ION RADICAL COMPLEXES IN THE GAS-PHASE - STRUCTURE AND MECHANISM IN THE FRAGMENTATION OF IONIZED ALKYL PHENYL ETHERS

The threshold fragmentation behavior of ionized alkyl (C2-C5) phenyl ethers has been investigated in detail. Their lowest energy dissociation products ionized phenol and an alkene. For some C5 homologues formation of an ionized alkene and neutral phenol becomes a competitive reaction. These low-energy, (metastable ion) decompositions all involve the formation of a complex consisting of a phenoxy radical and an alkyl ion. For secondary and tertiary alkyl phenyl ethers, an irreversible beta-H+ transfer in the complex yields the [C6H5OH].+ ion; the activation energy for these transfers is 145 +/- 6 kJ mol-1 for secondary ions and 83 +/- 1 kJ mol-1 for tertiary ions. The transition state for this process is discussed with reference to the theoretically calculated charge distributions in alkyl cations, in which the positive charge is mostly located in the groups attached to the formal charge-bearing carbon atom. With the exception of the ethyl analogue, primary alkyl ions undergo a rate-determining, single 1,2 H-shift producing a secondary or tertiary ion which then undergoes its characteristic beta-H+ transfer and rapidly dissociates. The activation energy for this rearrangement is 189 +/- 9 kJ mol-1. For a primary alkyl group having no beta-H atom (e.g., CH2C(CH3)3), a 1,2 methyl shift takes place in the complex (activation energy ca. 129 kJ mol-1), producing a tertiary ion which undergoes rapid beta-H+ transfer and then fragments. For ethyl phenyl ether, it is proposed that the transition state is best represented as a symmetrical H+ bridged species involving the nonclassical ethyl ion; For C5H11 isomers, new dissociation channels become competitive. The 1-pentyl and the 2-methyl-2-butyl phenyl ethers also showed loss of an ethyl radical in the metastable time frame producing the daughter ion C6H5OC+(CH3)2, the first ether after a rearrangement, and the second by a simple bond cleavage. The 3-methyl-1-butyl and 2,2-dimethylpropyl analogues displayed m/z 70, C5H10.+ ions, in their metastable ion mass spectra. These arose from specific beta-H atom transfer, competing with beta-H+ transfer, from the tertiary ion in the complex. The conclusions outlined above were reached with the aid of isotopic labeling, appearance energy measurements, and charge stripping mass spectrometry. For the C4 and C5 ions, more complex reactions come into play at higher internal energies, e.g., for ions which fragment in the ion source.