1 / 25

Methods Sensitive to Free Radical Structure

Methods Sensitive to Free Radical Structure. Resonance Raman Electron-Spin Resonance (ESR) or Electron Paramagnetic Resonance (EPR). Motivation. Absorption spectra of free radical and excited states are generally broad and featureless Conductivity is not species specific

farrah
Download Presentation

Methods Sensitive to Free Radical Structure

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Methods Sensitive toFree Radical Structure • Resonance Raman • Electron-Spin Resonance (ESR) or Electron Paramagnetic Resonance (EPR)

  2. Motivation • Absorption spectra of free radical and excited states are generally broad and featureless • Conductivity is not species specific • Conductivity is additive with respect to ionic content of the cell

  3. Specific Vibrations? • Now have vibrational spectroscopy in laser flash photolysis, usually in organic solvents • Water is a good filter of infrared and masks vibrational features of free radicals • Raman is weak, second-order effect • What about Resonance Enhanced Raman?

  4. LIGHT SCATTERING Medium Ei + - Incident light Emergent light 0 sScattered light Rayleigh s = 0 s = 0 mn Raman Pi = αij Ej P = Induced electric dipole moment E = Electric field of the electromagnetic radiation αij= Elements of polarizability tensor G.N.R. Tripathi

  5. ENHANCEMENT OF RAMAN SCATTERING e em 0 n mn m Imn = Const. I0 (0  mn)4 I(  )mn I2  (  ) mn = (1/h)  MmeMen/ (em 0 + i e) e + non-resonant terms (via αij) G.N.R. Tripathi Probability Amplitude

  6. RESONANCE RAMAN em >> 0 Normal Raman em - 0 ~ 0 Resonance Raman |(  ) mn 2 = Const. × (MmeMen)2 / 2 Enhancement up to 107-108 Pulse radiolysis time-resolved resonance Raman Identification, structure, reactivity and reaction mechanism of short-lived radicals and excited electronic states in condensed media Relevance: Theoretical chemistry, chemical dynamics, biochemistry, ,paper and pulp-industry, etc. G.N.R. Tripathi

  7. 2,2’- bipyridyl dppz = dipyridophenazine [Ru(bpy)2dppz]2+ bound to DNA NO DNA present DNA present 1000 1200 1300 1400 Raman shift (cm-1) http://www.lot-oriel.com/site/site_down/cc_appexraman_deen.pdf

  8. Two-slit experiment

  9. Selection Rulesfor the Amplitudes of Transitions Franck-Condon Factor Electronic Transition Elements (Dipole allowed) For Resonance enhancement both must be non-zero http://www.personal.dundee.ac.uk/~tjdines/Raman/RR3.HTM

  10. Relationship to Radiationless Transitions and Absorption dP(nm)/dt = (42/h) |Vmn|2 FC (Em) This is a probability. Quantum mechanics usually calculates amplitudes which are “roughly the square root” (being careful about complex numbers) Taking the square root, shows that the amplitudes for radiationless transitions are first-order in the interaction V Likewise, simple absorption and spontaneous emission are first-order processes with regard to an interaction Vrad

  11. Connection to Wavefunctions So we can use the path integral to see how one non-stationary state (f) at time ta propagates into another  at time tb In terms of the stationary states of the system

  12. Expansion of part of exponential for small potentials Putting this back into the Amplitude Kv(b,a) gives a perturbation expansion of the path integral

  13. Interpretation of First Term Represents propagation of a free particle from (xa,ta) to (xb,tb) with no scattering by the potential b V a

  14. Second Term

  15. Interpretation of Second Term b tb Particle moves from a to c as a free particle. At c it is scattered by V[x(s),s] = Vc. After it moves as a free particle to b. The amplitude is then integrated over xc, namely over all paths. t tc c ta a x

  16. Physical Meaning of 2nd Term Represents propagation of a particle from (xa,ta) to (xb,tb) that may be scattered once by the potential at (xc,tc) b V c a

  17. Interpretation of Third Term Represents propagation of a particle from (xa,ta) to (xb,tb) that may be scattered twice by the potential, once at (x(s),s) and once at (x(s),s) b V a

  18. Selection Rules (A-term) A-term:  Condon approximation - the transition polarizability is controlled by                the pure electronic transition moment and vibrational overlap                integrals The A-term is non-zero if two conditions are fulfilled: (i)    The transition dipole moments [mr]ge0 and [ms]eg0 are both non-zero. (ii)   The products of the vibrational overlap integrals, i.e. Franck-Condon         factors, <ng|e><e|mg> are non-zero for at least some values of         the excited state vibrational quantum number .

  19. Consideration of Franck-Condon Factors <ne|g> = 0 orthogonal <ne|g>  0 Non-symmetric Or Symmetric <ne|g>  0 <ne|g>  0 Totally Symmetric Vibrational Mode Totally Symmetric Vibrational Mode http://www.personal.dundee.ac.uk/~tjdines/Raman/RR4.HTM

  20. Why must these modes totally symmetric vibrations? He(Q) = Qg + DQ + (k/2)Q2 Hg(Q) = Qg + (k/2)Q2 All terms in the Hamiltonian must be totally symmetric, Therefore, the displacement DQ must also be totally symmetric

  21. G.N.R. Tripathi

  22. G.N.R. Tripathi

  23. G.N.R. Tripathi

More Related