step size of 20 meV between two data points. Again, a selected number of possible isomers was investigated computationally. Their structures and energies are summarized in Table D in the Supporting Information. For 1,1′,1″,1‴-(1,2ethanediylidene)tetrakis-benzene, (also called sym-tetraphenylethane) the product of a direct dimerization at the radical centers, a value of 7.99 eV was obtained by DFT, using the ωB97XD functional. The absolute energies for a number of other isomers were calculated to be higher, making a contribution to the spectrum in Figure 9 improbable. Note that no dimerization product of diphenylmethylene is visible in the mass spectrum. Because the benzhydryl radical might play a role in combustion chemistry, as discussed in the Introduction, its dimer might be also an intermediate on the way to polycyclic hydrocarbons, the precursors of soot.

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ASSOCIATED CONTENT

S Supporting Information *

A mass spectrum of fluorenyl at 7.0 eV, details of the calculations, and full refs 58 and 78 are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ingo.fi[email protected] (I.F.); patrick. [email protected] (P.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DFG through Grant FI 575/7-2 and the GRK 1221 and by the Swiss Federal Office for Energy (BFE Contract Number 101969/152433). The iPEPICO experiments were carried out at the VUV beamline of the Swiss Light Source, Paul Scherrer Institut. The calculations were performed at the Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften (LRZ Munich). We would like to thank Dr. A. Bodi for his experimental support.

SUMMARY AND CONCLUSION

The π-conjugated radicals fluorenyl and benzhydryl as well as the carbene diphenylmethylene were investigated by photoelectron photoion coincidence techniques. All three species are resonance-stabilized and might accumulate in high-energy environments like combustion engines or interstellar space. The reactive intermediates were produced by flash pyrolysis and imaging photoelectron photoion coincidence spectroscopy (iPEPICO) was applied. For fluorenyl, an IEad of 7.01 ± 0.02 eV was obtained. Some vibrational bands that contributed to the FC envelope were tentatively assigned to in-plane deformation modes. Adiabatic ionization energies of 7.07 (CBS-QB3) and 7.17 eV (G4) were computed in good agreement with the experiment. The experimental IEad agrees also well with a previous experimental value of 7.07 eV.13 The precursor of fluorenyl, fluoBr, was also investigated, and an IEad of 8.10 eV was found. An appearance energy for the fragmentation of the fluoBr cation to atomic bromine and a fluorenyl ion of AE0K = 9.4 eV was extracted from a fit to the breakdown diagram and the CoG plots. The information permits determination of bond dissociation energies; a BDE of 1.3 eV (125 kJ·mol−1) was determined for the C−Br bond of the fluoBr cation. The BDE of the C−Br bond of the neutral is around 2.4 eV (230 kJ·mol−1). We also studied the photoionization of the benzhydryl radical, C13H11. It was shown that the carbene diphenylmethylene is present as a side product. Due to overlapping mass signals (isotopes) and similar ionization energies, the two species cannot be separated from each other. From a joint fit to the photoelectron spectrum, we obtained IEad’s of 6.7 and 6.8 eV for the radical and the triplet carbene, respectively. Again, the values are in good agreement with calculations. However, the ionization energy of benzhydryl is significantly lower than the earlier value obtained from electron impact mass spectrometry. Furthermore, the dimerization products of the two radicals were detected. The efficient dimerization implies that both radicals might be important intermediates in soot formation. Ionization energies of 7.69 and 8.11 eV were extracted for C26H18, the dimer of fluorenyl, and C26H22, the dimer of benzhydryl. The structures of the dimerization products were derived based on calculations.

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