Collisional Stabilization and Thermal Dissociation of Highly Vibrationally Excited C9H12+ Ions from the Reaction O2+ + C9H12 → O2 + C9H12+

Abstract

Highly vibrationally excited n-propylbenzene cations, C9H12+*, were prepared by the charge transfer reaction O2+ + C9H12 → O2 + C9H12+* in a turbulent ion flow tube. The subsequent competition between fragmentation of C9H12+* into C7H7+ + C2H5 and stabilization in collisions with N2 was studied at temperatures in the range 423−603 K and at pressures between 15 and 200 Torr. Most of the C7H7+ is the aromatic benzylium isomer, while the fraction of the minor species, seven-membered-ring tropylium, increases with T, from 5 to 20%. Minor fragments are C6H6+, C7H8+, and C8H9+. Energy-transfer step sizes 〈ΔE〉 for collisional deactivation are obtained by combining the stabilization versus fragmentation ratios measured as a function of pressure in this study with fragmentation rates from the literature. The values are compared with related information for other excited molecular ions and are similar to those of their neutral analogues. At the highest temperatures, C9H12+ was also observed to pyrolyze after collisional stabilization. Employing unimolecular rate theory, the derived rate constants for thermal dissociation of C9H12+ are related to values derived from the specific rate constants k(E,J) for fragmentation. Good agreement is found between measured and predicted pyrolysis rate constants. This allows us to confirm the dissociation energy of C9H12+ into C7H7+ (benzylium) and C2H5 as 166.9 (±2.2) kJ mol-1 (at 0 K).