Fragmentation of deuteronated aromatic derivatives: The role of ion-neutral complexes

Abstract

The low-energy collision-induced dissociation reactions of the MD + ions of a number of alkyl phenyl ethers, alkylbenzenes, acetophenones and benzaldehyde have been studied as a function of collision energy to establish qualitatively the dependence of the fragmentation reactions observed on internal energy. Deuteronated alkyl phenyl ethers (ROC 6H 5·D +, R = C 3H 7, C 4H 9) fragment at low collision energies to form C 6H 5OHD + + (R-H), the thermochemically favoured products; with increasing collision energy (and, hence, internal energy) formation of the alkyl ion R + increases significantly in importance. Deuteronated alkylbenzenes (RC 6H 5, RC 6H 4R′, R = C 2H 5, C 3H 7) similarly form the deuteronated benzene (the thermochemically favoured product) at low collision energies with formation of the alkyl ion R + being observed at higher collision energies. The results for both systems are consistent with a fragmentation mechanism involving initial formation of an R +/aromatic ion/neutral complex. At low internal energies proton transfer occurs within this complex to form an ion/neutral complex consisting of the deuteronated aromatic and a neutral olefin; this complex fragments to the thermochemically favoured products. Since the transition state leading to these products is a “tight” transition state involving loss of rotational degrees of freedom, the proton transfer reaction is unfavourable entropically with respect to simple dissociation of the R +/aromatic complex to R + + ArD. Consequently, these products increase in importance as the internal energy is increased. The fragmentation of deuteronated aromatic carbonyl compounds can also be rationalized by similar mechanisms involving the intermediacy of ion/neutral complexes. Deuteronated acetophenone forms only CH 3CO + at all collision energies; this is both the thermochemically and entropically favoured product. However, deuteronated p-aminoacetophenone forms deuteronated aniline, the thermochemically favoured product at low collision energies with formation of CH 3CO +, the entropically favoured product increasing in importance with increasing collision energy. Deuteronated benzaldehyde forms C 6H 5D + + CO at low collision energies but HCO +, the entropically favoured product, is observed at higher collision energies.