Acceptor-proton-donor system

Note fhat in relation to the simple model of a hydrogen bond as an electrostatic dipole-dipole interaction, which is embodied in most modeling software, fhe D-H group does not lie along fhe dipole but points to the lone pairs. Note also fhat there is no absolute requirement for linearity of the acceptor-proton-donor system. 

In the Bronsted-Lowry theory all substances are considered, to some extent, ainphoteric. Theoretically, the strength of individual acids can be expressed through the equilibrium constant of process (I), K, called the "acidity constant" its reciprocal is termed the "basicity constant," /Tt,. Since neither acidity nor basicity of a system (system 1) manifest themselves without a proton acceptor or a proton donor (system 2), the individual constants and cannot be measured directly. One can, however, measure the equilibrium constant of the protolysis reaction (II) ... 

On the Brpnsted theory (p. 51), solutions with concentrations of H3O+ greater than that in pure water are acids (proton donors), and solutions rich in OH are bases (proton acceptors). The same classifications follow from the solvent-system theory of acids and bases... 

Table 1 The possible combinations of neutral and ionic proton donor/proton acceptor systems and the relationship between internal and external charge assistance... 

Amino groups may act not only as proton acceptor, but also as proton donor. Acidic N—H protons interact with basic solvents. In these cases an ortho-nitro group in an aniline system competes with the solvent by an internal hydrogen bond66, as depicted in 12. The stretching frequencies (by IR spectra in carbon tetrachloride) of vnh of complexes between A-methylaniline or diphenylamine (and some nitro-anilines66) and solvents depend on the proton accepting ability of the solvent (which is a moderate base)67. The frequency shifts are linearly related to the solvent s donor number (DN)3. 

In addition to monomeric linear dihydrogen-bonded complexes I shown in Figure 6.4, Alkorta and co-workers [9] have investigated associated systems II and III, where HX and HY are proton acceptors (LiH and NaH) and proton donors (see Table 6.2), respectively. According to the MP2/6-311-H-G(2d,2p)... 

A question arises, whether the various energy quantities derived by the SMO method could yield useful information on the monomer s role (proton donor or acceptor) in the weakly interacting water dimer system. 

In this work the separated representation was used to determine the energetic quantities for each contributing monomer in some water hexamer systems, too. On one hand, the results obtained for the monomers in these structures did not suggest to identify the same proton donor or acceptor nature as it was found for the linear water dimer. On the other hand, in three of the water hexamer structures that were investigated, some monomers showed a specific nature. These results confirm previous results [18] that there are extra interaction abilities in these hexamer structures (prism, w6t and w6q). 

To summarize, the four selected solvents represent different selectivity aspects DEA is a strong base, CHCI3 is a proton donor, EtAc is a dipole interactor and MeOH has both proton donor and proton acceptor properties. This means that the solvent systems which can be obtained by mixing these solvents can provide for a large variation in selectivity. 

To fully characterize and categorize the solute selectivities of GC stationary phases, Rohrschneider and McRe5molds pioneered one of the earliest characterization methods [5,6]. The Rohrschneider-McReynolds system is the oldest and widely accepted stationary phase classification systems that is based on the retention of five probe molecules namely, benzene, bufanol, 2-penfanone, nifropropane, and pyridine. Each probe molecule is used to represenf a disfincf or a combination of interactions with the stationary phase. Benzene measures dispersive interactions with weak proton acceptor properties butanol measures dipolar interactions with both proton donor and proton acceptor capabilities 2-pentanone measures dipolar interactions with proton acceptor but not proton donor capabilities nitropropane measures weak dipolar interactions and pyridine measures weak dipolar interactions with strong proton acceptor but not proton donor capabilities. 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

A Bronstcd-I.owry aeid is defined as a proton donor, a Bronsted-Loory base as a proton acceptor. The definitions apply generally toprotic systems those in which proton transfers can occur. A general equation expressing proton transfer in aqueous solution is ... 

Hydrogen bonding can occur in any system containing a proton donor group (X—H) and a proton acceptor... 

In a very thoughtful investigation of solvent systems to model membrane characteristics, Leahy et al. (1989, 1992) have argued that two receptors sited in different tissues (or membranes) could exist in environments that are very different in hydrogen bonding character one may be surrounded by amphiprotic groups (as in a protein) or by proton donors the other may be surrounded by proton acceptors (as in a phospholipid membrane). 

Buffering results from two reversible reaction equilibria occurring in a solution of nearly equal concentrations of a proton donor and its conjugate proton acceptor. Figure 2-19 explains how a buffer system works. Whenever H+ or OH- is added to a buffer, the result is a small change in the ratio of the relative concentrations of the weak acid and its anion and thus a small change in pH. The decrease in concentration of one component of the system is balanced exactly by an increase in the other. The sum of the buffer components does not change, only their ratio. 

Two especially important biological buffers are the phosphate and bicarbonate systems. The phosphate buffer system, which acts in the cytoplasm of all cells, consists of H2POT as proton donor and HPOf as proton acceptor ... 

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