Spontaneous deactivation

The other interpretation is based on the idea of electron tunneling from excited molecules. In this instance there occurs an overlap of the exponential decay at a rate constant k0 which refers to spontaneous deactivation of the Nh molecules from the excited to the ground state and of the logarithmic kinetics characteristic of electron tunneling reactions (cf. Chap. 4, Sect. 2)... 

As seen from Fig. 10, the non-exponential drop in the intensity of fluorescence for Nh-inverse kinetic isotope effect is observed as distinct from the process of spontaneous deactivation, for which a normal isotopic effect (td > ih) is observed. Note that the possibility of the abnormal isotopic effect for electron tunneling reactions follows directly from the theoretical concepts set out in Chap. 3, Sect. 6. The mean values of the parameter / obtained from experiments with various concentrations of CC14 proved to be p = (0.240 0.010) M 1 for Nh d8 and j = (0.205 0.010) M l for Nh. As the effect of the nuclear motion on W R) must be reflected more in the value of ve than in that of ae it seems natural to connect the difference observed between the values of P for Nh and those for Nh-d8 with the change in the parameter ve. At the value of ae 1 A typical of tunneling reactions, the difference observed in the values of P corresponds to an approximately 2.5-fold increase of ve upon naphthalene deuteration. With an increase in temperature from 77 to 140 K, the parameter / remained virtually unchanged, although the time, t, for spontaneous deactivation was markedly reduced. Thus, tunneling reaction (14) proceeds via a non-activated mechanism. 

In expression (4.64) can be included some spontaneous deactivation of the reactants. Then one obtains... 

The non-exponential decay of the triplet excited states of the photosensitizers is observed for Chi-, Phe- and ZnTPP-containing vesicles [132-135], The reason for the non-exponentiality may be, first, a statistical distribution of the concentrations of porphyrin molecules in the membrane, and, second, a simultaneous decay of the triplet excited states via several parallel channels such as spontaneous deactivation, concentration quenching and triplet-triplet annihilation which are known to be characteristic of porphyrins in organic solvents [129]. For ZnTPP and Phe in vesicles, the process of triplet-triplet annihilation is indeed observed [56, 134], while according to [132] this process is surprisingly absent for Chi. 

In these expressions only some of the active products (marked ) have been included. Radiation is associated with the spontaneous deactivation of an active product according to (2) below, if it does not happen to activate a molecule of oxygen according to (1) —... 

Processes that are kinetically of first order with respect to the concentration of radicals within the reaction locus. These processes include exit from the locus into the external phase, termination by reaction with monomer within the locus, termination by reaction with adventitious impurities in the locus, and spontaneous deactivation. 

Spontaneous deactivation to give a stable molecule will also produce a hydride derivative of the catalyst, XVII, viz. 

The tendency for energy transfer between a pair of molecules in resists is characterized by a critical distance Rq at which the rate of energy transfer and the rate of spontaneous deactivation of the donor are equal. This leads to a con-... 

Here, ko is the unimolecular decay rate constant of the excited donor, and Rq is the critical distance between D and A, at which the probabilities of spontaneous deactivation and of energy transfer are equal. Typical Rq values are listed in Table 1.3, which also includes values for self-transfer [14]. The latter process is of relevance for down-chain energy transfer (energy migration), which is referred to below. 

A spontaneous deactivation can also occur when a growing macroion and a low-molar-mass gegenion isomerize into an inactive compound (see Chapter 18). 

This relationship is interesting historically, since it shows how a normal decomposition reaction involving collision-type activation can transform from a unimolecular to a bimolecular type of mechanism. For example, if collisional deactivation of activated reactants is much more rapid than spontaneous deactivation (fcan 2), then -dnjdt = kik2/k3)n, i.e., the reactor is unimolecular. However, if the opposite is true, and if (1 - a) is not excessively... 

The system Co-imidazole-aminoacid is active for many cycles (more than 100) and is one of the most effective carriers of both gases. Moreover it is characterized by a favourable temperature range (0-50°C) which determines the existence of the free and oxygenated species at 1 atmosphere of oxygen pressure, as indicated by the values of the equilibrium constants (14). Oxygenated species of these complexes are also relatively resistant (especially in the case of proline and sarcosine) to the spontaneous deactivating intramolecular oxidation processes. 

In this equation is the fluorescence intensity of the donor emission and is the extinction coefficient of the acceptor at the wavenumber v. The distance between the donor and acceptor species at which excitation transfer and spontaneous deactivation of the donor are of equal probability is known as the critical transfer distance, Rq. An expression for Rq was also derived by Forster [5,6] and is given in equation (3). 

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