Generation and Reactivity of State Selected Atomic and Size Selected Transition Metal Cluster Ions

Abstract

Listen

Translate article

In this chapter, two distinct studies in the general field of gas phase transition metal ion chemistry are presented. In the first, atomic metal ions of the first row transition series are generated by electron impact on various volatile organometallic reagents. These ions are then mass selected and injected at low energies into a pressure and temperature variable drift reactor. After drifting through the reactor, filled with He buffer gas, the ions exit, are mass analyzed by a quadrupole and detected. By pulsing the ion beam as it enters the drift reactor, arrival time distributions (ATD) at the detector can be obtained. Many of the ATD’s were found to be bimodal or trimodal, indicating the atomic metal ion beams contained multiple components that interacted significantly differently with the He buffer gas. Analysis of the data showed the largest splittings in the ATD’s were due to ion electronic states with different electronic configurations (either 3dn or 4s3dn-1). In favorable cases electronic states within the 4s3dn-1 configuration could be resolved. Clustering of state selected M+ with He was studied. It was found that only M+ ions with 3dn configurations formed the M+-He adduct. Equilibrium constant measurements yielded values of ΔH between 1 and 4 kcal/mol and ΔS between 13 and 18 cal/mol°K. Deactivation of excited states was also observed only for metal ions with 3dn configuration excited states and 4s3dn-1 ground states. Deactivation rate constants were measured for two systems and a mechanism proposed. In the second study, size selected Nbn + ions (1 ≤ n ≤ 6) were injected into the drift cell and reacted with O2, NO, C2H6, C3Hg and C6H6 along with deuterated analogues. Several reactions of NbO+ are also reported. Both total rate constants and product branching ratios were measured. Examples were chosen to highlight the size selective behavior of the clusters and to point out that truly surprising reactions are observed. Thermodynamic and mechanistic conclusions are drawn where appropriate.