Unimolecular and Photoinduced Dissociations of Aromatic C₈H₁₀^+• Molecular Ions

Abstract

Production of C 7 H 7 + in the unimolecular and photoinduced dissociations of ethylbenzene; methylcycloheptatriene; and o-, m-, and p-xylene ions has been investigated using mass-analyzed ion kinetic energy spectrometry. Both the benzylium and tropylium ions were produced near the reaction threshold, whereas only the benzylium ion was observed at higher internal energy achieved by photoabsorption. Experimental data suggest that interconversion between ethylbenzene and methylcycloheptatriene ions occurs more rapidly than their dissociations near the reaction threshold, whereas xylene ions undergo rate-determining isomerization to the methylcycloheptatriene structure, and dissociation occurs from the ethylbenzene/methylcycloheptatriene ion mixture. The processes occurring at higher internal energy are quite similar, except that the tropylium channel cannot compete against the benzylium channel because either the ethylbenzene ion-to-methylcycloheptatriene ion conversion or the dissociation of the methylcycloheptatriene ion to the tropylium ion is slower than the direct dissociation of the ethylbenzene ion. The above mechanistic picture is in disagreement with the prediction of a dynamic model reported previously. A series of investigations performed for the production of C 7 H 7 + from alkylbenzene ions shows an interesting trend of increasing benzylium production with the increase in the total number of carbon atoms in the alkyl substituents.

Classical Trajectory and Statistical Adiabatic Channel Study of the Dynamics of Capture and Unimolecular Bond Fission. VII. Thermal Capture and Specific Rate Constants k(E,J) for the Dissociation of Molecular Ions

“…In any case, the weak J-dependence illustrated in Fig. 7 indicates that the measured k(E) from [37] and [38] are not different from k(E, J = 0) within experimental error.…”

“…we find, for C8H 10 -* -+C 7 H 7 + ± CH 3 , that k(E) could well be represented with A = 1.6 x 10's-', Eo/hc = 14070 cm-1, Eref/hc = 32420 cm-', and s* = 5.962, using the measured k(E) at Eref from [37] as the reference fitting point and using the experimental results from [38] for fixing s*. As seen in Fig.…”

“…Finally, the SACM/CT calculations of the present work have been complemented by product energy distributions in [22] which are internally consistent with the present results for k(E, J) and kcap(T). Appendix: Molecular parameters a) C8HIo+ --C 7 H 7 + + CH 3 (parameters taken from [37] and [38] …”

“…(3.8) was varied until an optimum experimental value of k(E) was reproduced. Choosing the experimental k(E) near 32 000 cm-1 from [37] for this purpose, the fit parameters Esw/hc,ýý 38 cm-' and C 2 /D • 74 are obtained. In this way the complete k(E, J) is predicted as shown in Figs.…”

“…[8,37,39]. Because of the large number of adjustable frequencies, this approach can obviously also reproduce measured k(E).…”