Spectroscopic evidence for the formation of pentalene + in the dissociative ionization of naphthalene †

Although acetylene loss is well known to constitute the main breakdown pathway of polycyclic aromatic hydrocarbon (PAH) species, the molecular structure of the dissociation products remains only poorly characterized. For instance, the structure of the C 8 H 6 product ion formed upon acetylene loss from the smallest PAH naphthalene (C 10 H 8 ) has not been experimentally established. Several C 8 H 6+ isomers are conceivable, including phenylacetylene, benzocyclobutadiene, pentalene as well as a number of a-cyclic products. Here we present infrared (IR) spectroscopic evidence for the formation of the (anti-aromatic) pentalene structure using a combination of tandem mass spectrometry and IR laser spectroscopy. The formation of pentalene is suggestive of facile 6-to 5-membered ring conversion, which possibly has implications for the PAH/fullerene interrelationship in energetic settings such as the interstellar medium and combustion environments.

Spectroscopic evidence for the formation of pentalene + in the dissociative ionization of naphthalene † Jordy Bouwman,* a Arjen J. de Haas a and Jos Oomens ab Although acetylene loss is well known to constitute the main breakdown pathway of polycyclic aromatic hydrocarbon (PAH) species, the molecular structure of the dissociation products remains only poorly characterized.For instance, the structure of the C 8 H 6 product ion formed upon acetylene loss from the smallest PAH naphthalene (C 10 H 8 ) has not been experimentally established.0][11][12] However, these MS-based studies do not provide structural information on the fragmentation products, leaving mechanistic details of the breakdown chemistry largely unknown.We do not know whether the hexagonal framework remains largely intact with ethynyl group(s) on the periphery, or whether more substantial rearrangements take place.This study addresses this question for the smallest PAH naphthalene.
4][15][16][17] In an attempt to elucidate the molecular structure of the C 8 H 6 + product formed in the electron impact ionization of naphthalene, Schroeter et al. 15 employed a variety of mass spectrometric methods.Based on their charge reversal (CR) data they dispute the formation of the phenylacetylene cation (PA + ) and suggest that the benzocyclobutadiene cation (BCB + ) is formed, as was also suggested by Ling et al. 14 However, the possible formation of other structures is not excluded. 15To date, no unambiguous identification of these reaction products has been reported.[18] A summary of the PES reported by Dyakov et al. 17  West et al. 12 studied the dissociation of naphthalene employing both tandem mass spectrometry and imaging photoelectron photoion coincidence spectrometry (iPEPICO).From their measurements they found that H-loss and C 2 H 2 -loss are the lowest energy dissociation channels.iPEPICO measurements have recently also been reported on quinoline and isoquinoline dissociation and it was suggested that either PA + , or BCB + are the HCN-loss products that are formed from their dissociative ionization. 20An unambiguous identification of the C 8 H 6 + isomer is however not possible from these experiments.
Here, we apply IR spectroscopy to identify the structure of the C 8 H 6 + product that is formed in the dissociative ionization of naphthalene.The measurements have been performed in an ion trap mass spectrometer coupled to the free electron laser for infrared experiments (FELIX).2][23][24][25][26] Briefly, the C 8 H 6 + dissociation product is formed by two-photon dissociative ionization of naphthalene vapor at 193 nm (ArF wavelength) and mass isolated in a Paul type quadrupole ion trap by means of a tailored RF (SWIFT) pulse. 24,23The IR spectrum of the mass-isolated product ion is then recorded by infrared multiphoton dissociation (IRMPD) spectroscopy. 27ig. 2 shows the mass spectrum of dissociatively ionized naphthalene (top), together with that of the subsequently massisolated m/z = 102 fragment ion (middle).The fragment signal is significantly enhanced when helium buffer gas is admitted to the trap at a pressure of 5 Â 10 À5 mbar.Furthermore, the SWIFT pulse that is used to isolate the fragment ion excites the secular frequency of the naphthalene cation, causing further collisional activation and an enhancement of the integrated fragment ion intensity by a factor of two.The infrared spectrum of the isolated C 8 H 6 + ion is constructed by plotting the fragmentation yield in the mass spectrum as a function of IR laser wavelength (see Fig. 3).The fragmentation yield is determined as the integrated fragment ion intensity divided by the total integrated ion intensity, i.e. the sum of the fragment and parent ion intensity.Also shown in Fig. 3 are B3LYP/6-311+G(d,p) computed infrared spectra of three C 8 H 6 + isomers, PE, BCB and PA.Calculated frequencies are scaled by a factor of 0.9679 to correct for anharmonicities. 26The computed stick spectrum is convolved with a 30 cm À1 Full Width at Half Maximum (FWHM) Gaussian profile to facilitate comparison with the experimental spectrum.
Predicted spectra of the three isomers that are possibly formed exhibit vibrational modes that are very diagnostic.Clearly, the computed spectrum of PE + resembles the experimental spectrum closely.The main discrepancy is the computed absorption band at B1460 cm À1 that is observed only very weakly in the experimental spectrum.This is possibly due to the nonlinear nature of the IRMPD technique, although an incorrect prediction of the band intensity is also possible.A weak absorption band is observed in the experimental spectrum between 1600 and 1700 cm À1 and is possibly caused by a small fraction of the product ions having the BCB + isomeric structure, which is predicted to possess an absorption band in this wavelength range (C-C stretch of the aromatic ring), however, none of the other strong modes for BCB + are observed.Hence, PE + is identified as the dominant naphthalene dissociation product.
The detection of PE + as a product of the dissociative ionization of naphthalene can be rationalized based on the summary of the C 10 H 8 + PES shown in Fig. 1.The formation of PE + occurs after isomerization of naphthalene + to azulene + .The energy available for isomerization in the experiment reported here is 2 Â 193 nm (295.8 kcal mol À1 ) less the ionization energy of naphthalene (187.8 kcal mol À1 ), 28 which equals 108.0 kcal mol À1 .This energy would be barely sufficient to cross the rate limiting transition state (TS4) that leads to PE + , but not yet sufficient to cross the barrier to form PA + .Collisions with helium induced by driving the secular frequency of the metastable naphthalene ion likely gives the ions just enough energy to dissociate.The rate limiting transition state to the formation of BCB + is slightly higher than the barriers leading to PE + and PA + .This hypothesis is supported by the

Several C 8
H 6 + isomers are conceivable, including phenylacetylene, benzocyclobutadiene, pentalene as well as a number of a-cyclic products.Here we present infrared (IR) spectroscopic evidence for the formation of the (anti-aromatic) pentalene structure using a combination of tandem mass spectrometry and IR laser spectroscopy.The formation of pentalene is suggestive of facile 6-to 5-membered ring conversion, which possibly has implications for the PAH/fullerene interrelationship in energetic settings such as the interstellar medium and combustion environments.
computed at the G3(MP2,CC)//B3LYP level of theory showing only the highest transition state barriers referenced to the naphthalene cation is shown in Fig.1.Solano and Mayer19 determined product branching fractions for the dissociation of the naphthalene cation from ab initio calculations in conjunction with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations and predicted that the pentalene cation (PE + ) is the main C 2 H 2 -loss product at low internal energies of the naphthalene cation, followed by PA + and BCB + .The contributions of PA + and BCB + grow as the internal energy of the naphthalene cation rises.These predictions await experimental verification.

Fig. 1
Fig. 1 Summary of the possible C 2 H 2 loss channels on the C 10 H 8 + potential energy surface constructed from the data reported in Dyakov et al.17

Fig. 2
Fig. 2 Mass spectra of naphthalene dissociatively ionized at 193 nm (top trace), the SWIFT isolated fragment at m/z = 102 (middle trace) and the SWIFT isolated fragment dissociated with intense IR radiation at 1350 cm À1 .

Fig. 3
Fig. 3 Infrared multiphoton dissociation spectrum of C 8 H 6 + formed spectrum of C 8 H 6 + formed from the dissociative ionization of naphthalene (top) shown together with the computed spectra of the C 8 H 6 + isomers pentalene, benzocyclobutadiene and phenylacetylene.
The mass spectrum of the C 8 H 6 + fragment upon resonant IR irradiation at 1350 cm À1 is shown in the bottom panel.IR-induced dissociation yields weak but clearly observable signals at m/z = 76 and 50, corresponding to C 6 H 4 + and C 4 H 2 + fragment ions that are formed by C 2 H 4 and C 4 H 4 -loss, respectively.