Multicollision chemistry of gas-phase transition-metal ions with small alkanes: rate constants and product branching at 0.75 torr of helium

Abstract

We have studied the reactions of the gas-phase first transition series metal ions Sc+ through Zn+ with CH4, C2H6, and C3Hg in the multicollision environment of He buffer gas at 0.75 Torr. Laser vaporization of pure metal targets creates reactant metal ions M+ in a fast flow reactor. For each primary M+ + alkane reaction step we obtain quantitative effective bimolecular rate constants and product branching fractions. The multicollision rate constants provide a strikingly different view of M+-alkane interactions than previous single-collision studies. At 0.75 Torr, all 10 M+ species react with all 3 alkanes with the single exception of the Mn+ + CH4 pair. Stabilization of long-lived adduct ions of stoichiometry M(alkane)+ by third-body collisions with He is the dominant process in most reactions, although H2 and CH4 elimination channels do occur for the same M+-alkane pairs that undergo single-collision elimination reactions. The rate constants vary widely and nonmonotonically across the transition series for a given alkane in a pattern that is remarkably similar for CH4, C2H6, and C3Hg. This similarity argues against direct insertion of M+ into C-C bonds as the initial step of the M+-alkane interaction, even for those reactant pairs that yield CH4 elimination products. From a simple collisional quenching model we obtain estimates of the time scale for the decay of long-lived hot adduct ions both backwards to reactants and forwards to elimination products. The experimental results suggest that the M+-alkane attractive forces that control the lifetime of the hot adduct ion involve either a donor-acceptor interaction or C-H bond insertion of the metal ion, i.e., strong chemical interactions rather than simple electrostatic forces. For the relatively inert ions Mn+ and Zn+, the dissociation energies Z)°(M+-alkane) exceed 10 kcal/mol. A qualitative model including the interplay of M+ size effects on long-range repulsive forces, of potential surface intersections, and of orbital symmetry and electron spin conservation effects can explain much of the existing M+-alkane reaction data in both multicollision and single-collision environments.

A Comprehensive Theoretical Study on the Reactions of Sc+ with CnH2n+2 (n=1–3): Structure, Mechanism, and Potential‐Energy Surface

“…For reactions M CH 4 , the energetically most favorable process observed by experiment is dehydrogenation to form MCH 2 H 2 at low energy. The proposed two main mechanisms for the process are shown in Scheme 1 (top).…”

“…[11±15] Although the reaction mechanism of Sc with CH 4 was also reported by Musaev et al, [16] comprehensive theoretical studies on the reactions of early first-row transition-metal ions with small alkanes are still necessary, since the electronic configurations of early transitionmetal ions and the strength of spin ± orbit interactions clearly differ from those of late firstrow transition-metal ions. Most of the d orbitals are occupied in late transition-metal ions, while some of them are empty in early transition-metal ions.…”

“…In the reactions of transition-metal ions with CH 4 , C 2 H 6 , and C 3 H 8 , the products of Equations (1) ± (19) (19) Here, M denotes a transition-metal ion. For a given metal ion only some of these reactions may be involved under certain conditions.…”

“…One involves a four-centered transition state (TS), and the other a dihydrido species. In reactions of M with C 2 H 6 and C 3 H 8 , the oxidative addition of C À H and C À C bonds to the metal center are two fundamental steps from which a series of products results, such as small neutral molecules (H 2 , CH 4 , C 2 H 4 , etc.) and fragmentation ions.…”

“…Apparently, much more effort has been devoted to experimental research than to theoretical studies on these reactions, especially the detailed examination of the PES. Recently, several theoretical studies on the reactions of late first-row transition-metal ions with small alkanes such as CH 4 , C 2 H 6 , and C 3 H 8 were reported.…”