UV Spectroscopy Dr Harisadhan Ghosh Department of Chemistry, Surendranath College University Syllabus Introduction; types of electronic transitions, end absorption; transition dipole moment and allowed/forbidden transitions; chromophores and auxochromes; Bathochromic and Hypsochromic shifts; intensity of absorptions (Hyper- /Hypochromic effects); application of Woodward’s Rules for calculation of

λmax for the following systems: conjugated diene, α,β-unsaturated aldehydes and ketones(alicyclic, homoannular and heteroannular); extended conjugated systems (dienes, aldehydes and ketones); relative positions of λmax considering conjugative effect, steric effect, solvent effect, effect of pH; effective chromophore concentration: keto-enol systems; benzenoid transitions. Prepared by H.G UV Spectroscopy

UV Spectrophometer UV Spectrum @H.G Fundamentals of UV absorption UV spectroscopy involves absorption spectroscopy where molecules interact with UV radiation and produce absorption spectra in the range of 200nm to 400nm. UV spectroscopy is a molecular spectroscopic method arising due to transition of valence electrons in a molecule from the ground state energy (Ee) to the higher excited state (Ee).

Then the difference in change of energy is ΔE = Ee- Eg = hν

Prepared by H.G Types of Electronic Transition Energy absorbed in the UV region by valence electrons causes transition from ground state to excited state.

The valence electrons are exited The energy required for various from bonding to an antibonding transitions are in the following order. orbitals (or HOMO to LUMO) σ → σ* > n → σ* c→ π* > n→ π* Prepared by H.G ABSORPTION CURVE

UV Spectra

Prepared by H.G 30 Selection Rules for Electronic Transition

Spin Selection Rule- Change of Spin Quantum Number is FORBIDDEN

Transition between same spin states allowed: singlet -> singlet, triplet -> triplet, others are forbidden: singlet -> triplet, doublet -> singlet, etc.

Prepared by H.G Selection rules*

spin multiplicity MS = 2S+1 S = Ss = n/2 (total spin quantum 1.Selection Spin selection rules* rule S = 0 or MS = 0 number)

(Transition between same spin states S Selection rules* spin multiplicity M = 2S+1 allowed: singlet -> singlet, triplet -> triplet, othersS = Ss are= n /2 (total spin quantum 1.forbidden: Spin selection singlet rule-> triplet, S = 0doublet or M -S> = singlet 0 number), spinetc.) multiplicity MS = 2S+1 S = Ss = n/2 (total spin quantum S 1. Spin2+ selection rule S = 0 or M = 0 number) [(TransitionMn(H2O)6] between same spin states allowed: singlet -> singlet, tripletPaul i-P>ri ntripletciple , others are (Transition between same spinno tstates obeye d forbidden: allowed:singlet singlet-> triplet, -> singlet, doublet triplet -> -> singlet triplet, ,others etc.) are forbidden: singlet -> triplet, doublet -> singlet, etc.) 2+ [Mn(H2O)6] h 2+ [Mn(H2O)6] Pauli-Principle noPta oubli-ePyriendciple not obeyed

S = 5/2 h h S = 5/2 S = 3/2

S = 1, forbidden

S = 5/2 S = 5/2 S = 3/2 1 1 S = 5/2 emax < 1 M cm .. only one electron is involvedPrepared in anyby H.G Stransition = 5/2 S = 3/2 S = 1, forbidden * were developed for metal atoms and ions (where they are rigorously obeyed), not complexes S = 1, forbidden e < 1 M1cm1 .. only one electron is involved in any transition max e < 1 M1cm1 .. only* were one developed electron for metal is atoms involved and ions in (where any they transition are rigorously obeyed), not complexesmax * were developed for metal atoms and ions (where they are rigorously obeyed), not complexes Symmetry Selection Rule

Whenever electrons change molecular orbitals due to excitation, e.g. in UV spectroscopy, the symmetry of the two involved orbitals is the criterion for a transition. We speak of symmetry-allowed and symmetry-forbidden transitions.

If the molecule has a centre of symmetry, transitions within a given set of p or d orbitals (i.e. those which only involve a redistribution of electrons within a given subshell) are forbidden.

Prepared by H.G Take the molecular orbitals of trans-butadiene as a first example. Figure 1 is a schematic representation of the energies of these orbitals.

There is Laporte's rule which states that, for any molecule possessing one centre of inversion, only transitions between g and u or vice versa are allowed. For Details see http://www.pci.tu-bs.de/aggericke/PC4e/Kap_V/Lage_Dipolmoment.htm Transition Dipolemoment https://en.wikipedia.org/wiki/Transition_dipole_moment

Prepared by H.G Absorption phenomenon for a,b-unsaturated carbonyl system

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