Kamlet-Taft hydrogen-bond donor

Decreasing Kamlet-Taft hydrogen-bond donor (HBD) index a... 

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., Doherty, R. M., Carr, P., Abraham, M. H., Marcus, Y Taft, R. W. Linear solvation energy relationships. 46. An improved equation for correlation and prediction of octanol-water partition coefficients of organic non-electrolytes (including strong hydrogen bond donor solutes)./. Phys. Chem. 1988, 92, 5244-5255. 

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). 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

Taft RW, Kamlet MJ (1976) The solvatochromic comparison method. 2. The a-scale of solvent hydrogen-bond donor (HBD) acidities. J Am Chem Soc 98 2886-2894. 

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. 

Kamlet, M., R.M. Doherty, M.H. Abraham, Y. Marcus, and R.W. Taft. 1988. Linear Solvation Energy Relationships. 46. An Improved Equation for Correlation and Prediction of Octanol/Water Partition Coefficients of Organic Nonelectrolytes. (Including Strong Hydrogen Bond Donor Solutes). J. Phys. Chem. 92, 5244-5255. 

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). 

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 a scale proposed to measure solvent hydrogen bond acidity, i.e. the ability of a bulk solvent to act as hydrogen bond donor toward a solute, was derived from 16 diverse properties involving 13 solutes as averaged values [Taft and Kamlet, 1976 Kamlet et al., 1983]. 

Taft, R.W. and Kamlet, MJ. (1976). TTie Solvatochromic Comparison Method. 2. The a-Scale of Solvent Hydrogen-Bond Donor (HBD) Acidities. J.Am.Chem.Soc., 98, 2886-2894. 

The solvatochromic shifts obtained for the BPHT electronic absorption and fluorescence emission maxima were also applied to evaluate the contributions of the polarity/polarizability of the solvent and its hydrogen-bond donor (HBD) ability to solute-solvent interactions in the ground and first excited singlet states of BPHTs. For this purpose, a simplified form of the classical Kamlet-Abboud-Taft solvatation energy relationship was used as follows [87,88] ... 

More recent solvatochromic studies [14-20] employ specialised dyes reflecting specific microscopic molecular interactions. For example, the Abboud-Kamlet-Taft solvent parameters [21-23] a, 3 and n give information on hydrogen bond donor and acceptor properties and the polarisability of a compound, respectively. For ionic liquids with [C4mim]-cations, the following order of [3- values was estab-... 

Predictions based on Abboud-Kamlet-Taft parameters lead to better results, since the anion s basicity (0-value) and the cation s hydrogen bond donor ability (a-value) are assessed separately. For example, in the nucleophilic substitution of methyl-p-nitrobenzene sulfonate and a halide (Fig. 11) the hydrogen bond donor ability of the cation should be reduced in order to eliminate strong interactions with the halide. This was impressively demonstrated for the rates of reaction, which decreased according to [C4mpyr][BTA] > [C4dmim][BTA] > [C4mim][BTA] [149,150],... 

According to Kamlet and Taft [86] for protic solvents the charge transition energy can be correlated with the hydrogen bond donor strength. The transition energy Et(30) can be calculated from the position of the absorption maximum of Reichardt s standard dye at 25 °C and 1 bar according to... 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

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