Effect of Alkali Metal Coordination on Gas-Phase Chemistry of the Diphosphate Ion: The MH2P2O7 Ions

Systematic experimental and theoretical studies on anionic phosphate species in the gas phase are almost nonexistent, even though they could provide a benchmark for enhanced comprehension of their liquid-phase chemical behavior. Gaseous MH2P2O7? ions (M=Li, Na, K, Rb, Cs), obtained from electrospray ionization of solutions containing H4P2O7 and MOH or M salts as a source of M+ ions were structurally assayed by collisionally activated dissociation (CAD) mass spectrometry and theoretical calculations at the B3LYP/6-31+G* level of theory. The joint application of mass spectrometric techniques and theoretical methods allowed the MH2P2O7? ions to be identified as having a structure in which the linear diphosphate anion is coordinated to the M+ ion (I) and provides information on gas-phase isomerization processes in the [PO3???MH2PO4]? clusters II and the [P2O6???M???H2O]? clusters IV. Studies of gas-phase reactivity by Fourier transform ion cyclotron resonance (FTICR) and triple quadrupole (TQ) mass spectrometry revealed that the MH2P2O7? ions react with selected nucleophiles by clustering, proton transfer and addition–elimination mechanisms. The influence of the coordination of alkali metal ions on the chemical behavior of pyrophosphate is discussed.