Reflection spectrum Solvent shift

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

A misconception that we commonly encounter is that a spectrum can be a mixture of the salt and the free base. This is an excuse that is often used by chemists to explain an inconveniently messy looking spectrum Don t be tempted by this idea - proton transfer is fast on the NMR timescale (or at least, it is when you use a polar solvent ) and because of this, if you have a sample of a compound that contains only half a mole-equivalent of an acid, you will observe chemical shifts which reflect partial protonation and not two sets of signals for protonated and free-base forms. It doesn t happen - ever ... 

These points arc illustrated by the elegant studies of Nelson and Hummel (1962) on the kinetics of denaturation of ribonuclease by urea. As is shown in Fig. 16, the difference spectrum generated with ribonuclease using 7 M urea versus aqueous reference solvent is markedly time-dependent. Difference spectra taken shortly after mixing protein and urea solvent reflect the refractive index red-shift of urea on the tyrosyl and phenylalanyl groups of the protein in its native conformation. As the urea denaturation... 

The effect of the electrolyte on the solution structure is not confined to the solvation of the ions, but may also play a role in altering the structure of the solvent itself. Hence, these two effects appear together in the NMR shift. This is well reflected by studies on the system AgN03-water-acetonitrile [Sc 66, Sc 76]. The chemical shift of water in this system yields relatively little information on its selective solvation effect, since the addition of acetonitrile causes the hydrogen-bonded structure of the water to break down this causes a shift in the NMR spectrum in the opposite direction to that due to the coordination of water to silver. More meaningful information on the composition of the solvate is provided by the chemical shift of the acetonitrile. In agreement with the results of potentiometric titrations, this shows that two acetonitrile molecules are coordinated in the first coordination sphere of the silver. 

---