Proton transfer, strongly basic molecule

A key issue in the chemistry and catalysis of basic molecules reacting in acid zeolites is the extent to which proton transfer occurs from the Bronsted site to the basic molecule. For strongly basic molecules like ammonia or pyridine, infrared spectroscopy clearly identifies the protonated adduct (NH4+ or PyH+) from its characteristic vibrational frequencies. For trimethylphosphine, also a strong base, both infrared and NMR evidence for complete proton transfer are convincing[37]. For molecules which are less strongly basic, the question is not so easily answered. 

Ammonia is among the smallest strongly basic molecules, and its diffusion is hardly affected by the porous structure. This makes it the most commonly used probe in calorimetry. One proton is transferred from the zeolite hydroxyl site to ammonia upon adsorption, i.e. ammonia is adsorbed as an ammonium ion. The heat of ammonia adsorption depends both on the proton mobility and on the affinity of ammonia for the proton. Ammonia adsorption isotherms are of type I. 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

First of all, what was considered were bare hydronium H3+0 ions with three equivalent protons, a hydrated hydronium ion with three strongly bound water molecules (i.e., Eigen cluster H904+), and the symmetric H502+ complex in which a proton is shared between two water molecules (i.e., the Zundel ion). Many intermediates or more-complex states of the hydrated proton, H+(H20) , may also exist. All clusters have a finite lifetime and transform between each other during charge transport. Due to the variation of the relative abundance of these three basic states, proton transfer may occur via different pathways. 

The last case concerns the solvent molecules with large dielectric constants or strong basicity the ions can be rapidly solvated (in the bulk or in large clusters) and proton transfer occurs. Since the emission arises from the transferred state, the Stokes shift is important (typically around 9000 cm"1 with a large bandwidth). The 1-naphtholate fluorescence in neutral water or a mixture of polar solvents... 

Besides the solvent, other species may participate in the proton abstraction from the analyte molecules. For instance, O2 has a strong basicity in the gas phase and may react with other molecules from the solvent or the analyte by proton transfer. These molecules must have acidity lower than 1451 kJ mol-1, this value corresponding to the gas-phase acidity of the H02 species ... 

Despite the presence of a formally divalent carbon atom, CO is not in fact a particularly reactive molecule and much of its chemistry depends on the use of either extreme conditions, energetic reagents or some form of catalysis. Perhaps the simplest examples of such catalysis are found in the reactions of carbon monoxide with protic reagents such as alcohols or secondary amines, affording esters or amides of formic acid. These reactions are catalyzed by alkoxide or amide anions, respectively, and, as shown in Scheme 1, the key step is nucleophilic attack on CO by the catalyst to give a strongly basic alkoxyacyl or aminoacyl anion which is immediately trapped by proton transfer from the alcohol or amine, so generating the catalytic species. 

Satchell and Satchell" reported a kinetic study on the aminolysis of p-nitrophenyl isothiocyanate with primary amines and anilines in diethyl ether and isooctane as solvents. The detailed analysis reveals that the aminolysis occurs via a zwitterionic intermediate, T , which undergoes subsequent proton transfer catalyzed by a second amine molecule (equation 12). Added carboxylic acids form inactive 1 2 amine-acid complexes with strong basic amines and inhibit aminolysis, but with weak bases the acids form only a negligible amount of complex and they catalyze the aminolysis. 

Strong bases are protonated by acidic surface hydroxyls the products of this reaction are conjugated acids and bases. The stabilization of the conjugated acid on the surface plays an important role for the progress of the reaction. In the following sections, we shall consider the proton transfer from OH groups to ammonia, pyridine, and some substituted molecules. It is important to bear in mind that sometimes proton transfer occurs as a result of the cooperative action of two or more basic molecules. 

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