Parallels between Decompositions of Even- and Odd-Electron Ions: Structures from Dissociation of 3-Chloro- n -Propyl Phenyl Ether

Radical cations of aliphatic phenyl ethers [from electron ionization (EI)] and their conjugate acid ions [from chemical ionization (CI)] exhibit similar decomposition mechanisms. The M^•+ and MH⁺ ions of the title compound (1-phenoxy-3-chloropropane, 1) both expel chloropropene, with hydrogen transfer coming from all chain positions. This result has ample precedent and is taken as proof of the intermediacy of ion–neutral complexes in both EI and CI. Isomerization of 1^•+ to PhOCH(CH₃)CH₂Cl^•+ prior to metastable chloropropene expulsion can be ruled out on the basis of isotope effect measurements, despite the prediction of this rearrangement by ab initio calculations. The results also preclude collapse of ion–neutral complexes back to their isomeric molecular ions, even though there appears to be no energy barrier to reforming a covalent bond between the ion and its neutral partner. Competing pathways for dissociation of 1^•+ via distonic ions are assessed by means of deuterated analogues. In addition, M^•+ and MH⁺ ions both dissociate to form C₉H₁₁O⁺ fragments. Ion–molecule reactions and collisional activation (CAD) studies indicate that these are mixtures of isomers (which may be interconverting). Deposition of H⁺ onto a ring-deuterated analogue is followed by loss of DCl, demonstrating that ring protonation precedes hydrogen chloride loss. This interpretation is confirmed by the predominant loss of HCl from MD⁺ ions of chain-deuterated analogues. Other decomposition pathways reveal evidence of transposition of positions 1 and 3 of the methylene chain, suggesting possible intervention of intermediates with 4-membered rings.