Study 5.2. Mechanistic photochemistry cyclopentane-1,3-diyl biradicals

Case Study 5.2 Mechanistic photochemistry - cyclopentane-1,3-diyl biradicals 

To understand this trend, we need to look at the requirements permitting SOC through admixture of the lower zwitterionic (hole pair) state to the covalent singlet ground state wavefunction. Two conditions must be met 320 the molecule must bend 

Case Study 5.3 Mechanistic photochemistry - the photo-Favorskii reaction of p-hydroxyphenacyl compounds 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

The equilibrium constant for the hydration of 31 has been determined as pATR = —15.9,470 Hence the equilibrium constant for the heterolytic dissociation of 9-fluorenol in water is Kdiii = KR/KW = 1019 9, which amounts to a ground-state free energy difference of ArG° = 114 kJ mol  

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