What are Chromophore?
Although the absorption of ultraviolet radiation results from the excitation of electrons from the ground to excited states, the nuclei that the electrons hold together in bonds play an important role in determining which wavelengths of radiation are absorbed.
The nuclei determine the strength with which the electrons are bound and thus influence the energy spacing between ground and excited states.
As a result, the characteristic energy of a transition and the wavelength of absorbed radiation are properties of a group of atoms rather than of electrons themselves.
The group of atoms producing such absorption is called a chromophore.
As structural changes occur in a chromophore, the exact energy and intensity of the absorption are expected to change accordingly.
Very often, it is extremely difficult to predict from theory how the absorption will change as the structure of the chromophore is modified, and it is necessary to apply empirical working guides to predict such relationships.
What are Auxochrome?
The attachment of substituent groups in place of hydrogen on a basic chromophore structure changes the position and intensity of an absorption band of the chromophore.
The substituent groups may not give rise to the absorption of the ultraviolet radiation themselves, but their presence modifies the absorption of the principal chromophore.
Substituents that increase the intensity of the absorption, and possibly the wavelength, are called auxochromes.
Typical auxochromes include- methyl, hydroxyl, alkoxy, halogen, and amino groups.
Electronic Transitions in UV Spectroscopy
According to molecular orbital theory, the excitation of a molecule by UV-visible radiation absorption involves the promotion of its electrons from a bonding (n) orbital to an antibonding orbital.
σ and π Bonding orbitals are associated with σ* and π* antibonding orbitals, respectively.
Non-bonding (n or p) orbitals are not associated with antibonding orbitals because the electrons in them do not form bonds.
For molecules, such as alkanes, that contain nothing but single bonds and lack atoms with unshared electron pairs, the only electronic transitions possible are of the σ -> σ* type.
These transitions have such high energy that they absorb ultraviolet energy at extremely short wavelengths—far shorter than the wavelengths that can be measured experimentally with standard spectrophotometers.
The excitation of the σ-bonding electron to the σ*-antibonding orbital is depicted at the right.
Alcohols, Ethers, Amines, and Sulfur Compounds
In saturated molecules that contain atoms bearing nonbonding pairs of electrons, transitions of the n->σ* type become important.
They are also rather high-energy transitions, but they do absorb radiation that lies within an experimentally accessible range.
Alcohols and amines absorb in the range from 175 to 200 nm, while organic thiols and sulfides absorb between 200 and 220 nm.
Most of the absorptions are below the cutoff points for the common solvents, so they are not observed in solution spectra. Figure 10.7 illustrates an n->σ* transition for an amine.
The excitation of the nonbonding electron to the antibonding orbital is shown at the right.
Alkenes and Alkynes
With unsaturated molecules, π -> π* transitions become possible.
These transitions are of rather high energy as well, but their positions are sensitive to the presence of substitution.
Alkenes absorb around 175 nm, and alkynes absorb around 170 nm.
The below figure shows this type of transition.
Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n->π* transitions.
These are perhaps the most interesting and most studied transitions, particularly among carbonyl compounds.
These transitions are also rather sensitive to substitution on the chromophore.
The typical carbonyl compound undergoes an n->π* transition around 280 to 290 nm.
Most n->π* transitions are forbidden and hence are of low intensity.
Carbonyl compounds also have a π -> π* transition at about 188 nm.
Below figure shows the n->π* and π -> π* transitions of the carbonyl group.
Absorption and Intensity Shifts in UV Spectroscopy
Other substituents may have any of four kinds of effects on the absorption:
Bathochromic shift (redshift)
The shift of an absorption maximum to a Longer wavelength due to the presence of an auxochrome, or solvent effect is called a bathochromic shift or redshift.
Hypsochromic shift (blue shift)
The shift of an absorption maximum to a shorter wavelength is called hypsochromic or blue shift. This is caused by the removal of conjugation or change in the solvent polarity.
An effect that leads to an increase in absorption intensity εmax is called the hyperchromic effect. The introduction of an auxochrome usually causes a hyperchromic shift
An effect that leads to a decrease in absorption intensity εmax is called the hypochromic effect. This is caused by the introduction of a group that distorts the chromophore.