Kamlet-Taft hydrogen-bond acceptor

P Kamlet-Taft hydrogen bond acceptor parameter... 

Recently, Kamlet and Taft introduced new elaborate parameters in order to explain the linear energy relationship for the formation of the hydrogen bond between HBA (hydrogen bond acceptor) and HBD (hydrogen bond donor). They treated several sulphoxides as HBA. The detailed presentations and actual treatments of these parameters have been described in their recent review article72. 

Kamlet, M. J., A. Solomonovici, and R. W. Taft. 1979. Linear Solvation Energy Relationships. 5. Correlations between Infrared Ar Values and the (3 Scale of Hydrogen Bond Acceptor Basicities. J. Am. Chem. Soc. 101, 3734. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Kamlet MJ, Taft RW (1976) The solvatochromic comparison method. 1. The /6-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc 98 377-383. 

Investigation of stable simple enols has recently been undertaken with a series of p,P-dimesityl-a-ethenols [29] and [30] in which Mes = 2,4,6-tri-methylphenyl. In this work, 5(OH) and /hcoh were measured for a series of compounds with various a-substituents and in a range of solvents (Biali and Rappoport, 1984 Rappoport et al., 1988). The nmr parameters were shown to be linearly related to the Kamlet and Taft P-variable, a solvent property that is related to its ability to act as a hydrogen-bond acceptor (Kamlet and Taft, 1976 Taft et al., 1985). 

Parameters of the Kamlet-Taft solvatochromic relationship. These parameters measure the contributions to overall solvent polarity of the hydrogen bond donor, the hydrogen bond acceptor, and the dipolarity/polarizability properties of solvents. 

This interpretation is also supported by the results of a study correlating the observed diastereomeric excess with the Kamlet-Taft (3 parameter [29], a measure of the hydrogen-bond acceptor strength. A linear free energy relationship was found to exist between P and the diastereomeric excess for those solvents having a (hydrogen bond donor strength) = 0 [28],... 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

P empirical parameter of solvent hydrogen-bond acceptor basicity (Taft and Kamlet) ... 

Kamlet-Taft parameters have also been obtained for some perfluorinated solvents.They are not hydrogen bond donors (a is typically 0.0) and are typically extremely poor hydrogen bond acceptors ( 3 is small and negative). They are extremely difficult to polarize, and this leads to a strongly negative tt parameter. This last property explains why they typically form biphasic systems with organic solvents. 

The activity coefficient at infinite dilution of a solute y°° is related to the relative strength of an intermolecular interaction with the ionic liquid. The choice of the model solutes was based on their Abbout-Kamlet-Taft parameters [4, 75, 76], to exhibit prevailing forces with regard to the hydrogen bond acceptor, donor and polarisability properties. Similar probe molecules have been used in computational studies [62, 63],... 

One such methodology is the Kamlet-Taft Solvatochromic parameter approach. In this methodology, a solvent can be characterized by three parameters, tt, a measure of the polarity and polarizability of the fluid, a, the acidity or hydrogen bond donor capability and P, the hydrogen bond acceptor capability or basicity. Each of these parameters is determined from the shift in UV-visible absorbance of a series of select indicator species dissolved in the solvent. Rather than depending on the bulk properties of the fluid, as is the case with the cohesive energy approaches, the solvatochromic parameters are derived from the interactions between the indicator solute and the immediate solvent shell, in effect they are a measure of how a solute sees the solvent. In each case, the scale of values has been normalized to between 0.0 for cyclohexane... 

From the values of the Kamlet-Taft solvatochromic parameters (Table IV), R-134a is seen to behave as a moderately polar, weakly polarizable fluid with little or no basicity and weak acidity (of the order of dichloromethane). The negative value for P has been commented on previously (J) and may be an artifact of the original scale definition, the regression for which did not include heavily fluorinated species. In any case, the value suggests that R-134a is a poorer hydrogen bond acceptor than, for example, hydrocarbons. 

Kamlet-Taft polarity/polarizability, hydrogen-bond donor (HBD), and hydrogen-bond acceptor (HBA) solvatochromic parameters taken from [138,148] 57 and p for TMP were obtained respectively from the correlations AN = 1.04 -I- 15.4(tr - 0.088) -I- 32.6a [149] and DN (kj/ mol) = — 3.8 -I- 163.9)3 [151], where 5 in this case only is a correction factor (not the Hildebrand solubility parameter) equal to zero forTMP. 

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