On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

Abstract

The intrinsic acidity of chalcocyclopentadienes (CpXH; X=O, S, Se, Te) is investigated by high-level G3B3 and G2 ab initio as well as B3LYP DFT calculations, which show that, independent of the nature of the heteroatom, all chalcocyclopentadienes are stronger acids in the gas phase than cyclopentadiene. However the acidity does not increase regularly down the group, and the acidity enhancement for Te derivatives is five times larger than for O derivatives, but only twice that of S-containing compounds. The most favorable deprotonation process corresponds to loss of the proton attached to the heteroatom, with the sole exception of the 5-substituted 1,3-cyclopentadienes, for which the O and S derivatives are predicted to behave as carbon acids. No matter the nature of the heteroatom, the 1-substituted 1,3-cyclopentadienes are the strongest acids. The intrinsic acidity of all isomers, namely, 1-substituted, 2-substituted, and 5-substituted 1,3-cyclopentadienes, increases with increasing aromaticity of the anion formed on deprotonation, and therefore the Te compound is the strongest acid for the three series. However, the intrinsic acidity of chalcocyclopentadienes is not dictated by aromaticity, so that, in general, the most stable deprotonated species do not coincide with the most aromatic ones.

Can Supported Reduced Vanadium Oxides form H2 from CH3OH? A Computational Gas-Phase Mechanistic Study

“…However, CH 3 OH has been widely studied and utilized as a smart chemical probe molecule to establish the fundamental structure–activity relationships for metal oxide catalysts. − Despite its vast use, there is still a lack of understanding on the nature of the reaction mechanisms involved in the catalytic transformations, due mainly to the difficulty for obtaining representative model systems on which to perform detailed chemical and kinetic analyses. In this context, extensive research efforts have been made both experimentally and theoretically to search for suitable catalysts to improve chemical processes concerning alcohol transformations; , likewise, gas-phase experiments combined with theoretical works − have provided a wealth of information of elementary steps and intermediates for various catalytic reactions, − and they provide ideal model systems for local reactive sites of condensed-phase catalysts. Therefore, small transition-metal oxides clusters may well be effective models of real surface species or active sites over the bulk transition-metal oxide materials, which are capable of providing a conceptual framework for modifying or controlling the chemical properties of mixed-metal oxides and for engineering industrial catalysts.…”