Lewis acceptor number

Desimoni et al. initially advocated the Acceptor Number (AN) as the dominant solvent parameter" The AN describes the ease with which a solvent can act as an electron pair acceptor (Lewis acid) and... 

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

The thermodynamic tendency of a substance to act as a Lewis acid. The strength of a Lewis acid depends on the nature of the base with which the Lewis acid forms a Lewis adduct. Hence, comparative measures of Lewis acidities are given by equilibrium constants for the formation of the adducts by a common reference base. See Lewis Acid Electrophilicity Hard Acids Soft Acids Acceptor Number... 

FLUORESCENCE DONOR NUMBER LEWIS ACIDITY Acceptor Number,... 

DEGREE OF DISSOCIATION LEWIS ACIDITY ACCEPTOR NUMBER ELECTROPHILICITY HARD ACID LEWIS ACID LEWIS BASE LEWIS BASICITY SOFT ACID Lewis adduct,... 

Raman spectroscopy has up to now mainly been applied to elucidate conformational forms and associated conformational equilibria of the IL components. Yet other applications are appearing in these years. One example is the characterization of metal ions like Mn, Ni Y Cu Y and Zn + in coordinating solvent mixtures by means of titration Raman Spectroscopy [118]. Another issue is the study of solvation of probe molecules in ILs. In such a study [118], for example, acceptor numbers (AN) of ILs in diphenylcyclopro-penone (DPCP) were estimated by an empirical equation associated with a C=C / C=0 stretching mode Raman band of DPCP. According to the dependence of AN on cation and anion species, the Lewis acidity of ILs was considered to come mainly from the cation charge [119]. 

The converse of the donor number is the acceptor number, AN, of a solvent. This is a measure of its power to accept electron pairs as a Lewis acid. There are a number of ways of assessing the acceptor ability. One convenient way is to use NMR to measure the interaction between a standard reference Lewis base, (C2H5)3PO, and the solvent by looking at the shift (eqn. 3.9). 

The acceptor number (AN) [10, 11] of solvent A (Lewis acid) is obtained by measuring the 31P-NMR chemical shift (AS, ppm) of triethylphosphine oxide (Et3P=0,... 

In ion solvation, the solvent molecules approach a cation with their negative charge and approach an anion with their positive charge (Fig. 2.1). Therefore, cation solvation is closely related to the electron pair donor capacity or Lewis basicity of solvents and tends to become stronger with the increase in donor number (DN). On the other hand, the anion solvation is closely related to the electron pair acceptability or Lewis acidity of solvents and tends to become stronger with the increase in acceptor number (AN). 

Figure 2.11, on the other hand, shows the acceptor numbers of mixtures of water and aprotic solvents. Because water is protic and selectively interacts with Et3P = 0 (strong Lewis base), many of the relations curve upward. However, with HMPA, the relation curves downward, because HMPA is a strong base and easily interacts with water to weaken the interaction between water and Et3P = O. The acidity and basicity of mixed solvents are influenced not only by the acidity and basicity of the constituent solvents but also by the mutual interactions between the molecules of constituent solvents. At present, however, this cannot be treated theoretically. 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

Many physical chemists have embraced the concepts of donicity (donor numbers, DN) and acceptor numbers (AN) as developed by Gutmann and his co-workers [12], The DN is measured by the heat of reaction of the donor solvent and antimony pentachloride in a 1 1 ratio as a dilute solution in 1,2-dichloro-ethane. It is taken to be a measure of the strength of the Lewis base. The AN is measured as the relative shift of the 31P NMR peak in triethylphosphine oxide dissolved in the sample solvent. Hexane is given the value of zero on the scale, and antimony pentachloride is given the value of 100. The AN is taken to be a measure of the strength of the Lewis acid. The applications of the concepts have... 

Acceptor number (or acceptivity), AN — is an empirical quantity for characterizing the electrophilic properties (-> Lewis acid-base theory) of a solvent A that expresses the solvent ability to accepting an electron pair of a donor atom from a solute molecule. AN is defined as the limiting value of the NMR shift, S, of the 31P atom in triethylphosphine oxide, Et3P=0, at infinite dilution in the solvent, relative to n-hexane, corrected for the diamagnetic susceptibility of the solvent, and normalized ... 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

A detailed discussion and comparison of all these and further solvent softness scales can be found in references [173, 238, 239]. For other Lewis acid/base parameters of EPD and EPA solvents, derived from calorimetric measurements [e.g. Gutmann s donor and acceptor numbers), see reference [65] and Section 2.2.6. 

The data collected by Gritzner [79] and his analysis led him to the conclusion that different interactions of hard and soft cations with donors cannot be accounted for by using only one parameter. Gritzner [79] also tried other correlations. No correlation was found with the acceptor number and the other Lewis acidity parameter, x, introduced by Dimroth and Reichardt [19, 83]. Only those parameters which represent donor properties of solvents are correlated with the change of E /2 potentials for the electroreduction of cations. 

The acceptor number, AN, of a solvent is a measure of the Lewis acid acceptor power of the solvent.3 Experimental evaluation of the AN involves observing the frequency changes induced by a solvent on the 31P NMR spectrum when triethylphosphine oxide (Et3PO) is dissolved in the solvent. Elexane was used as the reference solvent and has an AN of zero, at the other extreme is trifluoroacetic acid for which AN = 105.3. 

The electrophilicity of a solvent, and thus its Lewis acid character, can be quantified in terms of the acceptor number AN. ... 

Another scale for measuring solvent acidity was formulated by Mayer et al. [43]. It is called the solvent acceptor number (AN) and is based on the relative values of the NMR chemical shifts produced by a given solvent with a strong Lewis base, triethylphosphine oxide (fig. 4.13). The data were normalized so that the acceptor number of hexane is zero and that for the 1 1 adduct with the strong Lewis acid, SbCls, 100 when dissolved in 1-2 dichloroethane. The attractive feature of this scale is that it varies over a wide range for the polar solvents con-... 

The chemical shifts in triethylphosphine oxide measured in polar solvents is used to define the acceptor number scale for solvent acidity [25]. In this case, the oxygen atom in the P=0 bond acts as an electron pair donor to the solvent as a Lewis acid. The resulting inductive effect lowers the electron density at the phosphorus atom and results in a chemical shift which depends on solvent acidity. 

Solvent-induced frequency shifts have also been studied for dimethylsulfoxide (DMSO), which is a strong Lewis base. Most solvents behave as Lewis acids and interact with DMSO via the electron density on the — S=0 group. This interaction leads to a red shift for this band which is as large as 56 cm in the presence of acetic acid [34]. The magnitude of the shift correlates well with the solvent s acceptor number giving the relationship... 

Acetone provides another interesting system for studying intermolecular interactions. The basicity of this solvent, whose DN is 17.0, is close to that of water. On the other hand, it is a very weak Lewis acid with an AN equal to 12.5. Thus, one expects the C=0 stretching frequency V3 to shift with change in the acidity of the solvent in which acetone is dissolved. A good correlation is found between the frequency of this band in 22 solvents and the solvent s acceptor number AN, with the frequency shifting in the red direction as AN increases. The equation describing the correlation is... 

AN, the acceptor number of Lewis acids, was defined as the relative P NMR shift obtained when triethylphosphine oxide (EtsPO) was dissolved in the eandidate acid. The scale was normalized by assigning an AN value of 0 to the NMR shift obtained with hexane, and 100 to that obtained from the SbCLrEtsPO interaetion in dilute 1,2-dichloroethane solution. However, the total shift of P NMR in two-eomponent systems has an appreciable contribution of van der Waals interactions that must be accounted for in correlating spectral shifts with heats of acid base interactions. Riddle and Fowkes [7] corrected the P NMR shifts for van der Waals interactions and proposed a new scale of acceptor numbers. The new AN values (AN—AN ) in ppm are converted into AN in kcal/mol units by... 

Figure 18 Uptake ( /oS) of Lewis acids by PMMA and ArMCU versus the acidic character of the solutes. The %S is the solute per repeat unit molar ratio in the case of PMMA and the solute per nitrogen atom in the case of ArMCU. The solutes were characterized by AN, the Gutmann acceptor number CCI4, 2.3 l-2,dichloroethane (DCE), 6.4 dichloromethane (DCM) (CH2CI2), 13.5 trichloromethane (TCM), (chloroform) 18.7 hexafluoroisopropanol (HFIP), 66.3 and tri-fluoroacetic acid (TFAA), 111.

Gutimnn s Acceptor Number (AN). Mayer, Gutmann, and Gerger (112) have used infinite dilution P-nmr shifts of triethylphosphine oxide (52) as the basis for what they describe as Acceptor Number (AN), a quantitative empirical parameter for the electrophilic properties of solvents (the conversion factor is —5"" = AN/2.349). For protic solvents AN is intended to serve as a measure of HBD acidity for nonprotic solvents it is seemingly intended as a measure of Lewis-type acidity. Compared with a values which range from 33.5 to 41.3 for the aliphatic alcohols, AN values of representative non-HBD solvents are THF, 8.0 ethyl acetate, 10.8 DMSO, 19.3. Thus, the latter solvents are... 

Acidic/Basic Lewis acidity/basicity determines the solvent s abihty to demate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this acid/base pre ierty was proposed by Gutman (DN and AN -donor and acceptor number, reflectively) based on caleuimetric determination. The complete proton transfer reaction with femnatiem of protonated ions is determined by proton affinity, gas phase acidity, acid or base dissociation constants. Both concepts differ in terms of net chemicsd reactiem. 

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. 

Much attention has been given to correlating A// against properties of the interacting species. Two of the most common are the Gutmann [96] donor number, DN, or acceptor number, AN, scales and the Drago [97] E C formulation. The donor number, DN, quantifies the Lewis basicity of a variety of solvents and was defined for a given base B as the exothermic heat of its reaction with the reference acid, antimony pentachloride, in a 10 - M solution in a neutral solvent (1,2-dichloroethane). The units are (kJ/mol). Specifically ... 

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