Reactions of Laser-Ablated Zirconium Atoms within a Supersonic Expansion: Insertion versus Radical Mechanism

Abstract

In a laser ablation type microreactor followed by supersonic expansion, zirconium atoms have been reacted with methyl fluoride, CH(3)F (MeF), and mixtures of MeF and dimethylether, CH(3)-O-CH(3) (DME) seeded in He. With both mixtures, only a number of simple fluorinated products are formed, and they have been identified by one-photon ionization. All products can be linked to radical reactions either with F atoms, CH(3), or ZrF(1, 2, 3) radicals. No insertion products of the Grignard reagent type, F -Zr-CH(3) could be identified with or in the absence of DME. On the other hand, evidence has been found for the presence of organometallic compounds of the type ZrC(2)H(n=2, 4, 6), which could result from radical attack. Thus, even in conditions where intense solvation is at work, induced by clustering with polar DME molecules, which can act as stabilizing agents, a direct insertion mechanism into the C-F bond involving barrier suppression is not at work in our conditions. The reactivity due to radicals is very effective in this type of reactor, and the products that are efficiently formed can be quickly stabilized in the expansion. The radical attack supersedes, in the case of zirconium solvated by DME, the metastable mechanism with Zr(4d)(3)(5s)(1), that is certainly energetically impossible in the absence of strong reaction barrier suppression by a solvent. High level ab initio calculations performed at the CASPT2 level of theory are used for characterizing the electronic and geometric structure of the inserted products. They also reveal striking features of the reaction mechanism that support the absence of observation of inserted products within solvated clusters of zirconium.

A two‐step reaction scheme leading to singlet carbene species that can be detected under matrix conditions for the reaction of Zr(3F) with either CH3F or CH3CN

“…In accordance with the data provided in Table 3, the calculated energy for the 1 A 0 electronic state was only slightly less than the energy values corresponding to the triplet states, whereas the energy for singlet state 1 A 00 was approximately 26 kcal/mol greater than that of the most stable singlet state. Interestingly, these energy values differed markedly from those reported by Soorkia et al [16] For example, the energy difference reported by these authors for the 1 A 0 and 3 A 0 electronic states was 72 kcal/ mol.…”

“…The channels leading to the radical products can be useful for rationalizing the experimental data for the reaction of ablated zirconium atoms with CH 3 F within a supersonic expansion because the ZrF· and ·CH 3 species can be linked to the products detected by the gas‐phase determinations . However, they do not provide a plausible description of the process leading from the reactants to the H 3 CZrF and H 2 CZrHF complexes detected under the matrix conditions (as well as theoretically).…”

“…For instance, the Zr + CH 3 F reaction was investigated within a supersonic expansion by Soorkia et al The products detected by these authors, which can be linked to the radical species F, CH 3 , and ZrF, do not match the inserted structures H 3 CZrF and H 2 CZrHF determined for the same interaction under cryogenic conditions by Cho and Andrews . The detected products of the Fe + CCl 4 reaction also vary depending on the experimental conditions.…”

“…However, stable structures for the inserted singlet species H 3 CAZrAF and H 2 C@ZrHF were also detected in that investigation, even when connecting paths for the reactants and the products were not found. [16] The interaction of Zr( 3 F) 1 CH 3 CN has been modeled through DFT calculations. [15,19,20] In all these studies, it is assumed the existence of intersystem crossings (ISC) between the potential energy surfaces belonging to the low-lying triplet and the singlet channels, such that the products detected under cryogenic conditions can be linked to the reactants through a single pathway.…”

Rationalizing the Low‐Spin Products Observed for the Reactions M+CH3CN (M=V, Nb,Ta) Through a Non‐Spin Flip Scheme.

“…Another surprising fact is that the products observed for reactions occurring under matrix‐isolation conditions often differ from those determined in less constrained media. For example, the products detected for the reaction of fluoromethane with zirconium atoms within a supersonic expansion can be linked to the radical species F, CH 3 and Zr−F, whereas the products observed for the same reaction under matrix confinement are the inserted structures H 3 C−Zr‐F and H 2 C=ZrHF . Likewise, the products obtained by dropping Fe atoms into CCl 4 deposited on a cold FeO(111) surface, C 2 Cl 4 , C 2 Cl 6 , OCCl 2 , CO, FeCl 2 and FeCl 3 , support a radical mechanism, whereas the same reaction under matrix conditions yields high‐oxidation state complexes containing Fe−C bonds.…”