N and transitions non bonding electrons lone pairs

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n * and * transitions: non-bonding electrons (lone pairs), wavelength ( max ) in the 150-250 nm region. n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Any of these require that incoming photons match in energy the gap corrresponding to a transition from ground to excited state. Energies correspond to a 1-photon of 300 nm light are
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What are the nature of these absorptions? Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W): HOMO u bonding molecular orbital LUMO g antibonding molecular orbital h 170nm photon Example for a simple enone π π n π π n π* π π n π* π* π* π* π* π* π* π* - *; max =218 =11,000 n- *; max =320 =100
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Chromophore Excitation max , nm Solvent C=C * 171 hexane C=O n→ * * 290 180 hexane hexane N=O n→ * * 275 200 ethanol ethanol C-X X=Br, I n→ * n→ * 205 255 hexane hexane
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Orbital Spin States Singlet state (S):Most molecules have ground state with all electron spin paired and most excited state also have electron spin all paired, even though they may be one electron each lying in two different orbital. Such states have zero total spin and spin multiplicities of 1, are called singlet (S) states. Total Spin Multiplicities
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Orbital Spin States For some of the excited states, there are states with a pair of electrons having their spins parallel (in two orbitals), leading to total spin of 1 and multiplicities of 3. Total Spin Multiplicities
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Orbital Spin States For triplet state: Under the influence of external field, there are three values (i.e. 3 energy states) of +1, 0, -1 times the angular momentum. Such states are called triplet states (T). According to the selection rule, S→S, T→T, are allowed transitions, but S→T, T→S, are forbidden transitions.
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Selection Rules of electronic transition Electronic transitions may be classed as intense or weak according to the magnitude of ε max that corresponds to allowed or forbidden transition as governed by the following selection rules of electronic transition: Spin selection rule : there should be no change in spin orientation or no spin inversion during these transitions. Thus, S→S, T→T, are allowed, but S→T, T→S, are forbidden. ( S=0 transition allowed )
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Terms describing UV absorptions 1. Chromophores : functional groups that give electronic transitions. 2. Auxochromes : substituents with unshared pair e's like OH, NH, SH ..., when attached to π chromophore they generally move the absorption max. to longer λ. 3. Bathochromic shift : shift to longer λ, also called red shift.
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