Question

define and provide expamples for each of the following

1) qualitative photochemistry

2) quantitative photochemistry

Answer 1

1.Qualitative photochemistry

Qualitative photochemistry based on molecular orbital theory and when EMR of ultra or visible light interact with the sample electronic transition always takes place from HOMO to LUMO.

The type of absorption of Frquency dependon separation gapbetween HOMO to LUMO.

IF molecule is more conjugated difference will be lower and absorption band shifted to longer wavelength(Bathochrmic shift).

If auxo chrome + Chromophore both present than batho + hyperchromic shift also involved andcompound will be deepen in colour. Picric acid (yellow colour)

Types of excitation based on molecular structure

Sigma to sigma star (Alkanes)

n-sigma star (alcohol)

pi-pi star (alkene)

n-pi star(carbonyl)

It is based on Born-oppenheimer approximationand when EMR intearct with molecule all electrons are paired it is called ground singlet state(Jablonski diagram).

After abs. of light excitation without spin inversion takes place is called excited singlet S1

And process is called internal conversion no change in spin multiplicity.

Then some loss of energy (5 K cal/mol) it will in T1 excited triplet state both spin of e will be parallel, multi.=3and process is called inter system crossing.

During abs. of light molecule may undergo reactions with other molecule at any state and may show phosphorescence and fluorescence and other property like photo reduction, photosenstization, (benzophenone can transfer its triplet energy to butadiene and Butadiene can undergo triplet and give the reaction of its state).

Barton reaction also occur in the presence of light.

Quantitative photochemistry is based on how much amount of light is absorbed and which type of frequency is interact to molecule and responsible for excitation.

Firstly Grothus Draper states light absorbs give chemical change in the molecule.

Second Law, Einstein photo equivalence law states that one quantum absorption of light excited one molecule.

E= nhC/Lembda

n= number of quanta

h = planck constant

lambda = wavelength of light

therefore we can calculate energy of absorbed light also.

Quantum yield: With help of quantum yield we can determine the reaction mechanism.

Φ = Number of molecule undergoing the reaction / number of photons absorbed by the reactant

  

If Q. Y. = 1, it means by absoption of 1 quantum light one reactant molecule will be converted into one product, there is no involvement of secondary process.

If Q.Y. > or< 1 ,it means secondary process can be involved.

Ex: Formation of HCl

H₂ + Cl₂    2HCl(in the presence of light)

Q. Y. = 10⁵ , very high due to formation of free radical Cl, which reacts further with hydrogen gas in sec. process and chain reaction will be taken.

It can be determine how manynumber of photons absorbed by Actinometer.

Beer-Lambert law:

With help of Beer-Lambert law we can determine conc. of the sample and aborbance and absorption of light wavelength.

lnI_o/I_t = A =εcl    A= absorbance ; ε= molar absorbtivity

c= concentration and l= path length of cuvatte

1.find lambda max through spectrophotometer

2. calibration chart preparation

3. find absobance of unknown sample, we can calc.conc.