Bronsted localization

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

We can also crudely estimate the basicity of the carbonyl oxygen atom. Since the HOMO is strongly localized to the oxygen atom (the coefficient of 2pQ is close to 1), and the oxygen atom is monocoordinated but uncharged, one should expect the Lowry-Bronsted basicity to be less than that of alkoxides, which are monocoordinated but charged. 

Depending on niobium location, the Nb-containing catalysts can reveal Bronsted acid, Lewis acid, or redox properties. Niobium oxide cationic species (NbOn(5-2n)+), which occupy the extra lattice cation positions, play the role of the Lewis acid sites and may exhibit the redox properties. Nb localized in the framework of mesoporous MCM-41 sieves provides the Lewis acidity [3,4] and the oxidizing properties [5,12]. 

We can consider decarboxylation reactions in terms that are analogous to those in proton transfer reactions the reactivity of the carbanion in carboxylation reactions is analogous to internal return observed in proton transfer reactions from Bronsted acids. Kresge61 estimated that the rate constant for protonation of the acetylide anion, a localized carbanion (P A 21), is the same as the diffusional limit (1010 M s1). However, achieving this rate is highly dependent on the extent of localization of the carbanion. Jordan62 has shown that intermediates in thiazolium derivatives are also likely to be localized carbanions, which implies that protonation of these intermediates could occur at rates approaching those of other localized carbanions. 

The postulate that only the free water should be counted as possible sites for the surplus proton, introduced to help rationalize the position of H+ on Bronsted plots (Bell, 1943 Kresge et al., 1967) makes the values of AS for this acid even harder to understand because it reduces the localization entropy to about — 6 cal mole-1 deg-1. 

In our opinion, the main factor which governs the acidity of bridged OH groups in zeolites is the chemical one. If the local nature of Bronsted sites is taken into account, the following rational classification can be proposed for the bridged OH groups of zeolites with regard to their acidity and Si/Al ration in the framework (34). 

It has been claimed [114] that since a limiting Bronsted exponent is observed for the ionization of chloroform this carbon acid is fully normal in its proton transfer behaviour. This behaviour is explained by arguing that the carbanion is tetrahedral and the charge is localized on carbon [114]. Hence results are obtained which are similar to those for oxygen and nitrogen acids which also ionize to give a base with the electrons localized on one atom. Phenylacetylene shows similar behaviour and the carbanion may also possess an electron pair localized on carbon [143]. The results for all carbon acids will be compared in Sect. 5 and this point will be discussed in more detail. 

In a recent study [175] of the methoxide ion catalysed tritium exchange in methanol of weakly acidic fluorinated bicycloalkanes (pIT ca. 20), comparison of the rates of exchange for six compounds, with ion-pair pK values determined in cyclohexylamine containing caesium ions, gave a Bronsted plot with slope (a) within experimental error of unity. The isotope effects for proton compared with deuteron transfer (feH/feD = 1.2) are similar to those observed with chloroform. The charge on these polyfluorocarbanions may also be localized on carbon [175]. 

Moreover, a Bronsted correlation with a = 0.62 has been obtained for 2-phenyl-amino-l,2-diphenylethanol radical cation [219]. Comparison of this value with that obtained for the C-C bond cleavage of 4-MeOC6H4CH(OH)rBu + catalyzed by a series of bases in aqueous solution f = 0.4) [170], suggests that localization of the positive charge at nitrogen results in a significantly later transition state for fragmentation. 

Decationation of the process of conversion of the Bronsted to Lewis acid sites that takes place in the temperature range of 400-500° as a result of the evolution of water can be visualized as shown in Fig. 8. The Bronsted acid sites are shown by (A), the Bronsted base by (B), and the Lewis acid site by (C). The specific molecular locale of the Lewis acid site is the three-coordinated aluminum. It can act as an acceptor of H or of one electron. As an acceptor of the H ion it may play an important role in initiating the carbonium type of reactions of the hydrocarbons by facilitating carbonium ion formation. On the other hand, it is also an electron acceptor. Turkevich and Stamires have shown this when they studied the ESR of triphenylamine (an electron donor) on a variety of zeolites. They found that the amount of electron transfer increased... 

The inactivity of Graphon in the contacts with the white solids despite the near equivalence of its work function with that of BPL demonstrates an absence of coupling of the delocalized tr electron system of Graphon with the localized Bronsted and Lewis sites of the white solids. It is to be noted that electron transfer between the tt electron systems of different carbon blacks occurs quite readily. The oxide structures of carbon blacks are seen to play a fundamental role in this viewpoint at the microscopic level akin, for example, to the critical importance of the molecular structures of the adsorbates in chemisorption from the gas phase onto metals (41, 42) and metal oxides(43). 

Figure 22.6 CPMD simulation (BLYP functional) ofthree HjO molecules per one Bronsted site of H-CHA [11]. Left characteristic distances along a 4 ps trajectory for the hydrogen bond between the Bronsted site and a terminal HjO molecule of the trimer and between the terminal and central HjO molecules of the trimer (bottom). The bottom right insert shows the protonated HjO trimer in the gas phase with the HjO+ in the center, and the corresponding structure in H-CHA, which is a local minimum. Distances are given in A.

From the perspective of Molecular Orbital Theory, the energy of the lowest unoccupied antibonding orbital (LUMO) has been estimated at 3.8 eV, indicates the high electron affinity with respect to the central carbon atom, hence it is susceptible to attack by nucleophile and to the reduction while that of highest occupied molecular orbital (HOMO) is susceptible to attack by electrophile due to its high localized electron density as oxygen inplane lone pairs. It also interacts weakly with Lewis and Bronsted acids [21, 22a]. 

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