Coefficients, geminal

Experiments on 1 -CO using benzene in place of CTAB were also done to examine the effects solvent and environment on the photodissociation. None were found. The photointermediates arrived at the same time, had the same peak wavelengths, extinction coefficients and band shape. In so far as the dynamics observed in these experiments are independent of CO pressure and since there is no detectable geminate CO recombination, it is reasonable to expect effects on the photodissociation due to solvation to be minimal as diffusion has not yet occurred on the time scale studied. 

In the group of nonfluorescing alkanes the lifetimes are very short, x < 0.3 nsec an upper limit of i < 3 x 10 sec is estimated. The absence of fluorescence for these compounds may have two causes the low R, i.e., low rate coefficient of the Si Sq radiative transition and the short lifetime, i.e., the very fast chemical decomposition. In the C5 and C7-C10 cycloalkanes the ring strain, which is mainly caused by the repulsive interaction of their unfavorably displaced H atoms, may enhance the rate coefficient of the chemical decay by C-H decompositions. In the excited sates of the geminally branched... 

If it is assumed, in a first approximation, that the activity coefficient ratios /ha/ /a- and /ArCHo//ArCH(OH)o do not differ very much, the difference between H and J for a given solution would be expressed by H- — J = —log flH2o-However a calculation of the difference by the latter formula, using the only available information (33) about the activity of water in water-DMSO mixtures, leads to values much smaller than those found in practice. This would indicate that the ratio /ha//a-> for the H indicator acids, is considerably larger than the ratio of /ArCHo//ArCH(OH)o- for the benzaldehydes used as J indicators. Probably this difference in activity coefficient ratios is caused by a larger extent of charge delocalization in the A anion compared to the geminal diol anion, which causes an extra stabilization of A- (manifested by a decrease in f ) by dispersion interaction with DMSO. 

Ionization of solute molecules by photolysis or high-energy radiolysis produces electrons which may recombine with their geminate radical (ion), react with an impurity (usually an oxidant) or be trapped in a solvent vacancy. The properties of the solvated electron resemble those of anions (e.g. a diffusion coefficient of 4.9 x 10-9 m2 s-1 in water). It is... 

K. They noted a decay over timescales 95 and < 35 ns, respectively, which was attributed to geminate ion-pair recombination (see Fig. 33). The decay of the optical absorption is independent of the dose of radiation received and continues for about lps. Rather than displaying a dependence on time as eqn. (153), i.e. at f 3/2, the experimental results are more nearly represented by either at f 1 decay to an optical density about one tenth of the maximum or by a decay as t 1/2 to zero absorption. These effects may be the recombination of ions within a spur (or cluster of ion-pairs), which is more nearly like a homogeneous reaction. The range of electrons in propane at 100 K is 10 nm [334] and the extrapolated diffusion coefficient is 10 11 m2 s 1 [320]. The timescale of recombination is 10 ps. The locally greater concentration of ions within a spur probably leads to a faster rate of reaction and is consistent with the time-scale of the reaction observed. Baxendale et al. [395] observed the decay of the infrared optical absorption of the solvated electron in methylcyclo-hexane at 160 K. They noted that the faster decay occurring over < 50 ns was independent of dose and depended on time as t 1/2, i.e. the reaction rate decays as t 3/2, see eqn. (153). It was attributed to recombination of... 

A reaction between solutes A and B in a solvent occurs at a rate k(t) [A] [B] when both reactants are distributed randomly throughout the solution. However, when A and B represent the result of bond fission (by photolysis or radiolysis), the distance to which geminate A and B pairs separate may be very small compared with the separation between pairs of A and B, unless very intense pulses of light or radiation were used. A very marked correlation in the distribution of A about B exists from the moment that recombination begins. This affects the subsequent rate of reaction and the probability that A and B will survive recombination. In Fig. 41, two initial distributions and their respective rate coefficients are shown. With the possible exception of some ESR techniques, such as 3-pulse electron spin echo, there are no methods for determining the initial distribution of reactant pairs. Indeed, as was mentioned in Chap. 6, Sect. 2 and Chap. 7, Sect. 2, the rate of reaction and survival probability of... 

Until the geminate pairs start to mix, i.e., at relatively short times r relative diffusion coefficient, the monomolecular kinetics reads n(t) = n(0)u>(t), with n(0) = nA(0) = ne(0) being initial particle concentration. The distinctive feature of this stage is the linearity of the recombination kinetics n(t) with respect to the irradiation dose n(0). 

Another effect of crystal anisotropy is that the diffusion coefficient D turns out to become the diffusion tensor D [73-75]. It results in the asymmetric escape probabilities of geminate pairs in different directions. 

Studying the ESPT of hydroxy aromatic sulfonates, Huppert and co-workers [40-44] suggested an alternative model based on the geminate proton-anion recombination, governed by diffusive motion. The analysis was carried out by using Debye-Smoluchowskii-type diffusion equations. Their ESPT studies in water-methanol mixtures showed that solvent effects in the dissociation rate coefficient are equal to the effects in the dissociation equilibrium constant [45], 4-Hydroxy-1-naphthalenesulphonate in a water-propanol mixture as the solvent system has been found to behave somewhat differently than water-methanol or water-ethanol media, with a possible role of a water dimer [46,47],... 

Fink made no attempt to convert these semi-quantitative results to absolute numbers on the hydroxyl content. If one assumes that the hydroxyl content on a silica, pretreated at 1073 K is 0.7 OH/nm2 (cfr. chapter 4), that the adsorption coefficient of the free and geminal hydroxyls does not differ significantly, and if one neglects the 3730 cm 1 and 3718 cm 1 contributions to the total signal, absolute numbers can be derived (figure 5.5). 

The one-center energy components have no clear correspondence in the standard MM setting. In our approach the one-center contributions E- arise due to deviations of the geminal amplitude related ES Vs (7>P and 41 ) from their transferable values. These deviations interfere with hybridization. The derivatives of E f s with respect to the angles Land uji, taken at the values characteristic for the stable hybridization tetrahedra shapes which appear in the FATO model, yield quasi- and pseudotorques acting upon the hybridization tetrahedron. In evaluating these quantities we notice that all the hybridization dependence which appears in the one-center terms is that of the matrix elements of eq. (2.71). In the latter, the only source of the hybridization dependence is that of the second and fourth powers of the coefficients of the s-orbital in the HOs. Since they do not depend on the orientation of the hybridization tetrahedra, we immediately arrive at the conclusion that no quasitorques caused by the variation of electron densities appear in the TATO setting ... 

Tanaka et al. studied the decay reactions of PVB radical anions produced by electron pulses in MTHF [47]. At low concentration ( < 0.05 base-mol dm - 3) of polymers the decay reaction followed a simple second-order kinetics. The charge neutralization reaction is responsible for the decay curve as is the case of biphenyl radical anions. However, the rate constant of the polymer anions was only a half or one-third of that of the biphenyl anion, because of the small diffusion coefficient of the polymer ion in solution. At high concentration of the polymer, a spike was observed in the time-profile of the PVB anion this was attributed to the retarded geminate recombinations within micro-domains where the polymers were entangled with each other. 

Femtosecond flash photolysis studies on Q-state CdS [107] indicate that reaction (4a) proceeds via two recombination processes a 50 ps decay at low excitation intensities, postulated to correspond to geminate e h+ recombination, and a faster 2 ps decay at higher flash fluences, corresponding to non-geminate or possibly three body Auger charge carrier recombination. Other studies by Nosaka and Fox [118] indicate that the second order rate coefficient for electron-hole recombination within CdS particles is of the order 9 x 10 t7 m3 s l. 

Cage effect dynamics or kinetics of geminate recombination was observed for the first time under photodissociation of aC C dimer of aromatic radicals in a viscous media. A suggestion has been made that at least in a number of smdied cases the mumal diffusion coefficient of radicals in the pair is approximately 10 times lower than the sum of macroscopic diffusion coefficients of the individual species. In other words, a geminate recombination proceeds considerably longer than expected. 

Table 6-2. Solvent effect on the geminate singlet and triplet recombination yields ( (t)s and (t>r) and the yield of the ion radicals ((()i) for Ks = 6.176 A/ns and Kt = 18.53 A/ns. Relative diffusion coefficients defined by D = D(Py) + D(DMA) and dielectric constants (e) for various solvents are employed by the values at 25 °C. (Reproduced from Ref. [27] by permission from The American Institute of Physics)...

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