Kinetics and thermodynamics for the bonding of benzene to 20 main-group atomic cations: formation of half-sandwiches, full-sandwiches and beyond

摘要

An inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic study of room-temperature reactions of benzene (C_6H_6 or C_6D_6) with 20 main-group atomic cations (non-transition metals). These include the Group 1 ions K~+, Rb~+ and Cs~+, the Group 2 ions Ca~+, Sr~+ and Ba~+, the Group 12 ions Zn~+, Cd~+ and Hg~+, the Group 13 ions Ga~+, In~+ and Tl~+, the Group 14 ions Ge~+, Sn~+ and Pb~+, the Group 15 ions As~+, Sb~+ and Bi~+, the Group 16 ion Te~+ and the Group 17 ion I~+. The atomic ions were produced at ca. 5500 K in a plasma ion source and allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction in He at 0.35 Torr and 296 K. Benzene addition was observed to proceed rapidly, k > 3 * 10~(-10)cm~3 molecule~(-1)s~(-1) with most ions. Hg~+ and I~+ reacted by electron transfer and electron transfer was observed to compete with addition (in a ratio of 1:3) in the reaction with Zn~+. Most of the other cations added two benzene molecules, but the larger Group 2 (s~1) ions Ca~+, Sr~+ and Ba~+ added three benzene molecules in rapid succession. Equilibrium was achieved among the first additions only for Group 1 cations and for most of the second benzene additions that were observed. Standard free energy changes derived from the corresponding equilibrium constants are reported and compared with predictions based on B3LYP density functional theory. Both the experiments and theory show a large free energy change for the first ligation with benzene (more negative than -14kcal mol~(-1)), except for the Group 1 cations which have a rare-gas electronic configuration. A large decrease in the absolute free energy change for the addition of the second molecule of benzene is also observed. Theory has provided structural parameters for half-and full-sandwich structures and periodic trends in these structures are rationalized in terms of the valence shell electron (VSEC) configuration of the atomic ion.