Solvatochromic shifts hydrogen-bonding solvents

Laurence et al. [23, 24, 135] measured two solvatochromic scales for about 200 bases, each based on one reference hydrogen-bond donor, 4-nitrophenol or 4-nitroaniline, one standard state, the pure base, and one physical property, the enhanced bathochromic shift of the longest wavelength n n transition (with intramolecular charge-transfer character) upon hydrogen bonding. The solvatochromic comparison method was improved and inert solvents were carefully chosen [135] to fix the comparison lines of Equations 4.37 and 4.38 ... 

The colour of indigo depends dramatically upon its physical state and environment for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity... 

X0 is the value of the property in the gas phase. (In practice, X and X0 are often the logarithm of the property in question.) The parameters a and p are measures of a solvent s ability to donate and accept hydrogen bonds, respectively, and tt is an index of its polarity/polarizability. They were initially assigned on the basis of ultraviolet spectral shifts of certain dyes in a variety of solvents, and hence were labeled solvatochromic parameters.186"188... 

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. 

Effects of Hydrogen Bonding. In a solvatochromic plot of transition energy hv versus solvent polarity [e.g. the f D) function], the effect of hydrogen bonding between the solute and the solvent is an anomalous blue shift or red shift, of which an example is shown in Figure 3.53. This is the... 

The situation is quite different for n- r transitions. The lone electron pair is particularly well stabilized by polar and particularly by protic solvents so it becomes energetically more difficult to excite. Figure 2.45 shows the spectrum of N-nitrosodimethylamine in different solvents. Results of calculations indicate that the negative solvatochromism of carbonyl compounds can be explained on the basis of the structural changes due to the formation of hydrogen bonds (Taylor, 1982). Molecular dynamics simulations, however, indicate that the net blue shift is primarily due to electrostatic interactions (Blair et al., 1989). A large number of water molecules around the entire formaldehyde are responsible for the total blue shift the first solvation shell only accounts for one-third of the full shift. 

Af-Butyl-4-nitroaniline (30) was also shown42 to be able to donate a hydrogen bond to HBA solvents, hence could be used as a solvatochromic probe, but has not been explored further in this respect. Drago29 reported the wavenumbers of the lowest energy absorption bands of three /V-alkyl-substituted nitroanilines 27, /V-ethyl-4-nitroanilinc (31) and N-ethyl-3-nitroaniline (32). Their spectral shifts relative to that in dioxane are shown in Figure 2 as a function of / Kt and a fair correlation is seen. 

Dey and Dogra96 studied the solvatochromic properties of 2-(aminophenyl)benzthiazole (79), with the amine group in positions 2, 3 or 4, in 12 solvents ranging from cyclohexane to water in polarity. The largest shift was found for the meta isomer from 26320 cnr1 for cyclohexane to 19880 cm-1 for water. The 2- and 4-isomers have a quinoid resonance form and smaller spectral shifts, the least being for the 2-isomer that has an internal hydrogen bond. 

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