A study by spectroelectrochemical FTIR and density functional theory calculations of the reversible complexing ability of an electroactive tetrathiafulvalene crown.

We report on the study of the electrochemically targeted complexation/expulsion of a metal cation (Ba2+) by a crown ether tetra(thiomethyl)tetrathiafulvalene derivative (crown-TTM-TTF). Real time, in situ FTIR spectroelectrochemistry was used to obtain spectroscopic evidence of this electrochemically triggered phenomenon. Density functional theory calculations allowed the spectral information collected to be assigned. Both experimental and theoretical results clearly show that neutral crown-TTM-TTF complexes well Ba2+. Complexation is evidenced by a significant downshift of the frequency corresponding to the asymmetric stretching of the C-O-C ether groups. Concerning the cation crown-TTM-TTF, the spectroscopic signal of the complex form was difficult to identify, first because of the rather low value of the complexation constant and second because the vibration modes involving the oxygen atoms (which are the most affected by the complexation) were found by calculation to occur in the lower spectral region (<1000 cm(-1)), which is not accessible in our experimental conditions. In the case of the dication crown-TTM-TTF, it is now clear that the complex form does not exist, which means that the electrochemical formation of the dication necessarily involves the expulsion of the barium ion.

Reaction intermediates in acid catalysis by zeolites: prediction of the relative tendency to form alkoxides or carbocations as a function of hydrocarbon nature and active site structure.

The mechanism of protonation of ethene, propene, and isobutylene adsorbed on seven different Brønsted acid sites of mordenite has been studied at the ONIOM (B3PW91/6-31G(d,p):MNDO) theoretical level to assess the influence of olefin size and local geometry of the active site on the species and energies involved. The activation energies for olefin protonation are determined by short- and medium-range electrostatic effects and reflect the order of stability of primary, secondary, and tertiary carbenium ions. On the other hand, the stability of covalent alkoxides depends linearly on the AlO(b)Si angle value in the complex, which in turn is determined by the corresponding value in the deprotonated zeolite. It is also shown that the mechanism of protonation of isobutylene is different from that of ethene and propene and involves a free tert-butyl carbenium ion as a true reaction intermediate. Whether this carbenium ion is converted into a covalent alkoxide depends on the T position on which the Al is located. All these findings allow us to predict, on the basis of the position and local geometry of the Brønsted acid site, whether the reaction intermediates of olefin protonation will be covalent alkoxides or free carbenium ions.

Cluster and periodic calculations of the ethene protonation reaction catalyzed by theta-1 zeolite: influence of method, model size, and structural constraints.

The protonation of ethene by three different acid sites of theta-1 zeolite was theoretically studied to analyze the extent and relevance of the following aspects of heterogeneous catalysis: the local geometry of the Brønsted acid site in a particular zeolite, the size of the cluster used to model the catalyst, the degree of geometry relaxation around the active site, and the effects related to medium- and long-range interactions between the reaction site and its environment. It has been found that while the reaction energy is very sensitive to the local geometry of the site, the activation energy is mainly affected by the methodology used and by electrostatic effects on account of the carbocationic nature of the transition state.