Abstract
The theoretical analysis of electronic structure and bonding properties of anhydrous alkali metal oxalates, based on the results of DFT FP-LAPW calculations, Bader’s QTAIM topological properties of electron density, Cioslowski and Mixon’s topological bond orders [reported in the first part of this paper by Koleżyński (doi: 10.1007/s10973-013-3126-z )] and Brown’s Bond Valence Model calculations, carried out in the light of thermal decomposition pathway characteristic for these compounds are presented. The obtained results shed some additional light on the origins of the complex pathway observed during thermal decomposition process (two stage process, first the formation of respective carbonate and then decomposition to metal oxide and carbon dioxide). For all structures analyzed, strong similarities in electronic structure and bonding properties were found (ionic-covalent bonds in oxalate anion with C–C bond as the weakest one in entire structure and almost purely ionic between oxalate group and alkali metal cations), allowing us to propose the most probable pathway consisting of consecutive steps, leading to carbonate anion formation with simultaneous cationic sublattice relaxations, which results in relative ease of respective metal carbonate formation.
Highlights
Presented results show that despite the different crystal structure than in case of lithium and sodium oxalates, the properties of electronic structure, electron density topology and the results of BVM analysis are very similar to those obtained for previous two cases, which allow us to assume the same pathway of thermal decomposition process of anhydrous potassium oxalate as earlier: first C–C bond breaks, which due to excessive thermal energy leads to breaking free of remaining COO- anions and as a result of their translational and rotational movements leads to the formation of unstable [OCOCO2]2- anions, which afterward decompose to CO32- anions and carbon oxide(II) molecules [31,32,33]
Theoretical analysis of the results of FP-LAPW ab initio calculations of electronic structure, total electron density topology, and bond valence (Brown’s BVM approach) carried out for anhydrous alkali metal oxalates presented in this paper allows us to formulate the following conclusions
These difficulties follow mostly the constraints of the methods used in our approach, which despite the complex treatment of the problem of bonding properties in crystal structure allow analyzing properties permitting to propose the most probable sequence of structure decomposition, but does not allow in a simple way taking into account the bond formation processes, which takes place during thermal decomposition of anhydrous alkali metal oxalates
Summary
Anhydrous alkali metal oxalates M2C2O4 form an interesting group of compounds with very similar layered crystal structures and well-defined oxalate anions as an independent entity surrounded by metallic cation and decompose thermally via two step process with respective carbonate as an intermediate product [2,3,4,5,6,7,8,9,10,11]: M2C2O4 ! MCO3 þ CO ! MO þ CO þ CO2 which differ significantly from the thermal decomposition pathway characteristic for anhydrous transition metal oxalates (one-step process following either M2C2O4 ! MO þ CO þ CO2 or M2C2O4 ! M þ 2CO2 pathway).There is still lack of consistent theoretical description and explanation of the pathways of thermal decomposition of anhydrous metal oxalate, despite many experimental results available, but it seems plausible that the electronic structure and chemical bonding properties decide which pathway during thermal decomposition process given compound will follow and theoretical studies of these properties should provide us with some important information about the relations between electronic and crystal structure and bonding properties and a pathway of thermal decomposition process and in principle allow us to predict (or at least explain) the pathway experimentally observed for given compound.
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