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Reactive Intermediates

Reactive Intermediates. Want to see time development of excited states and free radicals Excited states and free radicals act as individual chemical species during their existence. They are species of particular interest because of their high energy content.

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Reactive Intermediates

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  1. Reactive Intermediates • Want to see time development of excited states and free radicals • Excited states and free radicals act as individual chemical species during their existence. • They are species of particular interest because of their high energy content. • If you can capture their energy content, you can do chemistry that you cannot do in ground states.

  2. How to Utilize the Energy Content? • If excited states channel their energy into specific bonds, then photochemistry can occur. • If scavengers or quenchers can find the excited state or free radical in time, then the electronic or chemical energy can be captured by the, ordinarily, stable scavenger or quencher.

  3. Different Actions of Scavengers • Direct capture of free radicals. • Repair of damage caused by radicals. • This second mechanism is important for the repair of damage by free radicals in biological systems.

  4. Motivations • Oxidative stress • Alzheimer’s disease • Biological aging • Basic issues • Neighboring-group effects • Details of oxidative scheme

  5. Radical RepairandAntioxidants R-S-H 1 H

  6. N-Acetyl-L-Methionine Amide

  7. STABILIZATION OF SULFURRADICAL CATIONS VIA INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND FORMATION Gly-Met Met-Gly

  8. Characterizing Excited States • Excited states are not stationary states when consideration is made of the electromagnetic field. • Therefore, excited-state processes are of primary significance.

  9. What happens to the energy when matter absorbs sunlight or UV? • Gin and tonic glows under “black light” - fluorescence • Things heat up when sitting in the sun – radiationless transitions • Some objects, like TV screens, glow after use or after the light is turned off – phosphorescence • Objects appeared colored under visible light – differential absorption

  10. Excited-State Processes (Intramolecular) • Fluorescence (fast radiative process) • Phosphorescence (slow radiative process) • Radiationless Transitions • Internal Conversion (transition with no spin flip) • Intersystem Crossing (transition with spin flip) • Vibrational Relaxation (heat produced) • Photochemistry (bonds broken) • Photoionization (no bonds broken, e- ejected)

  11. ISC IC excited singlet state S1 S1 singlet ground state S0 T1 ISC fluorescence vibrational relaxation (heat) phosphorescence excited triplet state S0 T1 Photochemistry Jabłonski - diagram State Picture Orbital Picture

  12. Radiationless TransitionsShowing Nuclear Contributions

  13. “Stokes” shiftAbsorption vs Emission E = hc /   

  14. Lifetimes & Quantum Yields • Triplet states have much longer lifetimes than singlet states • In solutions, singlets live on the order of nanoseconds or 10’s of nanoseconds • Triplets in solution live on the order of 10’ or 100’s of microseconds • Triplets rarely phosphoresce in solution (competitive kinetics)

  15. Competitive Kinetics Intramolecular decay channels isc T p S0 Intermolecular decay channels T + Q  S0 + Q’

  16. Intermolecular Excited-State Reactions • Energy Transfer A* + B  A + B* • Electron Transfer D* + A  D+ + A D + A*  D+ + A • Hydrogen Abstractions Note: Have to have excited states that live long enough to find quenching partner by diffusion

  17. Transient Absorption Absorpcja przejściowa – diagram Jabłońskiego.

  18. Important Types of Organic Excited States • ,* states, particularly in aromatics and polyenes • n,* states, particular in carbonyls S2 1,* isc S1 T2 1n,* 3,* T1 3n,* Example: Lowest electronic states of Benzophenone S0

  19. Weak Spin Interactions • Triplet states, spin interactions are weak so excited triplet states can live for some time • But they have lower energy than their corresponding singlet states with the same orbital configurations • By Pauli Exclusion principle - no two electrons in the same system can have the same quantum numbers • By Pauli Exclusion principle like spins avoid each other – correlation hole around each electron

  20. Photochemistry of Triplet States • 3n,* states are particularly good in H-abstractions: they act like free radicals • Molecules in excited states are generally more reactive in electron-transfer reactions than are their ground states

  21. Excited-State Electron Transfer • Because of the “hole” in the HOMO of the ground state, the excited states have a low-lying orbital available for accepting electrons • Because of the electron in the highly excited orbital, e.g the LUMO of the ground state, the excited state is a good donor for elect. transfer LUMO e D HOMO e LUMO A HOMO

  22. Why Triplet Quantum Yield is high inBenzophenone? S2 1,* isc S1 T2 1n,* 3,* T1 3n,* Lowest electronic states of Benzophenone S0 • 1n,* states have small krad because of small orbital overlap • (2) kisc is large because of low-lying 3,* and El-Sayed’s Rule

  23. Selection Rules for ISC • El-Sayed’s Rule • Intersystem crossing between states of like orbital character is slower than ISC between states of different orbital character.

  24. Energy Gap Law • The rate of radiationless transitions goes as the exponential of the energy gap between the 0-0 vibronic levels of the two electronically excited states

  25. Rational of Energy Gap Law • Related to the probability of undergoing muliphononic events which gets more difficult as the number of phonons increases • Franck-Condon factors get smaller as the difference in nuclear excitations between electronically excited states increase

  26. Classical Franck-Condon Factor Demonstrated for absorption

  27. Quantum MechanicalFranck-Condon Factors Demonstrated for absorption

  28. Deuterium Effect • Radiationless transitions, e.g. intersystem crossing, slow down with deuterium substitution • Franck-Condon Factors again are involved

  29. Relevance to Kasha’s Rule • Tend to go to lowest levels in each multiplicity • Relatively small gaps between higher singlet states, so IC is fast • Within a particular electronic manifold, the vibrational relaxation is fast – here you can go one phonon at a time and still almost conserve energy within the molecule

  30. Robinson-Frosch FormulaRadiationless Transitions dP(nm)/dt = (42/h) |Vmn|2 FC (Em) Probability per unit time of making a radiationless transition from the initial electronic state n, to a set of vibronic levels in the electronic state m is equal to the product of three factors (1) The square of the electronic coupling element (2) The Franck-Condon factor between the vibrational levels of the initial electronic state and the final one (3) The density of final vibronic states

  31. Implications of Robinson-Frosch • Electronic matrix elements are usually a factor of a thousand less for transitions that change spin • Franck-Condon factors favor small changes in vibrational quantum numbers • Higher density of states favors faster transitions dP(nm)/dt = (42/h) |Vmn|2 FC (Em)

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