Interannular proton exchange and fragmentation of carbonyl‐protonated benzophenones

Benzopbenones a initially protonated at the carbonyl group were prepared by electron‐impact induced dissocation of 1,1‐diphenylpropanols (compounds 1‐5). These protonated ketones decompose in the ion source and the second field‐free region of a reversed geometry mass spectrometer by proton migration to one of the phenyl groups and subsequent elimination of benzene. In the case of derivatives substituted by methoxy groups and trifluoromethyl groups respectively, the proton migrates predominantly to the more bask benzene ring, resulting in the elimination of anisole in the former case and of benzene in die latter case. A study of protonated benzopbenones labelled at the phenyl ring and at the carbonyl group shows that only a few interannular H/D exchange steps precede the fragmentation. This is observed not only for metastable ions in the magnetic sector instilment but also for ions of long lifetimes investigated by Fourier‐transform‐ion cyclotron resonance (FT‐ICR) Spectrometry. This is in contrast to the arene elimination from protonated 1,ω‐dipbenylalkanes and related polyphenylalkanes which fragment by complete positional exchange of all hydrogen atoms at the aromatic rings. The special behaviour of protonated benzophenones is attributed to a low barrier for the decomposition of a chemically activated arenium ion b , which arises from the initial proton transfer. Once b is formed, it decomposes quickly without much interannular proton exchange.