Threshold Photoelectron Spectroscopy of Cyclopropenylidene

For example, it was discovered in Centaurus A (NGC 5128), a strong radiogalaxy and in ..... In particular, the C1−C2 bond length deviates by more th...
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J. Phys. Chem. A 2010, 114, 11269–11276

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Threshold Photoelectron Spectroscopy of Cyclopropenylidene, Chlorocyclopropenylidene, and Their Deuterated Isotopomeres† Patrick Hemberger,‡ Bastian Noller,‡ Michael Steinbauer,‡ Ingo Fischer,*,‡ Christian Alcaraz,§ Ba´rbara K. Cunha de Miranda,§,| Gustavo A. Garcia,⊥ and He´loı¨se Soldi-Lose⊥ Institute of Physical and Theoretical Chemistry, UniVersity of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, Laboratoire de Chimie-Physique, UMR 8000 CNRS & UniVersite´ Paris-Sud 11, F-91405 Orsay Cedex, France, Laborato´rio de Espectroscopia e Laser, Instituto de Fı´sica, UniVersidade Federal Fluminense, 24210- 340, Nitero´i, RJ, Brazil, and Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin - BP 48, F-91192 Gif-sur-YVette, Cedex, France ReceiVed: May 3, 2010

Cyclopropenylidene (c-C3H2), chlorocyclopropenylidene (c-C3HCl), and their deuterated isotopomers were studied by the threshold photoelectron-photoion coincidence (TPEPICO) technique using VUV synchrotron radiation. The carbenes were generated via flash pyrolysis. In all species a change in geometry is visible upon ionization, with significant activity in the CdC, CsC-stretching mode and, in the case of c-C3H2/D2, the CsH-bending mode. The electron is removed from an sp2 like hybrid orbital centered on the carbene C atom. The mass selected threshold photoelectron (TPE) spectra were fitted by a Franck-Condon simulation, yielding the equilibrium geometry of the cation ground state (1A1). The adiabatic ionization energy IEad of c-C3H2 was determined to be 9.17 eV, in good agreement with calculations and literature values. Two vibrational wavenumbers of the cation were determined experimentally (ν3+ ) 1150 cm-1 and ν2+ ) 1530 cm-1). Chlorocyclopropenylidene was also studied by TPE spectroscopy and has a similar IEad of 9.17 eV. The spectrum also shows a vibrational progression that corresponds to the CdC- and CsC-stretching modes of the cation. The equilibrium geometry was also determined by a Franck-Condon fit. The IEad of the deuterated isotopomers, c-C3D2 and c-C3DCl, were also determined to be 9.17 eV. The spectra confirm the assignments for the nondeuterated species. Introduction In this manuscript we report an investigation on the photoionization of cyclopropenylidene, c-C3H2, chlorocyclopropenylidene, c-C3HCl, and their deuterated isotopomers, c-C3D2 and c-C3DCl, using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. Cyclopropenylidene is the thermochemically most stable of three isomers of the composition C3H2 and is well characterized by its microwave spectrum.1 It has C2V symmetry and a 1A1 neutral electronic ground state. Due to its comparably high stability it is one of the most abundant hydrocarbon intermediates in interstellar space. For example, it was discovered in Centaurus A (NGC 5128), a strong radiogalaxy and in the Horsehead nebula.2,3 However, since it is kinetically unstable toward reactions with other molecules, the carbene as well as its ion can be expected to play an important role in interstellar hydrocarbon-forming chemistry. This triggered several studies on its behavior in ion-molecule reactions.4–6 Recently cyclopropenylidene was also discovered in cyclopentene/oxygen flames and was identified by its photoion efficiency (PIE) curve.7 Finally, carbenes with their large number of low-lying electronic states, in both the neutral and cation, constitute model systems for nonadiabatic interactions and serve as benchmarks for †

Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Corresponding authors. E-mail: I.F., [email protected]; C.A., [email protected]. ‡ University of Wu¨rzburg. § UMR 8000 CNRS & Universite´ Paris-Sud 11. | Universidade Federal Fluminense. ⊥ Synchrotron SOLEIL.

computational methods.8,9 This great interest from different areas motivated us to investigate the gas-phase spectroscopy and photochemistry of the C3H2 isomers in more detail. We recently investigated the photochemistry of the B 1B1 state of c-C3H2 on the nanosecond time scale and the dynamics of the C 1A1 state of the linear isomer propadienylidene, l-C3H2, in the nanoand femtosecond domain.10–12 A conventional photoelectron spectrum (PES) of c-C3H2 has been recorded by Clauberg et al. using 10.49 eV photoionization. The authors reported an ionization energy of 9.15 eV. Thermochemical information on the carbene as well as the singlet triplet gap energy was derived in this study.13–15 Computational work yielded values of 9.164 eV for the IE, using coupled cluster calculations.16 However, in the experiments a low-energy band was observed that could not be unambiguously assigned. Due to the importance of the molecule, we decided to study the photoionization of the carbene by TPEPICO spectroscopy, a method that in contrast to conventional PES yields mass-selected photoelectron spectra, employing synchrotron radiation as the tunable light source. We showed previously that this approach is well suited to characterize the photoionization of reactive intermediates.17–20 As a second intermediate, we studied the c-C3HCl carbene, chlorocyclopropenylidene. Chlorinated carbenes are supposedly also important in combustion modeling and in astrophysics.21 There is only little information concerning these species available in the literature. Meier et al. produced the chlorinated carbene by flash pyrolysis of c-C3H2Cl2, trapped the carbene in an argon matrix, and recorded its infrared spectrum.22 Chlorocyclopropenylidene is also of theoretical interest because the

10.1021/jp104019d  2010 American Chemical Society Published on Web 06/15/2010

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SCHEME 1: Cyclopropenylidene and Chlorocyclopropenylidene Produced by HCl Elimination from the Precursors

Figure 1. Mass spectra of c-C3H3Cl (upper trace) and the pyrolysis products c-C3H2 and c-C3HCl (lower trace). Both spectra were averaged for 171 s and measured at a photon energy of 10 eV.

chlorinated isomers of carbenes possess different relative stabilities as compared to their nonchlorinated counterparts.23 In earlier theoretical studies the ionization energy of this carbene was computed on different levels of theory and found to lie between 8.77 and 9.06 eV.21 Experimental Setup The experimental setup called SAPHIRS,24 located at the SOLEIL storage ring (St. Aubin, France), was used. A detailed description of this apparatus is given elsewhere.25 In brief, an undulator (OPHELIE 2) provides synchrotron radiation in an energy range between 5 and 40 eV.26 A normal incidence monochromator (6.65 m, 200 gr/mm) was used to select the wavelength.27 A monochromator exit slit of 100 µm was utilized, which gives a photon resolution of around 5 meV at 9 eV. Argon (p ) 0.25 mbar) was employed in a gas filter28 to suppress the higher harmomics originating from the undulator. The SAPHIRS setup consists of two differentially pumped vacuum chambers, which are equipped with a velocity map imaging (VMI)29 spectrometer and a Wiley-McLaren TOF spectrometer (DELICIOUS II).25 Electrons are extracted by static fields of 95 V cm-1 and detected by an imaging detector with a delay line anode. Ions are further accelerated with a field of 380 V cm-1 and are detected on multichannel plates (MCP). A continuous (cw) jet of carbenes (in Argon) was generated by flash pyrolysis. A heated SiC tube was mounted on a 50 µm orifice augmented by a water cooling system.30,31 Two electrodes are connected to the SiC tube and 20-40 W were applied for heating purposes. We used 1-chlorocycloprop-2-ene and 1-chlorocycloprop-2-ene-d3 as precursors for the generation of the cyclopropenylidene (c-C3H2) and cyclopropenylidene-d2 (cC3D2) in the pyrolysis nozzle (R cleavage of HCl and DCl according to Scheme 1). Both compounds were synthesized according to the literature.32 The generation of c-C3HCl and c-C3DCl originates from c-C3H2Cl2 and c-C3D2Cl2 that are byproducts of the synthesis. In all experiments the heating power of the SiC tube was adjusted to get a full conversion of the precursor. The quality of the molecular beam was controlled by measuring ion velocity map images of Ar, which allow the optimization of the supersonic/thermal background ratio by aligning the nozzle with respect to the skimmer. Mass selected threshold photoelectron spectra (TPES) were obtained by scanning the photon energy (step size 3-5 meV) while recording the ion signal in coincidence with threshold electrons, which were selected with an energy resolution

between 3 and 10 meV. The spectra were typically averaged between 10 and 30 s per point and normalized by the photon flux, measured with a photodiode (AVUV from IRD). Computational Methods Density functional theory (DFT) calculations, using the B3LYP exchange correlation functional with a 6-311++G** basis set were performed by employing the Gaussian 03 suite of programs.33,34 For all carbenes and cations the geometry of the neutral and ionic ground state was optimized by applying the GDIIS (geometry optimization using direct inversion in the iterative subspace) algorithm, tight convergence criteria, and an ultrafine grid.35 The adiabatic ionization energy (IEad) was calculated from the difference of the sum of electronic and zero point energy of the cation and the neutral at their equilibrium geometry. Additionally, the geometry was optimized on a higher level of theory utilizing the CCSD method and a cc-pVTZ basis set.36–39 Furthermore a three points complete basis sets (CBS) extrapolation was performed to improve the accuracy of the IEad calculation and obtain a complete basis set energy ECBS, according to eq 1.40–43

E(x) ) ECBS + Be-Cx

(1)

Therefore single point calculations with the CCSD(T) or CCSD method and cc-pVnZ (n ) D, T, Q corresponding to x ) 1, 2, 3) basis sets were carried out.44–47 The amount of spin contamination was always negligible (S2 ∼ 0.75 for cations). The zero point correction was carried out using vibrational frequencies computed with a cc-pVTZ basis set. All vibrational frequencies were used unscaled. The Franck-Condon factors were calculated with the program fcfit27.48 For the initial simulation, the carbene and the cation geometry from ab initio calculations were employed, as well as the force constant matrices. In this first step vibrational bands were assigned. In the next step the line intensities were fitted to the experimental spectrum by varying the geometry of the cation. The details of the fitting procedure and the simulation are described by Spangenberg et al.48 The computed line spectrum was subsequently convoluted with a Gaussian function to facilitate a comparison with the experimental spectrum. Results and Discussion (a) Mass Spectra. In a first step, mass spectra were recorded to check whether the conversion of the precursor, c-C3H3Cl, to the carbene was complete. In the upper part of Figure 1, a mass

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Figure 2. Threshold photoelectron spectra of c-C3H3Cl (black curve) and c-C3H2Cl2 (red curve). Approximate adiabatic ionization energies of 10.15 and 10.05 eV, respectively, were determined for the two molecules.

spectrum of the precursor (m/z ) 74 and 76) at 10 eV is depicted, which was accumulated for 171 s. The peaks at m/z ) 108 and 110 were identified as byproducts (c-C3H2Cl2) from the synthesis. No contributions from dissociative ionization were observed at this energy. When the pyrolysis was turned on (lower trace of Figure 1), the precursor signal vanished completely and a strong signal at m/z ) 38 appeared, which can be assigned to the cyclic isomer of C3H2 (see Scheme 1). In addition, another signal at m/z ) 72 and 74 can be observed. Since c-C3H2Cl2 appears as a byproduct of the synthesis, which can also cleave off HCl according to the lower trace of Scheme 1, we assign this peak to c-C3HCl. The precursors can be present in two isomeric forms, that were both employed in previous measurements to generate this species.22 No other side products like recombination products were detected. (b) Photoionization of the Precursors. When studying reactive intermediates, one has to rule out the formation of their cations by dissociative photoionization (DPI) of the precursors, because such fragment ions could falsely be assigned to originate from the reactive intermediates. Therefore the photoionization of the precursors was studied. In Figure 2 the threshold photoelectron signals of c-C3H3Cl (black line) and c-C3H2Cl2 (red line) as a function of the photon energy are depicted, with the pyrolysis turned off. The c-C3H3Cl signal increases slowly at about 9.9 eV. We assign this small signal at low energy to either hot bands or sequence bands. The onset of the major peak at around 10.15 eV is assigned to the adiabatic ionization energy (IEad). The signal maximizes at 10.6 eV, while another band is visible at around 11.2 eV. The latter band might be due to an electronically excited state of the cation. The ionization energy of C3H2Cl2 is slightly shifted to the red. There is also a small hot band contribution in the TPE spectrum and the adiabatic ionization energy (IEad) is assigned to 10.05 eV. Note that two isomers differing in the position of the two Cl atoms are possible for this molecule (cf. Scheme 1) that we cannot distinguish. We did not observe the dissociative photoionization (DPI) of the precursors. Therefore this process can be neglected below 10 eV and the carbenes observed are pyrolysis products. (c) Threshold Photoelectron Spectra of the Carbenes. Once the photoionization of the precursors is understood, one can study the photoionization of the carbenes. The TPE spectra of c-C3H2, c-C3HCl, c-C3D2, and c-C3DCl were measured under the conditions represented by the mass spectra in Figure 1.

Figure 3. TPE spectrum of c-C3H2 showing a strong vibrational progression, which is attributed to the ν3+ and ν2+ modes (CdC- and CsC-stretching vibrations). Two FC fits were carried out with the 0-0 transition set to 9.17 (a) and 9.02 eV (b).

c-C3H2. In the TPE spectrum of cyclopropenylidene (Figure 3a) a long vibrational progression is visible, indicating a change in geometry upon photoionization. The signal is increasing at about 8.8 eV. Four large peaks at 9.02, 9.17, 9.32, and 9.47 eV are visible, contributing to a vibrational progression. Another feature is a series of small peaks displaced from the major ones, which represent a second vibronic progression. In previous experiments on CH3 we found a rotational temperature of about 400 K.20 This fairly high temperature contributes to the peak width. The experimental resolution is about 7 meV (60 cm-1), calculated by the square root of the sum of the photon resolution squared and the electron resolution squared. The rather high temperature was also observed in former continuous beam studies on radicals produced by pyrolysis.18,20,49 Interestingly, the unassigned low-energy band (9.02 eV) is more pronounced in our spectrum compared to the PES from Clauberg et al.13–15 When we follow the assignments of the previous work, the IEad is 9.17 eV with an estimated error of 15 meV. To aid the interpretation, we performed ab initio calculations on the CCSD cc-pVTZ level of theory. Cyclopropenylidene has a singlet ground state (1A1); the highest occupied molecular orbital (HOMO) is centered on the carbene center and can be approximated as a sp2 hybrid orbital. Removing an electron from the HOMO is therefore expected to induce a significant

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TABLE 1: Geometries and Vibrational Modes of c-C3H2 and Its Fully Deuterated Isotopomersa

a

Literature (lit.),1,14,50,51 calculations (calc), and fitted (FC fit) values are given.

geometry change of the three membered ring upon ionization to the X+ 2A1 state. Calculations show an expansion of the C2-C1-C3 angle, a contraction of the C1-C3 bond, an elongation of the C2-C3 bond, and a small increase of the H-C2-C3 angle (for numbers see Table 1). This change in geometry is consistent with the valence shell electron pair repulsion (VSEPR) model. The positive charge can be delocalized in the C2dC3 double bond and subsequently the C1sC3 and C1sC2 bonds will contract, because removing an electron from the C1 atom reduces electron repulsion on this site. After ionization the C1sC2 and C1sC3 bonds obtain a strong double bonding character, whereas the C2dC3 bond order decreases. For an easier comparison we nevertheless used the neutral ground state labels for the carbene and cation. Activity in the three A1 modes, namely, the C1sC2/3stretching mode (ν2+(calc) ) 1668 cm-1), the C2dC3-stretching mode (ν3+ (calc) ) 1232 cm-1), and the CsH-bending mode (ν4+ (calc) ) 915 cm-1) can be expected. In a next step a Franck-Condon (FC) simulation was performed.48 Since the geometry of the neutral singlet ground state is experimentally known, we used this geometry for our calculations.1 To improve the agreement with the experimental spectrum, the line intensities were optimized, varying the geometry of the cationic ground state. The calculated stick spectrum is represented in Figure 3a (blue sticks). The convolution (red line) of the stick spectrum with a Gauss function (full with at halfmaximum (fwhm) ) 20 meV) is in good agreement with the experimental spectrum. As a result, one gets the equilibrium structure of the cation, which is also depicted in Figure 4 and Table 1. Note that identical results were achieved using the B3LYP calculated geometries as a starting point. The optimized structure is in agreement with the structure found by Clauberg et al.14 Once a satisfactory fit is achieved, the peaks can be assigned following the literature nomenclature (see Table 1). The spectrum shows a progression in the ν3+ mode, which corresponds to the C2dC3-stretching motion of the cation. This

Figure 4. Comparison of the Franck-Condon fit obtained geometries with high-level ab initio calculations. Note that the structure of the IEad ) 9.02 eV deviates more than 2 pm from the CCSD results in the C-C bond lengths.

progression dominates the shape of the TPES. Further peaks can be assigned to the ν4+ and ν2+ fundamentals. For the ν3+ and the ν2+ modes, wavenumbers of 1150 and 1530 cm-1 were measured. Excitation in the ν2+ ) 1 state is underestimated by the simulation. A reason for this could be the participation of autoionizing states, which enhance this transition.52,53 Since autoionizing states do not generally play a role in conventional PE spectra, such a perturbation is not present in the literature spectrum.14 One has to ask whether the 9.02 eV peak could be the ionization onset. As a next step, we therefore modeled the TPE spectrum with the assumption that the low-energy band at 9.02 eV corresponds to the 0-0 transition. In Figure 3b the convolution (red line) of the line spectrum with a Gaussian function and the experimental spectrum (black line) is depicted. Comparing the fitted structure at IEad ) 9.02 eV and IEad ) 9.17 eV with high-level ab initio calculations (CCSD/cc-pVTZ) shows that the latter is much closer to the ab initio geometry (see Figure 4). In particular, the C1-C2 bond length deviates by more than 2 pm in the fit assuming IEad ) 9.02 eV. This is outside the error bars of such a high-level calculation. Coupled cluster calculations concerning the c-C3H2 system were already performed by Lau and Ng, who obtained an IEad

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Figure 5. TPE spectrum (black line), the line spectrum obtained by a FC fit (blue sticks), and the convolution with a Gaussian function (red line) of c-C3D2. Also the fitted geometry of the cation is given.

of 9.164 eV.16 Our own CCSD(T) calculations with cc-pVnZ (n ) D, T, Q) basis sets yielded, for a complete basis sets (CBS) extrapolation, an IEad of 9.151 eV. This strongly supports an IEad of 9.17 ( 0.015 eV, in agreement with the earlier value of 9.15 eV. The half width at half-maximum of the 0-0 band was taken as the error. The band at about 9.02 eV has thus to be assigned to either a hot band of unusually high intensity or to another species with m/z ) 38. To match the intensity of this peak in a hot band simulation, we had to assume an unusually high vibrational temperature of 1800 K to get sufficient population in the symmetric C1-C2/3-stretching vibration of the neutral (see Figure 3a, green line). Such a high temperature seems highly unlikely, compared to vibrational temperatures of 400-500 K that were recently determined for the methyl radical.20 In addition, the shape of the peak is not well represented. We therefore suggest that the peak at 9.02 eV results from a contribution of the propargylene isomer, HCCCH. Taatjes et al. observed HCCCH in a fuel rich cyclopentene/ oxygen flame and derived an ionization energy of 8.96 eV.7 Nevertheless, isomerization has to proceed over a barrier of 52.5 kcal/mol (2.27 eV).54 A contribution of the l-C3H2 isomer is not possible due to its high ionization energy of 10.43 eV.15 c-C3D2. To verify the assigned vibrational bands in the TPE spectrum of c-C3H2, we synthesized the deuterated isotopomer of the cylopropenylidene, c-C3D2, via flash pyrolysis of c-C3D3Cl. Especially the CH-bending vibration ν4+ (a1) should undergo a red shift and thus be observed more easily in the spectrum (for the band assignment see Table 1). A TPE spectrum, depicted in Figure 5 was recorded with a step size of 5 meV and an acquisition time of 10 s per data point. As for the cyclopropenylidene, one notices a vibrational progression of about 150 meV (1200 cm-1), which corresponds to a C2dC3-stretching vibration of the cation (ν3+). The small peak at 9.02 eV can again be interpreted as a contribution of the triplet isomer, DCCCD. The IEad was determined to be 9.17 ( 0.015 eV. Ab initio calculations (B3LYP) yielded in an IEad of c-C3D2 of 9.15 eV, which is in good agreement with the experimental value. The effect of deuteration on the ionization energy is thus small. We again performed a Franck-Condon fit of the spectrum, adjusting the cation geometry on the TPE spectrum. The resulting fit is depicted in Figure 5 (red line), the obtained equilibrium geometry is summarized in Table 1. The members

Figure 6. TPE spectrum of c-C3HCl. (a) shows the experimental spectrum and the calculated spectrum (red line) with IEad set to 9.17 eV, while in (b) the simulation if IEad ) 9.02 eV is depicted.

of the ν3+ progression are also indicated in the experimental spectrum. Its appearance is consistent with the calculation, where one observes a change in the C2sC3sC1 angle and in the C2dC3 bond length upon ionization. The ν2+ and ν4+ modes are also observed in the low-energy part of the TPES, where both lead to a broadening of the ν3+ ) 1 band. This pattern is also visible at higher photon energies. The ν3+ ) 2 band is surrounded by combination bands with the ν2+ and ν4+ mode (3141 /2131). The expected red shift of the ν4+ mode cannot unambiguously be observed in the spectrum, because it is masked by the strong ν3+ progression. Also the rotational broadening due to an inefficient cooling in the cw experiment leads to difficulties in the observation of the ν4+ band. The vibrational wavenumber of the ν3+ can be determined to 1150 cm-1. The negligible influence of deuteration on the wavenumber of this mode confirms that it is indeed a motion confined to the carbon ring. c-C3HCl and c-C3DCl. As visible in the mass spectrum (Figure 1 lower trace), peaks at 72/74 appear when the pyrolysis is turned on. The isotopic ratio of 100/32 is consistent with the 35 Cl/ 37Cl pattern and with the molecular formula C3HCl. To get more information on this species, a TPE spectrum (Figure 6a) was recorded. At around 8.8 eV the signal increases; a first strong peak appears at 9.17 eV. A dominant progression with almost equal spacing between the members is visible.

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Figure 7. Energy diagram of the C3HCl isomers. Relative energies were calculated with the B3LYP functional and 6-311++G** basis set. The most stable isomer is c-C3HCl.

Similar to the c-C3H2, three constitutional C3HCl isomers are possible. Ab initio calculations on the B3LYP level of theory were performed and the geometries of the neutrals, the cations, and the relative energies of all three isomers were determined. B3LYP was chosen because results for c-C3H2 were very similar to CCSD. In Figure 7, the structures of the three isomers, relative energies, and adiabatic ionization energies are depicted. The most stable isomer is the c-C3HCl, whereas the l-C3HCl and HCCCCl are about 0.4 eV less stable. The IEad differ strongly. Only for c-C3HCl is a reasonable agreement of the calculated (IEad ) 9.124 eV) and the measured IE found. Three arguments thus support chlorocyclopropenylidene as the carrier of the spectrum: (1) c-C3HCl is the most stable of the three isomers, (2) the ionization potential agrees well with the experimental one, and (3) c-C3H2Cl2 is a byproduct of the c-C3H3Cl synthesis. As already mentioned, the pyrolysis of c-C3H2Cl2 is known in the literature and has been shown to provide c-C3HCl by R cleavage of HCl.22 To optimize the geometry of the cation, we again carried out a FC simulation of the TPES. To obtain a good starting point, CCSD calculations were performed showing a strong change in geometry upon photoionization of c-C3HCl (see Table 2). As in c-C3H2, the HOMO is localized on the carbene C atom. The C2sC1sC3 angle increases, and thus the C2dC3 double bond is extended while the C2sC1 bond and the C3sC1 bond lengths are reduced. Note the lower symmetry compared to c-C3H2, Cs instead of C2V. The peak distance in the experimental spectrum shows a vibrational progression of 1160 cm-1 that can be assigned to a C2dC3-stretching vibration (ν3+). A Franck-Condon fit was carried out, setting the IEad to 9.17 eV. Note that we used the calculated geometry of the neutral, because no experimental data are available. Only the geometry of the cation was adjusted to get the best fit with the experimental spectrum. Since for c-C3H2 we observed good agreement between experimental and computational geometry of the neutral ground state, we consider this approach reasonable. The resulting stick spectrum (blue sticks) and its convolution (red line, fwhm ) 40 meV) are depicted in Figure 6a, which shows participation of two modes of the C1sC2/3-stretching (ν4+) and C2dC3-stretching (ν3+) vibrations. Combination bands of the ν3+ and ν4+ modes lead to a broadening of the peaks when going to higher photon energies. This feature is also visible in the experimental spectrum, where the signal converges to a continuum. At higher

Hemberger et al. photon energies the calculated peak positions deviate from the experimental ones. This is caused by anharmonicity, which is not considered in the fit. The spectrum was alternatively fitted with the assumption that IEad ) 9.02 eV, depicted in Figure 6b. A reasonable agreement with the experimental spectrum can also be achieved. A closer look at the optimized geometry yields again a more pronounced difference between the CCSD calculation and the fit (see Figure 8). Note that agreement is not as good as in the case of cyclopropenylidene, because no experimental ground state geometry was available. Especially the C1-C2 bond length deviates by almost 5 pm from the computed one. We thus assign the peak at 9.17 eV to the ionization threshold because the geometry is in better agreement with the CCSD calculations. The small signal at lower energies is due to hot band or sequence band transitions. To further improve the agreement, we performed IE calculations on different levels of theory. The CCSD geometry was utilized for single point calculations with the cc-pVnZ (n ) D, T, Q) basis sets, to carry out a CBS extrapolation. This procedure yielded IEad ) 9.149 eV. The less accurate DFT calculations with the B3LYP functional used in Figure 7 resulted in an IEad of 9.124 eV. Earlier calculations yielded values between 8.77 and 9.06 eV, depending on the method.21 Interestingly, the experimentally observed ionization energy is within the error bars the same as the one for c-C3H2. This shows that the p-orbital on the carbene center is the important site for ionization. The small influence of the chlorine atom is almost identical in the neutral and ionic ground states. This view is supported by the computed IE’s of 9.151 and 9.149 eV for c-C3H2 and c-C3HCl, respectively. Again, we also investigated the deuterated c-C3DCl isotopomer. A TPE spectrum was recorded in the energy range from 8.8 to 10 eV with a step size of 5 meV and an averaging time of 10 s per point as depicted in Figure 9. It shows a weak onset at about 8.8 eV and a first maximum at 9.17 eV; further maxima appear at 9.32, 9.47, 9.59, and 9.73 eV. The signal decreases at around 9.8 eV. Vibrational structure similar to that for c-C3HCl is visible. We assign the first strong peak at 9.17 eV as the IEad. Again, B3LYP calculations were performed and a FC fit was carried out to simulate the spectrum. The result (red line) is also given in Figure 9/Table 2 and is in good agreement with the experimental spectrum. The vibrational frequency of the ν4+ C2dC3-stretching vibration was determined to be 1200 cm-1. Summary The photoionization of several carbenesscyclopropenylidene, c-C3H2, chlorocyclopropenylidene, c-C3HCl, and their deuterated isotopomersswas investigated using threshold photoelectronphotoion coincidence spectroscopy in combination with VUV synchrotron radiation. It was shown by time-of-flight mass spectrometry that the carbenes were produced with high efficiency via flash pyrolysis. Upon ionization, all carbenes experience a geometry change and activity in several vibrational modes is observed. The equilibrium geometry of the cations was derived from a Franck-Condon fitting procedure. Due to the mass selectivity of the TPEPICO technique, a contribution of electrons from other masses to the spectra can be excluded. The TPES recorded for cyclopropenylidene shows the C2dC3 (ν3+)- and the symmetric C1sC2/3 (ν2+)-stretching vibrations, which were determined to be 1150 and 1530 cm-1, respectively. The ionization energy IEad of c-C3H2 was measured to be 9.17 eV, which is in good agreement with previous experimental and calculated values.

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TABLE 2: Geometries and Vibrational Modes of c-C3HCl and Its Fully Deuterated Isotopomersa

a

Literature (lit.),21 calculations (calc), and fitted (FC fit) values are given.

propargylene-d2. The alternative assignment to a hot band would require an unrealistically high vibrational temperature of 1800 K.

Figure 8. Comparison of the Franck-Condon fit obtained geometries for c-C3HCl with high-level ab initio calculations. Note that the structure of the IEad ) 9.02 eV deviates significantly from the ab initio data in the C-C bond lengths.

Mass spectra also show peaks at m/z ) 72/74 when the pyrolysis is turned on. This isotopic pattern points to the chlorine carrying molecule of the formula C3HCl. We assign these peaks to chlorocyclopropenylidene (c-C3HCl), which is the most stable isomer of this composition. A vibrational mode of 1160 cm-1 dominates the spectrum. Again, the TPES of c-C3DCl was recorded in addition. The fact that the adiabatic ionization energies of both c-C3H2 and c-C3HCl lie at 9.17 eV can be understood by taking into account that the electron is removed in both species from the same molecular orbital, located at the carbene center. Obviously, the influence of the Cl atom is small and similar in the neutral and the ion. Ab initio computations yielded ionization energies in good agreement with the experimental values.

Figure 9. TPES of c-C3DCl (black line) and a convoluted line spectrum (red line) obtained from a Franck-Condon fit. Also the fitted geometry is given.

To verify the band assignment, the deuterated isotopomere c-C3D2 was studied as well. A negligible influence of deuteration on the ionization energies was observed. The ν3+ and ν2+ wavenumbers do not change significantly, confirming their assignment to the motion of the three-membered ring. In both spectra, a low-energy band is present, which most likely originates from the propargylene (HCCCH) isomer and

Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (contract Fi575/7-1). Travel subsidies were provided by SOLEIL through the European Commission programme “Transnational access to research infrastructures” and by the German-French binational PROCOPE program. B.N. and M.S. acknowledge fellowships by the “Fonds der Chemischen Industrie”. C.A. and B.M. acknowledge financial support from the RTRA “Triangle de la Physique” (Project “Radicaux” 2009-007T) and from the CAPES-COFECUB program No. 525/06 (France/Brazil). We also thank Michael Schmitt (University of Du¨sseldorf) for his help with the program “fcfit27”, Dominik Gehrig for his contributions to the experiment, and Reinhold Fink, Thomas Schmidt, Volker Settels, and Bernd Engels for useful discussions on ab initio computations and for giving us the opportunity to use their computers for calculations.

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