Isomerization and Dissociation of n-Butylbenzene Radical Cation

Jan 9, 2012 - Stéphanie Halbert and Guy Bouchoux*. Laboratoire ... Mohamad Akbar Ali , V. Tyler Dillstrom , Jason Y. W. Lai , and Angela Violi. The J...
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Isomerization and Dissociation of n-Butylbenzene Radical Cation Stéphanie Halbert† and Guy Bouchoux* Laboratoire des Mécanismes Réactionnels, Ecole Polytechnique, CNRS, 91128 Palaiseau, France S Supporting Information *

ABSTRACT: Fragmentation mechanisms of ionized butylbenzene to give m/z 91 and m/z 92 fragment ions have been examined at the G3B3 and G3MP2B3 levels of theory. It is shown that the energetically favored pathways lead to tropylium, Tr+, and methylene-2,4-cyclohexadiene, MCD•+, ions. Formation of m/z 91 benzyl ions, Bz+, by a simple bond fission (SBF) process, needs about 30 kJ/mol more energy than Tr+. Possible formation of C7H8•+ ions of structures different from the retro-ene rearrangement (RER) product, MCD•+, has been also considered. Comparison with experimental data of this “thermometer” system is done through a kinetic modeling using Rice−Ramsperger−Kassel−Marcus (RRKM) and orbiting transition state (OTS) rate constant calculations on the G3MP2B3 0 K energy surface. The results agree with previous experimental observation if (i) the competitive formation of Tr+ and Bz+ is taken into account in the m/z 91 pathway, and (ii) the stepwise character of the RER fragmentation is introduced in the m/z 92 fragmentation route.

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bond elongation leading to the benzyl cation, Bz+ (m/z 91) plus a n-propyl radical (simple bond fission, SBF, Scheme 1),

INTRODUCTION Neutral and ionized alkylbenzenes are considered as prototypical systems in a number of physicochemical and biophysical problems. It constitutes for example simple systems where a flexible aliphatic chain may interact with an aromatic ring by non-covalent forces.1 From this point of view, conformational preferences and associated energetics of neutral and ionized butylbenzene were recently examined theoretically.2 In mass spectrometric studies, ionized alkylbenzene have been explored by several research groups for 3 decades in order to shed light on the structure, energetics, and mechanisms of formation of C7H8•+ (m/z 92) and C7H7+ (m/z 91) fragment ions.3 A prototypical example, probably the most extensively studied, is ionized butylbenzene.4−25 For this system, a strong dependence of the branching ratio of m/z 91 over m/z 92 fragment ion formations upon the internal energy of the precursor molecular ion has been observed. Earlier experiments involved ionbeam4−7 and trapped-ion photodissociation,8 charge exchange ionization,9 collisional activation,10−14 chemical and thermal activation,13 and photoelectron−photoion coincidence technique.15 Combining these various experiments, the internal energy range of butylbenzene molecular ion extends from ca. 2 to ca. 6 eV. At low internal energy, E, only the m/z 92 signal is detected, whereas the ratio [91]/[92] increases markedly to attain ∼8 when E = 6 eV. Statistical modeling of the two competitive reaction rates has been done by several authors generally assuming simplified fragmentation scheme.6,12−15 Indeed, competitions between simple bond fission and rearrangement are ubiquitous events in gas-phase ion chemistry.3 Prototypes of such competition are the allylic cleavage/retro-ene fragmentations of ionized alkenes and the homologue benzylic cleavage/retro-ene fragmentations of ionized alkylbenzenes.3 In the case of n-butylbenzene, the two competitive channels may be attributed to (i) the C−C © 2012 American Chemical Society

Scheme 1

and (ii) the retro-ene rearrangement producing ionized methylene-2,4-cyclohexadiene, MCD•+ (m/z 92), and propene (retro-ene rearrangement, RER, Scheme1). The structures of the C7H8•+ and C7H7+ fragment ions (Scheme 2) originating from dissociation of ionized nbutylbenzene have been tentatively identified more than 30 years ago by using various mass spectrometry techniques.16−21 In 1977, Dunbar and Klein16 used photodissociation spectra to characterize C7H8•+ ions resulting from fragmentation of nbutylbenzene ionized by electrons of 16 and 40 eV kinetic energy. After comparison with reference spectra of ionized toluene, TOL•+, and cycloheptatriene, CHT•+, the authors find that the C7H8•+ fragments population consists of more than 75% of a third structure, probably ionized MCD•+ the Received: December 5, 2011 Revised: January 9, 2012 Published: January 9, 2012 1307

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was concluded that ionized toluene, TOL•+, and tropylium, Tr+, ions were formed, at threshold, from n-butylbenzene.17,20 In fact, more elaborated treatment of the appearance energies measurements23,24 and consideration of more recent thermochemical data lead to ΔfH°298(C7H8•+) = 931 kJ/mol and ΔfH°298(C7H7+) = 872 kJ/mol. The former value is more properly compatible with ionized methylene-2,4-cyclohexadiene (ΔfH°298(MCD•+) = 912−958 kJ/mol) rather than toluene (ΔfH°298(TOL•+) = 903 kJ/mol). Indeed, the experimental ΔfH°298(C7H7+) threshold value of 872 kJ/mol is fully compatible with a tropylium cation (ΔfH°298(Tr+) = 881 kJ/ mol). In summary, fragmentation of ionized butylbenzene leads (i) to m/z 91 ions not only of Bz+ but also of Tr+ structure, particularly at threshold, and (ii) to m/z 92 ions of MCD•+ structure at the threshold, but polluted at high internal energy by a residual ( k6 > k1, k5, and k4, k3 > k2 hold in a large part of the internal energy range and that, consequently, eqs 6 and 7 reduce to 8 and 9:

k 91 ∼ k1 + k5

(8)

k 92 ∼ k 2k4 /(k3 + k4)

(9)

Figure 3 shows that k91 is close to k1 when E is larger than ∼2.2 eV and that it follows the representative curve of k5(E) below this limit. This is obviously expected from eq 8 since the 1313

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consequently expected to be only observed at high internal energy, as already noted. Competitive fragmentation of ionized butylbenzene is considered as a “thermometer” system since the ratio of ion abundances [91]/[92] is strongly sensitive to the internal energy of the precursor ion. A number of careful experiments has provided [91]/[92] ratio and overall reaction rate constants k91 and k92 in the internal energy range of 2−6 eV. A kinetic modeling is presented here which includes (i) the competitive formation of Tr+ and Bz+ in the m/z 91 pathway and (ii) the stepwise character of the RER fragmentation in the m/z 92 fragmentation routes. Individual rate constants of each reaction step are estimated using Rice−Ramsperger−Kassel−Marcus (RRKM) and orbiting transition state (OTS) rate constant calculations on the G3MP2B3 0 K energy surface. A good agreement is observed between the experimental data points and the theoretical overall rate constants k91 and k92 provided by the kinetic modeling.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing G3MP2B3 and G3B3 geometries, H°0 and H°298, of all species considered therein, B3LYP/6-31+G(d,p) geometries and energies of complexes c1−c5, parameters used in the kinetic modeling, and full ref 26. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. Calculated and experimental rate constant for m/z 92 fragment ion formation from ionized butylbenzene.

crossing point of the two k1(E) and k5(E) occurs at E ∼2.2 eV. This peculiar internal energy dependence of k91 explains also why previous modeling of k91, neglecting the Tr+ formation route (i.e., neglecting k5 in eq 8), lead to ΔH°0 sligthly lower than ΔH°0(1•+→Bz+ + n-propyl•). Considering now Figure 4, the global rate constant k92(E) and the individual rate constant k2(E) curves are clearly merged at high E values (>ca. 2.5 eV). By contrast, when the internal energy decreases, the overall rate constant k92(E) becomes increasingly lower than the individual isomerization rate constant k2(E). This phenomenon is due to the fact that k4 decreases more rapidly with decreasing E than does k3. Consequently, the factor k4/(k3 + k4) in eq 9 would reduce more efficiently k92, with respect to k2, at low internal energy. This dramatic role of the factor k4/(k3 + k4), i.e., of the explicit consideration of a two steps process for the RER fragmentation, also explains why the previous modeling based on a single rate constant (i.e., k92 ∼ k2) lead to ΔH°0 significantly lower than ΔH°0(1•+→ MCD•+ + propene).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 33 1 69 33 48 42. Fax: 33 1 69 33 48 03. E-mail: [email protected]. Present Address †

Institut Charles Gerhardt, Université Montpellier 2, CNRS 5253, cc 1501, Place E. Bataillon, 34095 Montpellier, France.

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ACKNOWLEDGMENTS This work was granted access to the HPC resources of [CCRT/CINES/IDRIS] under the allocation x2010085107 made by GENCI (Grand Equipement National de Calcul Intensif). The authors thank Carine Clavaguéra, Gilles Frison, and David Semrouni for their help during the setup of the computational procedures.

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CONCLUSION The present work reconsidered in detail the competitive fragmentations of ionized butylbenzene to give m/z 91 and m/z 92 ions. Energies and structures of the various critical points on the 0 K energy surface have been examined at the G3B3 and G3MP2B3 levels of theory. It is shown that formation of m/z 91 benzyl ions, Bz+, by a simple bond fission (SBF) process, as usually assumed, is not the energetically favored pathway. Rather, a stepwise process involving 1,2-H migration followed by ring closure/ring-opening reactions and finally leading to m/ z 91 tropylium ion, Tr+, is favored by 30 kJ/mol. More classically, the methylene-2,4-cyclohexadiene, MCD•+ fragment ion, expected to result from a retro-ene rearrangement, corresponds indeed to the lowest energy route leading to m/ z 92 ions. Formation of C7H8•+ ions of structures different from MCD•+ (namely, ionized toluene,TOL•+, ionized cycloheptatriene, CHT•+, distonic benzenium ion DBI•+ and ionized norcaradiene NCD•+) would need ∼1.5 eV more energy and is

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