Excited-state intramolecular H-atom transfer

I. Introduction Excited-State Intramolecular H-Atom Transfer in Hypericin-Like Perylene Quinones... 

I. INTRODUCTION EXCITED-STATE INTRAMOLECULAR H-ATOM TRANSFER IN HYPERICINLIKE PERYLENE QUINONES... 

The major finding of this study is that a vibrational mode corresponding to H-atom translocation can been identified in hypericin by the joint contributions of synthetic, computational, and spectroscopic methods. Identification of this mode is only a first step in providing a direct demonstration of excited-state intramolecular H-atom transfer in hypericin and its analogs. There is considerable work to be accomplished. As indicated elsewhere [77], ab initio calculations predict that the normal modes in this region of the spectrum are close for the normal and the monotautomeric forms. The direct observation of the formation of the tautomer will require both adequate temporal and spectral resolution. 

It is proposed that 32 reacts from its nn excited state by the nitro-to-nitrite (33) inversion followed by nitrite homolysis, when the naphthoxy radical must diffuse away from the cages to obtain the dimerization intermediate 35. However, the source of oxidizing agents is not identified. In comparison, o-nitro-ferf-butylbenzenes 37 are excited to undergo intramolecular H-atom transfer and cyclization to give indol-IV-oxides 40 (equation 34)38. The discrepancy may arise from the nature of the excited state, e.g. that of 37 may react from its njr state. 

The interpretation of the experimental data for the kinetics of photoacid-solvent clusters is complicated by the substantial fragmentation of the clusters after the excited-state reaction. The heat of reaction is often sufficient to allow the evaporation of one or several solvent molecules [14,16]. This difficulty does not arise when the H atom transfer or proton transfer occurs intramolecularly along a solvent wire attached to a bifunctional chromophore. 

It must be remembered, furthermore, that the identification of the H-atom translocation mode is not equivalent to the identification of the reaction coordinate. We have attributed the absence of a deuterium isotope effect on the excited-state H-atom transfer (for the 10-ps component in hypericin and hypo-crellin A) to the zero-point energy in the proton coordinate lying above the barrier, with the H-atom being effectively delocalized between the two oxygen atoms. Consequently, the reaction coordinate for the excited-state H-atom transfer cannot be identified with the proton coordinate, and it must be concluded that other intramolecular motions are in fact responsible for the process. Temperature-dependent measurements indicate that these motions are extremely low amplitude, Ea 0.05 kcal/mol for hypericin [37]. Because the nature of this motion is not yet identified, we refer to it as the skeleton coordinate [48, 71, 82]. We propose that it is the time scale for this latter conformational change... 

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