Basic site

The Br0nsted basicity of a surface is related to its deprotonation ability, which can be probed by investigating the dissociative adsorption of protic molecules (Bailly et al., 2005a Chizallet et al., 2006). The 0Lc2 0Lc H transformation thus induced can be followed by PL, which is one of the few techniques able to simultaneously characterize oxide ions and their protonated forms. The same kind of equilibrium is also involved when a hydroxylated surface is undergoing thermal pretreatment (Section 2.1). PL is thus an interesting tool to evaluate the surface basic properties of alkaline earth oxides. 

Adsorption of propyne and methanol on MgO has been followed by PL and IR spectroscopies to determine the Brqnsted basicity (Chizallet et al., 2006). Gas in excess and physisorbed species were removed by evacuation at room temperature before PL spectra were recorded. 

No conclusion could be reached from the global quenching of the signal after adsorption of methanol, but propyne can discriminate oxide ions of low coordination. Indeed, PL results indicate that Che- and Che2 ions are involved in propyne dissociation. On the basis of PL and IR (semiquantitative evaluation of the number of molecules dissociated per square meter) characterizing dissociative adsorption of propyne and in line with theoretical results showing that acetylene does not dissociate on CLc 2 (Nicholas et al., 1998), it is inferred that Mg3C—O and Mg —03c ion pairs are involved in propyne dissociation (Chizallet et al., 2006). 

The adsorption of molecular probes, followed by infrared or NMR spectroscopy and thermal desorption studies, is the most commonly adopted way to study basic sites. Carbon dioxide is frequently used in infrared studies, particularly of cationic zeolites with added alkali metals.Chloroform is also suitable, since the interaction with the chlorine atom and subsequently the C-Cl stretching frequency is a measure of the basic strength. NMR studies of basic zeolites have concentrated on the use of C containing probes such as methyl iodide and chloroform. Addition of methyl iodide results in its decomposition and the formation of methoxy groups at framework oxygens. The shift of the methyl carbon is expected to be related to the basicity of the framework, that is the tendency of the framework oxygens to donate electrons - C MAS NMR of methoxy groups prepared in this way shows a clear distinction between basic zeolites, such as Cs-X, and acidic zeolites, such as H-ZSM-5. 

For cationic forms of zeolites, the observed trends in (mild) basicity are clear. Zeolites with higher aluminium contents in the framework are more basic than high silica zeolites. Similarly, the basicity of zeolites exchanged with cations increases in the order Na K Rb Cs. 

Finally, strong irreversible hysteresis has been observed in H2 adsorption/ desorption measurements on Ni2(4,4 -bipy)(N03)4 by the groups of Thomas and Rosseinsky, even to very low desorption pressures. This results from kinetic trapping of H2 molecules which can then only be released by raising the temperature to IlOK and above. 

In summary, the adsorption behaviour of MOFs is of interest because of their large pore volumes, and the novel properties of their organic-lined pores. Furthermore, the reactivity and flexibility of their structures gives rise to reversible uptake of H-bonding molecules with recrystallisation, breathing  

Diffusion of adsorbate molecules throughout the pore space of microporous solids is an essential step in many applications of microporous solids and determines their utility and selectivity in applications. Whereas the thermodynamics of the adsorption determines the equilibrium situation, the kinetics of an adsorptive or catalytic process is controlled by the diffusion rates. This is exemplified in their use in shape-selective catalysis, where molecules must reach and leave active sites distributed through the crystallites and therefore products that diffuse faster will be enriched in the molecular mix leaving the solid. 

---

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

Highly colored, they have been used to dye cellulose acetate (552) and acrylic fibers (553). Cationic dyes prepared from 2-azothiazoles by simple alkylation on the ring nitrogen (552) have been used increasingly with the introduction of polyacrylonitrile fibers with basic sites that can be colored with such dyes (554). 

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

The picolyl ester has been prepared from amino acids and picolyl alcohol (DCC / CH2CI2, 20°, 16 h, 60% yield) or picolyl chloride (DMF, 90-100°, 2 h, 50% yield). It is cleaved by reduction (H2/Pd-C, aq.= FtOH, 10 h, 98% yield Na/NH3, 1.5 h, 93% yield) and by basic hydrolysis (1 NaOH, dioxane, 20°, 1 h, 93% yield). The basic site in a picolyl ester allows its ready separation by extraction into an acidic medium. ... 

The mechanism for acid-catalyzed hydrolysis of amides involves attack by water on the protonated amide. An inqjortant feature of the chemistry of amides is that the most basic site in an amide is the carbonyl oxygen. Very little of the N-protonated form is present. The major factor that contributes to the stability of the O-protonated form is the... 

When a Br nsted plot includes acids or bases with different numbers of acidic or basic sites, statistical corrections are sometimes applied in effect, the rate and equilibrium constants are corrected to a per functional group basis. If an acid has p equivalent dissociable protons and its conjugate base has q equivalent sites for proton addition, the statistically corrected forms of the Br insted relationships are... 

The initial investigation focused on the use of threitol-derived auxiliaries with various substituent groups on the dioxolane ring (Table 3.3). However, it became evident that the oxygen atoms in the substituents had a detrimental effect on selectivity. Comparison of the diastereoselectivities for the ketals 69-71, which contain Lewis basic sites in the substituents at the 1 and 2 positions, with those from simpler diol derived ketals 72-74 demonstrates the conflicting effects of numerous coordination sites. The simpler, diol-derived ketals provide superior results compared to the threitol derived ketals. The highest diastereoselectivity is observed in the case of the 1,2-diphenyl ethane-1,2-diol derived ketal 74. 

At first glance the hydrogen bond basicity (3 is controlled solely by the anions, with basicity decreasing in the order Cl > [RS03] >[BF4] > [PF ] > [N03] > [SCN] . However, while the general trend is clear, this is not the order that one would have expected, and the cations are obviously playing a role. Again, this may be a consequence of competition for the basic site (anion) between the test solute and the acidic site (cation) of the ionic liquid. It is unfortunate that no study to date has used a common anion across all possible cations. 

Interestingly, alanine and other amino acids exist primarily in a doubly charged form called a zwitterion rather than in the uncharged form. The zwit-terion form arises because amino acids have both acidic and basic sites within the same molecule and therefore undergo an internal acid-base reaction. 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

Enzymes work by bringing reactant molecules together, holding them, in the orientation necessary for reaction, and providing any necessary acidic or basic sites to catalyze specific steps. As an example, let s look at citrate synthase, an enzyme that catalyzes the aldol-like addition of acetyl CoA to oxaloacetate to give citrate. The reaction is the first step in the citric acid cycle, in which acetyl groups produced by degradation of food molecules are metabolized to yield C02 and H20. We ll look at the details of the citric acid cycle in Section 29.7. 

The mechanism of the C02 transfer reaction with acetyl CoA to give mal-onyl CoA is thought to involve C02 as the reactive species. One proposal is that loss of C02 is favored by hydrogen-bond formation between the A -carboxy-biotin carbonyl group and a nearby acidic site in the enzyme. Simultaneous deprotonation of acetyl CoA by a basic site in the enzyme gives a thioester eno-late ion that can react with C02 as it is formed (Figure 29.6). 

The straightforward construction of substituted pyrone 4 proceeded as follows (see Scheme 6c). Alkylation of the monoanion of 2,4-pentanedione (8) with methyl iodide furnishes 3-methyl-2,4-pentanedione. Conversion of this substance into the corresponding dianion with sodium amide followed by selective carboxylation of the more basic site provides intermediate 7. Pyrone 4 is obtained after cyclization with l,l -carbonyldiimidazole and methylation of the resulting enol with dimethyl sulfate. 

If p is the number of equivalent acidic protons in BH+, and q the number of equivalent basic sites in its conjugate base B, then a more precise form for the Brpnsted relation is... 

In this paper, the selective conversion of methane to C2 hydrocarbons over ternary Ca0-Mn0/Ce02 catalysts in the CO2 OCM process are presented. The synergistic effect between catalyst reducibility and distribution of basic sites are highlighted. The most promising catalyst was then tested towards its stability. 

---