Formation of [M−H]⁺ of 4-Substituted-1-(methoxymethyl)benzene Derivatives under Positive Fast Atom Bombardment Ionization

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The appearance of the characteristic peak of the hydride-eliminated molecule [M−H]⁺ under a positive ion mode (positive) fast atom bombardment (FAB) ionization condition and liquid-assisted secondary ion mass spectrometry (LSIMS) conditions is known for some compounds and the mechanism of its formation has been investigated. In this study, we investigated the formation mechanism of the hydride-eliminated molecule [M−H]⁺ from 4-substituted-1-(methoxymethyl)benzene under a positive FAB ionization condition. The mass spectra of 4-methoxy-1-(methoxymethyl)benzene (1), 4-methoxy-1-(methoxymethyl-d₂-)benzene (1-d₂), and 4-methoxy-1-(methoxymethyl-d₃)benzene (1-d₃) were measured under the positive FAB conditions. [M−H]⁺ was observed for 1 and 1-d₃, and [M−D]⁺ for 1-d₂, indicating that the site of hydride elimination was the methylene of the 1-(methoxymethyl) moiety. Since [M−H]⁺ was hardly observed under the conditions of positive electron ionization and positive chemical ionization in the gas phase, the hydride elimination is a reaction specific to positive FAB ionization. To examine the contribution of the 4-substituent to the hydride elimination reaction, the mass spectra of (methoxymethyl)benzene (2) and 4-nitro-1-(methoxymethyl)benzene (3) were measured using the positive FAB. The ordering of the relative peak intensity of [M−H]⁺ for [M+H]⁺ in the FAB mass spectra was 1 > 2 > 3, and the results suggest that the electron-donating power of the substituents is an important factor in the formation of [M−H]⁺.