Reduction of acetonitrile by hydrated magnesium cations Mg +(H2O)n (n≈20-60) in the gas phase

Ion-molecule reactions of Mg⁺(H₂O)_n (n≈20-60) with CH₃CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH₃CN initiates the formation of MgOH⁺(H₂O)_n-1 together with CH₃CHN^. or CH₃CNH^., which is similar to the reaction of hydrated electrons (H₂O) _n ^- with CH₃CN. In subsequent reaction steps, three more CH₃CN molecules are taken up by the clusters, to form MgOH⁺(CH₃CN)₃ after a reaction delay of 60seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH₃CN allows the unpaired electron that is solvated out from the Mg center to localize in a π(C-N)-like orbital of the bent CH₃CN^.-, which undergoes spontaneous proton transfer to form CH₃CNH^. or CH₃CHN^., with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH₃CNH ^. or CH₃CHN^. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH₃CN molecules taken up by the resulting MgOH⁺(H₂O)_n clusters coordinate directly to the first solvation shell of the MgOH⁺ core, as revealed by DFT calculations. © 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.