Solvents hydrogen-bond measurements

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 this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

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). 

Neumark and co-workers [56] pointed out the similarity of the cluster results to the transient behavior in aqueous 1 solution, which has been studied via ultrafast pump-probe measurements [50]. Bradforth and co-workers [50] observed IR (800 nm) transient absorption after UV (255 nm) excitation with 50 fs time resolution. In 1 solution, a promptly arising transient disappears within 50 fs, and absorption due to solvated electron rises with a 200 fs time constant. For longer time-scales, the trapped electron shows a biexponential decay with time constants of 8 and 60 ps, which is due to recombination with the nearby iodine atom. The close resemblance of time-scales for the rise of the solvated electron and isomerization in I (water) ( = 5 and 6) implies that the electron trapping pathway in solution can be modeled as a rearrangement of the solvent hydrogen-bond network in gas-phase clusters. 

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]. 

The Koppel-Paju B scale was proposed to measure solvent hydrogen bond basicity, based on solvent shifts of the IR stretching frequencies of the free and hydrogen bonded OH group of phenol in CCI4 [Koppel and Paju, 1974]. 

Electron Transfer. Neta and coworkers have worked extensively with halogen-substituted methyl peroxyl radicals (X H COO , where X = Cl, Br or F) in aqueous and non-aqueous media, using combinations of solvents in different ratios to change the polarity of the mixture. They describe the mechanism for the reaction of the water-soluble antioxidant Trolox with their peroxyl radicals as H-mediated electron transfer , having determined that the rate of the reaction increases with an increase in solvent polarity. They examined solvent polarity in terms of the dielectric constant of the solvent, e, and solvent basicity, reported as either the coordinate covalency parameter, f, which is a measure of solvent proton-transfer basicity, or the value, which is a measure of solvent hydrogen bond basicity . 

Tbe latter has been interpreted as a measure of a solvent s ability to interact with a proton donor in a solute-to-solvent hydrogen bond. ... 

For the analysis of SN1 solvolyses, Abraham et al. (9) have proposed an equation (equation 3) based on sensitivities toward solvatochromatic properties. In equation 3, tr is a measure of solvent dipolarity-polarization, a is a measure of solvent hydrogen bond donor acidity, and P is a measure of solvent hydrogen bond acceptor basicity. More recently, a term governing cavity effects has been added, and this term is considered to represent an important contribution (10, 11). The cavity term can be directly related to the square of the Hildebrand solubility parameter (10-12). A similar analysis by Koppel and Palm (13, 14) involves terms governed by solvent polarity, solvent polarizability, electrophilic solvation ability, and nucleophilic solvation ability. Recently, a cavity term has also been added to this analysis (12). 

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