Photo-fragmentation of alkyl phosphates in the gas-phase

Alkyl phosphates are experiencing an ever increasing use due to the current arising of new applications. This implies their increasing presence in the environment so their stability and reactivity under high-energy photons, which are still unknown, need to be clarified. In this study, a mass spectrometric investigation of the ionization and fragmentation processes of four representative alkyl phosphates (dibutyl, tributyl, bis-2-ethylhexyl, and tris-2-ethylhexyl phosphate) induced by vacuum ultraviolet (VUV) radiation has been carried out. The experimental data show that fragmentation occurs through a stepwise cleavage of the bonds between the phosphate group and the alkyl chains leading to the formation of esters with a lower number of alkyl chains and the protonated orthophosphoric acid as ending product. Interestingly, the presence of charged species with mass-over-charge (m/z) values larger than the parent ion is also observed in dibutyl and bis-2-ethylhexyl phosphate, suggesting the existence of their dimers in the gas phase. This has been rationalized, with the aid of ab-initio DFT calculations, in terms of the capability of these molecules to form H-bonds. The photo-induced reactivity in the gas phase has been clarified, and its similarity with the natural bacterial degradation has been pointed out.