Donor number definition

Some liquid covalent halides can act as nonaqueous solvents " based on Lewis acid-base behavior, according to the donor-acceptor definition. The self-dissociated ions consist of a cation formed by subtraction of a hahde ion from the neutral compound, while the anion is formed by its addition (equation 24). Salts derived from such covalent halides can be considered as titration products of the parent acidic and basic compounds (equations 25 and 26). In such cases, both the cation and the anion usually possess a stable coordination number with a high geometrical symmetry. 

Schreiber et al. [57] proposed also another definition of acceptor and donor numbers (assigned here as ANsch and DNsch, respectively) not related to Gutmann s scale and defined as follows ... 

It is clear that the frequency of the metal-solvent band decreases with increase in the size of the alkali metal, that is, with decrease in the field due to the charge on the ion. When ion pairing is absent, the value of the frequency is independent of the nature of the anion. This is definitely the case in DMSO, which strongly solvates cations because of its high donor number. In addition, the relatively high permittivity of this solvent reduces the influence of long-range ion-ion interactions. 

Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous solutions are subsumed by the Lewis definitions. In addition to the previous discussion of the dual polarity character of Lewis acids and bases, it should be noted that many of them are amphoteric, by definition. Donor number, DN, was developed in order to correlate the behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative measurement of the basicity of a solvent D is given by the enthalpy of its reaction with an arbitrarily chosen reference acid (SbCls in the Gutmann s scale). Latter Mayer introduced an acceptor number, AN, as the relative P NMR shift induced by triethylphosphine, and relative to acidic strength (AN=0 for hexane and 100 for SbCls). In 1989, Riddle and Fowkes modify these AN numbers, to express them, AN ", in correct enthalpic unit (kcaLmol). Table 10.2.3 gathers electron acceptor number AN and AN " and electron donor number DN for amphoteric solvents. 

These findings lead to the definition of the donor-number" for each of the solvents. The donor number is defined as the numerical quantity of the —/IHd sbcu value26. 

The Gutmann approach was to allocate a donor number (DN) for bases, and an acceptor number (AN) for acids, corresponding to the definitions first proposed by Lewis. The DN was determined by measuring in a calorimeter the heat of reaction between the base under investigation and antimony pentachloride in a dilute 1,2 dichloroethane solution. The AN was determined from the P NMR shift of triethylphosphine oxide (TEPO) when dissolved in the acid under investigation. The number allocated was determined from a linear scale in which -hexane was designated as zero and antimony pentachloride was designated as 100. 

Partitioning or cell-based methods provide an absolute measure of the chemical space covered by a collection of compounds. They are based on the definition of a low-dimensional chemistry space, for example, one based on a small number of physicochemical properties such as molecular weight, calculated logP, and number of hydrogen bond donors [45]. Each property defines an axis of the chemistry-space. The range of values for each property is divided into a set of bins, and the combinatorial product of all bins then defines the set of cells or partitions that make up the space. 

PSA. Hence, we only mention here that the definition of PSA is similar to that of Na and N. Thus, PSA possesses the same disadvantages when compared to thermodynamic H-bond descriptors. In fact, it has been estimated that there is a strong linear relationship between the calculated static PSA and the calculated dynamic polar surface [32, 33]. A further possible simplification using only the number of H-bond forming atoms and PLS statistics was proposed [35]. Excellent correlahons of PSA with the number of H-bond donors and acceptors were published [35, 36]. 

This electrophile/nucleophile dichotomy can be looked upon as a special case of the acid/base idea. The classical definition of acids and bases is that the former are proton donors, and the latter proton acceptors. This was made more general by Lewis, who defined acids as compounds prepared to accept electron pairs, and bases as substances that could provide such pairs. This would include a number of compounds not previously thought of as acids and bases, e.g. boron trifluoride (39),... 

Perhaps the greatest area in which the Lewis acid-base approach is most useful is that of coordination chemistry. In the formation of coordination compounds, Lewis acids such as Cr3+, Co3+, Pt2+, or Ag+ bind to a certain number (usually 2, 4, or 6) of groups as a result of electron pair donation and acceptance. Typical electron pair donors include H20, NH3, F , CN , and many other molecules and ions. The products, known as coordination compounds or coordination complexes, have definite structures that are predictable in terms of principles of bonding. Because of the importance of this area of inorganic chemistry, Chapters 16 through 22 in this book are devoted to coordination chemistry. 

It has been shown in this chapter that the probability of electron tunneling from a donor to an acceptor in condensed media depends on a large number of factors. At present the theory gives quite definite ideas about the scale of influence of the various factors on the probability of tunneling. 

Complexes that show a pH dependent relaxivity around physiological pH values are of particular interest since they may afford a way to distinguish tumor tissue (pH = 6.9) from healthy tissue (pH = 7.4). Protonation of a donor atom (0 or N) can lead to competitive binding of a water molecule and thus affect the relaxivity. The development of a ligand exhibiting a pH dependent coordination number has not been successful so far [174], but is definitely a promising approach. 

Given a NO/3NO- couple potential of 0.39 V, the effecter molecule of NOS would thus be expected to be ONOO, not NO. However, the direct detection of NO in living systems by a number of techniques (17,18,26) as well as the observation that activation of NOS results in antioxidant chemistry [briefly reviewed in (160-162)] suggest otherwise. The definitive mediation of vascular tone by NO (18, 31) and the discrete effects of HNO and NO donors observed in vitro, in vivo, and ex vivo corroborate the implications that NO cannot be readily reduced to NO- under physiological conditions (147). Additionally, the oxidation of methyl viologen by O2 was determined to be two orders of magnitude... 

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