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

被引:8
作者
Audier, HE
Morton, TH [1 ]
机构
[1] Ecole Polytech, Lab Mecanismes Reactionnels, F-91128 Palaiseau, France
[2] Univ Calif Riverside, Dept Chem, Riverside, CA 92521 USA
基金
美国国家科学基金会;
关键词
chemical ionization; isotopic exchange; mass-analyzed kinetic energy (MIKE) spectroscopy; mechanism; steady state kinetics;
D O I
10.1016/S1387-3806(98)14139-8
中图分类号
O64 [物理化学(理论化学)、化学物理学]; O56 [分子物理学、原子物理学];
学科分类号
070203 ; 070304 ; 081704 ; 1406 ;
摘要
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 [(CD3)(2)CHOPh]D+ gives only m/z 97 (C6H5D2O+), but that [(CD3)(2)CHOPh]H+ gives a mixture of m/z 96 (C6H6DO+) and m/z 97. Exchange must arise via ion-neutral complexes that result from O-protonated ions, (CD3)(2)CHO(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 H2O and D2O CI of (CH3)(2)CHOPh, CH3(CD3)CHOPh, and (CD3)(2)CHOPh. 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 [(CD3)(2)CHOPh]H+ that turns out to be the same ratio as is measured experimentally. This validation of the data analysis corroborates the value of X less than or equal to 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. (C) 1999 Elsevier Science B.V.
引用
收藏
页码:393 / 399
页数:7
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