Chromophore, Auxochrome and different type of shift in UV Spectroscopy: Free Notes & MCQ

Chromophore

It is a covalently unsaturated group which is responsible for absorption of UV or visible radiation and may or may not impact in colour to the compound.

A compound which contains chromophore it is called chromogen.

In unsaturated linkage such as -c=c- ,

-N=N-, the π electron are loosely bound. These loosely bound electron required less energy for electronic transition and the absorption band occur in near UV region.

Example Acetylene possessed -C=C-  in structure it λmax is 175-180 nm.

Auxo chrome 

It is saturated and unsaturated group which consists of one or more pair of non-bonded electron.

This group is attached to Chromophore help in altering the wavelength by increasing the intensity of absorption and increase λmax.

Example of Auxochrome is -OH, -NH2,

-OR etc.

Type of Shift

1. Bathochromic shift ( Red shift ) :-

In this Types the absorption maximum shift towards longer wavelength.

When two or more chromophore present in a molecule is responsible for bathochromic shift.

Ex. Ethene which contain unconjugated double bond show λmax at 170 nm.

2. Hypsochromic shift ( Blue shift) :-

In this Type the absorption maximum shift towards shorter wavelength. This shift is occur due to removal of conjugation or due to change  in the polarity of the solvent.

Ex. Aniline show λmax at 280 nm because of on conjugation between lone pair of electron between nitrogen and benzene.

3. Hyperchromic shift :-

In this Type of shift increase intensity with increase of absorption of radiation.

This is due to the introduce Auxochrome.

Ex. Increase intensity of the absorption of DNA due to denaturation at the λmax 260 nm.

4. Hypochromic shift :

In this Type of shift decrease intensity of radiation because of decreased absorption.

It is due to presence of certain group which may cause deformation of structure of compound.

MCQ

1. Which statement is false about Chromophore ?

A. It is co-valent unsaturated compound.

B. It is contain π electron.

C. They required high energy for electron transition.

D. They required low energy for electron transition.

2. Which statement is true about Auxochrome ?

A. It is only saturated compound.

B. It is only unsaturated compound.

C. It is Decreased intensity of radiation.

D. They are saturated and unsaturated compound.

3. Which group act as a Chromophore?

A. -N=N-

B. -OH

C. -OR

D.- NH2

4.  DNA denaturation is example of which type of shift ?

A. Bathochromic

B. Blue shift

C. Hyperchromic shift

D. Red shift

5.  λmax increase with absorption of radiation in which shift ?

A. Red shift

B. Blue shift

C. A and B

D. None of this

6. Due to deformation of compound due to attached some other functional group it is example of

A. Red shift

B. Blue shift

C. Hyperchromic shift

D. Hypochromic shift

7. Auxochrome have which type of electron ?

A. Non bonded

B. π electron

C.σ electron

D. None of this

Answer key

1. C

2. D

3. A

4. C

5. A

6. C

7. A

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