ICP-SIFT/MS study of gas-phase reactions of lanthanide cations with benzene: Room-temperature kinetics and periodicities in reactivity

Abstract

The gas-phase reactivity of atomic lanthanide cations (excluding Pm+) has been surveyed systematically with benzene using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer with an added facility for collisional dissociation of product ions. Observations are reported with La+, Ce+, Pr+, Nd+, Sm+, Eu+, Gd+, Tb+, Dy+, Ho+, Er+, Tm+, Yb+ and Lu+ at room temperature (295±2K) in helium at a total pressure of 0.35±0.02Torr. All the Ln+ ions were observed to add benzene exclusively and rapidly with efficiencies ≥28%. Sequential addition of C6H6 was seen to saturate with the formation of Ln+(C6H6)3. Measured onset energies were high (>33kcalmol−1) for the dissociation of La+C6H6, Ce+C6H6, Pr+C6H6, Gd+C6H6 and Tb+C6H6 and low (<20 kcalmol−1) for the dissociation of the remaining Ln+C6H6 cations.The reaction efficiency for the formation of Ln+C6H6 and the measured onset of benzene removal from Ln+C6H6 by collisional dissociation were seen to correlate with the electron promotion energy required to achieve a two non-f electron configuration in Ln+. These results are interpreted in terms of schematic triple minimum potential energy profiles in which the electron promotion energy can act as a “gate” for the conversion between an electrostatically bound adduct, an adduct of the sd-hybridized lanthanide cation and one achieved by a CH bond insertion of Ln+. The ICP/SIFT results are compared with results of FT/ICR measurements at low pressure.