Excited state rates

Note that in all of these situations, one can begin to unravel the excited state rate constants if measurements of i° and of i can be made. There are several ways in which this can be done. The most common one is from the decay of the emission from A, following pulse excitation. Alternatively, it has been possible in several cases to observe the UV-visible absorption spectrum of A, and the decay of this absorption again gives x° or t (see, for examples, refs. 22-26). In rare cases, it has been possible to observe the rate at which the primary photoproduct P grows in, again following a pulse excitation it should do this with the lifetime of A. 27... 

The kinetic isotope effects shown in Fig. 11 (Forster, 1972) resemble those reported for 2-naphthol by Stryer (1966). Like 2-naphthylamine, 2-naphthol shows an increased quantum yield and protonation of Sj occurs at lower acidities in D20 than in H20. For 2-naphthol, p-KJSj )-values of 3 0 in H20 and 3-4 in D20 are calculated from the measured excited state rate constants in H20 k.j = 5-29 x 107 s 1 and k2 = 5-5 x 101 0 dm3 mole-1 s-1, while in D20 k1 = 1 3 x 107 s-1 and k2 = 3-5 x 1010 dm3 mole-1 s-1. These results confirmed the earlier p/ (S )-values calculated by Wehry and Rogers (1966) using the Forster cycle (Table 9), which show incidentally that the pK-values are closer by about 0 1 unit in the Sj state. 

Such is not found experimentally. If a retarding effect is to be quantitated it will likely come from a direct measurement of the excited state rate constants. Table II summarizes the key results to date. 

Other Photofragmentations - Photodissociation of tert-huty hydroperoxide at 266 nm gives OH radicals with dynamics which are similar to those found for OH from H2O2, and which are consistent with dissociation via a repulsive excited state. Rates of p-scission of the ter/-butoxy radical to acetone and methyl radicals have been determined in flash-photolysis experiments by monitoring its transient UV absorption and its laser-induced fluorescence. ... 

Bimolecular excited state rate constants for water quenching of several 9-alkylxanthyl cations and the parent xanthyl cation were determined by Stem-... 

Several singlet-excited cations have been shown to imdergo photoinduced electron transfer from aromatic donors. Fluorescence from the 9-phenylxanthyl, xanthyl, thioxanthyl, and 9-phenylthioxanthyl cations is quenched in the presence of aromatic compounds [9,10,15], Steady-state fluorescence quenching experiments gave bimolecular excited state rate constants for quenching of the cations by the aromatic donors (Table 7). The quenching rate constants increase... 

If the quantum efficiency is not unity then reaction can proceed from primary excited state. Rate of such reactions is determined by rates of radiationless processes and quantum yield of the formation of reactive excited state. 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Mavri, J., Berendsen, H.J.C. Calculation of the proton transfer rate using density matrix evolution and molecular dynamics simulations Inclusion of the proton excited states. J. Phys. Chem. 99 (1995) 12711-12717. 

Several VTST techniques exist. Canonical variational theory (CVT), improved canonical variational theory (ICVT), and microcanonical variational theory (pVT) are the most frequently used. The microcanonical theory tends to be the most accurate, and canonical theory the least accurate. All these techniques tend to lose accuracy at higher temperatures. At higher temperatures, excited states, which are more difficult to compute accurately, play an increasingly important role, as do trajectories far from the transition structure. For very small molecules, errors at room temperature are often less than 10%. At high temperatures, computed reaction rates could be in error by an order of magnitude. 

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