Chemical reactions and collisional quenching of the chromium atomic ion in a metastable excited state

Abstract

The kinetics of reaction between Cr' produced by 70-eV electron impact on Cr(C0)6 are described. Approximately 26% of the Cr+ ions react at a rate 4.4 times slower than the remaining Cr+ ions. This is interpreted in terms of an excited state that is long lived on the time scale of the experiment (2 s). The variation of ion abundance with time in a mixture of Cr(CO), and CH4 following a pulse of 70-eV electrons is analyzed. The analysis indicates that the excited state reacts with CHI to form CrCH2+ but that the primary result of collision between the excited Cr+ and CH4 is quenching of the excited state. If the quenching process produces ground-state Cr+ rather than another excited state, it is spin forbidden. Collision-induced decomposition of CrCH4+ supports the involvement of an excited state in the reaction which produces CrCH2+. The CrCH2+ ion was not observed to react with CO. The possibility that Fe+ formed by electron impact on Fe(CO), includes ions in metastable states is discussed. IntroductionRecently we described a study of the reactions of first-row transition-metal atomic ions with butanes in the gas phase.' Cr+ and Mn+ generated by electron impact ionization on Cr(C0)6 and Mnz(CO)lo were found to differ in their reactivity from the group 8 atomic metal ions. Fe+, Co+, and Ni+ generated by electron impact on Fe(CO)5, Co2(CO),, and Ni(CO), react with the butanes to form various metal-olefin Mn+ undergoes no observable reactions.' Cr+ reacted with the butanes to form metakolefin complexes, but it does not react with isobutane to form a metal-propene complex.' A metal-propene complex is the dominant product in the reactions of the group 8 metal ions with isobutane.' It is somewhat surprising that Cr+ with its very stable d5 electronic configuration should react at all. In fact, collision-induced decomposition studies of CrC,H complexes suggest a very weak interaction between Cr+ and the butane.]The observed reactivity of Cr+ led to studies to determine whether an excited state of Cr+ might be involved. Several conclusions of these studies were added to the report of the study of the atomic metal ion chemistry with butane as a "note added in proof'.' A full description of these studies is the purpose of the present report. First, the kinetics of reactions of Cr+ with Cr(C0)6 are considered. Next, the reaction of Cr+ with CHI and the collision-induced decomposition of CrCH4+ are discussed. The reaction of excited Cr+ with CH, produces CrCH2+. Since this reaction is endothermic for ground-state species, it is particularly useful for characterizing the excited state. Evidence that the

Activation of Silane by W+ in the Gas Phase: Fourier-Transform Ion Cyclotron Resonance and ab Initio Theoretical Studies

“…Some of the excited states, especially the 4 D and 4 G, exhibit reactivity with alkanes that differ vastly from that of the 6 S ground state. As was previously done by Ridge and co-workers, we have monitored charge exchange between Cr + (generated by EI from Cr(CO) 6 ) and Cr(CO) 6 . Whenever a mixture of states is present, the decay of the signal due to Cr + is a superposition of two or more exponential functions, corresponding to the reactions of the various states.…”

Gas-phase ion-molecule charge-exchange reactions of iron(1+) with iron pentacarbonyl: observation of higher lying metastable electronic states

“…15 In the same study, Ridge and co-workers determined that the radiative lifetime of the excited states is greater than or equal to 2 s. Ridge et al 15 In the same study, Ridge and co-workers determined that the radiative lifetime of the excited states is greater than or equal to 2 s. Ridge et al…”

“…14 For example, Ridge and co-workers estimated that approximately 75% of Cr+ ions formed by El (70 0022-3654/92/2096-5314$03.00/0 © 1992 American Chemical Society eV) of Cr(CO)6 are in metastable electronic states. 15 In the same study, Ridge and co-workers determined that the radiative lifetime of the excited states is greater than or equal to 2 s. Ridge et al also determined that 22.4 ± 1.4% of Mn+ ions produced by electron impact ionization of Mn2(CO)10 are formed in an electronically excited state with a radiative lifetime of greater than 5.8 ± 0.7 s. 14 A number of recent studies have sought to characterize the effects of electronic excitation on the ion-molecule reactivity of the first-row atomic transition-metal ions.1617 For example, Ridge et al have shown that Cr+ ions produced by 70-eV El of Cr(CO)6 react exothermically with CH4 to form CrCH2+ ionic product in a reaction that is endothermic for the ground-state Cr+ [6S(3d5)] under thermal conditions. 15 Armentrout studied the state-specific gas-phase ion-molecule reactions of Cr+ and other first-row transition-metal ions.…”

Reactions and Thermochemistry of Small Cluster Ions: Fe(CS2)n+(n= 1, 2)

“…The energy difference between the 4 F and 6 D states is only 0.25 eV, but their reactivities are quite different. Only 3% of Fe + formed is in a metastable state at least 0.5 eV above the ground state . A nearly pure beam of Fe + ( 6 D) can be produced by subjecting Fe + produced by electron ionization to a large number (∼10 3 ) of low-energy collisions with argon in a drift cell at an argon pressure of 0.25 Torr and in the time range 200−300 μs .…”