RESUMO
Lithium-sulfur batteries are considered one of the possible next-generation energy-storage solutions, but to be commercially available many drawbacks have yet to be solved. One solution with great potentiality is the use of lithium sulfide as cathode material since it can be coupled to Li-free anodes, such as graphite, Si or Sn. Nevertheless, Li2S, like sulfur, is electronically and ionically insulating, with a high activation potential for its initial oxidation step. To overcome this issue, different strategies have been explored, one of them being the use of catalytic surfaces. In the present article, we study using first principles calculations the effect of the dielectric constant of the solvent on the activation energy of the cleavage reaction of Li2S on different catalytic surfaces. To the best of our knowledge, this is the first time that such a study is undertaken. We find that the effect of the solvent should be twofold: on one side, it should decrease the interaction between the Li2S molecule and the surface. On the other side, since the species arising in the dissociation reaction are charged, the solvent should decrease the activation barrier for the dissociation of the Li2S molecule, when compared with the reaction in vacuum. These theoretical findings are discussed in connection with experimental results from the literature, where the behaviour of the Li-S cathode is studied in different solvents.
RESUMO
A detailed kinetic study has been carried out for the aminolysis of ionizable Fischer thiocarbene complexes (CO)5M[double bond, length as m-dash]C(SR)CH3 (M = Cr, W; R = iPr, nBu, cHex, tBu) with five primary amines and one secondary amine in aqueous acetonitrile solutions (50% MeCN-50% water (v/v)). The observed rate constants for the reaction with primary amines showed a first-order dependence on the amine concentration, while with morpholine, the rate constant has second-order dependence. The general base catalysis process was confirmed by the variation of the rate constants with the concentration of an external catalyst and the pH. The results agree with a stepwise mechanism where the nucleophilic addition to the carbene carbon to produce a tetrahedral intermediate (T±) is the first step, followed by a rapid deprotonation of to form the anion T- which leads to the products by general-acid catalysed leaving group (-SR) expulsion. In general, it was found that the chromium complexes are less reactive than the tungsten analogues. The obtained Brønsted parameters for the nucleophilic addition (ßnuc) indicate that C-N bond formation has made little progress at the transition state. By using Charton's correlation, the role that the steric factor plays throughout the mechanism has been unraveled. The nucleophilic addition to the thiocarbenes is less sensitive to steric effects than the alkoxycarbenes regardless of the nature of the metal centre. Conversely, the steric effects on the general-base catalysis can be strong depending on the volume of the catalyst and the metal centre. On the basis of the structure-reactivity coefficients ß and ψ and comparison with alkoxycarbene complexes, esters and thiolesters, insights into the main factors ruling the reactivity in terms of transition state imbalances are discussed.
RESUMO
Rate constants for the hydrolysis of Fischer carbenes (CO)5Cr=C(OR)Ph (R = n-propyl, neopentyl, isopropyl, and menthyl) in 50% MeCN-50% water (v/v) at 25 degrees C are reported. The rate constants for the addition of -OH to the carbene carbon are 5.3, 3.7, 0.84, and 0.01 M(-1) s(-1), respectively. These rate constants give linear correlations with the corresponding rate constants for the hydrolysis of esters such as acetate, benzoate, and formiate. The slopes of the plots of the observed rate constants for the carbenes vs the rate constants for the esters are 1.4 and 1.2 for acetate and benzoate, respectively, indicating that the factors that decrease the reactivity of the two types of compounds are similar, but the carbenes show higher sensitivity. The rate constants are well correlated with several steric parameters giving a value of -3.84 for the Charton's psi parameter. The results show that the steric bulkiness of the R group is the main factor determining the reactivity differences for these carbene complexes.