Gas-phase chemistry of ionized and protonated GeF4: a joint experimental and theoretical study

The gas-phase ion chemistry of GeF4 and of its mixtures with water, ammonia and hydrocarbons was investigated by ion trap mass spectrometry (ITMS) and ab initio calculations. Under ITMS conditions, the only fragment detected from ionized GeF4 is GeF3+. This cation is a strong Lewis acid, able to react with H2O, NH3 and the unsaturated C2H2, C2H4 and C6H6 by addition-HF elimination reactions to form F2Ge(XH)+, FGe(XH)2+, Ge(XH)3+ (X = OH or NH2), F2GeC2H+, F2GeC2H3+ and F2GeC6H5+. The structure, stability and thermochemistry of these products and the mechanistic aspects of the exemplary reactions of GeF3+ with H2O, NH3 and C6H6 were investigated by MP2 and coupled cluster calculations. The experimental proton affinity (PA) and gas basicity (GB) of GeF4 were estimated as 121.5 ± 6.0 and 117.1 ± 6.0 kcal mol−1, respectively, and GeF4H+ was theoretically characterized as an ion-dipole complex between GeF3+ and HF. Consistently, it reacts with simple inorganic and organic molecules to form GeF3+-L complexes (L = H2O, NH3, C2H2, C2H4, C6H6, CO2, SO2 and GeF4). The theoretical investigation of the stability of these ions with respect to GeF3+ and L disclosed nearly linear correlations between their dissociation enthalpies and free energies and the PA and GB of L. Comparing the behavior of GeF3+ with the previously investigated CF3+ and SiF3+ revealed a periodically reversed order of reactivity CF3+ < GeF3+ < SiF3+. This parallels the order of the Lewis acidities of the three cations.