Bathochromic or red shift

If the % energy level is decreased by attractive forces in polar solvents, it should be expected that the n n transition will also show a red shift in polar solvents. It does, but there is a much more important interaction that overcomes the red shift for the n - )t transition. 

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Absorption bands due to conjugated chromophores are shifted to longer wavelengths bathochromic or red shift) and intensified relative to an isolated chromophore. The shift can be explained in terms of interaction or delocalization of the 7t and % orbitals of each chromophore to produce new orbitals in which the highest ti orbital and the lowest k orbital are closer in energy. Figure 9.8 shows the conjugation of two ethylene chromophores to form 1,3-butadiene. The n—>n transition in ethylene occurs at 165 nm with an s value of 1500 whereas in 1,3-butadiene the values are 217 nm and 2100 respectively. 

Here the peak is shifted to the longer wave length (also called bathochromic or red shift)... 

The absorbance maximum of phenols is increased by the addition of base and formation of the phenolate ion, with an associated absorbance shift to longer wavelength (a bathochromic or red shift), whilst the absorbance maximum of anilines is decreased by the addition of acid, which causes protonation of the amine and loss of the lone pair overlap with the K-system, leading to an absorbance shift to shorter wavelength (a hypsochromic or blue shift). 

As one -eleetron is promoted to a more diffuse n orbital, whieh is more polarizable, the exeited Si state usually has a larger polarizability than the So ground state. Thus, the dispersion interaetion of the excited carbonyl solute with the solvent will be larger than the dispersion interaction of the less polarizable ground-state solute with the solvent. The eleetronie exeitation energy decreases, resulting in a bathochromic (or red) shift of the 71 absorption band. 

There is a general observation that many molecules that absorb radiation due to a tt rr transition exhibit a shift in the absorption maximum to a longer wavelength when the molecule is dissolved in a polar solvent compared to a nonpolar solvent. The shift to a longer wavelength is called a bathochromic or red shift. This does not mean that the solution turns red or that the absorption occurs in the red portion of the visible spectrum. 

Figure 5.30 The energy difference between the tt and ti levels is decreased in the polar solvent. The absorption wavelength therefore increases. This is a bathochromic or red shift.

The term "solvatochromism describes a significant change in position or intensity of an absorption or emission band of a compound in solution, when the polarity of the medium is changed. A bathochromic (or red) shift of the absorption band with increasing solvent polarity is known as positive solvatochromism The corresponding hypsochromic (or blue) shift is termed negative solvatochromism. 

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