Electronic Transitions of C6H4+ Isomers: Neon Matrix and Theoretical

Article pubs.acs.org/JPCA

Electronic Transitions of C6H4+ Isomers: Neon Matrix and Theoretical Studies Jan Fulara,*,† Adam Nagy,† Karol Filipkowski,† Venkatesan S. Thimmakondu,‡ John F. Stanton,*,‡ and John P. Maier*,† †

Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712-0165, United States

‡

S Supporting Information *

ABSTRACT: Three open-chain isomers of C6H4+ and two cyclic ones were detected following mass-selective trapping in 6 K neon matrixes. The open-chain cations 5-hexene-1,3-diyne (CH2CH−CC−CC−H)+ and cis- (cis-HCC−CHCH−CCH)+ and trans-3-hexene-1,5-diyne (trans-HCC−CHCH−CCH)+, possess two absorption systems commencing at 609 and 373, 622 and 385, and 585 and 373 nm, respectively. They are assigned to the 1 2A″ and 2 2A″ ← X̃ 2A″, 12A2 and 2 2A2 ← X̃ 2B1, and 1 2Bg and 2 2Bg ← X̃ 2Au electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 2A″ ← X̃ 2A″ electronic transitions of propargyl cyclopropene and 2 2B1 ← X̃ 2A2 of o-benzyne cation structures. The assignment of the electronic transitions is based on theoretical vertical excitation energies calculated with CASPT2 and EOMEE-CCSDT methods for 12 isomers of C6H4+. These have been carried out at the geometries optimized using several ab initio methods. Adiabatic excitation energies were calculated for the five identified isomers of C6H4+.

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INTRODUCTION A number of isomeric structures is possible with the C6H4 molecular formula.1 Among these, o-, m-, and p-benzynes have drawn attention due to their importance as reactive intermediates.2,3 These have been studied in rare gas matrixes by infrared spectroscopy,4−7 and the o- and m-isomers have also been studied in the gas phase by microwave spectroscopy.8,9 According to theoretical calculations, o-benzyne is the lowestenergy structure; the m- and p-forms lie 61 and 107 kJmol−1 above it.10 Upon thermal excitation, they can interconvert; furthermore, the p-isomer may undergo Bergman rearrangement to form Z-hex-3-ene-1,5-diyne.11,12 Benzyne anions have also been the target of theoretical13 and experimental studies.14−16 These revealed that the stability of the anions changes in a similar way as the neutral counterparts. Photoelectron studies of benzyne anions provide, in addition to the electron affinities, the energy of the first triplet state of the neutrals and some vibrational frequencies.17 In contrast, little is known about benzyne cations. Photodisscociation of Mg+ complexes with o-, m-, and p-difluorobenzenes lead to the formation of benzyne cations, and their subsequent fragmentation was studied by mass spectrometry.18 These experiments showed that, irrespective of the difluorobenzene isomer used for generation, the ions had the same structure. Time-of-flight studies of C6H4+ produced by femtosecond photolysis of dibromobenzenes led to the same conclusion. Upon excitation of benzyne cations above 2 eV, a hydrogen shift is expected.19 © XXXX American Chemical Society

At higher energies, p-benzyne cation undergoes ring-opening, whereas o-C6H4+ dissociates to neutral acetylene and the diacetylene cation. The ground and the lowest-energy excited electronic states of the three benzyne cations have been studied by theoretical methods.20 The lowest-energy electronic transitions of two open-chain isomers of C6H4+ have been observed in neon matrixes and in the gas phase.21,22 The geometry was established from the analysis of the rotational structure supported by theoretical calculations. The system with its origin at around 609 nm is the 2 A″ ← X̃ 2A″ transition of the 5-hexene-1,3-diyne cation, whereas the 580nm absorption is the 2Bg ← X̃ 2Au system of the trans-3-hexene-1,5-diyne cation.19 In this contribution, the electronic absorption spectra of C6H4+ isomers in 6 K neon matrixes recorded after mass selection from a variety of precursors in the ion source are presented. Apart from the known electronic transitions of the two hexenediyne cations, two higher-energy transitions of these species are reported. In addition, two electronic systems of cis3-hexene-1,5-diyne cation are observed. A reversible cis−trans photoisomerization of 3-hexene-1,5-diyne cation is evidenced Special Issue: Terry A. Miller Festschrift Received: July 30, 2013 Revised: September 24, 2013

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dx.doi.org/10.1021/jp407566h | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Chart 1. Structures and Optimal Ground-State Energies of the C6H4+ Cations Calculated at Five Different Levels of Theorya

a

The symmetries and relative energies without and with (italics) ZPE correction are given under the structures. The energies are obtained at UHFMP2/cc-pVDZ (in black; 1st row), ROHF-MP2/cc-pVDZ (in red; 2nd row), EOMIP-CCSD/cc-pVDZ (in green; 3rd row), and EOMEA-CCSD/ cc-pVDZ (in purple; 4th row) levels. The energy difference between a given structure and that of the most stable one within a particular level of theory is given first. The highest point group symmetry of the structure for which a true minimum has been found is indicated in each row. Zeropoint corrections are not included in cases where the zero-point corrections at the MP2 level suffered from near-instabilities of the wave functions27 and unreasonable values are obtained.

with two wavelength-specific CCD cameras. The spectrum was recorded in several overlapping sections in the 250−1100 nm range starting from the longest wavelength and continued to higher energies. After completing the scans in the UV, the spectrum was recorded again in the visible region to check whether the UV measurement caused a photoconversion of the trapped cations. Fluorescence experiments were carried out on the same matrix as that for the absorption. A pulsed, tunable laser excited the sample at a wavelength that coincided with a specific absorption band. The emission was collected perpendicular to the matrix surface (45° to the excitation beam) by a lens that collimated the light onto the entrance of the optical fiber. Wavelength-dispersed fluorescence was recorded in several overlapping sections starting at a wavelength 2 nm longer than that of the laser photons to avoid saturation of the CCD. At a later stage, the C6H4+ cations were irradiated at wavelengths of absorption bands in the UV to attempt photoconversion of the C6H4+ isomers. A high-pressure xenon lamp fitted with band-pass filters, or a laser, were used for this purpose. The matrix was photobleached with λ > 260 nm photons from a medium-pressure mercury lamp equipped with a water filter to distinguish the absorptions of the cations from neutral species. Computational. Twelve isomers of C6H4+ were chosen for a computational characterization (Chart 1). The lowest-energy structures as well as those plausible from a specific precursor or reaction for the production of the cations were selected. The ground-state geometries were first optimized using density

upon excitation into the UV system. The assignment of the electronic spectra of the different C6H4+ isomers is based on theoretical calculations of their excitation energies. An absorption system at around 420 nm is tentatively assigned to the electronic transition of the propargyl cyclopropenyl and the o-benzyne cations.

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METHODS Experimental Section. The experimental setup has been described;23 it combines mass selection with matrix isolation. A mixture of ions and neutrals was produced from precursors in a hot-cathode discharge ion source. Several acyclic (1,4-hexadiyne, 2,5-hexadiyne, acetylene + cis-1,2-dichloroethene, acetylene + trans-1,2-dichloroethene, and ethene) and cyclic (phenylacetylene, 1,2-dibromobenzene, 1,4-dibromobenzene, benzene-1,2-dicarboxylic acid, and biphenyl) precursors have been used to form C6H4+ ions. These were extracted from the source by lenses and guided through an electrostatic deflector to a quadrupole mass filter, where the ions with m/z = 76 were separated with a resolution better than 1 amu. They were then co-deposited with excess neon containing a small amount of CH3Cl (20 000:1) onto a rhodium-coated sapphire substrate held at 6 K for 3−5 h to form a ∼150 μm thick matrix. The matrix was probed by broad-band light from a halogen or a high-pressure xenon lamp guided through the thin side of the matrix parallel to the substrate. The light exiting was collimated into a bundle of quartz fibers, which then illuminated the input slit of a 0.3 m spectrograph equipped B



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Figure 1 shows an overview spectrum obtained at 6 K after co-deposition of mass-selected C6H4+ with excess neon diluted

functional theory (DFT) with the UB3LYP functional and the correlation-consistent polarized valence triple-ζ basis set (ccpVTZ).24 Later, geometry optimizations were also done with second-order Møller−Plesset (MP2) perturbation theory,25,26 equation-of-motion ionization potential coupled cluster with single and double excitations (EOMIP-CCSD),27 and with the EOM electron-attached CCSD (EOMEA-CCSD) approximation.28 As reference wave functions, spin-unrestricted Hartree− Fock (UHF) and spin-restricted open-shell HF (ROHF) determinants were chosen for the MP2 geometry optimizations. All calculations except EOMEA-CCSD were done in the frozen core approximation. In the latter, all electrons were correlated due to a software limitation. Except for DFT, the cc-pVDZ basis set24 was used in all calculations. The EOMIP-CCSD calculations were started from the corresponding closed-shell neutral molecule C6H4, whereas EOMEA-CCSD calculations were from the closed-shell dications (C6H42+). Therefore, the reference wave function was restricted HF (RHF) in both cases, and these methods are spin-adapted. Consequently, the results are free from spin contamination and problems due “near instabilities” of the HF wave function.29 While the ground-state geometry optimizations were done at all 4 levels of theory for the 12 isomers given in Chart 1, the excited-state geometry optimizations of a few selected isomers (B+, C+, D+, and T+) were also carried out with the EOMIPCCSD method. This was necessary not only to calculate the geometry changes and zero-point energy contributions that distinguish the vertical and adiabatic excitation energies (ΔEadi) but also to compute harmonic vibrational frequencies to check whether it is a minimum, transition state, or a higher-order saddle point. Vertical excitation energies (ΔEver) were computed at the MP2 geometries with equation-of-motion excitation energy EOMEE-CCSD using UHF and ROHF reference wave functions. The complete active space second-order perturbation theory (CASPT2) was also used with unrestricted orbitals for vertical excitation energy calculations. The active space comprised 13 electrons distributed over 13 orbitals. The isomers of C6H4+, for which the electronic transitions have been assigned, were studied at a yet higher level of theory, the full CCSDT approximation and EOMEE-CCSDT calculations using the MRCC program.30 Moreover, the adiabatic excitation energies were estimated using EOMIP-CCSD or EOMEACCSD (depending upon the electronic transition) and groundand excited-state geometries in order to estimate the geometry relaxation. Relaxation energies so computed were then combined with the high-level vertical energies to obtain the best estimates for the adiabatic energy difference (finalΔEadi). DFT calculations were carried out using the Gaussian 09 program suite,31 coupled cluster calculations were conducted with the CFOUR and MRCC program packages,32 whereas CASPT2 calculations were carried out with Molcas.33

Figure 1. Spectrum recorded after deposition of mass-selected C6H4+ in a neon matrix (blue trace) and after photobleaching with λ > 260 nm photons (red trace). The cations (m/z = 76) were produced from phenylacetylene in a hot cathode discharge source. Weak absorptions of the C6H4+ fragments: HC6H+, HC4H+ and H2C6H+ are observed, and their known electronic transitions are shown as green traces.

with CH3Cl, 1:20 000, (blue trace). The cations were produced in the source from phenylacetylene mixed with helium in the ratio of 1:4. The red trace is the spectrum measured after irradiation of the matrix with λ > 260 nm photons. Three clear systems with onsets at 609, 585, and 373 nm decreased in intensity upon photobleaching. The former two are the transitions of the D+ and B+ structures of C6H4+, respectively, identified in the earlier studies. Weak absorptions of fragment ions, HC6H+, HC4H+, and H2C6H+, have also been detected in the spectrum prior to irradiation. These are products of collision-induced dissociation of m/z = 76 cations during the growth of the matrix. Their known transitions34,35 are shown in the green traces in Figure 1. The absorption bands of the fragment ions behave in a similar manner to the three prominent systems (609, 585, and 373 nm) upon exposure to UV light. Therefore, it can be assumed that the new 373 nm system originates also from cations, most likely from C6H4+. A series of precursors was used for the generation of C6H4+ to deduce the structure of the carrier of the 373 nm system. These included 1,4-hexadiyne, phenylacetylene, 1,2-dibromobenzene, 1,4-dibromobenzene, and benzene-1,2-dicarboxylic acid. The spectra obtained are shown in Figure 2 (traces a− e). The same three main systems at 609, 585, and 373 nm are present; however, their intensities vary with the precursor used. To facilitate the comparison, the spectra were normalized to the 609 nm band. The intensity ratio of the B+ origin band at 585 nm to the one of D+ at 609 nm is smallest for 1, 4-hexadiyne and increases in the series a−e. The intensity of the 373 nm system origin band behaves in a similar way to the 585 nm origin of B+. It might be presumed that both systems originate from the same B+ structure, but in the case of the 1,4-hexadiyne precursor, the 585 nm band is too weak in comparison to that at 373 nm to be consistent with this interpretation. Moreover, the 373 nm system is composed only of three broader and three sharper bands; the latter do not belong to this system. Using other precursors the 373 nm band appears stronger with

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OBSERVATIONS AND DISCUSSION Strongest Systems of C6H4+. The electronic transitions of two isomers of C6H4+ were studied earlier in the gas phase and neon matrixes.21,22 Two systems with onset at 609 and 585 nm were reported in solid neon following a mass-selective deposition of m/z = 76 cations. The cations causing the absorptions were identified as isomers D+ and B+, respectively (Chart 1), on the basis of high-resolution gas-phase spectra. In this paper, we discuss further spectroscopic studies on C6H4+ isomers isolated in neon. C



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in this way are shown in the right panel of Figure 3. Table 1 contains the wavelengths of the absorptions constituting the spectra of the isomers. Table 1. Band Maxima (±0.1 nm) and Assignment of the Absorptions of C6H4+ Isomers in 6 K Neon Matrixes

Figure 2. Electronic absorption spectra of C6H4+ in neon matrixes obtained from different precursors (a) 1,4-hexadiyne, (b) phenylacetylene, (c) 1,2-dibromobenzene, (d) 1,4-dibromobenzene, and (e) benzene-1,2-dicarboxylic acid. The spectra are normalized to the same intensity of the 609 nm band.

a well-developed vibrational structure. This is better seen in Figure 3 where the spectra of C6H4+ obtained from the 1,4-

λ /nm

ν̃ cm−1

Δ cm−1

608.8 600.4 592.6 590.8 579.5 575.8 573.7 566.1 558.5 543.1 528.8 372.8 366.3 359.0

16426 16656 16875 16926 17256 17367 17431 17665 17905 18413 18911 26824 27300 27855

0 230 449 500 830 941 1005 1239 1479 1987 2485 0 476 1031

585.5 577.7 568.5 554.4 552.9 539.4 526.0 373.0 366.5 359.9 353.3 347.2 341.2

17079 17112 17590 18038 18086 18539 19011 26810 27285 27785 28305 28802 29308

0 231 511 959 1007 1460 1932 0 475 975 1495 1992 2498

622.0 385.1

16077 25967

0 0

assignment Isomer D+ 000 1 2A″ ← X̃ 2A″ ν16 ν15 ν14 ν14 + ν15 2 ν14 ν10 ν8 ν6 ν6 + ν14 000 2 2A″ ← X̃ 2A″ ν15 ν11 Isomer B+ 000 1 2Bg ← X̃ 2Au ν9 ν8 ν8 + 2ν9 2ν8 ν4 ν4 + ν8 000 2 2Bg ← X̃ 2Au ν8 2ν8 3ν8 4ν8 5ν8 Isomer C+ 000 1 2A2 ← X̃ 2B1 000 2 2A2 ← X̃ 2B1 Isomers T+ and K+

423.0 417.0 407.8 401.3 399.7

Figure 3. Comparison of the C6H4+ absorptions produced from 1,4hexadiyne (red trace) and phenylacetylene (blue trace). The systems of B+ and D+ are very distinct in the visible region, whereas in the UV, they overlap. Isomer D+ is predominately formed in the ion source from the 1,4-hexadiyne precursor. After scaling of the intensity of the origin bands of B+ and D+ at 585 and 608 nm and subtracting the absorptions in the UV, clean spectra of B+ and D+ are obtained and shown in the right panel.

23641 23981 24522 24919 25019

0 541 938 1038

000 1 2A″ ← X̃ 2A″ T+ and 2 2B1 ← X̃ 2A2 K+

B+ ↔ C+ Photoisomerization. The earlier spectroscopic studies on C6H4+ revealed that the intensity of the 585 nm B+ system decreased each time that the spectrum was measured in the UV range. This suggests that B+ possesses an electronic transition in the UV and that the cation photoconverts to another structure (or decomposes). To avoid this, the exposure of the matrix to the UV light during measurements was significantly reduced. The number of accumulations was limited to 10 scans, and the region below 320 nm (where no absorptions were detected earlier using another setup with a monochromatic light source and a photomultiplier) was not scanned. After taking these precautions, the UV absorptions could be measured without changing the intensity of the 585 nm system. The photoconversion of C6H4+ was investigated by exposing the matrix to UV light with the wavelengths where the strong

hexadiyne and phenylacetylene precursors (red and blue traces, respectively) are compared. One can conclude that the absorption commencing at 373 nm originates from two overlapping systems of two C6H4+ isomers. These are the B+ and D+ structures, as is argued below. If the two spectra in Figure 3 are scaled to the intensity of the origin band of the first isomer at 585 nm and subtracted, the spectrum of the second one in the visible and UV ranges is obtained. Repeating this procedure for the second isomer, the isolated spectrum of the first one is inferred. The UV transitions of B+ and D+ separated D



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seen in Figure 3, were obtained using the data from the irradiation experiments and processing them in the same way as described above. The structure of B+ (Chart 1) has been established as the trans isomer of C6H4+ from the analysis of the 585 nm system recorded in the gas phase.36 The present studies show that isomer B+ upon excitation into the 373 nm system photoconverts to C+, which possesses two electronic transitions with onsets at 622 and 385 nm. The structure of C+ can be guessed from the structure of B+ because one can expect that the latter should isomerize to the cis form, a commonly observed process for such systems. Photoisomerization of B+ to C+ has a reversible character, as seen in Figure 4. After the third irradiation (Irr3) with the same conditions as Irr1, the system ended up at the same stage as it was after Irr1 (green trace in Figure 4). Thus, the C6H4+ cations survive in the matrix several irradiation cycles at the wavelengths 385 and 374.6 nm. 417 nm System. Apart from the intense systems with onsets at 609, 585, and 373 nm and two weak transitions of C+ at 622 and 385 nm in the spectra of C6H4+ produced from different precursors (Figure 2), a moderately intense system commencing at 417 nm is present. The intensity of this absorption varies with the precursor and is largest for dibromobenzenes. The absorption system of C6H4+ produced from 1,2-dibromobenzene is shown in Figure 5. The intensity

absorptions are detected. When the 585 nm system decreased after UV photolysis at a specific wavelength, the absorptions commencing at 373 nm also diminished; moreover, a weak broad absorption centered at 622 nm appeared. This absorption was overlooked in the earlier study because such broad structures are often observed as an undulating background caused by interference of light probing the matrix. The results of the photoconversion studies are summarized in Figure 4 for C6H4+ generated from phthalic acid as a

Figure 4. Reversible photoisomerisation of B+ and C+ isomers of C6H4+ in a neon matrix. The blue trace shows the absorptions detected after deposition of mass-selected C6H4+ produced from phthalic acid. The red trace was recorded after irradiation into the 385 nm band of C+ (red arrow) with λ = 390 ± 5 nm photons. The spectrum in purple was obtained after laser irradiation into the 373 nm band of B+ at the wavelength marked with the purple arrow. Broad absorptions of C+ at 385 and 622 nm have been recovered to their intensity, as in trace AD. The absorptions obtained after the exposure of the matrix to λ = 390 ± 5 nm photons are in green.

precoursor for which the highest intensity ratio of the 585−609 nm bands was observed. The UV system and the section in the visible region where the origins of B+ and D+ are present are shown in the blue trace (AD). A weak, broad band centered at 622 nm marked with an arrow and label C+ is present. The red trace (labeled Irr 1) shows the sections in the same regions recorded after 30 min of irradiation with λ = 390 ± 5 nm; the wavelength of the irradiating photons is marked with the red arrow. The absorptions commencing at 585 and 373 nm increased in intensity, whereas the 385 and 622 nm bands vanished. The 609 nm system remained unchanged under these conditions. In the next step, the same matrix was irradiated for 10 min with λ = 374.6 nm photons from a pulsed laser. The photon’s wavelength is marked with a purple arrow and is located on the long-wavelength slope of the 373 nm band. The spectrum obtained after the second irradiation (Irr2) is shown in purple. The absorptions starting at 585 and 373 nm diminished to a lower level than in the spectrum recorded after deposition of the cations (AD); at the same time, the bands at 385 and 622 nm recovered their intensities. The results of these experiments confirm that the 585 nm system and part of the 373 nm one belong to the same isomer, B+. The latter system overlaps with the second transition of D+. The separated UV spectra of B+ and D+, identical with those

Figure 5. The 417 nm absorption systems of C6H4+ in a neon matrix. The cations were generated from 1,2-dibromobenzene in the discharge source.

of this system remained unchanged upon irradiation of the matrix with visible and UV light, except upon exposure to a medium-pressure mercury lamp equipped with a water filter only. Under such conditions, the absorptions of all cations decreased due to recombination with electrons photodetached from anions, which balanced the positive charge in the matrix. The anions were produced from the CH3Cl electron scavenger added to neon. One can presume that the 417 nm feature originates from a cyclic structure of C6H4+ as cyclic C6+ has been detected from the C6Br6 precursor.36 The possible structure of the carrier of the 417 nm absorptions is discussed in the Computational section. D+ Fluorescence. A structured emission commencing at 609 nm has been observed upon laser excitation of the C6H4+ cations generated from phthalic acid. The onset of the fluorescence coincides with the origin of the 609 nm system of D+. The emission spectrum is shown in Figure 6. It was E



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Table 2. Band Maxima (±0.1 nm) and Assignment in the Fluorescence Spectrum of Isomer D+ in a 6 K Neon Matrix λ /nm

ν̃ cm−1

Δ cm−1

608.8 617.1 627.9 635.6 648.3 650.8 656.6 669.4 691.7 702 716.5 727

16426 16205 15926 15733 15425 15366 15230 14939 14457 14245 13957 13755

0 221 500 693 1001 1060 1196 1487 1969 2181 2469 2671

assignment 000 1 2A″ → X̃ 2A″ ν16 ν14 ν12 2ν14 ν11 ν12 + ν14 ν8 ν6 ν5 ν6 + ν14 ν5 + ν14

MP2 with a UHF reference (black), MP2 based on ROHF orbitals (red), EOMIP-CCSD (green), and EOMEA-CCSD (purple). All energies are reckoned with respect to the propargyl cyclopropene cation (isomer T+), which is both classically stable (it is a resonance-stabilized radical, propargyl, substituted by an aromatic C3H2+ ring) and well-behaved theoretically. At all levels of theory, the most stable isomer of C6H4+ is L+, which is the cation of m-benzyne; the other two benzyne isomers J+ and K+ are not local minima at the EOMIP-CCSD level (which was used for the zero-point correction for all isomers as this level of theory is the most stable in the current application); therefore, it would seem that the second most stable structure is the doubly resonance stabilized T+ form. All other (acyclic) isomers are considerably higher in energy but are generally favored from the acyclic precursors that have ultimately led to the production of isomers B+, C+, and D+ in these experiments. The benzyne cation isomers J+, K+, and L+ have been studied previously using CASPT2 with an atomic natural orbital basis set.20 Because L+ is the most stable isomer of C6H4+ and benzynes are known to present difficulties for theory,37 it was felt worthwhile to carry out our own CASPT2 calculations. Accordingly, geometry optimization of the benzyne cations using the cc-pVDZ basis set and CASPT2 was carried out. The most stable is L+, and K+ and J+ lie 58.0 and 87.0 kJ/mol above it, respectively. 2A2 ground states for isomers K+ and L+ were obtained, the former in disagreement with ref 18 (but in accord with present DFT, MP2, and EOM-CCSD results) but the latter in agreement with the previously reported results for the most stable isomer L+. An interesting feature of L+ is that it adopts a bicyclic structure in the electronic ground state, with a relatively close transannular interaction between the two formally divalent carbon atoms. It should be noted that this question, “bicyclic” or “monocyclic”, also occurs in theoretical treatments of the neutral ground electronic state of mbenzyne.36 The theoretical issues surrounding these benzyne cation isomers are difficult, and it behooves one to be careful in interpreting them. Full documentation of the optimized structures, vibrational frequency calculations, and so forth may be found in the copious Supporting Information (Tables S1−S47). Excitation Energies of B+, C+, and D+. Though none of the B+, C+, and D+ structures were calculated to be the most stable at any level of theory, their transitions dominate the absorption spectrum of C6H4+ in this study. These univocally

Figure 6. Fluorescence (red) and absorption (green) spectra of isomer D+ of C6H4+ in a neon matrix. The spectra are plotted on a wavenumber scale to emphasize their mirror image nature. The onsets of fluorescence and absorptions are scaled to the same intensity. The asterisk marks the origin band of isomer B+.

recorded in 4 overlapping sections, each averaged over 1000 laser shots. The first segment 603−662 nm contains the onset of the fluorescence and was recorded upon excitation of the 590.8 nm absorption band of D+. The remaining sections were measured when the origin band of D+ at 608.8 nm was excited. The spectra were normalized by the bands in the overlapping regions. The fluorescence spectrum was compared with the absorptions of D+ produced from 1,4-hexadyine, where this isomer predominates. The spectra are plotted on a wavenumber scale to emphasize the mirror image nature. The strongest band in emission and absorption is the origin, and the next most intense one in both spectra is spaced at 500 cm−1 lower and higher energies from it. Fluorescence was detected in the spectral window containing the origin band at 609 nm following laser excitation at the wavelengths of all of the visible bands of D+. The laser was also scanned across the UV system commencing at 373 nm; however, no emission was detected in the 603−662 nm range or at wavelengths > 376 nm, nor upon excitation into the 417, 385, and 622 nm systems. It appears that only isomer D+ decays radiatively and that the excited electronic state that corresponds to the 609 nm system is the lowest-energy one above the ground state. Band maxima of the fluorescence together with the assignment based on calculated energies of normal modes (Supporting Information) of D+ are collected in Table 2. Frequencies of seven vibrations, ν16, ν14, ν12, ν11, ν8, ν6, and ν5, have been inferred from the spectra. The first two are in-plane bending deformations of the carbon chain, whereas ν6 and ν5 are triple bond stretches. Overtone and combination bands of these modes are discernible in the spectrum.

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THEORY AND EXPERIMENT Ground State of C6H4+ Isomers. The structures of isomers selected for the theoretical calculations are shown in Chart 1. The point group symmetries and relative ground-state energies calculated at four levels of theory, with (in italics) and without zero-point energy corrections, are shown below each structure. From top to bottom, the energies given are from F



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Table 3. Excitation Energies (in eV) of B+, C+, D+, K+, and T+ Isomers of C6H4+ Calculated at Different Levels of Theories Compared with the Neon Matrix Dataa CASPT2

transitions + ̃ 2 B X Au → 1 2Bg → 2 2Bg → 3 2Bg C+ X̃ 2B1 → 1 2B1 → 2 2B1 → 1 2A2 → 2 2A2 → 3 2A2 D+ X̃ 2A″ →1 2A″ → 2 2A″ → 3 2A″ → 4 2A″ T+ X̃ 2A″ → 1 2A″ → 2 2A″ → 3 2A″ → 4 2A″ K+ X̃ 2A2 → 1 2A2 → 2 2A2 → 2 2B2 → 3 2B2 → 4 2B2 → 2 2B1 → 3 2B1 → 4 2B1

UHF-EOMEECCSD

ROHF-EOMEECCSD

CCSDT

EOMEECCSDT

EOMIP or EOMEA-CCSD

UHF

ROHF

UHF

ROHF

ΔEvert

f

ΔEvert

f

ΔEvert

f

ΔEvert

ΔEvert

ΔEvert

ΔEvert

ΔEvert

ΔEadi

2.11 3.64 6.08 3.50 6.04 2.18 3.73

0.073 0.49 0.0001 0.054 0.014 0.065 0.32

2.16 3.65 4.26 5.04 3.35 5.03 6.27

0.16 0.0012 0.25 0.023 0.088 0.52 0.065

− 3.73 − 3.56 − 2.21 3.67 − 2.19 3.55

3.31

3.33

3.29

3.28

2.49 3.45 − 3.75 − 2.34 3.47 − 2.38 3.80 6.46 7.11 3.40

0.0003 0.0019 0.0001 0.060 0.41

0.10 0.56 0.0007 0.040 0.017 0.050 0.33 0.0003 0.14 0.033 0.26 0.070 0.076 0.0042 0.35 0.0002 0.019 0.054 0.0014 0.0004 0.0022 0.0006 0.0098 0.053

2.34 3.74 − 3.60 − 2.23 3.67 − 2.21 3.59

4.37 4.45 4.71 3.00 4.88

2.42 3.85 5.89 3.65 5.17 2.30 3.79 5.87 2.27 3.68 4.25 4.66 3.35 4.63 5.16 5.90 4.52 5.58 4.53 4.95 5.47 1.93 3.19 5.08

2.40 − − 3.67 − 2.27 − − − −

0.19

0.12 0.54 0.0008 0.044 0.020 0.062 0.32 0.0008 0.16 0.050 0.21 0.088 0.083 0.0045 0.35 0.0022 0.022 0.053 0.0014 0.0004 0.0023 0.0006 0.011 0.055

2.34 − − 3.72 − 2.21 − − − −

5.11

2.55 3.87 5.91 3.83 5.24 2.41 3.82 5.89 2.41 3.83 4.28 4.71 3.43 4.72 5.22 5.93 4.50 5.68 4.48 5.03 5.55 1.94 3.28 5.08

ΔEadi

expt.

2.29 2.93 − 3.51 − 2.10 2.94 − 2.21 3.58

2.14 3.22 − 3.36 − 1.99 3.14

2.12 3.32

2.04 3.37

2.04 3.33

3.13

3.04

2.97

fin

1.99 3.22

2.97

The electronic transitions of each isomer are indicated in the first column. Oscillator strengths ( f) and EOMEA-CCSD energies are marked in italics.

a

the 373−609 nm (2.04 eV) systems of D+ is estimated to be 2.4. Though the calculated intensity ratio (∼1.5) of the electronic transitions to the 3 2A″ and 1 2A″ states agrees well with observations, the predicted excitation energy is about 1 eV too high, which is outside of the range expected from the calculations that have been done for this unproblematic isomer. According to the calculations, there is a 2 2A″ state ∼3.65 eV above the ground state, an excitation energy that matches better with the laboratory data. However, the calculated oscillator strength of the electronic transition to this state is smaller than that to 3 2A″. The finΔEadi adiabatic energy, extrapolated in the same way as that for 1 2A″, is 3.37 eV, which is 0.04 eV higher than that deduced from the spectrum of D+. It is tentatively concluded, with due acknowledgment of the discrepancy in calculated intensity, that this is the observed transition. Calculations of the excitation energies of isomer B+ using all three methods give similar results. They predict the 1 2Bg ← X̃ 2Au vertical electronic transition in the 2.13−2.55 eV energy range, and the finΔEadi value is 2.14 eV, in agreement with the experimental origin band at 2.12 eV (585 nm). The second 2 2Bg ← X̃ 2Au transition according to theory is predicted to be 5−7 times stronger than the first and lies in the 3.64−3.87 eV energy range. The finΔEadi value is 3.22 eV, which is in good agreement with 373 nm (3.32 eV) from the spectrum. The intensity ratio of the 373 and 585 nm systems determined from the spectrum of B+ is 5.4 and is close to the theoretical prediction, a pleasing situation in contrast to the case for isomer D +.

reveal that each cation possesses two relatively strong electronic transitions located in the visible and the UV regions. The results of CASPT2 and EOMEE-CCSD with UHF and ROHF reference wave functions on the vertical excitation energies are compared in Table 3. The former have been done only with unrestricted orbitals. Transitions with oscillator strengths greater than 1 × 10−4 and excitation energies smaller than 6 eV are included in the table. All calculations predict one strong transition in the visible and the other in the UV for each cation. For example, the lowest-energy 1 2A″ ← X̃ 2A″ transition of D+ is predicted by CASPT2, EOMEE-CCSD (UHF), and EOMEE-CCSD (ROHF) at 2.16, 2.41, and 2.27 eV, respectively, with oscillator strengths of 0.16, 0.16, and 0.14. The origin of the red system of D+ is located in the gas phase at 2.05 (2.04 eV in neon). As the calculated excitation energies are the vertical ones, agreement with experimental data is acceptable. However, while comparing this value with the best-estimate adiabatic excitation energy, the comparison is even better. Augmenting the vertical excitation energy of 2.21 eV obtained at the UHFEOMEE-CCSDT level, with a 0.17 eV geometry relaxation and zero-point correction obtained with EOMIP-CCSD, the estimated adiabatic excitation energy (finΔEadi) is 2.04 eV, in excellent agreement with the experiment (Table 3). The CASPT2, EOMEE-CCSD (UHF), and EOMEE-CCSD (ROHF) calculations predict the second strong transition of D+ at 4.26, 4.28, and 4.25 eV, respectively, with oscillator strengths of 0.25, 0.21, and 0.26. The origin of the UV system of D+ is at 3.33 eV (373 nm) in a neon matrix, and the intensity ratio of G



Article

Table 4. Vertical Excitation Energies (in eV) of C6H4+ Isomers Calculated with CASPT2, EOM Using UHF, and ROHF Reference Wave Functionsa CASPT2 transitions A+ X̃ 2B1 → 1 2B2 → 2 2 B2 → 3 2 B2 → 4 2 B2 → 5 2 B2 → 6 2 B2 → 7 2 B2 J+ X̃ 2B3u → 1 2B1g → 2 2B1g → 1 2Ag → 2 2Ag → 3 2Ag → 1 2B2g → 2 2B2g + ̃ 2 L X A2 → 1 2A2 → 1 2 B2 → 2 2 B2 → 2 2 B1 → 3 2 B1 Q+ X̃ 2B1 → 1 2B1 → 2 2 B1 → 1 2A2 → 2 2A2 → 3 2A2 + ̃ 2 M X A″ → 1 2A″ → 2 2A″ → 3 2A″ → 4 2A″ + ̃ 2 P X A″ → 1 2A″ → 2 2A″ → 3 2A″ → 4 2A″ + ̃ 2 E X A2 →1 2A2 →2 2A2 →3 2B1 →1 2B2 →2 2B2 →3 2B2 →4 2B2 a

UHF-EOMEE-CCSD

ROHF-EOMEE-CCSD

ΔEvert

f

ΔEvert

f

ΔEvert

f

1.65 2.29 3.28 3.34 3.80 4.52 5.23 5.16 5.48 1.60 5.44 6.53 1.56 5.25 5.43 2.04 5.62 5.30 5.73 3.37 4.37 1.72 5.26 5.96 2.95 3.48 3.88 4.26 2.14 3.34 3.95 5.93 3.85 5.25 4.57 1.28 2.79 3.49 4.48

0.019 0.0071 0.021 0.018 0.20 0.013 0.13 0.019 0.0043 0.058 0.0016 0.0042 0.0009 0.0028 0.042 0.0074 0.064 0.0002 0.0015 0.017 0.2 0.069 0.15 0.0038 0.0028 0.0000 0.016 0.099 0.0072 0.52 0.13 0.0041 0.039 0.59 0.016 0.0062 0.0019 0.039 0.063

1.88 2.54 3.10 3.53 3.94 4.96 5.22 4.83 5.47 1.45 3.75 5.57 1.86 5.03 4.95 2.39 4.62 5.56 5.93 3.57 4.67 1.93 4.59 5.71 2.36 3.23 3.89 4.34 2.53 3.47 4.11 5.07 3.11 5.17 4.82 1.74 3.06 3.74 5.05

0.019 0.0012 0.069 0.13 0.031 0.046 0.5 0.0002 0.0010 0.03 0.0001 0.0001 0.0009 0.0007 0.0022 0.0072 0.0007 0.0010 0.0008 0.028 0.24 0.088 0.04 0.13 0.0059 0.0026 0.2 0.0004 0.0042 0.52 0.12 0.01 0.0079 0.29 0.014 0.0073 0.0051 0.011 0.08

1.85 2.48 2.94 3.51 3.78 4.71 5.11 4.89 5.43 1.40 3.75 5.56 1.80 5.06 4.89 2.25 4.50 5.49 5.82 3.51 4.62 1.95 4.58 5.75 2.34 3.05 3.75 4.36 2.27 3.51 4.00 4.89 3.07 5.06 4.85 1.71 3.10 3.74 5.06

0.016 0.0036 0.039 0.12 0.0062 0.0002 0.61 0.0001 0.0010 0.028 0.0001 0.0001 0.0008 0.0007 0.0027 0.0066 0.0008 0.0010 0.0007 0.023 0.24 0.087 0.058 0.11 0.0025 0.0014 0.18 0.0001 0.0021 0.54 0.12 0.0094 0.0040 0.26 0.013 0.0073 0.0048 0.011 0.078

In the first column, electronic states involved in transitions are indicated. Oscillator strengths are marked in italics.

Other Isomers of C6H4+ and the 417 nm System. To identify the cation that could be responsible for the 417 nm system (Figure 6), the excitation energies of the remaining nine isomers of C6H4+ were calculated using the same methods as those applied for the B+, C+, and D+ structures. The excitation energies and oscillator strengths are collected in Tables 3 and 4. All three methods, CASPT2, EOMEE-CCSD/UHF, and EOMEE-CCSD/ROHF, predict reasonably well the energy of the first excited state of the B+, C+, and D+ cations. Inspection of Table 4 allows an eliminatation of some isomers as the carrier of the 417 nm system. These are A+, J+, and Q+ because the calculations predict strong transitions in the near-infrared at ∼750 nm (1.6−1.7 eV) and no absorptions have been detected in this region. The bicyclic isomer L+ according to the CASPT2 calculations has a low-energy and weak transition at 2.04 eV with oscillator strength 0.0074 and two considerably stronger systems at around 5.43 and 5.62 eV.

Isomer C+ is the last among three for which the structure has been determined univocally by experiment. It is structurally similar to B+ (cis-modification of B+). Two transitions energies of 1.99 and 3.22 eV have been detected in solid neon; the latter is ∼2 times more intense than the former. The calculations locate the first 1 2A2 excited state 2.2−2.4 eV (vertically) above the X̃ 2B1 ground state, which lies quite near to the red system of C+. The finΔEadi value is 1.99 eV, which is in agreement with the observation (1.99 eV). The next two electronic states 1 2B1 and 2 2A2 are predicted in the ranges of 3.50−3.83 and 3.73− 3.82 eV above X̃ 2B1, respectively. The calculated oscillator strength of the transition to the 1 2B1 state is almost an order of magnitude smaller than the ones to 2 2A2. Therefore, the 385 nm (3.22 eV) system is assigned to the 2 2A2 ← X̃ 2B1 electronic transition of C+. The finΔEadi value is 3.14 eV, acceptably close to the experimental origin at 3.22 eV. H



Article

The M+ isomer can also be excluded from consideration because the first predicted allowed electronic transition is located at a much higher energy (above 4.0 eV) than the observed one. There is an additional and compelling argument against M+; one can expect that the most efficient method of M+ production is dissociative ionization of 1,4-hexadiyne with elimination of neutral H2. One CH3 group of the precursor remains intact during dissociation. Therefore, the 417 nm system should be the strongest in the case of this precursor, in contrast to the observations. After the considerations outlined above are applied to the analysis, four structures K+, E+, T+, and P+ remain for further discussion. All of these are predicted to possess a strong transition in the proper energy range (∼3.3 eV). Three (K+, E+ and T+) contain rings with six, five, and three carbons, respectively. The last one is an open-chain structure and can probably be excluded because the 417 nm absorption has not been observed from acyclic precursors, particularly from 1,5hexadiyne, structurally similar to P+. The strongest absorptions at around 417 nm have been detected using dibromobenzenes for the generation of C6H4+. In the past, C6+ has been studied in a neon matrix by absorption spectroscopy. Linear and cyclic C6+ were produced from C6Cl6+, whereas only the cyclic isomer from C6Br6 was produced under similar conditions to those in the present experiments.38 Less energy is needed to cleave a C−Br bond than that for C−Cl, and discharge conditions in the ion source are milder in the former case. The formation of cyclic C6+ from C6Br6 suggests that a similar process occurs during generation of C6H4+ from C6H4Br2. One can expect that K+, L+, or J+ is chiefly formed in the ion source depending on which isomer of dibromobenzene (o-, m-, or p-) is used for the production. However, this supposition is in conflict with the observations. An equally intense 417 nm system was observed regardless of which isomer of dibromobenzene was used for generation of cations. It is concluded that K+, L+, and J+ isomerize in the source to the same form (presumably L+ because it is found to be most stable computationally and the other two isomers are not predicted to be local minima, although the caveat noted earlier about interpreting the theoretical results is operative here), or perhaps, all of them undergo more significant rearrangement. A similar effect was observed in a femtosecond photodissociative ionization of o-, m-, and p-dibromobenzens, which led to the C6H4+ ions. The C6H4+ cations decayed with the same rate irrespective of the precursor, suggesting production of the same isomer.19 Also, formation and dissociation of C6H4+ produced upon UV irradiation of Mg+ complexes with three isomers of difluorobenzene led to the same isomer of C6H4+ with all precursors used.18,39 It was suggested that this is the obenzyne cation K+. The most stable cation among these is calculated to be L+ (Chart 1); however, it was excluded from consideration on the basis of calculated excitation energies. The excitation energy of isomer K+ from the 2A2 ground state fits well with the position of the origin of the 417 nm system. The excitation energies of K+ have also been calculated with CASPT2 also for the X̃ 2B1 ground state found in ref 18. The electronic transitions predicted are at 3.05, 4.98, and 5.58 eV, with oscillator strengths of 0.057, 0.0053, and 0.017, respectively, and they do not differ much from those starting in the X̃ 2A2 state (Table 3). In the present experiments, C6H4+ ions produced in the source have enough excess energy to undergo isomerization, as

alluded to above. Besides ring opening (B+, C+, and D+ were also observed from benzene derivatives) and a hydrogen shift, they may also undergo a ring rearrangement to form five and three carbon ring structures. It is well-known that electronically excited benzene isomerizes to higher-energy structures (fulvene, benzvalene, and others).40 It cannot be excluded that C6H4+ with a six-membered carbon ring geometry rearranges to a five- (E+) or three-membered one (T+) if a small energy barrier separates them. According to our calculations (Chart 1), the resonance-stabilized cation T+ is clearly the lower-energy structure; E+ lies considerably higher in energy. The most intense vibrational band of the 417 nm system of C6H4+corresponds to a frequency of 541 cm−1, close to the 580 and 530 cm−1 fundamentals in the excited state of K+ and T+, respectively (Supporting Information). Calculations for the ground state of E+ predict 660 cm−1 for the vibration, significantly different from the observed spectrum. A Franck−Condon (FC) simulation for the 1 2A″ ← X̃ 2A″ transition of isomer T+ was carried out using the ground-state geometry and harmonic force field obtained with EOMIPCCSD and the cc-pVDZ basis set and an upper-state geometry and harmonic force field calculated with EOMIP-CCSD with the same basis set. The result of this simulation is shown in Figure 7 along with the observed spectrum near the 417 nm

Figure 7. The 417 nm absorption systems of C6H4+ in a neon matrix, superimposed onto a FC simulation of the 1 2A″ ← X̃ 2A″ electronic transition of isomer T+.

feature. It is clear that the agreement is very good, which lends support to the idea that isomer T+ is the carrier of this series of bands. While isomer K+ is not a minimum on the EOMIPCCSD potential energy surface and therefore not suitable for a similar FC simulation, we conservatively remind the reader that this is a challenging isomer for theory (unlike T+, which is welltreated at the present level of theory) and that it cannot be logically discarded from consideration simply because the FC simulation of the isomer is in rather good agreement with the observations. Taking into account all of foregoing arguments, the 417 nm system is tentatively assigned to the 2 2B1 ← X̃ 2A2 electronic transition of K+ or alternatively the 1 2A″ ← X̃ 2A″ electronic transition of T+, with a preference for the latter assignment. It cannot be excluded that both cations have contributions in this region because a weak, not assigned band at 423 nm (Figure 5) could be the origin of the electronic transition of another I



Article

isomer of C6H4+. Further experimental and theoretical studies would be helpful in disentangling this problem.

(2) Alabugin, I. V.; Manoharan, M. Radical-Anionic Cyclizations of Enediynes: Remarkable Effects of Benzannelation and Remote Substituents on Cyclorearomatization Reactions. J. Am. Chem. Soc. 2003, 125, 4495−4509. (3) Zhang, F.; Parker, D.; Kim, Y. S.; Kaiser, R. I.; Mebel, A. M. On the formation of ortho-Benzyne (o-C6H4) under single collision condition and it’s role in interstellar chemistry. Astrophys. J. 2011, 728, 141/1−141/10. (4) Sander, W.; Exner, M.; Winkler, M.; Balster, A.; Hjerpe, A.; Kraka, E.; Cremer, D. Vibrational Spectrum of m-Benzyne: A Matrix Isolation and Computational Study. J. Am. Chem. Soc. 2002, 124, 13072−13079. (5) Winker, M.; Sander, W. Matrix Isolation and Electronic Structure of Di- and Tridehydrobenzenes. Aust. J. Chem. 2010, 63, 1013−1017. (6) Wenk, H. H.; Balster, A.; Sander, W.; Hrovat, D. A.; Borden, W. T. Matrix Isolation of Perfluorinated p-Benzyne. Angew. Chem., Int. Ed. 2001, 40, 2295−2298. (7) Radziszewski, J. G.; Hess, B. A.; Zahradnik, R. Infrared Spectrum of o-Benzyne: Experimental and Theory. J. Am. Chem. Soc. 1992, 114, 52−57. (8) Godfrey, P. D. Microwave Spectroscopy of Benzyne. Aust. J. Chem. 2010, 63, 1061−1065. (9) Kukolich, S. G. Molecular Structure of o-Benzyne from Microwave Measurements. J. Phys. Chem. 2004, 108, 2645−2651. (10) Li, H.; Yu, S.-Y.; Huang, M.-B.; Wang, Z.-X. The S1 States of o-, m-, and p-Benzyne Studied Using Multiconfiguration Second-Order Perturbation Theory. Chem. Phys. Lett. 2007, 450, 12−18. (11) Zhang, X.; Maccarone, A. T.; Nimlos, M. R.; Kato, S.; Bierbaum, V. M.; Ellison, G. B.; Ruscic, B.; Simmonett, A. C.; Allen, W. D.; Schaefer, H. F., III. Unimolecular Thermal Fragmentation of orthoBenzyne. J. Chem. Phys. 2007, 126, 044312. (12) McMahon, R. J.; Halter, R. J.; Fimmen, R. L.; Wilson, R. J.; Peebles, S. A.; Kuczkowski, R. L.; Stanton, J. F. Equilibrium Structure of cis-Hex-2-ene-1,5-diyne and Relevance to the Bergman Cyclization. J. Am. Chem. Soc. 2000, 122, 939−949. (13) Nash, J. J.; Squires, R. R. Theoretical Studies of o-, m-, and pBenzyne Negative Ions. J. Am. Chem. Soc. 1996, 118, 11872−11883. (14) Wenthold, P. G.; Hu, J.; Squires, R. R. o-, m-, and p-Benzyne Negative Ions in the Gas Phase: Synthesis, Authentication, and Thermochemistry. J. Am. Chem. Soc. 1996, 47, 11865−11871. (15) Wenthold, P. G.; Hu, J.; Squires, R. R. Gas-Phase Reactions of the Benzyne Negative Ions. J. Mass Spectrom. 1998, 33, 796−802. (16) Wenthold, P. G.; Squires, R. R.; Lieneberger, W. C. Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p-Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o-, m-, and p-Benzyne. J. Am. Chem. Soc. 1998, 120, 5279−5290. (17) Leopold, D. G.; Miller, A. E. S.; Lineberger, W. C. Determination of the Singlet−Triplet Splitting and Electron Affinity of o-Benzyne by Negative Ion Photoelectron Spectroscopy. J. Am. Chem. Soc. 1986, 108, 1379−1384. (18) Liu, H.-C.; Wang, C.-S.; Guo, W.; Wu, Y.-D.; Yang, S. Formation and Decomposition of Distonic o-, m-, and p-Benzyne Radical Cations from Photolysis of Mg+(o-, m-, p-C6H4F2). J. Am. Chem. Soc. 2002, 124, 3794−3798. (19) Diau, E. W.-G.; Casanova, J.; Roberts, J. D.; Zewail, A. H. Femtosecond Observation of Benzyne Intermediates in a Molecular Beam: Bergman Rearrangement in the Isolated Molecule. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 1376−1379. (20) Li, H.; Huang, M.-B. The o-, m-, and p-Benzyne Radical Cations: A Theoretical Study. Phys. Chem. Chem. Phys. 2008, 10, 5381−5387. (21) Mitsunori, A.; Motylewski, T.; Kolek, P.; Maier, J. P. Electronic Absorption Spectrum of a Nonlinear Carbon Chain: trans-C6H4+. Phys. Chem. Chem. Phys. 2005, 10, 2138−2141. (22) Araki, M.; Linnartz, H.; Cias, P.; Denisov, A.; Fulara, J.; Maier, J. P. High-Resolution Electronic Spectroscopy of a Nonlinear Carbo Chain Radical C6H4+. J. Chem. Phys. 2003, 118, 10561−10565. (23) Freivoge, P.; Fulara, J.; Lesson, D.; Forney, D.; Maier, J. P. Absorption Spectra of Conjugated Hydrocarbon Cation Chains in Neon Matrices. Chem. Phys. 1994, 189, 335−341.

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CONCLUSIONS C6H4+ cations were produced in a hot cathode discharge source from several cyclic and acyclic precursors and after mass selection trapped in 6 K neon matrixes. Three open-chain isomers of C6H4+, 5-hexene-1,3-diyne D+ and cis- and trans-3hexene-1,5-diyne C+ and B+, and two cyclic structures obenzyne K+ and propargyl cyclopropene T+ cations, have been detected based on their electronic absorption spectra. Two electronic transitions in the visible and UV regions were detected for each D+, C+, and B+ cations. The UV system of D+ overlaps with absorptions of B+. Structured fluorescence was observed upon excitation into the visible system of D+. A reversible cis−trans isomerization of the 3-hexene-1,5-diyne cation was observed upon excitation into the UV systems of B+ and C+. Two overlapping blue systems, a moderately intense and weak one, were detected for C6H4+ produced from the dibromo- and dicarboxylic-acid-substituted benzene precursors, which are assigned to o-benzyne K+ and/or propargyl cyclopropene T+ cations. The assignment is based on calculated excitation energies of C6H4+. Twelve isomers of C6H4+ were characterized computationally. Geometries in their ground states were optimized with DFT/B3LYP, MP2 (with UHF and ROHF wave functions), EOMIP-CCSD, and EOMEA-CCSD methods. According to these, the most stable form of C6H4+ is the m-benzyne cation L+. The next in energy is the propargyl cyclopropene cation T+. Vertical excitation electronic energies of the 12 cations have been computed using CASPT2 and EOMEE-CCSD methods with UHF, and the later was also computed with ROHF determinants. These calculations allow the assignment of the observed electronic transitions to the five C6H4+ cations B+, C+, D+, K+, and T+ and reject the other seven considered structures. The five isomers detected in the present laboratory studies have been comprehensively studied with the theoretical methods. Their geometries were optimized also in the excited states, which provided more accurate adiabatic excitation energies. These agree well with the observations.

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ASSOCIATED CONTENT

* Supporting Information S

Tables showing energetics, optimized, ground, and corresponding excited electronic states of the 12 isomers studied. This material is available free of charge via the Internet at http:// pubs.acs.org.

■ ■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Swiss National Science Foundation (Project No. 200020-140316/1) and U.S. National Science Foundation (Grant CHE-1012743).

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REFERENCES

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Electronic Transitions of C6H4+ Isomers: Neon Matrix and Theoretical

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