Ring versus oxygen protonation in metastable ion decompositions of protonated isopropyl phenyl ether

Abstract

Protonated alkyl phenyl ethers possess more than one stable tautomer. A debate has arisen over whether only one of them gives rise to the principal dissociation pathway observed in their mass-analyzed ion kinetic energy (MIKE) spectra. Alkene loss constitutes the major (often the exclusive) metastable ion decomposition, yielding protonated phenol ions. A hydron deposited by chemical ionization exchanges with some of the alkyl hydrogens (but none of the ring hydrogens) prior to fragmentation. Previously published MIKE spectra have shown that [(CD 3) 2CHOPh]D + gives only m/ z 97 (C 6H 5D 2O +), but that [(CD 3) 2CHOPh]H + gives a mixture of m/ z 96 (C 6H 6DO +) and m/ z 97. Exchange must arise via ion-neutral complexes that result from O-protonated ions, (CD 3) 2CHO(H)Ph +. Current controversy centers around the contribution of ring-protonated ions to the production of unexchanged fragment ions. Here we determine the mole fractions of ring-protonated ( X) and O-protonated (1 − X) parent ions using m/ z 95: m/ z 96: m/ z 97 MIKE ion abundance ratios from H 2O and D 2O CI of (CH 3) 2CHOPh, CH 3(CD 3)CHOPh, and (CD 3) 2CHOPh. Data from the first two compounds give unbiased assessments of X and four other relative rate constants that are obtained using a steady-state kinetic model that gives a set of five equations in five unknowns. The values calculated from the data predict an m/ z 96: m/ z 97 ratio of 4.7 for [(CD 3) 2CHOPh]H + that turns out to be the same ratio as is measured experimentally. This validation of the data analysis corroborates the value of X ≤ 0.01 extracted from the experimental results. The contribution of ring-protonated parent ions to the MIKE spectra of chemically ionized isopropyl phenyl ether is therefore negligible.