Excited electronic states, hydrogen transfer

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

Figure 3.35 PE profiles for excited-state hydrogen transfer (full curve) and proton transfer (dashed curve) of 7HQ-(NH3)3. The calculated energies of the electronic ground state of the enol and keto forms are also indicated. The molecular orbitals contributing dominantly to the excited-state wavefunctions are shown for the minima along the hydrogen transfer and proton transfer paths. The energies have been calculated with the CIS method. (Reprinted from C. Manca, C. Tanner and S. Leutwyler, Int. Rev. Phys. Chern., 24, 457-488. Copyright (2005), with permission from Taylor Francis).

Excited electronic states have also been considered to explain NR dissociations of heterocyclic radicals in which low-energy losses of hydrogen atoms compete with high energy ring-cleavage dissociations. The latter reactions were interpreted as starting from excited electronic states of the radicals [27,28]. Formation of excited electronic states upon collisional electron transfer has also been studied with smaller molecular systems, e.g. CHn [45], 02 [46], and H3 [47,48],... 

In excited state hydrogen transfer occurring on sub-100 fs time scales, Raman-active low-frequency modes that couple strongly to the electronic Sq-Sj transition. 

Excited-state absorption (ESA) is a type of electronic spectroscopy for which only a few theoretical studies have been performed (for a recent study on the singlet-singlet ESA spectrum of azulene, see Ref. 130 and for some benchmark calculations on small molecules, see Ref. 131). From the experimental point of view ESA is a very important topic in photophysical (kinetic) investigations of energy, electron or hydrogen-transfer processes. Although in principle all excited states of a molecule show a distinct electronic absorption spectrum, only the ESA spectra of the lowest excited state in each multiplicity (i.e.. Si or Ti) are usually accessible experimentally. Because of the short... 

Ultrafast studies on tautomerism concentrate on compounds that can exhibit hydrogen transfer in the electronically excited state. Hydrogen transfer is a very typical reaction for the interconversion between different tautomeric forms. It converts enol to keto, amino to imino, imino to enamino, and lactim to lactam forms, to name some examples. For time-resolved experiments, excited-state intramolecular proton transfer (ESIPT) is particularly well suited since a short laser pulse in the visible or ultraviolet (UV) spectral region can trigger this process by promoting the molecule into the electronically excited state and initiating the transfer in this way [3]. The vast majority of experiments on tautomerism with ultrafast time resolution are therefore done on compounds exhibiting ESIPT. 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Ne is metastable neon produced by electron impact. Ne transfers its excitation to hydrogen molecules. The hydrogen molecules participating in these energy transfer collisions are produced in highly excited preionized states which ionize after a time lag sufficient to permit the initial neon and hydrogen collision partners to separate. The hydrogen ion is formed in the v = 5 or 6 quantum states and reacts with a second neon... 

Finally, we note the informative work of Garcia-Garibay and co-workers, who have extensively studied QMT in hydrogen transfer reactions in the excited triplet states of ort/zo-alkylarylketones, for example, 6 —> 7, which are electronically similar to radical rearrangements. 

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