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Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

"Molecular Photochemistry - how to study mechanisms of photochemical reactions ? ". Bronis l aw Marciniak. Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland. 2012/2013 - lecture 7. 5. Examples illustrating the investigation of photoreaction mechanisms:

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Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

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  1. "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 7

  2. 5. Examples illustrating the investigation • of photoreaction mechanisms: • -   photochemistry of 1,3,5,-trithianes in solution

  3. TT (1,3,5-trithiane) TMT (2,4,6-trimethyl-1,3,5-trithiane) ISOMER b (cis-cis), ISOMER a (cis–trans) TPT (2,4,6-triphenyl-1,3,5-trithiane) ISOMER b (cis–cis), ISOMER a (cis–trans)

  4. Trithiane structures TT TMT TPT

  5. Isomers of the trithianes a-form (cis-trans) b-form (cis-cis) R = CH3, C6H5

  6. Ground-state absorptions of trithianes in MeCN

  7. 254 nm photolysis of TT in MeCN

  8. Stable products (GC, GCMS, HPLC, UV) For TT: primary product secondary product

  9. HPLC following 254 nm photolysisof TT in MeCN

  10. 254 nm photolysis of TT in MeCN

  11. 313 nm photolysis of TT in MeCNpreirradiated at 254 nm for 12 minutes

  12. 254 nm photolysis of -TMT in MeCN

  13. Stable products (GC, GCMS, HPLC, UV) For -TMT: -TMT primary product primary product secondary product

  14. HPLC following 254 nm photolysisof -TMT in MeCN

  15. Extrapolation of Fto zero time

  16. Quantum yields  TT -TMT -TMT -TPT -TPT Trithiane disappearance 0.54 0.38 0.43 0.19 0.48 Thioester formation 0.49 0.22 0.32 0.14 0.44 Isomer formation – 0.01 0.10  0.01  0.01 Thioester formation from laser flash photolysis 0.52 0.25 0.32 0.17 0.52 Steady-state photolysis at 254 nmLaser flash photolysis at 266 nm

  17. 266 nm laser flash of TT in MeCN

  18. 266 nm laser flash of -TMT in MeCN

  19. Mechanism forTrithiane = TT, a-TMT, or b-TMT

  20. 266 nm laser photolysis of b-TPT in MeCN

  21. Laser-intensity dependencea-TPT in MeCN

  22. Mechanism forTrithane = a-TPT or b-TPT

  23. Solvent effect

  24. Table: Quantum yieldsa of trithiane disappearance (fdis) anddithioester formation (fprod) in various solvents a All quantum yields were extrapolated to zero irradiation times; estimated error is equal to 10%. b Sum of 0.34 + 0.12 for RC(=S)SCH(R)SCH2R and RC(=S)SCH2R, respectively.

  25. Decay time (tdecay) of intermediate I, growth time (tgrowth) of the dithioesters absorbing at 310 nm, and rate constant (kII) of I with CH3OH a Previously measured [9]. b No growth observed. c Determined from a growth/decay fitting function d The decay lifetime of the shorter component of a biexponential decay. e Previously measured [6].

  26. 266 nm laser flash of b-TPT in MeOH 60-100 ns 600-700 ns 1.4-1.6 ms 6-8 ms 

  27. 266 nm laser flash of b-TPT in MeOH  = 1.3 ms

  28. Initial spectra of 266-nm photolysis of b-TPT in various solvents Open circles: CH3CN, filled circles: CH3OH, squares: cyclohexane

  29. Quenching of intermediate, I, by methanol, following 266-nm laser excitation of b-TPT in acetonitrile kqII= 7.8 × 103 M-1 s-1

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